Reading Exercise 3: - HRD Press
|Reading Exercise 3: |
|Maintaining Control |
| |Functions such as breathing, thinking, reflexes and memory are|
| |all controlled by the central nervous system. It is comprised |
| |of highly specialized cells called neurones or nerves. These |
| |cells make electrical impulses and transmit them in a similar |
| |way to wire carrying electricity. The central nervous system |
| |comprises four parts. The brain and spinal cord are the main |
| |pathways by which an organism's cells communicate. The |
| |receptor cells detect a stimulus, and make up the five |
| |senses—sight, smell, touch, taste and hearing. The sensory |
| |nerves carry messages from the receptor cells to the spine and|
| |brain. The motor nerves (/) carry messages to the effectors; |
| |these are glands or muscles that respond to a stimulus. A |
| |reflex, such as flinching at a pain, is a response to a |
| |danger. |
| | |
| | |
|( | |
| |Humans have a large brain compared to other mammals. In an |
| |adult the brain has about 100 billion neurones. Brain cells |
| |cannot be replaced once they are damaged; this is why brain |
| |damage is generally permanent. The cerebrum is the movement |
| |center of the brain. The area is highly folded, which |
| |increases the surface area of its cells and therefore the |
| |number of messages that it can process simultaneously. The |
| |neurones (/) do not touch each other and the gap between them |
| |is called the synapse. Impulses cross the gap via |
| |neurotransmitters, which are subsequently |
|( | |
| |removed by biological catalysts called enzymes. Some toxins |
| |produced by snakes and spiders act by disrupting the enzymes. |
| |Over fifty neurotransmitters have been identified. Dopamine is|
| |one of them and is responsible for emotional behavior. The |
| |brain also produces pain killing neurotransmitters, such as |
| |encephalin and endorphins. Heroin and morphine are artificial |
| |forms of these neurotransmitters. |
| |Some drugs act in ways similar to neurotransmitters and are |
| |called psychoactive for this reason. Some are legal, such as |
|( |tobacco, alcohol and prescription drugs. Others, such as |
| |heroin and marijuana, are illegal. A person can come to |
| |believe that they need a particular drug to function and |
| |become psychologically dependent on a drug. The body itself |
| |can become physically dependent on a drug. This can occur when|
| |the neurotransmitters in the brain become conditioned to the |
| |presence of the drug and can’t function correctly if the drug |
| |is withdrawn. On the other hand, the brain may build up a |
| |tolerance to the drug and the user finds that a larger amount |
| |of it is required to get the same response. |
| | |
| |Another of the body's message networks is the endocrine |
| |network. The endocrine glands include the pituitary glands |
| |thyroid gland, pancreas, adrenal glands, and the gonads. These|
| |glands produce hormones that lock onto the outside of a target|
| |cell and induce a change in that cell. While the pituitary |
|( |gland produces some hormones, its |
| |main role is to direct the other endocrine glands. The thyroid|
| |gland produces two hormones involved in the homeostasis of |
| |calcium in the blood. Homeostasis is a constant level of a |
| |substance in the system; calcitonin removes soluble calcium |
| |and stores it in the insoluble form in bones, while thyroxin |
| |removes it from the bones and returns it to the blood. |
|( |Pheromones are the chemical messengers used by organisms of |
| |the same species to attract a mate. While insects and other |
| |mammals are known to use these, no human equivalent has been |
| |identified. |
| |The regulation of cellular respiration is controlled by the |
| |supply of the simple sugar glucose. After eating, glucose |
| |levels in the blood rise. The pancreas excretes insulin to |
| |turn the excess glucose into glycogen, which is then stored by|
| |the liver. If glucose is consumed during exercise, the |
| |pancreas releases glucagons to turn glycogen into glucose |
| |again. In this way the pancreas maintains homeostasis of |
| |glucose levels in the blood. |
|( | |
| |The regulation of cellular respiration is controlled by the |
| |supply of the simple sugar glucose. After eating, glucose |
| |levels in the blood rise. The pancreas excretes insulin to |
| |turn the excess glucose into glycogen, which is then stored by|
| |the liver. If glucose is consumed during exercise, the |
| |pancreas releases glucagons to turn glycogen into glucose |
| |again. In this way the pancreas maintains homeostasis of |
| |glucose levels in the blood. |
|( | |
| |In general, diabetes is caused by the destruction of the cells|
| |in the pancreas that produce insulin. Insulin-dependent |
| |diabetics must inject themselves daily with the hormone and |
| |follow a strict diet and exercise plan. This helps them avoid |
| |hypoglycemia, where the blood sugar level drops dramatically. |
| |The treatment used to avoid confusion, shaking or coma is to |
| |give the sufferer some sugar. Prior to 1980, the insulin used |
| |to treat diabetics was extracted from the pancreases of cows |
| |and pigs. Today, the insulin is produced in bulk from |
| |genetically engineered bacteria. Other treatments include the |
| |implantation of donor pancreases, or the pancreas of a |
| |genetically altered pig. The other option is to inject the |
|( |cells responsible for insulin production. If another type of |
| |cell can be induced to produce insulin, they can be introduced|
| |then into the pancreas to replace the destroyed cells. |
| |The kidneys are responsible for removing most of the waste |
| |produced by the body. Urea is the broken down amino acids |
| |produced by the liver. The kidney treats this to conserve |
| |water, salts, glucose, and complete amino acids, while |
| |removing toxic waste. Kidneys are involved in maintaining the |
|( |homeostasis of water and salts in the body and can produce |
| |more concentrate or dilute urine to do so. Illness, injury, or|
| |overuse of some drugs can cause kidney failure. While a human |
| |can survive with the use of one fully functioning kidney, |
| |anything less would require the person to undergo dialysis |
| |two to four times a week. Dialysis is when a machine is used |
| |to perform the same function as the kidneys. It takes |
| |approximately four hours, and most people on dialysis live in |
| |hope of a kidney transplant. |
|( |Reptiles and insects are ectodermic—the external environment |
| |regulates their body temperature. Mammals, birds and fish are |
| |endotherms, maintaining a stable body temperature through |
| |internal metabolism. The human body needs to stay around 37(C,|
| |and hypothermia will result if the temperature drops |
| |dramatically. On the other hand, an elevated body temperature |
| |causes hyperthermia. The body cools itself by sweating, or |
| |through vasodilatation. The blood vessels close to the skin |
| |expand to increase the volume of blood flowing through and so |
| |heat is lost to the environment. Heat is generated through |
| |shivering, vasorestriction or through goose bumps. Goose bumps|
| |raise the hair on the skin, trapping warm air between fur and |
| |skin. On relatively hairless humans, this contributes little |
| |to heat generation. |
| | |
| | |
| | |
|( | |
| |Plants also respond to stimuli, but in a simpler fashion. |
| |These responses are called tropisms. Phototropism is the |
| |response to light—this helps the plant to measure the length |
| |of the day, and hence flower or die at the correct time of |
| |year. Gravitropism is the response to gravity—when a seed |
| |germinates it needs to point its roots down and its shoots up.|
| |The hormone responsible |
| |for this is called auxin. For example, it causes cells on the |
|( |dark side of the plant to elongate, so bending the plant to |
| |the light source. A plant will remove all food from a leaf, |
| |and transfer waste there before releasing a growth-inhibiting |
| |hormone to cause the leaf to drop off. The gas ethylene is |
| |involved in the ripening of fruit, so fruit can be picked |
| |before it ripens and is caused to do so at a more desirable |
| |time. (©bbc.co.uk) |
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