Renal VI: Hormonal Regulation of Body Water

Question 1

Where does ADH originate?

Answer 1

It is synthesized as a peptide prohormone in the neurons of the hypothalamus, travels down the axons, and terminates in the posterior pituitary. It is released in the circulation in response to EC hyperosmolality and volume depletion.

Question 2

Which sensors stimulate vs. inhibit ADH release?

Answer 2

• stimulate: hypothalamic osmoreceptors

- inhibit: aortic arch and carotid baroreceptors

Question 3

What does ADH target in the kidney?

Answer 3

extracellular ADH receptors in the distal tubules and collecting duct

Question 4

Does angiotensin II stimulate or inhibit ADH release?

Answer 4

stimulate

Question 5

Does atrial natriuretic peptide (ANP) stimulate or inhibit ADH release?

Answer 5

inhibit (because ANP acts to reduce BP and ADH acts to increase BP)

Question 6

What is our thirst threshold?

Answer 6

It is near 298 mOsm/kg H2O (the plasma osmolality at which we begin feeling thirsty)

Question 7

How much change in blood volume/pressure is required before AVP secretion is stimulated?

Answer 7

5-10% decrease

Question 8

What are some ADH disorders?

Answer 8

• central (pituitary) diabetes insipidus: inadequate ADH release, often due to hypothalamic damage
• nephrogenic diabetes insipidus: ADH has trouble exerting effects due to mutation in V2 receptor
• syndrome of inappropriate ADH secretion (SIADH): too much ADH secretion

Question 9

Total body water is about what percentage of body weight?

Answer 9

60%

Question 10

What is the extracellular fluid composed of?

Answer 10

interstitial fluid and plasma

Question 11

Does ICF or ECF make up more of total body water?

Answer 11

ICF (2/3 of TBW, whereas ECF is 1/3 of TBW)

Question 12

What are the major cations and anions of the ICF and ECF?

Answer 12

• ICF cations: K+ and Mg2+
• ICF anions: proteins and inorganic phosphates
• ECF cations: Na+
• ECF anions: Cl- and HCO3-

Question 13

The anion gap represents the difference between what?

Answer 13

measured cations and measured anions in plasma or urine [Na+ - (Cl- + HCO3-)]

Question 14

How are fluid volumes measured by the dilution method (what is the equation)?

Answer 14

V space = (Amt. given - Amt. lost) / equilibrium in space

Question 15

What is ADH released in response to?

Answer 15

hyperosmolality and volume depletion

Question 16

How do we measure the different fluid volumes?

Answer 16

• TBW: w/ D2O
• ECF water: w/ radiosodium or radiosulfate
• Plasma water: w/ Evans Blue
• IC water: TBW - EC water
• IS water: EC water - Plasma water

Question 17

Example of isosmotic volume contraction?

Answer 17

diarrhea

Question 18

Example of hyperosmotic volume contraction?

Answer 18

water deprivation

Question 19

Example of hyposmotic volume contraction?

Answer 19

adrenal insufficiency (insufficient aldosterone, which normally increases sodium reabsorption)

Question 20

Example of isosmotic volume expansion?

Answer 20

infusion of physiologic saline (isotonic NaCl)

Question 21

Example of hyperosmotic volume expansion?

Answer 21

high NaCl intake, like gulping sea water

Question 22

Example of hyposmotic volume expansion?

Answer 22

syndrome of inappropriate ADH (SIADH)

Question 23

What is the major way in which the anion gap is used in clinical practice?

Answer 23

to identify the cause of metabolic acidosis (low blood pH)

Question 24

What is the anion gap mainly composed of?

Answer 24

albumin (but also can contain phosphate, sulfate, and other anions)