IPHY 3430 10/27/11

Materials filtered into Bowman’s capsule

WaterIonsglucose, amino acidswastes (NH3, urea, etc)a few plasma proteinseverything else in plasmahopefully no cells

Tubular reabsorption involves transport of molecules in filtrate back into the blood.

Passive diffusionActive transportPinocytosis

Tubular Reabsorption

Glucose (cotransport with Na+; active)Amino Acids (cotransport with Na+; active)Na+ (active--about 67% reabsorbed in proximal

tubule)All other positive ions (Ca++, K+, etc) activeSome negative ions (sulfate, phosphate) active

Cl- passivewater--passive (by osmosis) following movement of

other molecules Proteins = pinocytosisWastes = some urea diffuses back into blood

Tubular reabsorption

Example: glucose (under 320 mg/ml filtered load)

tubule bloodAll glucoserestored to bloodAnd none left in filtrate by end of proximal tubule

None

Tubular reabsorptionExample: glucose over 325 mg/min filtered load

tubule blood

Carrier molecules for glucose saturated, soSome glucose left in filtrate by end of proximal tubule

Glucoseleft

At the end of the proximal tubule: all glucose, amino acids, many ions except some of the Na+, Cl-, almost all protein, 65% of water, 50% urea have been reabsorbed back into blood

Remaining in filtrate: about 35% of water, wastes, the rest of Na and Cl, excesses of any ions, toxins

Loop of Henle

Sole purpose is to conserve water

Depends on an extracellular gradient of Na and Cl concentration

Descending LoopPermeable to water

300400

400500500600

filtrate

700 600

800 700

900 800

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1200 11001200

Concentration offiltrate rises due towater leaving andconcentrating solute

At bottom of Loop, another 15-20% of water reabsorbed back into blood

Direction of flow

Ascending LoopNot permeable to waterConcentration of filtrate changes due to active

transport of Na and Cl from filtratefiltrate

1200

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200 100

NaCl

NaCl

NaCl

Direction of flow

By the end of the loop of Henle, 15-20% more water reabsorbed back into blood

Distal tubule

Active secretion of K+ if necessaryActive secretion of H+ if necessary

CO2 + H20 --> H2CO3--> H+ + HCO3-(carbonic anhydrase)

Collecting DuctSecretes a variably concentrated urine depending on needs of the body

filtrate200

400600

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1200

Dehydration:Maximal vasopressinsecretion

filtrate

200

200

200

200

200

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1200

Overhydration:Minimal vasopressin secretion

Regulation of body water (colloid osmotic pressure)

cop --> hypothalamus--> posterior lobe of pituitary releases--> vasopressin --> permeability of collecting duct to water--> water reabsorption from filtrate into blood--> excretion of dilute, large volume urine --> water content of blood --> cop

Regulation of body water (colloid osmotic pressure)

cop --> hypothalamus--> posterior lobe of pituitary releases--> vasopressin --> permeability of collecting duct to water--> water reabsorption from filtrate into blood--> excretion of concentrated, small volume urine

--> water content of blood --> cop

Effect of alcohol on vasopressin secretion

Renin-angiotensin system helps vasopressin conserve water, if necessary,for regulation of body water and blood pressure

Angiotensin II also causes:

aldosterone release from adrenal gland ---> Na+ uptake from urine --> water uptake from urine --> blood volume --> venous return --> stroke volume --> cardiac output

net effect of angiotensin II --> BP

Additional Na+ regulation

blood Na+ --> adrenal gland--> aldosterone secretion --> active uptake

of Na+ from filtrate in collecting duct --> blood Na+

and….

Even more Na+ regulation

blood Na+ --> heart atrium --> secretion of atrial natriuretic peptide --> absorption of Na+ from filtrate in collecting duct --> blood Na+