OCR 21st Century Science - B4 - Homeostasis

Notes on homeostasis

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  • Created on: 13-02-12 20:30
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B4 ­ Homeostasis ­ Answers
1. Homeostasis: the maintenance of a constant internal environment so that cells can function properly, through
automatic control systems. The body must maintain:
body temperature at 37°C
the correct levels of water and salt
control the amounts of nutrients
take in enough oxygen for respiration
get rid of toxic waste products
2. Strenuous exercise, or living in a hot or cold environment, scuba diving and mountain climbing affect our body
temperature, blood oxygen levels, and hydration and salt levels.
3. First, we need receptors to detect when things such as temperature change. Then we need a processing centre
to receive this information and coordinate our response. Finally, we need effectors to produce a response that
ensures our body temperature stays at 37°C. This is the case for both artificial systems, such as an incubator as
well as our body system.
4. Negative feedback ensures that, in any control system, changes are reversed and returned back to the set
level. Negative feedback keeps our body temperature at a constant 37°C. If we get too hot, blood vessels in our
skin vasodilate (become larger) and we lose heat and cool down. If we get too cold blood vessels in our skin
vasoconstrict (become smaller), we lose less heat and our body warms up. The other factors also controlled in
the body by negative feedback are blood oxygen levels and salt levels.
5. Diffusion is the movement of particles from an area of high concentration to an area of low concentration. This
is described as moving down a concentration gradient. Chemicals that move in and out of cells by diffusion
include oxygen, carbon dioxide and dissolved food.
6. Osmosis is the overall movement of water from a dilute solution to a more concentrated solution through a
partially permeable membrane. This is still like diffusion, as the water is moving from a higher concentration of
water to a lower concentration of water. If red blood cells are placed in pure water, water enters them by
osmosis and the red blood cells swell up and burst. If cells are placed in a concentrated solution, water leaves
them by osmosis and they are unable to function.
7. Active transport is the process by which dissolved molecules move across a cell membrane from a lower to a
higher concentration. In active transport, particles move against the concentration gradient - and therefore
require an input of energy from the cell. In humans, active transport takes place during the digestion of food in
the small intestine. Carbohydrates are broken down into simple sugars such as glucose. The glucose is absorbed
by active transport into the villi, to be passed into the bloodstream and taken around the body.
8. A constant body temperature is maintained by balancing energy gain and energy loss.
9. Although our core temperature must be 37ºC, our fingers and toes can be colder. This is because energy is
transferred from the blood as it travels to our fingers and toes. Extremities also have a larger surface area so
they lose more heat than our core.
10. Temperature receptors in the skin detect changes in the external temperature. They pass this information to the
processing centre in the brain, called the hypothalamus. The processing centre also has temperature receptors
to detect changes in the temperature of the blood. The processing centre automatically triggers changes to the
effectors to ensure our body temperature remains constant, at 37°C. The effectors are sweat glands and
muscles. If we are too hot or too cold, the processing centre sends nerve impulses to the skin, which has two
ways to either increase or decrease heat loss from the body's surface.
11. At a high body temperature:
More sweat is produced by sweat glands which cools the body when it evaporates
Blood vessels supplying the capillaries of the skin dilate (vasodilation) allowing more blood to flow through
skin capillaries which increase energy loss

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At a low body temperature:
The increased rate of respiration stimulated when muscles contract rapidly (shivering) results in some
energy transferred in respiration warming the surrounding tissues
Blood vessels supplying the capillaries of the skin constrict (vasoconstriction) restricting blood flow through
skin capillaries which reduces energy loss.
13. Heat stroke is an uncontrolled increase in body temperature.
14. Heat stroke is caused by:
High temperatures which cause an increase in sweating.…read more

Page 3

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For the cells of our body to work properly, it is important that their water content is maintained at the correct
level. This means our body must maintain a balance between the water we take in and the water we lose. This is
done by the kidneys.
23. Blood is brought to the kidneys to be filtered, and then returned, to be circulated around the body.…read more


Sasha Dean


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