Monday, May 7, 2012



The Digestive System and Nutrition
The Digestive System Brings Nutrients into the Body

    • The walls of the GI tract are composed of four layers
    • Five basic processes accomplish digestive system function
    • Two types of motility aid digestive processes
The Mouth Processes Food for Swallowing

    • Teeth bite and chew food
    • The tongue positions and tastes food
    • Saliva begins the process of digestion

The Pharynx and Esophagus Deliver Food to the Stomach
The Stomach Stores Food, Digests Protein, and Regulates Delivery

    • Gastric juice breaks down proteins
    • Stomach contractions mix food and push it forward
The Small Intestine Digests Food and Absorbs Nutrients and Water

Accessory Organs Aid Digestions and Absorption
    • The pancreas secretes enzymes and NaHCO3
    • The liver produces bile and performs many other functions
    • The gallbladder stores bile until needed           

The Large Intestine Absorbs Nutrients and Eliminates Wastes

How Nutrients are Absorbed

    • Proteins and carbohydrates are absorbed by active transport
    • Lipids are broken down, then reassembled
    • Water is absorbed by osmosis
    • Vitamins and minerals follow a variety of paths

Endocrine and Nervous Systems Regulate Digestion

    • Regulation depends on volume and content of food
    • Nutrients are used or stored until needed

Nutrition: You Are What You Eat
    • MyPyramid plan offers a personalized approach
    • Carbohydrates: a major energy source
    • Lipids: essential cell components and energy sources
    • Complete proteins contain every amino acid
    • Vitamins are essential for normal function
    • Minerals: elements essential for body processes
    • Fiber benefits the colon

Weight Control: Energy Consumed Versus Energy Spent

    • BMR: determining how many Calories we need
    • Energy balance and body weight
    • Physical activity: an efficient way to use Calories
    • Healthy weight improves overall health

Disorders of the Digestive System

    • Disorders of the GI tract
    • Disorders of the accessory organs
    • Malnutrition: too many or too few nutrients
    • Obesity: a worldwide epidemic?

Eating Disorders: Anorexia Nervosa and Bulimia

The Nervous System: Integration and Control
The Nervous System has Two Principal Parts

Neurons are the Communication Cells of the Nervous System

Neurons Initiate Action Potentials

    • Sodium-potassium pump maintains resting potential
    • Graded potentials alter the resting potential
    • An action potential is a sudden reversal of membrane voltage
    • Action potentials are all-or-none and self-propagating

Neuroglial Cells Support and Protect Neurons

Information is Transferred from a Neuron to its Target

    • Neurotransmitter is released
    • Neurotransmitters exert excitatory or inhibitory effects
    • Postsynaptic neurons integrate and process information

 Sensory Mechanisms
Receptors Receive and Convert Stimuli

    • Receptors are classified according to stimulus
    • The CNS interprets nerve impulses based on origin and frequency
    • Some receptors adapt to continuing stimuli
    • Somatic sensations and special senses provide sensory information
Somatic Sensations Arise from Receptors Throughout the Body

    • Mechanoreceptors detect touch, pressure, and vibration
    • Mechanoreceptors indicate limb position, muscle length, and tension
    • Thermoreceptors detect temperature
    • Pain receptors signal discomfort

Vision: Detecting and Interpreting Visual Stimuli

    • Structure of the eye
    • Regulating the amount of light and focusing the image
    • Eyeball shape affects focus
    • Light is converted into action potentials
    • Rods and cones respond to light
    • Rods provide vision in dim light
    • Cones provide color vision and accurate images
    • Visual receptors adapt

Disorders of Sensory Mechanisms

    • Disorders of the ears
    • Disorders of the eyes

Human Impacts, Biodiversity, and Environmental Issues
Pollutants Impair Air Quality

    • Excessive greenhouse gases lead to global warming
    • CFCs deplete the ozone layer
    • Pollutants produce acid precipitation
    • Smog blankets industrial areas

Pollution Jeopardizes Scarce Water Supplies

    • Water is scarce and unequally distributed
    • Urbanization increases storm water runoff
    • Human activities pollute freshwater
    • Groundwater pollution may impair human health
    • Oil pollution damages oceans and shorelines

Pollution and Overuse Damage the Land

·      Summary 5/7/12
The Digestive System and Nutrition
The Digestive System Brings Nutrients into the Body
The digestive system is made of a series of hollow organs that extend from the mouth to the anus: the mouth, pharynx, esophagus, stomach small intestine, large intestine, rectum, and anus. They form a hollow tube that we call the gastrointestinal tract. The space within this tube is the lumen. This system also carries four accessory organs – the salivary glands, liver, gallbladder, and pancreas. Each one of these organs share the function of getting nutrients into the body.

The walls of the GI tract are composed of four layers
There are four layers that form the walls of the GI tract from the esophagus to the anus:

  • The mucosa – innermost tissue made of mucous membrane (all nutrients must cross this to enter the bloodstream).

  • The submucosa – next to the mucosa layer and made of connective tissue, which contain blood vessels, lymph vessels, and nerves.

  • The muscularis – the third layer, which consists of two to three layers of smooth tissue is responsible for motility (movement).

  • The serosa – the outermost layer consisting of a thin connective tissue sheath. It surrounds and protects the other layers and attaches the digestive system to the body cavity walls.

The organs of the digestive tract are separated by sphincters, or thick rings of smooth muscle. When these contract, they close off the passageway between organs.

Five basic processes accomplish digestive system function
Click Here to View A Digestive Process Video 

The process of taking food apart so that the nutrients in the food can be absorbed into the body is done by these five basic tasks:

  1. Mechanical processing and movement -  (This is obviously chewing, which breaks food into smaller pieces and propels it forward.)
  2. Secretion – Along the digestive tract at various places, fluid, digestive enzymes, acid, alkali, bile and mucus are all secreted. We also have a few hormones that are secreted into the blood that regulate digestion.
  3. Digestion – the contents in the lumen break down into smaller and smaller particles until you have nutrient molecules.
  4. Absorption – nutrients pass over from the mucosal layer and into the blood or lymph.
  5. Elimination – anything that is not digested is eliminated via the anus.

Two types of motility aid digestive processes
There are two types of motion within the GI tract and they function very differently. Peristalsis propels food forward. It starts when a lump of food (bolus) stretches part of the GI tract, which causes the smooth muscle in front to relax and the muscle behind to contract. This motion pushes the food forward.

Segmentation, on the other hand, mixes the food. There are sections of smooth muscle that randomly contract and relax, mixing up the contents. This eventually causes absorption and primarily takes place in the small intestine. 5/7/12
The Mouth Processes Food for Swallowing
Teeth bite and chew food
Our mouth basically functions as a food processor. This is where it all starts. We have four types of teeth and each has a special function. Incisors cut the food, canines tear it, premolars and molars grind and crush.

