Tonicity, volume, RAAS

Question 1

what stimulates vasopressin secretion

Answer 1

Increased effective osmotic pressure of plasma.
Decreased ECF volume.
Pain, emotion, stress, exercise.
Nausea and vomiting.
Standing.
Carbamazepine.
Angiotensin II.

Question 2

what decreases vasopressin secretion

Answer 2

Decreased effective osmotic pressure of plasme.
Increased ECF volume.
Alcohol.

Question 3

define tonicity

Answer 3

Refers to the effective osmolality.
The concentration of solutes that exert an osmotic force across cell membranes, mainly Na, glucose and urea.

Question 3

what is a normal plasma osmolality

Answer 3

280-290mOsm/kg

Question 4

what stimulates thirst

Answer 4

1. increased plasma osmolality - Detected by osmoreceptors in the hypothalamus
2. ↓ ECF volume / pressure - Detected by baroreceptors in the heart, great vessels, and kidneys (via RAAS); stimulates thirst indirectly via angiotensin II.
3. dry mouth, psychological/anticipatory

Question 5

threshold for activation of thirst and ADH

Answer 5

Thirst is triggered when plasma osmolality increases by ~2–3%.

ADH release begins at ~1% increase in osmolality.

Volume depletion needs ~10% or more blood loss to strongly trigger thirst.

Question 6

where is renin released

Answer 6

From juxtaglomerular (JG) cells in the afferent arteriole of the kidney

Question 7

what stimulates renin release

Answer 7

• ↓ Perfusion pressure
• ↓ Na⁺ in distal tubule (sensed by macula densa)
• ↑ Sympathetic activity (β₁ receptors)

Question 8

what are the actions of angiotensin II

Answer 8

Vasoconstrictor.
- ↑ TPR via vasoconstriction (especially of arterioles)
- Constricts efferent arteriole → maintains GFR during low perfusion

Stimulates aldosterone secretion → ↑ Na⁺ reabsorption in distal nephron

Stimulates thirst and ADH release via hypothalamus

Directly enhances Na⁺ uptake in proximal tubule

Question 9

what is aldosterone’s role

Answer 9

Acts on the distal convoluted tubule and collecting duct

Promotes Na⁺ reabsorption and K⁺/H⁺ excretion

Result: Water follows Na⁺, increasing ECF volume

Question 10

what stimulates renin secretion

Answer 10

↓ Renal perfusion pressure (renal baroreceptor)

↓ NaCl delivery to macula densa

↑ Sympathetic stimulation (β₁)

Question 11

what inhibits renin secretion

Answer 11

Angiotensin II (negative feedback)

ANP (Atrial Natriuretic Peptide)

Increased Na⁺/Cl⁻ at macula densa