Lecture 34 and 35: Tubular Reabsorption and secretion

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Nephron

--- must reabsorb 99% of the filtrate
-Proximal convoluted tubule does most reabsorption
-The rest of nephron is just doing some fine-tuning
-Solutes reabsorbed by active & passive processes
-Water follows solutes by osmosis
-Small proteins move across into the blood by pinocytosis

Secretion

Tubular --- transfers materials from blood into tubular fluid
-helps control blood pH because of secretion of H+
-helps eliminate certain substances (NH4+, creatinine, k+)

Paracellular reabsorption

Reabsorption routes: ---
-50% of reabsorbed material moves between cells by diffusion in some parts of tubule

Transcellular reabsorption

Reabsorption routes: ---
-material moves through both the apical and basal membranes of the tubule cell by active transport

Transport mechanisms

---
-Apical and basolateral membranes of tubule cells have different types of transport proteins
-Reabsorption of Na+ is important
-several transport systems exist to reabsorb Na+
-Na+/K+ ATPase pumps sodium from tubule cell cytosol through the basolateral membrane only

osmosis

Water is only reabsorbed by ---

obligatory

--- water reabsorption occurs when water is "obliged" to follow the solutes being reabsorbed

facultative

--- water reabsorption occurs in collecting duct under control of antidiuretic hormone

Isomotic reabsorption

Reabsorption in the PCT: ---
-Na+ symporters help reabsorb materials from the tubular filtrate (Glucose, amino acids, lactic acid, water-soluble vitamins)
-Intracellular sodium levels are kept low due to Na+/K+ pump on basolateral side

Glucosuria

---
-Renal symporters cannot reabsorb glucose fast enough if blood glucose level is above 200 mg/mL (the renal threshold)
-some glucose remains in the urine(---)
-Common cause is diabetes mellitus because insulin activity is deficient and blood glucose is too high

Juxtamedullary nephrons

Thick ascending limb is impermeable to water

Symporters

--- in the loop of Henle:
-Thick ascending limb of loop of Henle has Na+ k- Cl- symporters that reabsorb these ions
-K+ moves back into filtrate through K+ leak channels
-Na+ is pumped out on basolateral side
-Cl- diffuses across cell
-Cations passively move to the vasa recta
-Drawn to negative charge in capillaries

dilute urine

Formation of ---- ---:
Ex: drinking water -> decreased blood osmolarity -> ADH inhibited
Goal is to remove excess fluid from blood by producing dilute ---

dilute urine

Formation of --- ---:
-Blood plasma has 300 mOsm/L concentration
-Filtrate osmolarity increases as it moves down descending loop of Henle
-Filtrate osmolarity increases as it moves up ascending loop of Henle
-Filtrate osmolarity decreases in collecting duct -> dilute urine
-Collecting duct is impermeable to water

concentrated urine

Formation of --- ---:
Ex: water deprivation -> increased blood osmolarity -> ADH stimulated
-Goal is to prevent water loss and make concentrated urine

Counter current mechanism

--- --- ---:
Descending Limb of loop of Henle is very permeable to water
-Higher osmolarity of interstitial fluid outside the descending limb causes water to move out of the tubule by osmosis; at hairpin turn, osmolarity of filtrate can reach 1200 mOsm/liter
-Ascending limb is impermeable to water, but symporters remove Na+ and Cl- so osmolarity of filtrate drops to 100 mOsm/liter, but less filtrate is left in the tubule

ADH

--- actions:
-Stimulte Na+ K- Cl- symporters in thick limb of ascending loop of Henle -- this builds osmotic gradient in interstitial fluid
-Stimulate water reabsorption in upper collecting ducts
-Stimulate water reabsorption and urea recycling in lower collecting ducts -- this builds osmotic gradient in interstitial fluid
-End result: concentrated urine

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