Urinary System, Lecture 3

Question 1

Hormonal Regulation

Answer 1

renin - angiotensin (plasma protein) - aldosterone (RAA) system
- several pathways to JG cells; all start with decreased plasma volume
- renin secretion and release into blood triggers a cascade of activations resulting angiotensin ll
- control release of renin mainly
- renin to angiotensinogen to A1; and then angiotensin converting enzyme converts A1 to A2 (these changes just occur, so mainly the release of renin controlled through three pathways)

Question 2

Hormonal Regulation (2)

Answer 2

RAA system - continued
- angiotensin ll circulates binds adrenal gland cells triggering secretion/release aldosterone into blood
- aldosterone release also from high blood potassium
- aldosterone binds collecting duct cells:
◦ increases number apical membrane channels (
purple/blue) and basolateral primary pumps
◦ results in ion movement (red)(sodium
reabsorption, potassium secretion) and more
water following sodium

Question 3

Hormonal Regulation (vasopressin affect on aquaporin and water)

Answer 3

vasopressin (also known as ADH; antidiuretic hormone) -> controls number of water channels that aquaporin can get through
- number of aquaporins in distal tubule and collecting duct variable; virtually absent in this part of renal tubule unless vasopressin present (controlled mainly by hormones now)
- vasopressin causes insertion of aquaporins on apical membrane
facultative water reabsorption - adapting to need
- variable amount; hormone regulated
- under normal hydration conditions:
◦ 19% reabsorption
◦ 1% urinary excretion (1-2 L/day)

-> dehydration - 19.8% water reabsorbed (0.2% urinary excretion); largely due to vasopressin controlled increase in number of aquaporins (open more water channels)
-> overhydration - as low as 0% water reabsorbed (20% urinary excretion); largely due to vasopressin controlled decrease in number of aquaporins (no facultative/vasopressin has been released so minimal water channels which will maximize elimination)

• facultative “adapting to need” can range from 0-19.8%
• we never make urinary excretion 0 (can be minimal/always some moving down

Question 4

Vasopressin - dehydration

Answer 4

vasopressin - dehydration/sweat losses
- vasopressin high, greater water reabsorption
- maximize it if we have been losing a lot of water
- 19.8% of water reabsorbed
- main stimulus excretion to increase vasopressin is levels in plasma volume changes
- resulting excretion of low volume concentrated urine
* diagram to go with this

Question 5

Hormonal Regulation - overhydration

Answer 5

vasopressin - overhydration
- vasopressin low, collecting duct relatively impermeable to water (minimal aquaporins)
- as low as 0% to water reabsorbed
- main stimulus to decrease vasopressin levels is extracellular osmolarity changes
- resulting excretion of high volume dilute urine
* diagram to go with this

Question 6

Hormonal Regulation - atrial natriuretic peptide (ANP)

Answer 6

atrial natriuretic peptide (ANP)
- released in response to stretch of atria of heart
- alters arteriole radius and sodium reabsorption
- decrease sodium reabsorption mainly in proximal tubules and collecting ducts
- will increase urinary excretion as water follows sodium, resulting in decreasing plasma volume
- can also inhibit the release of aldosterone and vasopressin