The tongue positions and tastes food
The tongue is skeletal muscle and is enclosed by mucous membrane. It makes chewing more efficient, positioning the food over the teeth and mashing it against the roof of the mouth. It also contributes to our sense of taste and speech.

Saliva begins the process of digestion
Our saliva glands produce saliva, making it easier to chew and swallow. Saliva contains four main ingredients: mucin (protein) – holds the food together so we can swallow easier, salivary amylase (enzyme) – begins digesting carbohydrates, bicarbonate (HCO3) – maintains pH, and lysozyme (enzyme) – inhibits bacterial growth.

The Pharynx and Esophagus Deliver Food to the Stomach
Our tongue pushes food into the pharynx for swallowing, which causes a temporary halt in our breathing. It starts with a voluntary movement, but once in the pharynx, becomes an involuntary movement - the swallowing reflex. The esophagus is just beyond the pharynx, and connects to the stomach. The lining in the esophagus secretes a mucus that helps slide the food down. Once it hits the bottom of the esophagus, the sphincter opens very briefly and passes the contents into the stomach. It also prevents back-flow from the  stomach. (It is the malfunction of the sphincter that causes acid reflux).

The Stomach Stores Food, Digests Protein, and Regulates Delivery
The stomach is a muscular sac that expands and has three functions. It stores food, digests food and regulates delivery.

Gastric juice breaks down proteins
Gastric juice is a combination of hydrochloric acid, pepsinogen, which becomes pepsin (a protein digesting enzyme) once exposed to stomach acid, and other fluids. It has an acidic pH of 2, allowing the dissolving of connective tissue in food and the digestion of proteins and peptides into amino acids. This is then delivered into the small intestine and at this point is called chyme. The pyloric sphincter between the stomach and small intestine regulates the rate of transport.

accessed 5/7/12
Stomach contractions mix food and push it forward
After a meal, your stomach contractions stop and the stomach stretches to accommodate the food. This signals peristalsis to increase, which causes a mixing motion. With each contraction, approximately a tablespoon of chyme enters the small intestine. When you hear your stomach gurgling, that is peristalsis. It takes two – six hours for your stomach to empty completely. And when there is high fat or acid content in your stomach, the chyme triggers the release of hormones that slow down peristalsis so the small intestine have more time to absorb the nutrients.

The Small Intestine Digests Food and Absorbs Nutrients and Water
The two major functions of the small intestine are digestion and absorption. Protein digestion continues and carbohydrate and lipid digestion is added to the mix. This is where the acidic gastric juice begins to neutralize, when the pancreas and intestines add enzymes. 5/7/12
During absorption, all is broken down to a single amino acid, monosaccharide, fatty acid, or glycerol.  These are now small enough to transport across the mucosal cells and into the blood. Almost 90% of nutrients and water are absorbed here.

There are three regions of the small intestine. The duodenum is the first section. Almost 10 inches long, this is where most of the digestion occurs. Then the product of digestion is absorbed in the other two sections – the jejunum and ileum, which are approximately 10 feet long, combined.

It is the structure of the small intestine that make it suited for absorption. There are large folds covered in villi (microscopic projections), and these have even smaller projections, or microvilli. Between the folds, villi, and microvilli, the surface area of the small intestine can increase up to 500 times. This increases its ability to absorb.

Accessory Organs Aid Digestion and Absorption
Since we have already covered the salivary glands, now we need to look at the other three organs that play a role in digestion and absorption.

The pancreas secretes enzymes and NaHCO3
The pancreas has both endocrine and exocrine functions. It secretes hormones that regulate blood glucose. But it also secretes digestive enzymes and sodium bicarbonate. 5/7/12
The enzymes include proteases (trypsin, chymotrypsin, and carboxypeptidase), which digest proteins; pancreatic amylase, which continues digestion of carbohydrates; and lipase, which digests lipids.

Sodium bicarbonate works to neutralize stomach acid.

The liver produces bile and performs many other functions
The liver is located in the upper right abdominal cavity and has many functions. It’s primary digestive function is producing bile, Bile is a watery mixture of electrolytes, cholesterol, bile salts, lecithin, and pigments (mostly bilirubin).

One of the most important GI tract features is the hepatic portal system. This system carries blood from one capillary bed to another. It takes nutrient-rich blood directly from the digestive organs to the liver via the hepatic portal vein. The blood is then returned to general circulation. The location of the liver is ideal for processing and storing nutrients.

The following is a list of the other functions of the liver.
  • Stores fat-soluble vitamins (A,D,E, and K) and iron.
  • Stores glucose as glycogen after a meal, and converts glycogen to glucose between meals.
  • Manufacturing plasma proteins like albumin and fibrinogen from amino acids.
  • Synthesizing and storing lipids.
  • Inactivating many chemicals, including alcohol, hormones, drugs, and poisons.
  • Converting ammonia (a toxic waste of metabolism) into urea.
  • Destroying worn-out red blood cells. 5/7/12
The gallbladder stores bile until needed
Bile leaves the liver via ducts to the gallbladder, where water is removed. The concentrated bile is stored here until after a meal, when it is released into the small intestine through the bile duct, which joins the pancreatic duct.

The Large Intestine Absorbs Nutrients and Eliminates Wastes 5/7/12
Most of the water and nutrients have been absorbed by the time it reaches the large intestine. Whatever does remain is absorbed by the large intestine and now you have nearly solid waste. This intestine is larger in diameter, but only half as long as the small intestine. It begins with a pouch, called the cecum, where chyme is received from the small intestine. There is a small fingerlike pouch extending from the cecum, the appendix, which has no known digestive function. There are four regions of the large intestine, or colon – the ascending colon on the right side of the body, the transverse colon that crosses over to the left, the descending colon passing down the left side, and the end, or sigmoid colon. The sigmoid colon stores feces until defecation when they pass through the rectum to the anus.

How absorption occurs depends on the type of nutrient.

  • Proteins and carbohydrates are absorbed by active transport – once enzymes have broken down proteins into amino acids, they are actively transported into mucosal cells. Then they move via diffusion into the capillaries. What remains of carbohydrates become monosaccharides, which are transported the same way only by different active transport proteins.
  • Lipids are broken down, then reassembled – fatty acids and monoglycerides are the products of lipid digestion. They dissolve quickly in micelles (small droplets made of bile salts and lecithin). The micelles transport to the outer surface of the mucosal cells, where they are absorbed. Once the fatty acids and monoglycerides are in, they recombine and become triglycerides.
  • Water is absorbed by osmosis – when nutrients are absorbed in the small intestine or when you drink large amounts of water, the concentration of water in the lumen becomes higher than in the intestinal cells or in the blood. This causes the diffusion of water through the epithelial layer of cells of the small intestine and into the blood. The capacity of water absorption by the small intestine is almost limitless.
  • Vitamins and minerals follow a variety of paths – Fat soluble vitamins are dissolved in micelles and absorbed by diffusion across the lipid membrane. Water-soluble vitamins are absorbed by active transport or diffusion through channels or pores. Minerals are also absorbed via active transport or diffusion also, via transport proteins, pores, or channels.

