New sections for the final exam Lesson 19 20 1 Components of the nephron a Renal corpuscle a spherical structure consisting of a glomerular Bowman s capsule a cup shaped chamber and a capillary network b Renal tubule a long tubular passageway that begins at the renal corpuscle the regions of the renal tubule are i Proximal convoluted tubule ii Nephron loop Loop of Henle a U shaped tube that extends partially into the medulla but in the juxtamedullary nephrons it extends deeper into the medulla iii Distal convoluted tubule Filtration occurs in the renal corpuscle as the blood pressure against the capillary wall forces water and dissolved solutes out of the glomerular capillaries and into the capsular space The solutes include glucose free fatty acids amino acids vitamins metabolic wastes and excess ions Reabsorption occurs in the renal tubule to take up useful organic nutrients and more than 90 of the water in filtrate back into the bloodstream Secretion also occurs in the renal tubule where waste products that didn t enter the renal corpuscle through filtration at the glomerulus can enter the urinary system to be excreted 2 The proximal convoluted tubule contains simple cuboidal epithelium and microvilli This allows for the reabsorption of organic nutrients ions and water These substances enter the peritubular fluid and diffuse into the peritubular capillaries Some secretion of hydrogen ions ammonium ions drugs and toxins also occurs at the PCT The Loop of Henle Nephron loop has two segments The descending limb functions in water reabsorption and the ascending limb functions in sodium and chloride reabsorption The distal convoluted tubule does not have microvilli and is smaller in diameter than the PCT The DCT s processes include the selective reabsorption of water depends on ADH the selective reabsorption of sodium depends on aldosterone and calcium depends on PTH and Calcitriol from peritubular fluid and the active secretion of ions such as hydrogen potassium and ammonium and drugs and toxins At the collecting duct fluid composition is adjusted to determine the final osmotic concentration and volume of urine Na is reabsorbed in exchange for the secretion of K controlled by aldosterone HCO3 is reabsorbed in exchange for the secretion of Cl water depending on ADH and urea is reabsorbed and H is secreted in exchange for HCO3 is the peritubular fluid is too acidic Aldosterone increases the number of Na K exchange pumps at the DCT and collecting duct to reabsorb Na is exchange for K Water is then reabsorbed by osmosis ADH increases the number of water channels aquaporins in the apical cell membranes of the DCT and collecting duct It concentrates 100mOsm L at the DCT to 1200mOsm L by the time it reaches the minor calyx 3 Filtration at the glomerulus is governed by the balance between hydrostatic pressure and colloid osmotic pressure of the material in the solution of either side of the capillary walls The efferent arteriole leaving the glomerulus has a smaller diameter than the afferent arteriole that enters the glomerulus This creates a relatively high hydrostatic pressure that pushes water and solute molecules out of the plasma and into the filtrate 4 Cortical nephrons make up 85 of all nephrons and are located mostly within the superficial cortex of the kidney The nephron loop is relatively short Juxtamedullary nephrons make up 15 of all nephrons are their nephron loops extend deep into the medulla The peritubular capillaries are also connected to the vasa recta which is a capillary network of juxtamedullary nephrons The vasa recta functions to return solutes and water reabsorbed in the medulla to the general circulation without disrupting the concentration gradient Some solutes that are absorbed in the descending portion do not diffuse out in the ascending portion Also more water moves into the ascending portion than is moved out of the descending portion In juxtamedullary nephrons the ascending limb creates high solute concentrations in the surrounding peritubular fluid and functions to concentrate the urine 5 The juxtaglomerular complex JGC is an endocrine structure formed by macula densa and juxtaglomerular cells The macula densa are epithelial cells of the DCT near the renal corpuscle The juxtaglomerular cells are smooth muscle fibers in the wall of the afferent arteriole that are associated with the cells of the macula densa The JGC secretes a The hormone erythropoietin when arteriole blood is low in oxygen It increases RBC production in the bone marrow to increase the blood oxygen levels b The enzyme renin when blood volume and pressure is low Renin activates the plasma protein angiotensinogen to angiotensin I Angiotensin I is converted to angiotensin II by angiotensin converting enzyme ACE in the lung capillaries Angiotensin II causes vasoconstriction of the arterioles and the precapillary sphincters to increase the systemic blood volume and blood pressure 6 There are two ways of expressing osmotic concentration It is important to express the concentration because urine is four times as concentrated as blood plasma The first is by osmolarity or the total number of solute particles per liter This is expressed in osmoles per liter Osm L or milliosmoles per liter mOsmol L Most body fluids have an osmotic concentration of about 300 mOsm L The second way is by the concentrations of large organic molecules in milligrams per unit volume of solution mg dL or g dL 7 The net filtration pressure is the average pressure forcing water and dissolved materials out of the glomerular capillaries and into the capsular space NFP GHP glomerular hydrostatic pressure CsHP capsular hydrostatic pressure BCOP This number should never be negative because then no filtration would occur 8 The glomerular filtration rate is the amount of filtrate that kidneys produce each minute There are three levels of control that occur to control the GFR a Autoregulation the GFR is maintained at the local level despite changes in local blood pressure and blood flow This occurs through three ways i Changes in the diameter of the afferent arterioles efferent arterioles and glomerular capillaries ii Reduction in blood flow and glomerular blood pressure which triggers the dilation of the glomerular capillaries and afferent arteriole and the constriction of the efferent arteriole iii A rise in the renal blood pressure that stretches the walls of the afferent arterioles and causes the smooth muscle cells to contract
View Full Document
Unlocking...