BSCI440 Exam 4 Chapter 19 and 20 Textbook Notes Singer Chapter 19 The Kidneys Most important function of the kidneys homeostatic regulation of the water and ion content of the blood 1 Regulation of extracellular fluid volume and blood pressure Extracellular fluid volume decreases blood pressure also decreases Kidneys work with the cardiovascular system to ensure blood pressure and tissue perfusion remain in the normal range 2 Regulation of osmolarity blood osmolarity 300 mOsM 3 Maintenance of ion balance Body integrates kidney function with behavioral drives such as thirst to maintain Kidneys keep concentrations of key ions within normal ranges by balancing dietary intake with urinary loss Na is the major ion involved in regulating ECF 4 Homeostatic regulation of pH If ECF becomes too acidic kidneys remove H and conserve bicarbonate ions HCO3 When ECF becomes too alkaline the kidneys remove HCO3 and conserve H Metabolic wastes include creatine from muscle metabolism and the nitrogenous 5 Excretion of wastes wastes urea and uric acid Hormones 6 Production of hormones Kidneys play important roles in three endocrine pathways synthesize erythropoietin release renin enzyme that regulates production of hormones involved in sodium balance and blood pressure homeostasis renal enzymes convert vitamin D3 into a hormone that regulates Ca2 balance Kidneys Kidneys are the site of urine formation Kidney has two layers outer cortex and inner medulla 80 of nephrons are located in the cortex cortical nephrons and 20 are contained in the medulla juxtamedullary nephrons Renal arteries supply kidneys with blood renal veins carry blood from kidneys to the inferior vena cava Blood flows from the afferent arteriole into the glomerulus blood leaving the glomerulus flows into the efferent arterioles then into the peritubular capillaries that surround the tubule In the juxtamedullary nephrons the peritubular capillaries are called the vasa recta The function of the renal portal system filter fluid out of the blood and into the lumen of the nephron at the glomerular capillaries then to reabsorb fluid from the tubule back into the blood at the peritubular capillaries Bowman s capsule surrounds the glomerulus renal corpuscle proximal tubule descending loop of Henle ascending loop of Henle distal tubule collecting duct renal pelvis urine Kidney Function Three basic processes occur in the nephron filtration reabsorption and secretion 1 Filtration movement of fluid from blood into lumen of nephron takes place only in the renal corpuscle filtrate becomes part of the body s external environment excretion removal in the urine unless reabsorbed back into body 180 Liters filtered in Bowman s capsule only 70 reabsorbed leaves 54 L 2 Reabsorption process of moving substances in the filtrate from the lumen of the tubule back into the blood flowing through the peritubular capillaries Proximal tubule cells transport solutes out of lumen and water follows by osmosis When filtrate goes through loop of Henle primary site to create dilute urine more solute is reabsorbed than water and filtrate becomes hyposmotic relative to plasma 100 mOsM 54 Liters 18 Liters Then flows to distal tubule and collecting duct fine regulation of salt and water balance 50 1200 mOsM 18 Liters 1 5 Liters Final volume and osmolarity of urine depend on the body s need to conserve or excrete water and solute 3 Secretion removes selected molecules from the blood and adds them to the filtrate in the tubule lumen typically uses membrane proteins to move molecules across epithelium Amount excreted amount filtered amount reabsorbed amount secreted Filtration Under normal conditions blood cells remain in the capillary so that the filtrate is composed of water and dissolved solutes Takes place in renal corpuscle the portion of the capsule epithelium that surrounds each glomerular capillary consists of specialized cells called podocytes which leave narrow filtration slits closed Filtration slit contains proteins such as nephrin and podocin in diseases where these proteins are absent or abnormal proteins leak across the glomerular filtration barrier into urine Capillary pressure drives filtration 1 Hydrostatic pressure PH the hydrostatic pressure of blood flowing through the glomerular capillaries forces fluid through the leaky endothelium 55 mmHg 2 Colloid osmotic pressure the osmotic pressure inside glomerular capillaries is higher than that of the fluid in Bowman s capsule due to presence of proteins in the plasma 30 mmHg 3 Fluid pressure Pfluid Bowman s capsule is an enclosed space so presence of fluid in the capsule creates fluid pressure that opposes fluid movement into the capsule fluid filtering out of the capillaries must displace the fluid already in the capsule lumen 15 mmHg Net filtration pressure 55 30 15 10 mmHg Glomerular filtration rate GFR the volume of fluid that filters into Bowman s capsule per unit time 180 L day influence by two factors The net filtration pressure determined by renal blood flow and blood pressure The filtration coefficient surface area of the glomerular capillaries available for filtration and the permeability of interface between the capillary and Bowman s capsule GFR is relatively constant over a wide range of blood pressures controlled by regulation of blood flow through the renal arterioles if renal arteriole resistance increases renal blood flow decreases Constriction of afferent arteriole hydrostatic pressure decreases on glomerular side of constriction and GFR decreases most regulation occurs here Constriction of efferent arteriole hydrostatic pressure increases in glomerular capillaries GFR is subject to autoregulation local control process in which the kidney maintains a relatively constant GFR in the face of normal fluctuations of blood pressure Myogenic response intrinsic ability of vascular smooth muscle to respond to pressure and GFR increases changes o When smooth muscle in arterioles stretch due to increased pressure stretch sensitive ion channels open and the muscle cells depolarize which opens Ca2 channels and vascular smooth muscle contracts and decreases filtration pressure o If blood pressure decreases the tonic level of arteriolar contraction disappears and the arteriole becomes maximally dilated Tubuloglomerular feedback a paracrine signaling mechanism through which changes in fluid flow through the loop of Henle influence GFR o The ascending tubule and arteriole walls are modified in the regions