Urine Concentration and Dilution Flashcards
Average urine produced per day
1.5L
When is angiotensin released?
Low salt, low water
Effect of angiotensin release
Constrict efferent arteriole preferentially
Increased glomerulus pressure, GFR
What do ACE inhibitors do?
Inhibit constriction of efferent arteriole
Location of osmoreceptor cells
Anterior hypothalamus
Does water come from metabolism?
Yes
From aerobic respiration
Effect of negative water balance on osmolarity
ECF osmolarity will increase
In what is a negative water balance most quickly detected?
ECF osmolarity, not plasma volume
Negative water balance spread through all tissues in the body, and there is more water in ECF than in blood
Effect of vasopressin
Promotes aquaporin recruitment into kidney tubule walls
What do aldosterone and angiotensin II affect?
Na balance
NOT water balance (at least not directly)
Hormones that result in more concentrated urine
Aldosterone
Angiotensin II
Vasopressin
Most important hormone for water retention
Vasopressin
Can make concentrated urine without aldosterone and angiotensin II, but not without vasopressin
Why can concentrated urine be made without aldosterone and angiotensin II but not without vasopressin?
Vasopressin makes the collecting duct permeable to water with aquaporins
Aldosterone increases Na reabsorption in collecting duct –> if duct is impermeable to water, osmotic gradient has no effect
Angiotensin II promotes Na reabsorption in proximal convoluted tubule. Water is reabsorbed here, but not in the collecting duct.
What increases osmolarity?
Loss of water from the body
Normal setpoint for body osmolarity
300mM
Role of proximal tubule
Bulk absorption
Role of ascending limb
Dilution of filtrate
Role of collecting ducts
Fine tuning of filtrate concentration according to needs
Necessary amount of solute to excrete each day
600mOsm
Necessary amount of water to excrete each day
500mL
Urine osmolarity range
3 - 1200 mOsm/Kg of water
Role of descending limb
Concentrates urine
Urine concentration after the loop of Henle
More dilute than when it entered loop of Henle
This can be modified in the collecting duct
Factors contributing to medullary concentration gradient
Na
Urea (from collecting duct)
Medullary concentration gradient
300mOsm at cortex
1200mOsm at base of loop of Henle in medulla
Why can’t urine exceed a concentration of 1200mOsm?
1200mOsm is the highest concentration in the medullary concentration gradient
State of fluid in distal convoluted tubule
Isoosmotic
What is reabsorbed in the descending limb?
Only water
What is reabsorbed in the ascending limb?
Only ions (no water)
State of fluid in thick ascending limb
Hypoosmotic
Cotransporter in the luminal membrane of thick ascending limb
Na/K/2Cl cotransporter
Transporters on the basal surface of thick ascending limb
Cl- transporter
K+ transporter
K+/Cl- costransporter
Na+/K+ antiport (active) (Na+ into interstitial fluid, K+ into cell)
Role of Na/K antiport in thick ascending limb
Helps drive K/Cl cotransporter
Moves Na out of the cell into the interstitial fluid
How does vasopressin work?
1) Vasopressin binds membrane receptor on basal surface of collecting duct
2) cAMP secondary messenger system activated
3) Aquaporin 2-containing vesicles fuse with luminal wall, deposit aquaporin 2
4) Water absorbed osmotically into blood (vasa recta)