Endocrine and Nervous Systems Regulate Digestion
Whereas most regulatory mechanisms are operating to maintain a constant internal environment or homeostasis, the regulation of the digestive system actually promotes rapid, efficient digestion and absorption, regardless of homeostasis. The digestive process actually alters the internal environment for a short time because absorption of nutrients entering the blood takes only a few hours.

Regulation depends on volume and content of food
Stretching of the stomach and proteins stimulate the stomach to release the hormone, gastrin, which triggers more gastric juice. Then when chyme arrives at the small intestine, the stretching of the duodenum increases segmentation to mix the chyme. The duodenum also secretes secretin and cholecystokinin (CCK) hormones. Acid triggers secretin and fat and protein trigger CCK.

Nutrients are used or stored until needed
Regulation of organic metabolism involves almost every organ in the body working together. Although the main two key players are the pancreas and the liver. What the body does with molecules, whether actively using them or storing them, depends on what is in short supply or excess at any given moment. When we consume more energy containing nutrients than what we use, our bodies store them for the future. This can cause weight gain over time. When we consume fewer energy-containing nutrients, our bodies draw on the stored nutrients. This can cause loss of weight over time.

Nutrition: You Are What You Eat
Because most nutrients enter the body via the digestive system, the phrase “you are what you eat” is very accurate. 5/7/12
MyPyramid plan offers a personalized approach
The My Pyramid plan is a website developed by the Center for Nutrition Policy and Promotions at the USDA. It is a fairly comprehensive, personalized approach that includes physical activity as well as nutrition. You can enter your age, gender, and activity level on the website, and this system will match you to the best plan for your needs. It is a good place to start for basic information on a healthy diet. In general, that will include: eating a variety of foods, maintaining a healthy weight, eating plenty of fresh fruits, vegetables, and whole-grains, keeping cholesterol and saturated fats at a minimum, using sugar, salt, and sodium in moderation, and drinking alcohol in moderation.

Carbohydrates: A major energy source – Many nutritionists recommend 45-65% of calorie intake come from carbohydrates. It is the body’s main source of energy. There are two types of carbs – simple and complex. Complex carbs are the more desirable because they release sugars more slowly and contribute fiber, vitamins, and minerals.  Simple carbs are sugars found in natural foods such as fruit and honey. Refined sugars, such as corn syrup and granulated sugar, have had most of their nutrients removed, and are far less nutritious. The commercials for “corn” sugar, are at best, misleading. “Sugar” is not “sugar”, as they claim.

Lipids: Essential cell components and energy sources – Lipids, including fats, are components of every living cell. Fat stores energy, cushions organs, insulates the body under the skin, and stores several vitamins. But most of us consume far more than what we need. They should account for no more than 20-35% of calories in our diet per day. Diets high in saturated fat, cholesterol, and trans fats place us at higher risk for cardiovascular disease and cancer.

Complete proteins contain every amino acid – Proteins are vital to every cell, just like lipids are. They form enzymes that direct metabolism, serve as receptor and transport molecules, build muscle fibers, and a few are hormones. Proteins are composed of 20 amino acids. Our bodies produce 12 of these. The other 8 (essential amino acids) must be ingested in food. A complete protein contains all 20. (Please see the chart). Approximately 15% of our calories should come from protein. It is critical during pregnancy and childhood that the amino acids are balanced. Any one amino acid missing from the diet can retard growth and alter mental and physical performance.

Vitamins are essential for normal function – There are at least 13 chemicals, or vitamins that are needed for proper function. Our bodies only produce a couple of these: vitamin D (skin synthesis when exposed to sunlight), and bacteria in the colon that produce vitamins K, B6, and biotin. All other vitamins must come from food. They fall into two categories: fat and water soluble. The difference is how they are absorbed. Fat soluble vitamins are absorbed along with fat and excess is stored for later use. Water soluble vitamins are absorbed more readily, but are only stored very briefly and then excreted in urine. So, we need to consume these on a regular basis.
Minerals: Elements essential for body processes – Minerals are the atoms of chemical elements and essential for body function also. They are the ions in blood plasma and cell cytoplasm, they are the chemical structure of bones, and they contribute to nerve and muscle activity. There are 21 minerals considered essential for animals. Nine of these are trace minerals – they make up less than 0.01% of your body weight.

Fiber benefits the colon – Doctors recommend eating 20-35 grams of fiber each day, which is more than what most of us get. Fiber is found in a variety of vegetables, fruit, and grains. It is an indigestible material, but is beneficial to our bodies. When you have a low-fiber diet, it causes chronic constipation, hemorrhoids, and diverticulosis. It is also associated with higher risk of developing colon cancer. Eat your fiber!

Weight Control: Energy Consumed Versus Energy Spent
Energy is measured via calories. Scientists use 1000 kilocalories to measure nutrient content. Calorie with a capital “C” is what denotes this.

BMR: Determining how many Calories we need
To maintain a stable body weight, the number of Calories must equal the number we use. Your basal metabolic rate (BMR) I what determines your caloric energy needs. This is the energy your body needs to breathe, maintain organ function, etc… BMR can be influenced by the following:
  • Gender and body composition
  • Age
  • Health
  • Stress
  • Food intake
  • Genetics

Energy balance and body weight
A healthy weight is a balancing act between energy intake and energy expenditure. Our excess is stored in fat cells. Studies have found that overweight people have two to three times more fat cells than a normal individual. So when they diet, they shrink their fat reserves in each cell, which is why their bodies respond as if they are starving. Dieting is difficult for chronically overweight people because they are fighting the body’s own weight-control system. 5/7/12
Physical activity: An efficient way to use Calories
Even though our BMR stays fairly constant, we can have a drastic effect on the amount of Calories we burn via exercise. To lose one pound of fat, we must use up about 3500 Calories. The best approach is a gradual one, decreasing caloric intake in small amounts while increasing physical activity gradually. Not only will exercise affect weight, it improves your cardiovascular system, strengthens bones, tones muscle, and promotes a general sense of well-being.

