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108 Cards in this Set
- Front
- Back
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Why is it essential to keep the ECF constant in terms of concentration of electrolytes?
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So that cells can function normally.
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What largely regulates the ECF sodium concentration?
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The volume of ECF
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What 2 factors control body water?
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1. Fluid intake
2. Renal excretion of water |
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What regulates fluid intake?
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Thirst
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hat regulates Renal excretion of water?
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Many factors which influence GFR and tubular reabsorption.
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What happens to body fluid osmolarity when there is excess water in the body?
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Osmolarity is decreased
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What does the kidney do if the body water is increased and plasma osmolarity is low?
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The kidney will lose more water to retain more solutes - producing a dilute urine.
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How low can the urine osmolarity go?
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As low as 50 mOsm/L
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How high can the urine osmolality increase?
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To 1200-1400 mOsm/L
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What happens to the solute mass in urine if it needs to produce dilute urine?
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The solute concentration WILL NOT change.
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How is it that the kidney can increase water excretion, but not necessarily solute?
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Because ADH, which regulates water reabsorption and secretion, Does not change solute transport.
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How does the ADH feedback system respond to increased ECF osmolarity?
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It is detected by chemoreceptors, stimulates posterior pituitary to secrete ADH; ADH increases the permeability of distal tubules and collecting ducts to reabsorb more water, decreasing the ECF osmolarity.
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HWhen there is a large excess of water in the body, how much can the kidney excrete?
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As much as 20 L/day with a concentration as low as 50 mOsm
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If you drink a liter of water how will it change your:
-Urine Osmolarity -Plasma Osmolarity |
-Urine osmolarity will go down from 600 to as low as 50
-Plasma osmolarity will hardly change |
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How does urine flow rate and solute excretion change after drinking 1 L of water?
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-Urine flowrate increases 6X
-Solute excretion DOES NOT CHANGE |
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What happens to tubular fluid as it flows through the proximal tubule?
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It remains isosmotic to the plasma.
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Why does tubular fluid in the PCT remain isosmotic?
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Because solutes and water are reabsorbed in equal proportions
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What IS the osmolarity of the fluid in the proximal tubule?
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300 mOsm
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What is the tonicity of the ISF around the renal medulla?
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Very high - 1200 mOsm
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What happens to the tubular fluid as it passes down the descending limb of LOH?
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Water goes where the salt is - in the ISF - via osmosis, so the tubular fluid reaches equilibrium and becomes very hypertonic.
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How is it that water can be reabsorbed in the descending limb of Henle's loop?
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The tight junctions are leaky there.
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What happens when fluid hits the thick ascending limb of the loop of henle?
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It becomes impermeable to water but solutes continue to be reabsorbed.
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How does the osmolarity of the tubular fluid change in the thick ascending limb of henle's loop?
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It becomes HYPOtonic - dilute.
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How hypotonic does the tubular fluid become at the early distal tubule?
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100 mOsm
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How does the body produce large amounts of dilute urine?
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-By ADH not being present so failure to reabsorb water
-Continued reabsorption of NaCl at the late distal tubule and collecting ducts |
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Where in the nephron does active NaCl transport absolutely NOT occur?
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-Thin descending limb of LOH
-Thin ascending limb of LOH |
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Why doesn't active transport occur in the thin limbs of LOH?
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Because there's no mitochondria so not enough energy
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What areas of the nephron are absolutely NOT permeable to water?
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-Thin ascending limb
-Thick ascending limb |
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What areas of the nephron depend on ADH for water reabsorption?
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-Late distal tubule
-Cortical collecting duct -Medullary collecting duct |
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What areas of the nephron are permeable to NaCl for passive diffusion?
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-Proximal tubule
-Thin desc/asc limb of LOH |
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What mainly occurs in the descending thin limb of LOH?
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Osmosis of water until the tubule fluid osmolarity equals the renal ISF osmolarity (1200)
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What mainly occurs in the ascending thin limb of LOH?
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-Passive reabsorption of NaCl
-And active secretion of Urea |
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Where is the majority of Urea reabsorption?
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In the inner medullary collecting duct under the control of ADH.
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What is the amt of mOsm we must excrete daily?
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600 mOsm / day
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What is the max concentrating ability of the kidneys?
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1200 mOsm/L
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What is the obligatory urine volume that we must excrete daily?
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0.5 L/day
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Why do we HAVE to excrete 0.5 L/day?
