Lecture 12: Osmoregulation

Question 1

2 sources of body water

Answer 1

1. Ingested
2. Organic nutrient oxidation

Question 2

Ways of losing body water

Answer 2

Insensible:
- Evaporation from skin, respiratory
Sensible:
- GI, menstrual, urine

Question 3

H2O reabsorption by tubule segment

Answer 3

• 2/3rds in PT following solutes
• 23% descend. limb
• 0% ascend. limb
• CD: AQP2 regulation by ADH (principal cells)

Question 4

Renal urea recycling

Answer 4

• 50% reabsorbed in PT
• Secreted in thin descend. limb
• Reabsorbed w/ H2O in DT/CD (UT1 transporters, stim. by ADH)

Question 5

Vasa recta countercurrent exchangers

Answer 5

• Medullary peritubular capillaries = vasa recta
• Capillary/venule hairpins

Question 6

Effect of dehydration on osmoregulation

Answer 6

Dehydration -> ADH secretion -> H2O/urea reabsorption -> low vol. of concentrated urine formed

Question 7

2 factors for ADH secretion

Answer 7

1. Plasma osmolarity
2. ECF/blood volume
   Sensed by baro/osmoreceptors signaling to hypothalamus

Question 8

Effects of ADH

Answer 8

1. Increase principal cell H2O permeability (AQP2 insertion)
2. Increase urea recycling via more medullary CD urea permeability
3. Increase thick AL NKCC transporters (more countercurrent multiplier)
4. Less vasa recta blood flow (less washout)
5. More Na+ reabsorption, K+ secretion in CD principal cells

Question 9

How does alcohol affect ADH?

Answer 9

Alcohol directly inhibits ADH pituitary secretion

Question 10

Diabetes insipidus

Answer 10

Loss of ADH action leading to polyuria

Question 11

Free water

Answer 11

H2O free of solutes

Question 12

Free water clearance

Answer 12

Amount of free water that kidneys excrete out of blood plasma per day; represents renal ability to dilute/concentrate urine

Question 13

Free water clearance equation

Answer 13

C_H2O = urine vol. - C_osm;
C_osm = clearance of all solutes = urine [osm] * urine vol. / plasma [osm]