Healthy weight improves overall health
The reason we worry about our weight is a direct correlation between obesity and health status. But the real reason is because $80 billion is spent annually on nutrition related health problems. So the government is concerned. Both the government and insurance companies regularly publish body mass index (BMI) charts, using a persons height and weight. But these numbers are a guideline only. There are other factors it does not account for.

Disorders of the Digestive System
There are many common digestive problems which are not necessarily life threatening. One of the most common is food poisoning, caused by contaminated food or beverage with bacteria or their toxic products. Food allergies is another common problem.

Disorders of the GI tract
The following is a list of fairly common disorders of the GI tract.

  • Lactose intolerance: difficulty digesting milk
  • Peptic ulcers: sores in the stomach
  • Celiac disease: gluten intolerance
  • Diverticulosis: weakness in the wall of the large intestine
  • Colon polyps: noncancerous growths

Disorders of the accessory organs

  • Hepatitis: inflammation of the liver
  • Gallstones: can obstruct bile flow 5/7/12
Malnutrition: Too many or too few nutrients
Malnutrition refers to conditions where human development and function are compromised by an unbalanced diet. It can be caused by either overnutrition or undernutrition. Overnutrition can lead to obesity, but the far greater problem is undernourishment. It is estimated that 800 million people worldwide are undernourished. Nearly 20 million people, most children, die every year of starvation or related diseases.

Obesity: A worldwide epidemic?
The rise in obesity, just in the U.S. has risen from 12.6% in 1990 to 34% in 2006. The collective gene pool cannot change that quickly. So we must look at environmental factors to account for this. Computers, cars, television, and desk jobs have all combined to produce a more sedentary lifestyle. Food has also become relatively cheap and readily available. So we evidently eat and drink more. Additional fats and oils in our diet, account for 42% of this increase. It seems to me it is time to take a look at the food industry and what they are selling to the public. 5/7/12

Eating Disorders: Anorexia Nervosa and Bulimia
Eating disorders are not truly digestive disorders, but involve the nervous system. With anorexia, a person diets excessively until they stop eating altogether. With bulimia, people binge and purge in a vicious cycle. These eating disorders seem to have deep roots in psychological and cultural factors. Both play havoc with the body and mind. It usually requires a team of professionals to treat these disorders, from medical, dental, and psychiatric, to nutritional needs. 5/7/12

The Nervous System: Integration and Control
Our nervous systems have four characteristics:

  • It receives information from many different senses simultaneously.
  • It integrates information – taking different pieces of information from many different sources and assembling it into something that makes sense.
  • It is very fast – receiving and integrating information and producing a response within tenths of a second.
  • It can initiate specific responses, including muscle contractions, glandular secretion, and conscious thought and emotions.

The Nervous System has Two Principal Parts
The nervous system is divided into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. It functions to receive, process, store, and transfer information. The PNS includes everything else that lies outside of the CNS. The PNS has two distinct subdivisions: the sensory division, which carries information from the brain and spinal cord; and the motor division, which carries information from the CNS to all other parts of the body.

The PNS if further divided according to function. There is the somatic division, which controls skeletal muscles; and the autonomic division, which controls smooth muscle, cardiac muscle, and glands. Then within the autonomic division, you have the sympathetic and parasympathetic divisions. These two divisions oppose one another for the most part, in order to accomplish automatic, subconscious maintenance of homeostasis.

Neurons are the Communication Cells of the Nervous System
Neurons are highly specialized for communication. They generate and conduct electrical impulses (action potentials), from one part of the body to another. They consist of a cell body, one or more dendrites, and an axon. There are three types of neurons:

  • Sensory neurons – these provide input to the CNS via the PNS. They are specialized to respond to stimulus such as light or pressure.
  • Interneurons – these transmit impulses within the CNS and influence the function of other neurons.
  • Motor neurons – of the PNS transmit impulses away from the CNS to all organs and tissues of the body.

Neurons Initiate Action Potentials
Sodium-potassium pump maintains resting potential
A neuron that is capable of action potential, but is not generating one at the moment, is a normal, or resting, membrane potential. This means the inside of the neuron is negatively charged compared to the outside (-70 millivolts). For every three sodium ions transported out of the cell, two potassium ions are transported in. Every time the sodium-potassium pump goes through a cycle, it results in the removal of one osmotic particle and one positive charge. This and the negatively charged protein molecules that are trapped in the cell create a slight excess of negative charge in the cell cytoplasm than the interstitial fluid. So, sodium is higher in concentration in the interstitial fluid than in the cytoplasm and the opposite if true for potassium. Sodium is always leaking into the cell and potassium is always leaking out via passive diffusion. It is the active transport of the sodium-potassium pump that balances the rate of leakage and maintains the membrane potential.

Graded potentials alter the resting potential
The resting potential of a neuron changes when an impulse arrives from another neuron. These signals may depolarize it, moving the voltage closer to zero; or hyperpolarize it, making it more negative. These changes are graded potentials because they can vary in size. A neuron can receive hundreds of incoming signals at the same time and can these can add up in space and time. This means that many incoming signals can produce bigger changes in the membrane than just one impulse alone, referred to as summation.

An action potential is a sudden reversal of membrane voltage
When the threshold is reached of all the graded potentials within the membrane, action potential results. Action potential is a sudden reversal of voltage difference across the cell membrane. It is the only way that information is transmitted long distance via the nervous system. Once threshold has been reached, voltage-sensitive ion channels open and close within the axon.

There are three steps involved in action potential:

  1. Depolarization: sodium moves into the axon.
  2. Repolarization: potassium moves out of the axon.
  3. Reestablishment of the resting potential.

While action potential is occurring, an axon cannot generate another action potential. This is referred to as the absolute refractory period.  This period ensures that action potential only travels in one direction. Right after this absolute period is a relative refractory period, in which it is harder than usual to generate the next action potential. And whether or not a neuron is generating an action potential, the sodium-potassium pump continues to maintain normal concentrations and resting potential.

Action potentials are all-or-none and self-propagating
Action potentials can be compared to firing a gun: a certain amount of pressure (threshold level), is required to make the gun fire. And pressing the gun too lightly or too hard does not make the bullet leave the gun any faster. This is the all-or-none of action potential – it either happens, or it doesn’t.

Action potential is also self-propagating in that it continues to propagate itself in the next region of the axon. While impulses move forward from one region, action potential brings the next region of the axon to threshold. Electrical current runs continuously down the axon, moving at a constant rate of speed and amplitude. Stronger stimuli generate more action potentials per time unit, rather than faster or bigger ones. And though speed is constant for a given neuron, the speed in different neurons varies from 5 to 250 mph. 5/7/12
Neuroglial Cells Support and Protect Neurons
Approximately 20 % of the cells in the human nervous system are neurons. The rest are neuroglial cells, providing support and protection to the neurons and helping to maintain concentrations of important chemicals in the fluid surrounding them. These cells do not generate or transmit impulses.

