The Vital Role of Blood Supply in Kidney Function

Blood and Nerve Supply of the Kidneys - Blood supply - Although kidneys constitute less than 0.5% of total body mass, they receive 20-25% of Resting cardiac output - Left and right renal artery enters kidney - Branches into arteries and arterioles - Each nephron receives one afferent arteriole - Divides into glomerulus – capillary ball - Reunite to form efferent arteriole Glomerular filtration - Glomerular filtrate – fluid that enters capsular space - Daily volume 150-180 liters – more than 99% returned to blood plasma via tubular Reabsorption - Filtration of water and small solutes - Prevents filtration of most plasma proteins, blood cells and platelets - Volume of fluid filtered is large because of large surface area, thin and porous Membrane, and high glomerular capillary blood pressure - Glomerular filtration rate – amount of filtrate formed in all the renal corpuscles of both Kidneys each minute - Homeostasis requires kidneys maintain a relatively constant GFR - Too high – substances pass too quickly and are not reabsorbed - Too low – nearly all reabsorbed and some waste products not adequately

Excreted - GFR directly related to pressure that determine net filtration pressure Juxtaglomerular Apparatus - Myogenic response - Like autoregulation in other systemic arterioles - Tubuloglomerular feedback - Paracrine control – chemical messengers - Hormones and autonomic neurons - By changing resistance in arterioles Secretion - Transfer of molecules from extracellular fluid into lumen of the nephron - Active process - Important in homeostatic regulation - Potassium plus and Hydrogen plus - Increasing secretion enhances nephron excretion - A competitive process - Penicillin competes with probenecid for secretion. Without probenecid, penicillin is 80% eliminated within a few hours Excretion - Excretion = filtration – reabsorption and secretion - Clearance - Rate at which a solute disappears from the body by excretion or by metabolism

- Non-invasive way to measure GFR - Insulin and creatinine used to measure GFR - Insulin is neither reabsorbed nor secreted so what goes into the nephron is Excreted – not natural - Creatinine is natural so it can be measured in humans but it is slightly secreted