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To get rid of metabolic waste products
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2 basic requirements for forming a concentrated urine:
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1. High amounts of ADH
2. High osmolarity in the renal medullary interstitial fluid |
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Why does the renal medullary interstitial fluid have to be so high in osmolarity in order to form a concentrated urine?
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Because even if ADH is present to insert AQP2 in the collecting ducts, you need the osmotic driving force for water reabsorption.
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What process makes the renal medullary interstitium hyperosmotic to the collecting duct fluid?
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The countercurrent multiplier system.
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What is the countercurrent mechanism dependent on?
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The special arrangement of loops of henle and vasa recta.
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What are vasa recta? What nephrons have them?
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Long peritubular capillaries wrapped around the long loops of henle in juxtamedullary nephrons.
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What %age of nephrons are juxtamedullary nephrons?
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25%
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What is the most important cause of the high medullary osmolality?
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Active transport of sodium and co-transport of K, Cl, and other ions from the thick ascending loop of henle.
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How high of a gradient can the Na/K pump in the thick ascending loop of henle establish?
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200 mOsm gradient
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What helps establish the high osmolarity of the renal medullary ISF?
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The passive reabsorption of NaCl in the thin ascending limb of Henle's loop.
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How does the countercurrent multiplier system work?
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1. Na/K ATPase pumps Na out of the thick asc. limb of LOH
2. Sets up a 200 mOsm gradient from tubule to ISF; limited by paracellular diffusion of ions back into the tubule 3. Higher ISF osmolarity is noticed by the descending limb so its water osmotically diffuses until the fluid equilibrates with ISF 4. Now isoosmolar fluid from desc limb moves to asc limb; this replaces the hypotonic fluid there, so more ions pumped into ISF. Repeat |
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What is the ultimate highest osmolarity at the bottom of the loop of henle that is reached in the ISF?
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1200 mOsm
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What is the tonicity of the fluid in the thick ascending limb?
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500 mOsm
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What is the tonicity of the fluid in the early distal tubule?
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100 mOsm
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How does the kidney form highly concentrated urine?
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By removing more water in the presence of ADH from the collecting ducts.
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How concentrated can urine GET?
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As concentrated as the inner medullary ISF is - 1200 mOsm
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What is the osmolarity of fluid as it leaves the loop of henle?
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100 mOsm
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What happens to the osmolarity of tubule fluid as it flows thru the early distal tubule? Why?
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It becomes more dilute - because it actively transports NaCl out, but no water permeability.
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What happens to the osmolarity of tubule fluid as it flows into the late distal tubule?
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It depends on the plasma conc of ADH to determine if water reabsorption can occur.
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Where will the majority of water be reabsorbed if ADH is high? Why is it important?
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IN THE CORTICAL COLLECTING DUCT because it helps keep the high osmolarity in the medulla.
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What happens to the water reabsorbed from cortical collecting ducts if ADH is present?
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It gets swept away by the rapidly flowing peritubular capillaries.
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Does any water reabsorption occur in the medullary collecting ducts?
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yes, but minimal compared to the cortical collecting ducts.
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Is NaCl reabsorption the only thing that contributes to the high medullary ISF osmolarity?
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NO - urea contributes 40-50% to it.
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How much of the absolute osmolarity is contributed by urea when the kidney is maximally concentrating urine?
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500-600 mOsm/L
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How is urea reabsorption different from NaCl?
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Urea is reabsorbed passively by diffusion.
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Where is the majority of urea reabsorption?
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In the renal medullary collecting ducts.
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What regulates urea reabsorption from the renal medullary collecting ducts?
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ADH
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What conditions will cause high urea reabsorption?
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-When there is water deficit
-High amts of serum ADH |
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What is the mechanism for max urea reabsorption?
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-ADH stimulates much water reabsorption from CORTICAL CDs
-Loss of water increases the urea concentration -ADH puts UT-1 on lumen memb -UT1+High Urea conc stimulate maximal diffusion to ISF. |
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Even though urea is being reabsorbed when ADH is high, what is its excretion in urine?
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Still high
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How do we know that urea helps the kidney's ability to concentrate urine?
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People on high-protein diets excrete a lot of urea and can really concentrate their urine.
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How does urea contribute to the countercurrent multiplier system?
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By getting trapped in the renal medulla.
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How does urea get trapped in the medulla?
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When reabsorbed from the inner medullary collecting duct it recirculates to the LOH and gets secreted BACK into the thin limb.
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What happens to the urea that gets secreted into the thin limbs of LOH?