In the PNS, or peripheral nervous system, there are specialized neuroglial cells that enclose and protect many neuron axons. These are referred to as Schwann cells. These cells produce myelin, which is a fatty, insulating material. These Schwann cells wrap themselves around short segments of an axon many times, forming a blanket or myelin sheath. Between these cells are uninsulated gaps, or nodes of Ranvier, where the surface of the axon is exposed.

The myelin sheath has three important functions:

  1. It saves the neuron energy.
  2. It speeds up the transmission of impulses.
  3. It helps damaged or severed axons of the peripheral nervous system regenerate.

In the central nervous system, or CNS, the myelin sheath is produced by a neuroglial cell called an oligodendrocyte. Unlike the Schwann cells, the sheath formed by the oligodendrocytes degenerates once the axon it protects is destroyed. Because of this, neurons of the CNS do not regenerate after injury.

Information is Transferred from a Neuron to its Target
After an action potential reaches the axon terminals of a neuron, the information must be converted to another form for transmission to its destination (muscle cell, gland cell, or another neuron). The action potential causes the release of a chemical that crosses a special junction between two cells, or the synapse. We call this substance the neurotransmitter, because it transmits the signal from the neuron to its target. This process is call synaptic transmission.

Neurotransmitter is released
At the synapse, the cell membrane of the neuron that is sending the information is called the presynaptic membrane. Postsynaptic membrane is the membrane of the receiving cell. The small fluid-filled gap between these two membranes is the synaptic cleft. Neurotransmitters are stored in a bulb at the end of the presynaptic neuron. Transmission follows this pattern:

  • An action potential arrives at the bulb, causing calcium channels in the presynaptic membrane to open and diffuse into the axon bulb.
  • The presence of calcium causes vesicles to fuse with the presynaptic membrane, causing the release of the neurotransmitter into the synaptic cleft. And because the cleft is so narrow, the neurotransmitter molecules diffuse into the postsynaptic membrane.
  • Certain gated channels open because molecules of neurotransmitters bind to receptors on the postsynaptic membrane.
  • Sodium ions diffuse inward, which produces graded depolarization of the postsynaptic membrane in the area of the synapse.

Graded potential causes the opening of chemically sensitive ion channels, rather than voltage-sensitive channels.

Neurotransmitters exert excitatory or inhibitory effects
The response of the postsynaptic cell depends on several factors: type of neurotransmitter, concentration in the synaptic cleft, and the types of receptors and chemically sensitive ion channels in the postsynaptic membrane. Scientists have identified more than 50 chemicals that can function as neurotransmitters. All of these are stored in vesicles within the axon bulb and released via action potential. There are excitatory and inhibitory neurotransmitters, as well as some that function both ways. Excitatory neurotransmitters depolarize the postsynaptic cell, which causes threshold approach or excess of threshold. This encourages new impulses in the postsynaptic neuron. Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell (the cell becomes more negative). This in turn, prevents action potential. Neurotransmitters that function as both are dependent on the type of receptor they bind with on the postsynaptic membrane.

Postsynaptic neurons integrate and process information
It is the conversion of the action potential (electrical) to the neurotransmitter (chemical), that allows the postsynaptic cell to integrate and process information. When one neuron receives input from many neurons, we call this convergence. It’s action potential then goes to many other neurons, known as divergence. Neurons do not interpret information (they do not have a brain), but the effect of convergence is that neurons integrate and process thousands of simultaneous incoming stimulatory and inhibitory signals before they generate and transmit their own action potentials. They also reroute information to many destinations. So, individual neurons cannot see, smell, or hear, but their combined actions allow us to experience these incredible sensations. 5/7/12
Sensory Mechanisms
Your body’s sensory mechanisms are constantly providing the brain with detailed information about the world around you, as well as the body itself. Try closing your eyes and listening to any sounds you can hear. Then try to judge how far away they are and what direction they are coming from. Lets explore how different kinds of sensory information are received by your body, converted to nerve impulses, and transmitted to the brain in a way that actually makes sense.

Receptors receive and convert stimuli
The sensory input that causes change within or without your body is referred to as stimuli. Stimuli can be heat, pressure, sound waves, or chemical. Receptors, structures specialized in receiving stimuli, accept and convert the stimuli energy into another form. Some receptors convert stimuli into graded potential and if powerful enough, generates an impulse within the sensory neuron. Other receptors are a part of cells that produce graded potentials and release a neurotransmitter, stimulating nearby sensory neurons. However it is done, the effect is the same, generating an impulse in a sensory neuron. When the CNS receives these impulses, many times we experience a sensation, becoming consciously aware of the stimulus.

Receptors are classified according to stimulus
Classification of receptors is done according to the type of stimulus energy they convert:

  • Mechanoreceptors: respond to forms of mechanical energy, such as sound waves, fluid pressure, physical touch, etc..
  • Thermoreceptors: response to heat and cold
  • Pain receptors: respond to tissue damage or excessive pressure or temperature.
  • Chemoreceptors: respond to the presence of chemicals.
  • Photoreceptors: respond to light.

Many receptors contribute to sensation. A few are “silent” receptors (we are not consciously aware of their actions). These function in negative feedback loops that maintain homeostasis inside our bodies. 5/7/12
The CNS interprets nerve impulses based on origin and frequency
Nerve impulses are transmitted from receptors to specific brain areas.  This is how the CNS can interpret and distinguish incoming impulses. For example, visual stimuli travel in sensory neurons whose axons go directly to regions in your brain associated with vision. All of these incoming impulses are interpreted as light, regardless of how they were initiated. So the central nervous system gets all the information it needs by monitoring where the impulse originates and their frequency.

Some receptors adapt to continuing stimuli
The CNS is able to ignore one sensation over another. And some receptors can ignore sensory input after a short time due to receptor adaptation – the sensory neuron stops sending impulses even though the stimulus is still present. Receptors in the skin and olfactory adapt fairly quickly. Some receptors such as pain, joint, muscle and all silent receptors, adapt very slowly or not at all. This is critical to our survival. It allows us to act appropriately when ill or injured and allows our bodies to maintain homeostasis.

Somatic sensations and special senses provide sensory information
Sensations provided by receptors are either somatic or special. Somatic sensations originate from receptors at more than one location in the body. These sensations include temperature, touch, vibration, pressure, pain, and awareness of body movement and position. There are five special senses (taste, smell, hearing, balance, and vision) that originate from restricted areas of the body. They deliver very specialized information.

