Filtration Pressure 23 1 Filtration Pressure The glom filtrate is normally almost protein free and has no significant COP 23 2 Filtration Pressure In most blood capillaries the BHP drops low enough at the venous end that net osmosis overrides filtration and the capillaries reabsorb fluid In the glomerular capillaries the BHP remains high enough throughout capillary that only have filtration The high blood pressure in the glomeruli makes the kidneys especially vulnerable to hypertension Hypertension ruptures glomerular capillaries and leads to scarring of the kidneys Over time hypertension can lead to renal failure 23 3 The Urinary System Urine formation glomerular filtration continued 23 4 Control of GFR GFR is the amount of filtrate formed per minute by both kidneys only a small portion of this filtrate is actually eliminated as urine glomerular filtration rate GFR must be precisely controlled to ensure proper filtration and reabsorption GFR is adjusted by changing glomerular blood pressure renal autoregulation sympathetic control hormonal mechanism renin and angiotensin Water balloon with tiny holes squeeze balloon and water pressure builds inside like the pressure builds inside glom causing water to squeeze out faster like water filters out of glom faster 23 5 Control of GFR In general constrict aff arteriole AA and or dilate eff arteriole EA lower pressure in glom which decreases GFR In general dilate AA and or constrict EA raise pressure in glom which increases GFR 23 6 Renal Autoregulation of GFR Renal autoregulation the ability of nephrons to adjust their own blood flow and GFR locally without external control Use autoregulation to maintain a relatively stable GFR even if BP changes myogenic mechanism the tendency of smooth muscle to contract when stretched tubuloglomerular feedback several structures at end of nephron loop 23 7 Renal Autoregulation of GFR Myogenic mechanism myogenic mechanism responds to immediate pressure changes at entrance to glomerulus to maintain the same pressure inside glomerulus In order to keep GFR constant in face of systemic BP fluctuations BP stretches the AA AA constricts in response increases resistance to oppose increased BP at entrance of glomerulus This lowers net BP and net blood flow into glom back down to normal BP R net BP entering glom 23 8 Myogenic mechanism BP less stretch of the AA AA dilates in response decreases resistance in response to decreased blood pressure at entrance of glomerulus This increases net BP and net blood flow into glom back up to normal net BP entering glom has been adjusted 23 9 Tubuloglomerular feedback A mechanism localized to whole nephron by which the glomerulus receives feedback on the downstream tubular fluid flow rate Involves a set of structures called the juxtaglomerular apparatus where the ascending limb of the nephron loop passes between the afferent and efferent arterioles Three types of special cells are found in the juxtaglomerular apparatus 23 10 Juxtaglomerular Apparatus Juxtaglomerular JG cells are enlarged smooth muscle cells found mostly in the afferent arteriole with a few in the EA across from the macula densa JG cells dilate or constrict arterioles and in response to a blood pressure drop will release renin The macula densa is a patch of slender epithelial cells on the side of the tubule facing the arterioles at the end of the nephron loop The macula densa detects variations in fluid flow and composition of tubular fluid In response it secretes chemical messengers that stimulate JG cells Mesangial cells are in the gaps between afferent and efferent arterioles and among capillaries of the glomerulus Communicate with macula densa and JG cells by gap junctions and paracrine secretions 23 11 Juxtaglomerular Apparatus blood pressure is not changing during TG feedback unlike the myogenic mechanism Pressure into glom is constant P R F if P is same then increase R decrease F Macula densa detects sodium levels in tubular fluid flowing past it A low GFR causes a low tubular flow rate in DCT this brings less Na past macula densa which detects this as low flow of tubular fluid in DCT A high GFR means more Na flows past macula densa which is detected as high flow in DCT 23 12 Sympathetic nerve fiber Juxtaglomerular cells Afferent arteriole Smooth muscle cells Macula densa Mesangial cells Efferent arteriole Nephron loop Juxtaglomerular Apparatus macula densa releases a messenger which causes JG cells to relax Decreases R into glom increases blood F into glom increases P if GFR has decreased dilates afferent arteriole inside glom increases GFR back up to normal if GFR has increased Macula densa releases a diff messenger JG cells contract which constricts afferent arteriole Increase R into glom decrease blood F into glom decrease P inside glom reducing GFR back down to normal mesangial cells may contract constricting the capillaries and reducing filtration 23 13 Negative Tuberoglomerular Feedback Control of GFR 23 14 Renal Autoregulation Renal autoregulation is not perfect Like any negative feedback regulated process GFR can vary within a certain range which maintains a dynamic equilibrium Renal autoregulation cannot adjust much for extremely high blood pressure or low BP For example at very low systemic BP during hypovolemic shock glomerular filtration and urine output cease 23 15 Sympathetic Control of GFR Sympathetic nerve fibers work in conjunction with epinephrine during strenuous exercise to stimulate afferent arterioles to constrict This reduces blood flow into glomerulus which reduces the GFR and urine production redirecting blood to the heart brain and skeletal muscles During intense exercise blood cannot be spared for filtering because needed to meet the demands of skel card muscle During shock when systemic BP is really low need to try to increase BP by constricting all blood vessels except to brain and heart Constriction of afferent arteriole lowers glomerular pressure causing a lower GFR which means less water and sodium excreted helping increase blood volume and blood pressure 23 16 Hormonal Control of GFR If total sodium and water levels in blood is lower which lower blood vol which lowers systemic BP which decrease GFR due to constriction on AA to decrease Na excretion A drop in blood pressure stimulates juxtaglomerular cells to secrete renin because either Baroreceptors cause medulla to increase symp output to stimulate JG cells JG cells are less stretched due to drop in BP Blood vol