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It goes through the relatively urea-impermeable cortical segments to be reabsorbed AGAIN at the medullary collecting ducts
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how much of the osmolarity in the renal medulla (papilla) is contributed by:
-NaCl? -Urea? |
NaCl = 700 mOsm
Urea = 500 mOsm |
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What is the PURPOSE of ADH/water/urea reabsorption?
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To concentrate urine and preserve water in times of need.
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What if there is water excess and you don't NEED to preserve it?
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The ADH levels won't be so high, so water and urea will be less permeable and more will be excreted.
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What is the goal of the vasa recta?
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To preserve the hyperosmolarity of the renal medulla
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2 special features of the renal medullary bloodflow that contribute to the preservation of the high solute concentrations:
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1. Sluggish medullary bloodflow
2. Vasa recta serve as countercurrent exchangers |
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What percent of the entire renal blood flow is in the medullary vasa recta?
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Less than 5%
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What happens to blood as it enters the medulla via the vasa recta capillaries?
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-Loses water as it enters the highly concentrated ISF
-Gains some solutes too |
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What is the tonicity of the BLOOD by the time it reaches the tips of the vasa recta?
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1200 mOsm - isotonic to the medulla ISF
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What happens to the blood as it moves from the inner medulla back out to the cortex?
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-Water moves back into it
-Solutes move back out |
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So what is the general role of the vasa recta in medullary hyperosmolarity?
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They prevent it from being dissipated.
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What will DECREASE the ability of the vasa recta to prevent washout?
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Increased medullary bloodflow - vasodilation
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So increased bloodflow will reduce the kidney's ability to:
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concentrate urine
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When the kidney creates highly concentrated urine, does that mean it contains a lot of NaCl?
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No
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What makes urine hyperosmolar when the kidney produces highly concentrated urine?
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Urea, creatinine, and other waste products
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What does it mean to say there is an obligatory urine volume?
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For the kidney to excrete a certain amt of solute, there is a minimal amt of water that HAS to be excreted along with it.
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What is the obligatory urine volume if you excrete 1200 mOsm of solute?
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1 L
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What type of urine will be produced when ADH is high?
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Concentrated
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When the kidney creates highly concentrated urine, does that mean it contains a lot of NaCl?
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No
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What makes urine hyperosmolar when the kidney produces highly concentrated urine?
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Urea, creatinine, and other waste products
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What does it mean to say there is an obligatory urine volume?
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For the kidney to excrete a certain amt of solute, there is a minimal amt of water that HAS to be excreted along with it.
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What is the obligatory urine volume if you excrete 1200 mOsm of solute?
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1 L
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What type of urine will be produced when ADH is high?
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Concentrated
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What are 3 factors that will inhibit the kidney from forming concentrated urine?
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1. Lack of ADH or inactivity of it
2. Lack of medullary ISF hyperosmolarity due to abnormal bloodflow 3. Lack of sodium transport in the thick ascneding limb |
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What causes the kidneys to form dilute urine?
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Lack of ADH
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What results from lack of ADH?
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-No urea reabsorption from the collecting ducts
-No water reabsorption from the collecting ducts -Urea's contribution to the medullary ISF osmolarity drains away |
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What happens to the urinary concentration of urea when the kidney forms dilute urine?
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It increases
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What happens to the concentration of sodium when the kidney forms dilute urine?
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It stays the same - the kidneys alter the VOLUME of urine, but not the solute AMOUNT.
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What happens to the osmolarity of the medulla ISF when ADH is at a minimum low?
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It becomes 600 mOsm instead of 1200
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What is the osmolarity of the urine excreted when ADH is at a minimal low?
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50 mOsm
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How long does it take to reestablish the urea gradient after washout during absence of ADH?
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8-12 hrs
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How much volume of filtrate starts out in the proximal tubule? How much ends as urine when ADH is present? When ADH is absent?
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Start: 125 ml
End w/ ADH: 0.2 ml End w/out ADH: 20 mL |
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How high can the urine osmolarity be?
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1200 mOsm/L
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What volume of filtrate is left at the end of the proximal tubule?
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44 mL
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How much volume of filtrate is left at the end of the loop of henle?
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25 mL
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Where does the biggest fluctuation of osmolarity take place in the nephron?
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Loop of Henle
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What is plasma sodium concentration closely linked to?
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ECF osmolarity
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What is the normal plasma sodium concentration? (units)
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140 mEq/L
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What is the normal plasma osmolarity?
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300 mOsm/L
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