KIDNEY FUNCTIONS URINE FORMATION

Learning Outcomes: Describe the function of each section of the nephron Describe the process of urine formation Describe how the kidneys maintain blood pH

Urine Composition Water Nitrogenous wastes (urea etc.) Salts May contain H+ and other ions May contain drugs, vitamins and other substances

Urine Formation 3 major steps: Glomerular filtration - in the glomerular capsule Tubular reabsorption - in the proximal tubule and loop of Henle Tubular secretion - distal tubule

Different parts of the nephron have different permeability characteristics Substances are filtered out of the blood by the nephron and excreted in urine Some substances are reabsorbed back into the blood from the nephron

Pressure Filtration Blood enters the glomerulus from the afferent arteriole Blood pressure forces small molecules out of the capillaries into the capsule

Substances that leave the blood: Water, salts, nutrients (glucose, amino acids), nitrogenous wastes (urea) Filtrate resembles plasma

Selective Reabsorption At the proximal tubule and loop of Henle Some substances must be reabsorbed back into the blood Examples: Water, nutrients (glucose, amino acids), salts

Na+ ions, glucose, amino acids - move back into the peritubular capillaries by active transport (why active transport?) Cl- ions follow passively Water is reabsorbed by osmosis

Absorption of Water at the Loop of Henle Function of the loop of Henle is to remove as much water as possible from the filtrate and return it to the blood

Descending limb is permeable to water but not salt Ascending limb is permeable to salt but not water Sodium is actively pumped out of the ascending limb

Medulla is hypertonic to the filtrate (contains high [Na+]) Water leaves the tubule by osmosis and enters the medulla, then the capillaries

Fig. 16.7

Fig. 16.6

Tubular Secretion At the distal tubule Excess H+ ions, drugs, and other substances move from the blood into the nephron by active transport Ions secreted or kept in the blood as needed, to maintain correct pH

Reabsorption of Water More water is reabsorbed into the blood from the collecting duct Collecting duct is in the hypertonic medulla so water is drawn out by osmosis

Fig. 16.6

Urine is now ready to be excreted from the body Collects in the renal pelvis before going to the bladder

Path of Blood Renal artery & arterioles Afferent arteriole Glomerulus Efferent arteriole Peritubular capillary network Venules to renal vein

Questions How is the composition of blood in the renal artery different from that in the renal vein? What is the importance of the hypertonic condition of the renal medulla? Why is glucose transported actively instead of passively at the proximal tubule?

Fig. 16.6

Fig. 16.6