Component 3.4 - Homeostasis/Ultrafiltration/Selective Reabsorption

Question 1

How does negative feedback work? (5)

Answer 1

1) Set point determined by control centre
2) A receptor monitors a condition
3) Receptor sends instructions to a co-ordinator
4) Co-ordinator communicates with effector(s) that bring about corrective procedures
5) Restores a set point

• corrected by negative feedback

Question 2

What is negative feedback?

Answer 2

A change in a system produces a second change which reverses the first change

Question 3

What is the difference between excretion and egestion?

Answer 3

Excretion is the removal of metabolic waste made by the body and egestion is the removal of waste not made by the body

Question 4

What is osmoregulation?

Answer 4

The control of the water content and solute composition of body fluids e.g blood, tissue fluid and lymph

Question 5

What parts of the nephron are in the cortex and which parts are in the medulla?

Answer 5

The glomerulus, proximal and distal convoluted tubules are in the cortex.
The loop of Henle and the collecting duct are in the medulla

Question 6

Why is the pressure in the afferent arteriole high?

Answer 6

1) Ventricular systole

Question 7

How is the blood in the glomerulus separated from the Bowman’s capsule?

Answer 7

1) Wall of the capillary (single layer of epithelium)
2) Basement membrane - is a selective barrier
3) The wall of the Bowmans capsule - made of podocytes which wrap around capillary pulling it closer

Question 8

What is ultrafiltration?

Answer 8

Filtration under high pressure

Question 9

What does not pass into the bowmans capsule?

Answer 9

Blood cells, platelets, large proteins

Question 10

How are molecules forced out of the glomerulus?

Answer 10

Afferent arteriole is wider than efferent arteriole. Creates high pressure, along with hydrostatic pressure (as water has not been lost yet) forcing molecules out.

Question 11

What is selective reabsorption?

Answer 11

The uptake of specific molecules and ions from the glomerular filtrate in the nephron back into the blood

Question 12

What is absorbed in the proximal convoluted tubule?

Answer 12

All glucose and amino acids, some urea, most water and sodium and chloride ions

Question 13

What are the adaptations of the proximal convoluted tubule?

Answer 13

1) large SA because it is long and there are millions of nephrons
2) Microvilli and basal channels which increases the surface area of the cuboidal epithelium
3) Many mitochondria
4) Close association with capillaries
5) Tight junctions preventing seepage of reabsorbed materials back into filtrate

Question 14

How are molecules reabsorbed in the PCT?

Answer 14

• mineral ions by active transport
• glucose and amino acids by secondary active transport using co-transport mechanism with Na+
• water leaves by osmosis
• urea and small proteins are reabsorbed by diffusion

Question 15

Why might there be a high concentration of glucose in the glomerular filtrate and/or it be present in the urine?

Answer 15

May be too few transport molecules in the membrane to absorb it all and this could be because:

1) The pancreas secretes too little insulin
2) The response of liver cells to insulin is reduced because insulin receptors in the surface membrane is damaged.

Question 16

What happens in terms of the mechanism of water absorption in the ascending limb?

Answer 16

• Walls are impermeable to water
• They actively transport sodium and chloride ions into the tissue fluid in the medulla (low water potential)
• As the filtrate climbs it contains fewer ions and becomes more dilute (higher water potential)

Question 17

What happens in terms of the mechanism of water absorption in the descending limb

Answer 17

• Walls slightly permeable to water
• Due to low water potential of tissue fluid caused by Na+ and Cl-, water diffuses out of the descending limb by osmosis and is carried away by vasa recta
• Contents of descending limb become more concentrated as they reach tip of loop of Henlé due to the loss of water

Question 18

How does having fluid flowing up in one and down in the other limb help?

Answer 18

It enables an osmotic gradient to be maintained down to the tip of the loop of Henlé and for the maximum concentration to be built up at the base of the loop. The mechanism is the counter-current multiplier.

Question 19

Why does water diffuse out the collecting duct?

Answer 19

Because it passes back down into the region of low water potential. Water therefore diffuses out by osmosis.

Question 20

What is homeostasis and why is it important?

Answer 20

Describes the mechanisms by which a constant internal environment is achieved e.g core body temperature

Important so cell’s of the body can function efficiently independent of fluctuations in external environment and even during levels of activity of the organism