Somatic Sensations Arise from Receptors Throughout the Body
The receptors for somatic sensations are located throughout your body in skin, joints, skeletal muscles, tendons, and internal organs. The sensory neurons that are linked to these receptors send impulses to the somatosensory area of the parietal lobe of the cerebral cortex in the brain. This area processes the information and sends it to the primary motor area in the frontal lobe. Then, if necessary, impulses are generated in motor neurons of the PNS to cause body movement.

Mechanoreceptors detect touch, pressure, and vibration
Mechanoreceptors are modified dendritic endings of sensory neurons. Any force that changes the form of the plasma membrane of the dendritic ending produces a graded potential. If the graded potential is large enough that is exceeds threshold, the sensory neuron initiates an impulse. These receptors vary in location, the degree to which they adapt, and the intensity of stimulus required to generate an impulse. Here are several examples of different types of receptors in the skin that detect somatic sensations:

  • Unencapsulated dendritic endings – around hairs and near the skin surface. Signal pain, light pressure and changes in temperature.
  • Merkel disks – located in the epidermis. Unecapsulated, detect light pressure and touch.
  • Meissner’s corpuscles – located just under the epidermis, at the top of the dermis. Encapsulated, detect the beginning and end of light pressure.
  • Ruffini endings – in the center of the dermis. Encapsulated, they respond to ongoing pressure.
  • Pacinian corpuscles – encapsulated endings in connective tissue within the dermis. Respond to deep pressure or high-frequency vibration.

Mechanoreceptors indicate limb position, muscle length, and tension
You can tell the positions of your limbs via mechanoreceptors located in joints (joint position), skeletal muscles (length), and tendons (tension). Best known are the muscle spindles, which monitor muscle length. Muscle length, for the most part, determines joint position because of the way it is attached to the bone. Mechanical distortion of the mechanoreceptors causes local graded potentials in the dendritic endings, and if threshold is passed, action potential is produced.

Thermoreceptors detect temperature
Thermoreceptors, near the skin surface, provide information about the external environment. They adapt quickly, which allows us to monitor changes in temperature, and yet adjust sensory input so it becomes more bearable. For example, when you step into a hot shower, it is uncomfortable at first, but your body adjusts to it after a few moments. There are other thermoreceptors located in the abdominal and thoracic organs that monitor internal temperature, maintaining homeostasis. 5/7/12
Pain receptors signal discomfort
There are two types of pain: fast pain (sharp or acute), occurs a tenth of second after the stimulus. The reflex withdrawal response to fast pain is strong and rapid. Slow pain may not appear until seconds or even minutes after injury. This is due to the activation of chemically sensitive pain receptors by chemicals released from damaged tissue. Referred pain, is slow pain from internal organs, often perceived as originating from an area of the body that is completely different from the source of origination. This happens because action potentials from internal pain receptors are transmitted via the same spinal neurons that transmit action potentials from pain receptors in the skin and skeletal muscles to the brain. Our brains have no way to determine the exact sourcem so it assigns pain to another location. Pain receptors do not generally adapt, which is beneficial for survival. However, that also means that people with chronic diseases or disabilities often experience constant discomfort.

Vision: Detecting and Interpreting Visual Stimuli
Electromagnetic radiation, or light, travels at a speed of 186,000 miles per second in waves. Our eyes actually allow is to receive and process light. We can detect objects from near or far, and from dim or bright sources. Light is collected and focused onto specialized cells within our eyes, called photoreceptors. 5/7/12
Structure of the eye
The sclera is the tough outer coat, or the “white of the eye”, and covers the outer surface except in front, where it is the clear cornea. Light passes through the cornea and aqueous humor (space filled with fluid that nourishes and cushions the cornea and lens). Light then hits either the iris (a colored, disk-shaped muscle), or passes through the pupil (an adjustable opening in the center of the iris). If light passes through the pupil, it strikes the lens, a transparent, flexible structure attached with connective tissue fibers to a ring of smooth muscle, or ciliary muscle. After light passes through the main chamber, it hits the retina, on the back and sides of the eye. The retina is made of photoreceptors, neurons, and blood vessels. At the back is the optic nerve, which carries information to the thalamus, and then on to the visual cortex for interpretation. The skeletal muscles surrounding the eye control movements, letting us look where we want to. The macula, or central region of the retina, has the highest density of photoreceptors.

Regulating the amount of light and focusing the image
There are two sets of smooth muscle that adjust the amount of light entering the eye. When there is bright light, the muscles arranged circularly around the pupil causes the pupil to contract. If this did not take place, daylight would overpower our photoreceptors and temporarily blind us. In dim light, smooth muscle arranged radially around the pupil, causes the pupil to dilate. It is our nerves that control these muscles. The cornea and lens focus the light that enters. Because our cornea is curved, it bends most of the incoming light. But because the curvature is not adjustable, the degree to which light is bent is controlled by the ciliary muscle. When this muscle contracts, the tension on the fibers attached to the lens is reduced, which allows focus on near objects. The reverse it true for focus on distant objects. This is referred to as accommodation. As light rays form each point of an object are bent and focused, the image on the retina is inverted. But our brain interprets it as right side up.

Eyeball shape affects focus 5/7/12
Our ability to focus properly can be affected by the shape of our eyeball. People with myopia (an inherited condition) have an eyeball slightly longer than normal. (Myopia can also be caused by the ciliary muscle contracting too strongly). This causes distant objects to focus in front of the retina, which means that distant objects are out of focus. This is referred to as nearsightedness and can be corrected with concave lenses. Hyperopia (farsightedness) occurs when the eyeball is too short. This causes nearby objects to focus behind the retina, so nearby objects are out of focus. Convex lenses will correct this. Then there is astigmatism (blurred vision), caused by irregular shape of the cornea or lens. This causes the light to scatter, not focusing evenly on the retina. Astigmatism can also be corrected with specially ground lenses.

Light is converted into action potentials
It is our retina that converts light (stimulus) into impulses. It allows us to see in color, perceive images, and adapt to different light intensities. The retina is made of four layers:

  • Outermost layer – pigmented cells and choroid. Absorbs light not captured by photoreceptor cells.
  • 2nd layer – photoreceptor cells (rods and cones).
  • 3rd layer – bipolar cells (neurons). Partially process and integrate information before passing it along to the 4th layer.
  • Innermost layer – ganglion cells (neurons). The long axons of the ganglion cells become the optic nerve running to the brain.

Rods and cones respond to light
Rods and cones are flattened disks that contain numerous photopigment molecules. These molecules are a light-sensitive protein. This protein changes shape when exposed to energy (in the form of light). This change in shape causes some of the sodium channels to close, which reduces the amount of neurotransmitter normally released. Neurotransmitter normally inhibits bipolar cells. So ultimately light increases bipolar cell activity, which then activates ganglion cells. There are approximately 120 million rods, 6 million cones, and 1 million ganglion cells with axons going to the brain.

