Regulation of Body Fluid Osmolarity- Rao

Question 1

Explain the body’s response to hyperosmotic plasma starting with the hypothalamic actions

Answer 1

Activation of osmoreceptors in thirst centers augments thirst drive; drink more water to decrease osmolarity

activation of osmoreceptors in supraoptic nucleus of hypothalamus increases synaptic activity with posterior pituitary, releasing AVP. This increases water permeability of distal nephron, excreting the hyperosmotic urine.

Question 2

Is AVP release more sensitive to hypervolemia or hyperosmolarity?

Answer 2

Hyperosmolarity; it takes about 10% dec in ECFV to activate AVP

Question 3

Explain how GI fluid loss in vomiting and diarrhea can lead to hyponatremia

Answer 3

Consumption of fluid but not food causes volume depletion, releasing AVP which concentrates urine and dilutes the plasma; the conservation of water dilutes the plasma and reduces osmolarity.

Question 4

What is the formula for osmolar clearance?

Answer 4

Cosm = (UF*Uosm)/Posm

Question 5

What is the formula for free water clearance?

Answer 5

CH2O = UF - (UF*Uosm)/Posm

Question 6

What are the features of the kidney responsible for developing medullary hyperosmolarity?

Answer 6

1. Anatomical arrangement of Loop, vasa recta, and peritubular capillaries
2. Active transport of Na and co-transport of K+/Cl- out of TALH into medullary ISF
3. Active transport of Na+ from CD to ISF
4. Passive diffusion of urea from medullary CD to medullary ISF
5. Diffusion of only a little water from medullary tubule to interstitium

Question 7

Why don’t medullary capillaries wash out the interstitial hyperosmolarity?

Answer 7

Low medullary blood flow

Vasa recta serves as countercurrent

Question 8

What is central diabetes insipidus?

Answer 8

Pituitary fails to release AVP, resulting in rapid dehydration. Rare congenital

Question 9

What is nephrogenic diabetes insipidus?

Answer 9

CD does not respond to AVP due to V2 receptor or AQP2 mutation.

Also may be caused by lithium and tetracycline

Question 10

What are some non-congenital causes of lost medullary hyperosmolarity?

Answer 10

Furosemide; excessive fluid delivery to LOH; decreased urea production; age and renal failure