IGCSE HUMAN BIOLOGY

Chapter Questions – Section 8

1) Explain the meaning of the following terms:

a) Homeostasis

Homeostasis is the name given to all the processes, which operate to maintain a stable internal environment within the body. This means that the body works constantly to prevent large fluctuations in temperature, blood sugar levels, water content of blood, acidity of the blood, amount of oxygen and carbon dioxide within the blood, the level of hormones in the blood, the heart rate and the activity of pathogens.

b) Excretion

Excretion means the removal of waste products that have resulted from chemical (metabolic) reactions within the body.

c) Ultrafiltration

Ultrafiltration is the process of separating the blood into molecules of different sizes under pressure in the kidneys.

d) Selective Re-absorption

This is the re-absorption of various substances in different amounts in the kidney tubule.

e) Endotherm

Humans are endotherms, which means that they generate heat from inside. Endotherms use heat from chemical reactions in its cells to warm its body.

2) The diagram shows a simple diagram of the kidney tubule (nephron).

a) What are the parts labelled X, Y and Z?

X – Glomerulus

Y – Bowman’s capsule

Z – Loop of Henle

b) Four places in the nephron and its blood supply are labelled A, B, C and D. Which of the four substances are found at each of these four places?

Water - B

Urea - D

Glucose - A

Salt - C

3) The hormone ADH controls the amount of water removed from the blood by the kidneys. Write a short description of the action of ADH in a person who has lost a lot of water by sweating, but has been unable to replace this water by drinking. Explain how this is an example of negative feedback.

On a hot day a person is likely to lose some water and salts through the process of sweating. Thus, the urine on a hot day would be more concentrated and less in volume that on a cold day. This is because the loss of water through sweating causes the concentration of water in the blood to decrease below its set point. This is detected by cells in the hypothalamus. These cells are sensitive to the solute concentration of the blood and cause the pituitary gland to release more ADH (anti-diuretic hormone). ADH travels in the bloodstream to the kidney, where it causes the collecting ducts in the kidneys to become more permeable to water, so that more water is re-absorbed back into the bloodstream. This makes the urine more concentrated, so the body loses less water and the blood becomes more dilute.

When the water content of the blood returns to normal, this acts as a signal to “switch off” the ADH production. The kidney tubules then start to reabsorb less water. Similarly, if someone drinks a large volume of water, the blood will become more dilute. This leads to lower amounts of ADH being released and the kidney tubules become less permeable and allow more water to pass out of the body in urine. In this way, through the action of ADH, the level of water and the internal environment is kept constant.

The action of ADH is an example of negative feedback. A change in the conditions in the body away from the set point starts a corrective process, which aims to return the conditions to normal. Once the desired internal environment is recreated, the corrective process is switched off.

4) The bar chart shows the volume of urine collected from a person before and after drinking a litre of distilled water. The person’s urine was collected immediately before the water was drunk and then at 30 minute intervals for four hours.

a) Describe how the output of urine changed during the course of the experiment.

Increased up to an hour into the experiment, and then decreased gradually as the body regulated the water content of the blood.

b) Explain the difference between the urine produced at 60 minutes and 90 minutes.

It has reduced from 300cm3 to 200cm3. This is because the level of water in the blood is being regulated by ADH. Initially, less ADH is secreted allowing the collecting ducts to be less permeable and allowing more water to pass out of the body in urine. As the water levels in the blood return to normal, more ADH is released and the collecting ducts start to become more permeable and less water is gradually lost.

c) The same experiment was repeated with the person sitting in a very hot room. How would you expect the volume of urine collected to differ from the first experiment? Explain your answer.

The volume of urine collected would be much less. This is because the body will conserve water by releasing ADH to make the collecting ducts more permeable to water and so more water is reabsorbed.

d) Between 90 and 120 minutes, the person produced 150cm3 of urine. If the rate of filtration in the glomeruli during this time was 125cm3 per minute, calculate the percentage of filtrate reabsorbed by the kidney tubules.

125cm3 x 30 minutes = 3,750cm3 (or 3.75dm3)

150cm3 / 3750cm3 = 4%

Therefore, 96% of the glomerular filtrate has been reabsorbed back into the blood.

5) Working on a computer, construct a table to show the changes taking place when a person is put in a hot or cold room.

|Changes taking place |Hot Environment |Cold Environment |
|Sweating |Sweat glands produce a greater amount of |Sweat glands produce less sweat. |
| |sweat, which is secreted onto the surface of | |
| |the skin. This evaporates, cooling the skin | |
|Blood flow through capillary loops |Vasodilation increases the blood flow through|Vasconstriction decreases blood flow through |
| |the surface capillaries so that more heat is |the surface capillaries so that less heat is |
| |radiated from the skin |radiated from the skin |
|Hairs in the skin |Hairs on the surface of the skin lie flat |Hairs of the surface of the skin stand |
| |against the skin’s surface |upright (contraction of the hair erector |
| | |muscles) and trap a layer of warm air next to|
| | |the skin surface |
|Shivering |N/A |Muscles of the body contract and relax |
| | |rapidly. This increased muscle action creates|
| | |heat within the muscles |
|Metabolism |Slows down in order to generate less heat |Speeds up in order to generate more heat |

6) Humans are able to maintain a constant body temperature, which is usually higher than that of their surroundings.

a) Explain the advantage of maintaining a high body temperature.

The advantage of maintain a high body temperature means that all chemical reactions in the human body can occur at a steady, predictable rate. It is also important that the body does not become too hot, as the enzymes in the body would become denatured and would not work.

b) The temperature of the blood is constantly monitored by the brain. If it detects a drop in blood temperature, the following things happen; the arterioles leading to the skin capillaries constrict, less sweat is formed and shivering begins.

i) Explain how each response helps to keep the body warm.

Vasoconstriction – They arterioles leading to the blood capillaries near the surface of the body constrict and blood flow to the surface of the skin is reduced, preventing heat loss through the skin.

Sweat formation ceases – Seating is a cooling process. If we are already too cold, the sweat glands cease sweat production in order to control the amount of heat lost via the evaporation process.

Shivering – This is the rapid contraction and relaxation of the muscles. The increased muscle activity generates heat in order to keep the body warm.

ii) Explain how the structure of the arterioles allows them to constrict.

As the arterioles are made of elastic fibres and smooth muscle, they are able to dilate and constrict allowing more or less blood to flow through them, as required. Vasoconstriction and vasodilation are brought about by rings of muscles called sphincter muscles

c) When the weather is hot, we produce less urine.

i) What is the name of the hormone that controls the amount of urine produced by the body?

Anti-diuretic hormone (ADH).

ii) Explain how the body produces less urine on a hot day.

On a hot day a person is likely to lose some water and salts through the process of sweating. Thus, the urine on a hot day would be more concentrated and less in volume that on a cold day. This is because the loss of water through sweating causes the concentration of water in the blood to decrease below its set point. This is detected by cells in the hypothalamus. These cells are sensitive to the solute concentration of the blood and cause the pituitary gland to release more ADH (anti-diuretic hormone). ADH travels in the bloodstream to the kidney, where it causes the collecting ducts in the kidneys to become more permeable to water, so that more water is re-absorbed back into the bloodstream. This makes the urine more concentrated, so the body loses less water and the blood becomes more dilute.

iii) Explain how the hormone above works in the kidney to produce less urine.

ADH changes the permeability of the collecting ducts, so more or less water is re0absorbed back into the body.