TABLE OF CONTENTS
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
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| resources.teachnet.ie/accessed 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
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:
- Mechanical processing and movement - (This is obviously chewing, which breaks food into smaller pieces and propels it forward.)
- 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.
- Digestion – the contents in the lumen break down into smaller and smaller particles until you have nutrient molecules.
- Absorption – nutrients pass over from the mucosal layer and into the blood or lymph.
- 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.
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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.
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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.
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| uvahealth.com/accessed 5/7/12 |
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.
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| arthursclipart.org/accessed 5/7/12 |
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.
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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
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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.
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| neatsolutions.com/accessed 5/7/12 |
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.
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| inlineskating.about.com/accessed 5/7/12 |
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
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| arabiangazette.com/accessed 5/7/12 |
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.
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| students.cis.uab.edu/accessed 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.
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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:
- Depolarization: sodium moves into the axon.
- Repolarization: potassium moves out of the axon.
- 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.
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| sciencephoto.com/accessed 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:
- It saves the neuron energy.
- It speeds up the transmission of impulses.
- 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.
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| online.wsj.com/accessed 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.
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| rci.rutgers.edu/accessed 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.
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| valadservices.com/axccessed 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.
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| well-health366.blogspot.com/accessed 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
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| lasikeye.com/accessed 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.
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| sciencephoto.com/accessed 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
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| tympanic memebrane/cornellent.org/accessed 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.
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| nfty.rjblog.org/accessed 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.
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| effectsofacidrain/gettyimages.com/accessed 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.
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| co.henry.ga.us/accessed 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
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| consciouslifestyleradioblog.com/accessed 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
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| katu.com/accessed 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.
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| desertification/oceanworld.tamu.edu/accesed 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.
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