IPHY /27/11

Materials filtered into Bowman ’ s capsule Water Ions glucose, amino acids wastes (NH3, urea, etc) a few plasma proteins everything else in plasma hopefully no cells

Tubular reabsorption involves transport of molecules in filtrate back into the blood. Passive diffusion Active transport Pinocytosis

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) active Some negative ions (sulfate, phosphate) active Cl- passive water--passive (by osmosis) following movement of other molecules Proteins = pinocytosis Wastes = some urea diffuses back into blood

Tubular reabsorption Example: glucose (under 320 mg/ml filtered load) tubule blood All glucose restored to blood And none left in filtrate by end of proximal tubule None

Tubular reabsorption Example: glucose over 325 mg/min filtered load tubule blood Carrier molecules for glucose saturated, so Some glucose left in filtrate by end of proximal tubule Glucose left

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 Loop Permeable to water filtrate Concentration of filtrate rises due to water leaving and concentrating solute At bottom of Loop, another 15-20% of water reabsorbed back into blood Direction of flow

Ascending Loop Not permeable to water Concentration of filtrate changes due to active transport of Na and Cl from filtrate filtrate 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 necessary Active secretion of H+ if necessary CO2 + H20 --> H2CO3--> H+ + HCO3- (carbonic anhydrase)

Collecting Duct Secretes a variably concentrated urine depending on needs of the body filtrate Dehydration: Maximal vasopressin secretion filtrate 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+