23B: Homeostasis - Osmoregulation

Question 1

Describe the 2 main functions of the Kidney:

Answer 1

• Excretion: filtration of blood to remove urea
• Osmoregulation: kidneys maintain blood at a constant water potential, despite wide variations in salt + water intake

Question 2

What are the 3 main regions of the Kidney?

Answer 2

• outer cortex
• medulla
• the pelvis: region draining urine into ureter

Question 3

Describe Ultrafiltration:

Answer 3

• the afferent arteriole is wider than the efferent arteriole, which creates a high hydrostatic pressure in the glomerulus (network of capillaries)
• water + small molecules (e.g glucose + ions + urea) are forced out of capillaries into the lumen of the Bowman’s Capsule, forming filtrate

Question 4

Which components are not found in the filtrate, why not?

Answer 4

• red blood cell, platelets & plasma proteins
• they are too large to fit through the filtration system

Question 4

Describe the Selective Reabsorption in Proximal Convoluted Tube:

Answer 4

1. sodium ions are actively transported out of the proximal convoluted cells into the blood
2. this maintains a low concentration of sodium ions in the proximal convoluted cells, which creates a concentration gradient between the filtrate from Bowman’s Capsule and then proximal convoluted cells
3. this allows glucose + sodium ions to be transported into proximal convoluted cells from the glomerulus filtrate, via specific co-transport carrier proteins (example of co-transport)
4. glucose leaves proximal convoluted cells into the blood via facilitated diffusion
5. the transport of sodium ions and glucose into proximal convoluted tube reduces water potential of the cells, so water moves from filtrate into proximal convoluted cells via osmosis

Question 5

Describe 2 adaptations of the cells lining the proximal convoluted tubes:

Answer 5

1. many microvilli: increases the surface area for a higher level of absorption
2. many mitochondria means that aerobic respiration can occur, so more ATP can be produced for the active transport of sodium ions into the blood from the proximal convoluted tube cells

Question 6

Describe the Role of the Loop of Henle:

- describe what happens in the descending limb

Answer 6

• carries filtrate down the medulla
• it is permeable to water, but impermeable to sodium ions
• therefore, water will leave the filtrate in the loop of henle by osmosis, into the tissue fluid
• filtrate in the loop becomes more concentrated so the water potential decreases

Question 7

Describe the Role of the Loop of Henle:

- describe what heppens in the ascending limb

Answer 7

• carries filtrate up the medulla
• it is impermeable to water, but permeable to sodium ions
• at the lower end of the limb, there is a high concentration of sodium ions, so they will move out of the filtrate into the tissue fluid of the medulla via facilitated diffusion
• at the upper end of the limb, there is a lower concentration of sodium ions, so sodium ions are actively transported out of the ascending limb into the tissue fluid of medulla via active transport
• this creates a concentration gradient of sodium ions down the medulla, with the lower end of limb having a high sodium concentration, and the higher end having a low concentration of sodium ion

Question 8

How is water reabsorbed in the Distal Convoluted Tubule and the Collecting Duct?

Answer 8

• the tissue fluid of the Medulla will be more concentrated at this stage of the reabsorption
• however, the cells of the distal convoluted tubule would have a high water potential due to having lost many sodium ions in the proximal convoluted tubule
• therefore in the distal convoluted tubule and the collecting duct, water moves out via osmosis into the tissue fluid of medulla
• this allows the water to be reabsorbed into blood capillaries (vasa recta)

Question 9

Describe the Control of Water Potential by Negative Feedback when there is a Decrease in Water Potential:

Answer 9

• osmoreceptors in the hypothalamus detect decrease in water potential
• hypothalamus sends nerve impulses to the posterior pituitary gland
• this is stimulated to release more ADH into blood
• this causes the cells of the collecting duct to be more permeable to water
• more water leaves the collecting duct, by osmosis, and is reabsorbed into the blood from the tissue fluid
• a low volume of highly concentrated urine is produced

Question 10

How does ADH work?

Answer 10

• increases transcription of the genes coding for aquaporins
• this means more aquaporins are produced, and inserted into the membranes of cells making up the wall of the collecting duct
• more water moved, by osmosis, from the filtrate in the collecting duct into the medulla and then reabsorbed into the vasa recta
• when ADH decreases, these aquaporins are removed from membranes, decreasing permeability

Question 11

Why are aquaporins vesicles produced?
Which organelle produced aquaporins?

Answer 11

• produced so aquaporins are available right away, more time-efficient
• golgi body