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) |

Total number of words in 1 minute: _______

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