145 - Kidney function, fluid and electrolyte balance 3 Flashcards
Osmolarity trough the kidney
~300 miliosmols/L at cortex to 1200 miliosmols/L near papilla (most-central part)
Significance of 300 miliosmols/L osmolarity at cortex of kidney
Same as osmolarity of the rest of the body
Role of the proximal convoluted tubule
iso-osmotically reabsorbs ~65% of filtrate (without changing osmolarity of fluid)
Osmolarity of fluid entering thick descending limb
Iso-osmotic (300miliosmolar/L)
Where is most glucose reuptaken?
Proximal tubule
Where is the concentration of renal filtrate the highest?
Loop of Henle
Type of epithelium in descending loop of Helne
Few mitochondria, thin membranes.
Very permeable to water.
Impermeable to almost everything else (lacks channels, transporters). Very lipid-soluble substances can still pass through wall.
Why is the descending limb water-permeable?
Aquaporins
Effect of impermeability of descending limb wall to everything but water.
Increase in concentration of renal filtrate (only water can leave).
Gradual increase in concentration of solutes of renal filtrate by a factor of four.
Solute responsible for a lot of the high osmotic pressure in the renal papilla
Urea
Permeabilities in the epithelium of the thin ascending limb
High permeability to sodium.
Low water permeability
Effect of permeabilities of thin ascending limb
Movement of sodium out of thin ascending limb
Proportion of sodium reabsorbed in thick ascending limb
~25%
Proportion of water reabsorbed in descending limb
~10-12%
Distal tubule sodium reabsorption
Regulated, according to body needs
Difference in sodium reabsorption between proximal tubule, descending limb and distal tubule
Proximal tubule and descending limb maintain the same amount of sodium reabsorption all the time.
Distal tubule is regulated according to body needs
Fluid entering the collecting duct
Dilute
Possible effects of collecting duct on filtrate
Travels through gradient of 300- 1200mOsM.
Can alter permeability to water.
This means that filtrate can be diluted or concentrated, depending on body needs.
What regulates aquaporins in collecting duct
Vasopressin or antidiuretic hormone (the same hormone, just different names)
Effect of antidiuretic hormone
Insertion of aquaporins into collecting duct.
This means that water is removed from filtrate, concentrating urine in collecting duct.
Effect of acute renal failure on urine concentration
Will be roughly equal to plasma.
Won’t be filtered, reabsorbed, etc very much.
When is kidney filtrate less concentrated that extracellular fluid?
At the end of the loop of Henle (just before the distal tubule)
Osmolarity of fluid in Bowman’s capsule
300mOsM
Osmolarity of fluid at the end of loop of Henle
100mOsM
Mechanism of tubular reabsorption
1) Na+/K+ ATPase pumps Na+ into extracellular fluid
2) This leads to diffusion of anions from filtrate following electrochemical gradient
3) Water moves by osmosis into extracellular fluid
4)
Examples of renal cotransporters
Na+/glucose
Na+/phosphate in proximal tubule
Why isn’t all filtered phosphate reabsorbed?
Na+/phosphate cotransporter in proximal tubule doesn’t have a very high capacity
How can chlorides and water be reuptaken in the kidneys?
From reuptake of Na+, there is a small negative charge in the lumen,
Cl- doesn’t have an epithelial channel, but can travel between epithelial cells of the tubule (through tight junctions). Paracellular route
Things that can be reabsorbed via paracellular route
Cl- and water
Broad routes of reabsorption
Paracellular and transcellular
Proportion of O2 absorbed by the kidney that is for active transport
80%
What is required for renal active transport?
Reabsorption of water, chloride, glucose, amino acids, urea
Secretion of potassium, hydrogen
NHE
Sodium-hydrogen exchanger.
Located on apical membrane of epithelial cells of renal tubule.
What makes Na+ reabsorption less efficient?
Reabsorbed Na+ can leak back into lumen through paracellular pathway
What increases concentration of K+ in renal epithelial cells?
K+/Na+ antiporter (3Na+ out of cell, 2K+ into cell)
What do sodium pumps drive reabsorption of?
K+, water, Cl-, HCO3-
