excretion and osmoregulation

Question 1

What is excretion?

Answer 1

Excretion is the removal from the body of waste compounds produced during the metabolism of cells.

Question 2

What are some excretory products and what are they excreted as?

Answer 2

Excess mineral salts and nitrogenous waste products: urea, uric acid and creatinine –> constituent of urine, constituent of sweat but in small quantities (for nitrogenous waste products)
Excess water –> main constituent of urine, sweat, water vapour in expired air
Carbon dioxide –> gas in expired air

Question 3

What are the 5 steps in the formation of urine?

Answer 3

1. Ultrafiltration in the renal (bowman’s) capsule
2. Selective reabsorption in the proximal convoluted tubule
3. Water conservation in the loop of henle
4. Blood pH and ion concentration regulation in the distal convoluted tubule
5. Water reabsorption in the collecting duct

Question 4

Describe the process of ultrafiltration in the renal (bowman’s) capsule.

Answer 4

In the glomerulus, water and relatively small molecules of the blood plasma (e.g. ions, mineral salts, urea, glucose, amino acids) pass out of the capillaries into the lumen of the capsule. This filtrate is known as the glomerular filtrate.
Red blood cells, white blood cells, platelets and proteins are retained in the glomerulus as they are too large to pass out.
(This process is described as ultrafiltration as it is powered by the high pressure of the blood)

Question 5

Describe the process of selective reabsorption in the proximal convoluted tubule.

Answer 5

In the longest section of the nephron, a large part of the glomerular filtrate is reabsorbed into the capillary network via diffusion and active transport (and most of the water is absorbed by osmosis, and also subsequently absorbed at the loop of henle).

Question 6

When is ADH produced / not produced?

Answer 6

When we have drunk a lot of water, the hypothalamus detects this and stops the posterior pituitary gland from secreting ADH.
When we have taken in little water, sweated excessively, eaten salty food, the hypothalamus detects this and directs the posterior pituitary gland to secrete ADH.

Question 7

What is the role of ADH in osmoregulation (water reabsorption in the collecting duct)?

Answer 7

When ADH is absent, collecting duct walls are less permeable. Water stays in the collecting duct and a lot of dilute urine is formed.
When ADH is present, collecting duct walls are permeable. Water diffuses into the medulla and a small quantity of concentrated urine is formed.

Question 8

How does a dialysis machine work?

Answer 8

1. Blood is drawn from the vein in the patient’s arm and pumped through dialysis machine tubing
2. Tubing is bathed in a specially controlled dialysis fluid, walls are partially permeable
3. Small molecules (e.g. urea and other metabolic waste products) diffuse out of the tubing and into dialysis fluid, large molecules (blood cells, platelets) remain in the tubing
4. Filtered blood is then returned to a vein in the patient’s arm