Rods provide vision in dim light
As seen above, there are about 20 times more rods than cones. So if we imagine all 120 rods converging on half of the ganglion cells (half a million), that means there would be 240 rods converging on a single ganglion cell. The convergence increases our ability to see in dim light, but without detail or accuracy. This is due to the fact that rods have photopigment called rhodopsin. Rhodopsin is more sensitive to light, than the photopigment in cones. So rods are primarily responsible for dim light vision, but they do not give us color vision, which explains why objects appear less colorful in dim light. Neither are rods and cones distributed evenly on the retina. Regions farthest away from the fovea have the highest ratio of rods to cones. So if you want to see a dim star at night, rather than looking directly at it, adjust your vision to look just off the side. 5/7/12

Cones provide color vision and accurate images
Cones allow us to see color because there are three different kinds: red, green, and blue. Each one contains a photopigment that absorbs energy from red, green, and blue light. The ability to distinguish various colors is due to the way our brain interprets the ratio of impulses that come from the ganglion cells that are connected to the cones. When all three are activated, we perceive white light. Whereas black is no light at all. It is the cones that are also responsible for visual acuity. Cones require stronger light to activate, which is why it is harder to distinguish color in dim light.

Visual receptors adapt
As we all know, vision adapts to changing light. It usually takes longer when going from bright to dim light than the other way around. This adaptation depends on rapid adjustment of the pupil by the iris and adaptation by the rods. The absorption of light via rhodopsin uses up the photopigment temporarily. Light energy breaks rhodopsin into two molecules. They can be resynthesized, but it takes a moment. When you have been in bright light, most of the rhodopsin has been broken down. So when you enter a dim room next, the cones are not functioning at first. When you go out into sunlight after being in a dim room, the light is very bright because you have the maximum amount of photopigment available in both the rods and cones. But then the rhodopsin is quickly used up, so you are using mainly cones in bright light.

Disorders of Sensory Mechanisms
Disorders of the ear
tympanic memebrane/ 5/7/12

  • Deafness (loss of hearing) – nerve deafness is caused by damage to the hair cells. Is usually caused by frequent exposure to loud sounds. Conduction deafness is caused by damage to the tympanic membrane or bones of the middle ear. Conduction deafness is most often due to arthritis of the middle ear bones.

  • Otitis media (inflammation of the middle ear) – a common cause of earaches. Usually due to upper respiratory tract infections.

  • Meniere’s syndrome (inner ear condition that impairs hearing and balance) – this is a chronic condition with cause unknown, though it may be due to excess fluid in the cochlea and semicircular canals. It affects balance and hearing. (I happen to have Meniere’s syndrome – it can be very debilitating).

Disorders of the eyes

  • Retinal detachment (retina separates from choroid) – most commonly caused by a blow the head. The retina tears and vitreous humor leaks through and peels the retina away from the choroid. Prompt surgery can usually repair the damage.

  • Cataracts (the lens becomes opaque) – can be congenital, but most often is age related or associated with diabetes. Delivery of nutrients to the lens becomes insufficient causing proteins to clump, making the lens less transparent and eventually opaque. Surgery to replace the lens with an artificial one can repair the problem, if successful.

  • Glaucoma (pressure inside the eye rises) – the canal of Schlemm, the drainage vessel for aqueous humor, becomes blocked. The excess fluid increases pressure and compresses blood vessels. If detected early enough, can be controlled with drugs or surgery before permanent damage is done, although any lost site prior to that will not be recovered.

  • Age-related macular degeneration (AMD) – visual impairment caused by detachment of the retina and degeneration of photoreceptor cells in the macular region of the retina. Can be caused by an accumulation of cellular debris between the choroids and retina or abnormal growth of blood vessels in the region. No effective cure yet, however, vascular growth factors and vitamin treatments are showing promise at slowing or delaying progression of the disease.

  • Color blindness (inability to distinguish the full range of colors) – most commonly caused by deficient numbers of a particular type of cone. Rarer, is the inability to perceive any color. This happens when two of the three cones are missing completely. Often inherited, red-green blindness is an X-linked recessive trait. 5/7/12
Human Impacts, Biodiversity, and Environmental Issues
The impact that we, as humans, have on our environment can be either destructive or beneficial. Our presence has definitely altered air, water, and land, both in local and global ecosystems. Let take a look at some of the issues…

Pollutants Impair Air Quality
Pollution is the trace amounts of thousands of chemicals in the air that have adverse effects on living organisms. The major concerns fall into four categories:

Excessive greenhouse gases lead to global warming
Inside a greenhouse, sunlight penetrates, is converted to heat, and becomes warmer because the heat cannot escape. The same thing is happening on earth in the upper layers of the atmosphere with certain gases.  This gas is made of water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), and halons that contain bromine. Together, these gases produce the greenhouse effect. This process is normal, maintaining earth’s surface temperature. But human activities have increased the levels of greenhouse gases, especially CO2.  This is raising the average global temperature, or, global warming.

The two things that contribute the most to global warming is the burning of fossil fuels for energy and deforestation. (Trees absorb CO2 from the air during photosynthesis. A large tree can store 50lb of CO2 every year). When we burn trees, it is double the damage because the carbon in the wood is released back into the atmosphere.

CFCs deplete the ozone layer
Ozone (O3) is in the troposphere and stratosphere. In the troposphere is the layer of pollution we see from automobile exhaust and industrial pollution. It is mildly toxic, causing plant damage and respiratory distress in all animal life, including humans. Higher up in the stratosphere, ozone is beneficial, creating a shield from ultraviolet rays. But in the early 1980’s, it was discovered that CFCs (a group of chemicals used in refrigerators, air conditioners, and aerosol sprays) had migrated upward, decomposed, and released chlorine atoms. These atoms combine with ozone, destroy it, and create oxygen. And because the chlorine atom can be reused in this reaction, one chlorine atom can destroy as many as 10,000 ozone molecules. By 1985, scientists began to see holes in the ozone layer. Quick action by the international community has halted the damage, but it will take 100-150 years before the ozone layer returns to normal.

effectsofacidrain/ 5/7/12
Pollutants produce acid precipitation
Acid precipitation is caused when sulfur dioxide is released into the air via burning coal and oil, and nitrogen oxide released via automobile exhaust. These oxides combine with water vapor in the air and become sulfuric and nitric acid, which then dissolve in raindrops, which fall as acid precipitation. This corrodes metal and stone and damages forests and aquatic ecosystems. There have been steps taken to remove sulfur from coal burning power plants and as a result, by 1985, sulfur in rainwater had declined by 33% across most of the U.S. Northeast. Recent regulatory actions are expected to eliminate most of the remaining sulfur emissions by 2014.

