Homeostasis: Osmoregulation

Question 1

Regulation of the composition of body fluids

Answer 1

• Water moves easily through plasma membranes so that any difference in osmotic concentration between the intracellular fluid and the extracellular fluid does not last very long.
• If an imbalance in osmotic concentration does occur in any tissue, osmosis normally restores the balance within seconds.

Question 2

Fluid Balance

Answer 2

• Most of the body fluid is obtained from water that is either taken in as a liquid or contained in food that is eaten.
• A small amount is obtained as a by-product of chemical processes occurring within the cells. This water is referred to as metabolic water.
• Fluids are lost from the body via the kidneys, through the skin, from the surface of the lungs and from the alimentary canal.

Question 3

Control of water loss by the kidneys

Answer 3

• The volume and composition of urine produced by the kidneys depends on how much water there is in the body fluids.
• Approximately 99% of the water filtered through the glomeruli of the kidneys is reabsorbed.
• This reabsorption occurs through the walls of the kidney tubules along their entire length.
• However, the reabsorption occurring at the proximal convoluted tubule and loop of Henle is by osmosis, while reabsorption at the distal convoluted tubule and collecting tubule is active reabsorption.
• The level of active reabsorption is controlled by a hormone known as antidiuretic hormone (ADH).
• ADH is produced by the hypothalamus and released from the posterior lobe of the pituitary.
• ADH increases the permeability of the walls of the distal convoluted tubule and collecting duct.
• When the concentration of ADH in the blood plasma is high, the tubules are very permeable to water, and thus water is able to leave the tubule and enter the surrounding capillary network.
• This reduces the volume of water in the filtrate and increases the concentration of the materials remaining.
• Conversely, when the concentration of ADH in the plasma is low, the tubules are not very permeable to water, and little water is reabsorbed into the plasma of the blood.
• In this situation, the fluid within the tubules remains fairly dilute, as its volume is not reduced to any significant extent.

Question 4

Water Balance - Hormonal Control

Answer 4

1. A decreased amount of water in the blood, results in the water concentration of the blood plasma to decrease. This means that the osmotic pressure of the blood is raised.
2. Osmoreceptors in the hypothalamus detect the increased osmotic pressure of the blood.
3. The hypothalamus stimulates the posterior lobe of the pituitary gland to release ADH into the bloodstream.
4. ADH is carried all over the body by the blood but it affects its target organs which are the nephron tubules in the kidney. The permeability to water of the distal convoluted tubules and the collecting ducts is increased.
5. More water is then reabsorbed into the blood plasma from the tubules and ducts.
6. The reabsorption of water increases the water concentration in the plasma so that the osmotic pressure of the blood is decreased.
7. The response is decreased osmotic pressure of the blood. This has eliminated or reduced the original stimulus that was increasing osmotic pressure of the plasma. Negative feedback has occurred.

Question 5

Regulating Water Intake

Answer 5

• Water is continually lost from the body in sweat, urine, faeces and exhaled breath.
• At times of strenuous activity this water loss can be quite high.
• As water is lost from the blood, the plasma becomes more concentrated, has a lower water content and hence a higher osmotic pressure.
• As a result, water moves from the intercellular fluid into the plasma by osmosis.
• Now the intercellular fluid is more concentrated and water diffuses out of the cells, so that the cells start to shrink from dehydration.

Question 6

Water Balance - Nervous Control

Answer 6

1. As water is lost from the various body fluids, there is a reduction in plasma volume and an increase in osmotic concentration of the extracellular fluid.
2. Osmoreceptors in a thirst centre in the hypothalamus detect the rising osmotic concentration of the blood. Other stimuli such as a dry mouth are also involved.
3. Stimulation of the thirst centre makes the person feel thirsty.
4. The conscious feeling of thirst stimulates the person to drink.
5. The fluid consumed is absorbed into the plasma from the alimentary canal.
6. As the blood circulates through the body, it enables the intercellular fluid and intracellular fluid to return to the proper osmotic concentration.
7. After drinking, the thirst centre is no longer stimulated and the desire to take in water ceases. This is another negative feedback loop.

Question 7

Too much and too little water

Answer 7

• Dehydration is when water loss exceeds water intake:
• Symptoms of dehydration become noticeable when a person has lost about 2% of the normal body water.
• The loss may be through sweating, vomiting or diarrhoea.
• Symptoms of dehydration include severe thirst, low blood pressure, dizziness and headache.
• It is possible to have too much water in the body. This is called water intoxication, or sometimes water poisoning:
• It occurs when body fluids become diluted and cells take in extra water by osmosis.
• This may happen if a person loses a lot of water and salts through sweating and replaces the loss with plain water.
• The first sign of water intoxication is usually light-headedness. Headache, vomiting and collapse may follow.