Exam #4: Urine Concentration & Dilution Flashcards
Explain the importance of urea recirculation in the medulla of the kidney.
The recirculation of urea into the medulla of the kidney provides the kidney with roughly half of its ability to concentrate urine
Where is urea reabsorbed?
1) 50 % of the filtered urea is reabsorbed in the proximal tubule (BUT, urea is later secreted back into the Loop of Henle)
2) 60% urea is reabsorbed again in the meduallary collecting ducts
*Note that roughly 40% of the filtered urea is excreted
Where is urea secreted?
1) Urea is secreted in the medullary thin tubule of the Loop of Henle
Explain why a high-protein diet can enable people to concentrate their urine better than people with low protein intake.
Concentrated urine requires two things:
1) ADH
2) Urea
Thus, people that have a high protein diet & more urea, concentrate their urine better. Conversely, the malnourished have difficulty concentrating their urine.
Identify the portions of the loop of Henle that reabsorb sodium and chloride.
1) Thick ascending limb reabsorbed Na+/K+/2Cl- by secondary active transport/ cotransport
Identify the portions of the loop of Henle that are permeable to water.
1) Proximal convoluted tubule
2) Descending limb–Loop of Henle (via aquaporin-1)
Identify the portions of the loop of Henle that are impermeable to water.
1) Thin ascending limb–Loop of Henle
2) Thick ascending limb–Loop of Henle
State which part of the nephron is responsible for countercurrent multiplication.
Loop of Henle
What is countercurrent multiplication?
Deposition of NaCl into the deeper regions of the kidney via repetition–or multiplication–of single effect & second step.
1) Single effect= Na+-K+-2Cl- reabsorbed WITHOUT water in THICK ASCENDING LIMB–>dilution of TUBULAR FLUID & increased osmolarity of interstitial fluid
2) Second step: tubular flow= descending limb of Loop of Henle IS PERMEABLE to water; thus, water flow INTO ISF to equilibrate osmolarity
Identify the kidney structure that performs countercurrent exchange.
Vasa Recta
Note that this is the process that helps MAINTAIN the corticopapillary osmotic gradient that was ESTABLISHED by countercurrent multiplication.
State if countercurrent exchange involves essentially passive or active transport.
Countercurrent exchange is purely a PASSIVE process (vs. active and passive processes involved in countercurrent multiplication).
How does the process of countercurrent exchange allow medullary blood flow and removal of some medullary interstitial fluid without causing the washout of the concentration gradients in the renal medullary interstitium?
Countercurrent multiplication and urea recycling constantly replace the solute that is carried away with the blood
Describe the effect of ADH (vasopressin) concentration on the cortical collecting ducts and the medullary collecting ducts
1) ADH increases the Na+/K+/2Cl- cotransporter in the THICK ASCENDING LIMB, increasing the “single effect” of countercurrent multiplication.
2) ADH increases water permeability in the cortical & outer medullary collecting ducts, BUT it DOES NOT increase UREA PERMEABILITY; water reabsorbed but urea left behind
3) In the inner medullary collecting ducts, BOTH water & urea are reabsorbed by the action of ADH; ADH adds to the corticopapillary osmotic gradient of the inner medulla
Which area of the nephron can develop the highest osmolarity in the presence of ADH?
Medullary collecting duct
Describe the compensatory responses to dehydration.
- Dehydration causes LOW blood volume and HIGH blood osmolarity, which BOTH increase ADH secretion
1) Decreased ANP
2) Increased sympathetics i.e. renin & angiotensin II
3) Increased osmolarity= decreased ADH