Smog blankets industrial areas
There are several pollutants that react with each other in sunlight and water vapor. The worst ones are nitrogen oxides and hydrocarbons. This forms a hazy brown or gray layer of smog (term from “smoke” and “fog”). Most smog is caused by the burning of fossil fuels and automobile exhaust. The chemicals in smog can irritate the eyes and lungs and may lead to chronic illnesses such as asthma and emphysema. Cleanup efforts have significantly improved some areas, but there are still cities that have not yet solved the problem.

Pollution Jeopardizes Scarce Water Supplies
There are three major activities that affect water quality and availability: excessive use, replacing natural vegetation with buildings and roads, and polluting sources of water.

Water is scarce and unequally distributed
Although water is a renewable resource in that it is always evaporating from oceans and falls on land as rain or snow, the freshwater on land and its aquifers, make up less than 1% of Earth’s total water. Ninety-seven percent is salty ocean and 2% are glaciers and polar ice caps. So it is somewhat scarce and it is not evenly disturbed throughout human populations. Industrialized nations use 10-100 times more water than less industrialized areas. There are already some desert and semiarid regions that have reached their capacity. Water rights are controversial and we have already had to make some choices between irrigating crops, supplying growing cities, and encouraging the reproduction of Pacific salmon. 5/7/12
Urbanization increases storm water runoff
Because urban areas have shifted to roads and buildings instead of fields and woods, stormwater runoff has become a problem. On the east coast in the U.S., stormwater combines with sewage (CSO), which causes sewage overflow, and this in turn, overwhelms streams and oceans. In the New York harbor, 28 billion gallons of CSO flow into it every year. It is a major source for bacteria, causing eye and ear infections, gastroenteritis, skin rashes, respiratory infections, and hepatitis in swimmers and kayakers. On top of that problem, pipes used to transport stormwater leads to stream overflow during storms and low water levels during dry periods. This is causing erosion of the streambeds and we are losing aquatic life. There are efforts underway to stabilize stream channels, reducing erosion and restoring life.

Human activities pollute freshwater
Many of our human activities pollute water or soil – untreated sewage, chemicals from factories, runoff from pesticides and fertilizers, and rubber and oil from city streets – all have to go somewhere. They either degrade chemically or they pollute water and soil. Some water pollutants are organic nutrients, coming from sewage treatment plants, food-packing plants or paper mills. When they degrade by bacteria, the rapid growth of the bacteria depletes water of oxygen, threatening wildlife. Then there are inorganic nutrients, for example, nitrate and phosphate fertilizers and sulfate in laundry detergent. These cause rapid growth of algae, which dies and is decomposed by bacteria. Rapid growth of plant life and the death of animal life in a shallow body of water via excessive organic and inorganic nutrients is termed eutrophication.

Then you have toxic pollutants such as polychlorinated biphenyls (PCBs), oil and gasoline, pesticides, herbicides, and heavy metals. These remain in our environment for a long time because they do not decompose. Because animals eat many times over their own weight in food, their tissues become more concentrated the higher up the food chain, or biological magnification. One example of this is the metal mercury. Mercury often ends up in aquatic ecosystems and is consumed by shark, tuna, and whales. And human consumption can be dangerous, especially in pregnant women and children. Overloading on mercury can cause loss of coordination, decreased memory and intellect, and poor immune function.

Other worldwide water pollutants include disease-causing organisms that can cause typhoid fever and hepatitis, sediments from soil erosion, nitrogen fertilizers, and heat pollution. Do you think we get the picture yet? 

Groundwater pollution may impair human health 5/7/12
The same pollutants that threaten surface water, also pollute groundwater. But there are two more additional concerns. One, groundwater is usually drinking water and may affect human health very quickly. And two, groundwater is a slow exchanging pool, so once polluted, it may stay that way for a long time. Right now, it is estimated that as much as 50% of all water systems and rural wells are contaminated with some sort of pollutant. Public officials suspect that pollutants contribute to miscarriages, skin rashes, nervous disorders, and birth defects. Of special concern is the disposal of radioactive waste. Radioactive waste remains radioactive for thousands of years. Some radioactive wastes are now incorporated into glass and then buried deep underground.

Oil pollution damages oceans and shorelines 5/7/12
In most years, although 2010 may be an exception, several million tons of oil enters the world’s oceans. About 50% comes from natural seepage, 30 % by oil disposal on land that is washed into the sea, and 20% from accidents at sea. In general, about a quarter of oil spilled at sea is evaporated, half is degraded by bacteria and the rest settles on the ocean floor. But in the short term, and especially if spilled close to shore, it causes major damage to marine and shoreline ecosystems. Even when we try to clean up a spill, salvaging the ecosystem somewhat, it ends up in either land, if buried, or the air, if burned. The largest oil spill in U.S. history in 2010 will be felt for decades to come. No one knows the full extent of the environmental and economic damage from just that one spill.

Pollution and Overuse Damage the Land
Although we pollute our land, the biggest concern may be overuse of it. We strip mountaintops to find coal, cut down forests for lumber or to clear space for crops, and dam river valleys to produce hydroelectric power. The U.S. alone consumes 22 tons of fuels, metals, minerals, and biomass (food and forest products) for every person, every year. Then add to that the amount of earth we move to build and find energy and erosion of soil due to agriculture and forestry, and now it is almost 88 tons per person, per year.

desertification/ 5/7/12
Human activities have altered nearly a third of Earth’s land mass. Cities expand to nearby farmland where it is fairly flat, even though only a small amount of the earth’s surface is suitable for farmland. Cities require huge quantities of water and power and generate waste and pollution in a fairly small area. In rural areas, more than half the population of the world lives in rural poverty. They cut down all trees for fuel and shelter and overgraze their lands with livestock. All of this leads to erosion and desertification – the transformation of marginal land into near-desert conditions. Every year, 15 million acres becomes desert, where it was once productive.

Wars also cause environmental damage ie… Iraq drained their marshlands, which resulted in loss of valuable farmland. And this is just one example. There is also the issue of how we dispose of our garbage. 
My Summary
In summary, what in the world have we done to our planet? (And continue to do, I might add). It seems to me that our efforts to salvage things has possibly come a little too late. We may be able to fix some things, but the impact of some of our pollution, I’m afraid, is going to have very far-reaching consequences for years and years to come. I am more than saddened by the fact that we have not taken very good care of something that I feel we were entrusted with. (I have cried each time I have gotten to this chapter) Sometimes, when I am out shooting photos, I sense that this earth is tired. I know that doesn’t sound too rational, but I feel it, nevertheless. And if I could, I would ask it and God’s forgiveness and heal it of all the damage we’ve done to it.

No comments:

Post a Comment