Lecture 36 4/11/24 Flashcards
What is the importance of osmolarity?
-cells of the body must be bathed with extracellular fluid with a relatively constant conc. of electrolytes
-must be precisely regulated to prevent cells from shrinking or swelling
What regulates ECF osmolarity and NaCl conc.?
the amount of extracellular water
What controls total body water?
-fluid intake
-renal water excretion
What happens when body fluid osmolarity increases above normal?
-post. pituitary secretes more ADH
-ADH increases permeability of distal tubules and collecting ducts to water
-water reabsorption is increased and urine volume is decreased without altering the rate of solute excretion
What is the maximum diluted urine the kidney can excrete?
20 L/day with a conc. of 50 mOsm/L
How does urine become dilute as it passes through the nephron?
-in proximal tubules, fluid remains isosmotic
-in thick ascending LOH, fluid becomes diluted (not permeable to water)
-in distal and collecting tubules, fluid becomes more dilute IF there is an absence of ADH (only permeable to water when ADH is present)
Why is it important that the kidney can form concentrated urine?
minimizes the fluid intake required to match what is lost via lungs/feces/skin/urine
How do the kidneys respond to a water deficit in the body?
excreting solutes while increasing water reabsorption and decreasing urine volume
What is obligatory urine loss?
the minimum volume of urine that must be excreted in order to get rid of metabolic waste and ingested electrolytes
What is urine specific gravity?
measure of the weight of solutes in a given volume of urine, determined by number and size of the molecules
When is it possible for urine specific gravity to falsely suggest a highly concentrated urine?
when there are significant amounts of large molecules in the urine, such as glucose, radiocontrast media, or some antibiotics
What are the basic requirements for forming a concentrated urine?
-high level of ADH
-high osmolarity of renal medullary interstitial fluid
Why is it important that the renal medullary interstitium is typically hyperosmotic?
when ADH levels are high, water will move through the tubular membrane via osmosis into the renal interstitium due to the conc. gradient
What are the components of the countercurrent mechanism?
-countercurrent multiplier system in the nephron loops of juxtamedullary nephrons
-recycling of urea in the medullary collecting ducts
-countercurrent exchanger in the vasa recta
What are the major factors that contribute to a high solute conc. in the renal medulla?
-active transport of Na+ and co-transport of K+, Cl-, and other ions out of the thick ascending LOH into the interstitium
-active transport of ions from the collecting ducts into the interstitium
-facilitated diffusion of urea from the inner medullary collecting ducts into the interstitium
-diffusion of only small amounts of water from the medullary tubules into the interstitium
Why is it important that the descending limb of the LOH is permeable to water?
allows for the tubular fluid osmolarity to quickly become equal to the renal medullary osmolarity
What is the countercurrent multiplier?
-repetitive reabsorption of NaCl by the thick ascending LOH
-continued inflow of new NaCl from the proximal tubule into the LOH
Why does the early distal tubule further dilute the tubular fluid?
it actively transports NaCl out of the tubule while being relatively impermeable to water
Why is it important that large amounts of water is reabsorbed into the renal cortex and not the medulla?
helps preserve the high medullary interstitial fluid osmolarity
What are the characteristics of urea?
-contributes 40-50% of the interstitial osmolarity in the renal medullar interstitium
-passively reabsorbed from the tubule
-large amounts are passively reabsorbed when ADH is high
What is the mechanism for reabsorption of urea into the renal medulla?
-little urea is reabsorbed in the ascending LOH and collecting tubules; segments are impermeable to urea
-with high ADH, water is reabsorbed rapidly from cortical collecting tubule; urea conc. increases rapidly
-more water reabsorption occurs in inner medullary collecting ducts, further concentrating urea
-high concentration of urea eventually causes urea to diffuse out of tubule and into renal interstitial fluid
What are the characteristics of urea transporters?
-facilitate urea diffusion
-activated by ADH
How does malnutrition impact urine concentration?
-malnutrition is associated with low urea concentration
-causes considerable impairment of urine-concentrating ability
What determines the rate of urea excretion?
-concentration of urea in the plasma
-GFR
-renal tubular urea reabsorption
