Lecture 17: Urine concentration-Exam 3 Flashcards
Excretion of dilute urine in ascending thick of loop of Henle
Na+,K+and Cl- avidly reabsorbed
Segment is impermeable to water
Tubular fluid is less concentrated
Whether ADH is present or not does not matter
Excretion of dilute urine in distal convoluted tubule
Additional reabsorption of NaCl
Impermeable of water in absence of ADH
Osmolarity reached 50 mosm/L
Excretion of concentrated urine
Kidney can produce a maximum urine concentration between 1200 to 1400 mosm/L
Required presence of ADH and high osmolarity of medullary interstitial fluid necessary for water reabsorption
Obligatory urine volume
70 Kg man must excrete about 600 mosm of solutes each day to get rid of waste material
max urine concentration=1200 mosm/L
Obligatory urine volume=600mosm/day/1200mosm/L=0.5 L/day
Actions of proximal tubule in urine formation
Reabsorb 65% of filtered electrolytes
Highly permeable to water
osmolarity= 300 mosm/L
Actions of descending tubule in urine formation
Highly permeable to water
Less permeable to sodium and urea
osmolarity increases to 1200 mosm when ADH concentration is high
Actions of thin ascending loop in urine formation
Impermeable to water
Reabsorbs NaCl and fluid becomes more dilute
Urea absorbed into interstitium from collecting tubules diffuses
Actions of thick ascending tubule
Impermeable to water
Large amounts of NaCl,K+ and other solutes transported from tubules into medullary interstitium
osmolarity=100mosm/L
Actions of early distal tubule
Similar to thick ascending tubule
tubular fluid more dilute to 50 mosm/L
Actions of late distal tubule and cortical collecting duct
Osmolarity depends on ADH
Urea is not very permeant
Actions of inner medullary collecting duct
Osmolarity depends on ADH and osmolarity of surrounding interstitial osmolarity
Urine specific gravity
Measure of the weight of solutes in a given volume of urine
The more concentrated the urine, the higher the urine specific gravity
Countercurrent multiplier mechanism
Major reason for high medullary osmolarity is active transport of sodium and cotransport of potassium, chloride and other ions from the thick ascending loop of Henle
Pump capable of establishing about 200 mosm
Fluid flows through the proximal tubule which has a concentration of 300 mosm/L
Solutes are reabsorbed from the thick ascending loop which caused the fluid to be more diluted. Drop of osmolarity to 200 mosm/L
Interstitial fluid rises to 400 mosm/L
as more fluid flows in the water is reabsorbed causing increasing osmolarity to 400 mosm/L
Role of urea in concentrated urine
Ascending loop of Henle and distal cortical collecting tubule are impermeable to urea
High concentration of ADH causes:
-water is reabsorbed from cortical collecting tubule and urea is not very permeant and fluid becomes more concentrated in tubule
-more water is reabsorbed from medullary collecting duct resulting in a higher concentration of urea
Higher concentration of urine results in diffusion of urea into interstitial fluid which is facilitated by UT-A1 and UT-A3 (ADH activated) transporters
Simultaneous movement of water and urea out of the inner medullary collecting ducts maintains a high concentration of urea in the tubular fluid and eventually in the urine even tho urea is being reabsorbed
Osmoreceptor-ADH feedback mechanism
Controls extracellular fluid sodium concentration and osmolarity
Increase extracellular fluid osmolarity causes shrinking of osmoreceptor cells in anterior hypothalamus which lead to action potentials
Action potentials release ADH which increases water permeability in distal nephron segments
ADH is formed in magnocellular neurons in supraoptic and paraventricular nuclei
Osmoreceptors cells in the vicinity of the AV3V region in the anterior region of third ventricle
