3.6.4.3 Control of blood water potential

Question 1

2 main functions of the kidneys

Answer 1

1. Removal of nitrogenous metabolic waste from the body (urea)
2. Osmoregulation - balance of water and dissolved solutes

Question 2

What is deanimation?

Answer 2

production of urea in the liver

NH2 group removed from amino acid, becomes ammonia then urea

Question 3

What is ultrafiltration?

Answer 3

filtration under high hydrostatic pressure created in the glomerulus

Question 4

How does ultrafiltration work?

Answer 4

• capillary endothelial wall has fenestrations
• pores in basement membrane
• podocyctes create filtration slits in the epithelial layer of capsule
• Small molecules able to pass through into bowman’s capsule as filtrate (urea, water, glucose, amino acids, salt ions, vitamins and minerals)
• large molecules too big to pass through (red blood cells, white blood cells, palms proteins, platelets

Question 5

How are cells of the PCT adapted for selective reabsorption?

Answer 5

• Microvilli - large surface area
• more RER /ribosomes / Golgi - make more transport proteins
• more membrane proteins - for more AT/FD/CO-T
• more mitochondria - more ATP for active transport

Question 6

What is reabsorbed and by what transport method?

Answer 6

• All glucose and amino acids by co transport with Na+
• Most water by osmosis
• Most salt ions by facilitated diffusion / active transport
• no urea reabsorbed

Question 7

How does the loop of Henle provide an osmotic gradient in the medulla for reabsorption of water?

Answer 7

• high concentration of salts with low water potential in the medulla
• reabsorption of water by osmosis from descending limb and collecting duct (reduces volume of filtrate)
• Ascending limb impermeable to water, Na+ actively transported into tissue fluid of medulla
• Descending limb permeable to water, water moves out of filtrate by osmosis down water potential gradient

Question 8

Long vs short loop of Henle

Answer 8

• Longer loop - looses more ions, creates a lower water potential in the medulla than other organisms
• e.g. low fresh water availability, lower water potential gradient, more water out of CD by osmosis, lower volume urine
• Shorter loop - lose less ions, creates a higher water potential in the medulla than other organisms
• e.g. large availability of fresh water, lower water potential gradient, less water moves out of CD by osmosis, higher volume urine

Question 9

Osmoregulation using ADH - how does increased salt / solute intake reduce blood water volume?

Answer 9

1. Osmoreceptors in hypothalamus detect a decrease in water potential of blood
2. hypothalamus sends impulse to posterior pituitary gland, which releases ADH into blood
3. ADH travels via blood to kidneys
4. ADH binds to receptors on membrane of collecting duct cells, increasing permeability of membranes by increasing number of aquaporins inserted into membrane
5. more water moved out of collecting duct into medulla by osmosis
6. urine volume decreases, urine concentration increases

Question 10

Describe what would happen if the water potential of blood increased

Answer 10

1. Increase detected by osmoreceptors in hypothalamus
2. Posterior pituitary gland releases LESS ADH
3. ADH travels through blood to cells of collecting duct
4. collecting duct cell membranes become less permeable - have less aquaporins
5. less water moves out of the collecting duct into the medulla by osmosis
6. urine volume increases, urine concentration increases