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Renal System, Water & Electrolyte Balance

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Physiology 1021
Sarah Mc Lean

MODULE 11 Renal System WaterElectrolyte BalanceThe renal system includes the kidneys ureters bladder and urethra The principal function of the kidneys is the regulation of water balance electrolyte levels pH of the blood and the longterm regulation of arterial pressure As we examine these functions you will be seeing many concepts from the first three modules including homeostasis negative feedback and volume and composition of body fluid compartments as well as membrane transport mechanism and osmosis Functions of the KidneysThe basic function of the kidneys is to remove nonessential substances from the plasma including waste metabolites excess water and electrolytes and to recover any essential substance like glucose In doing so the kidneys play a major role in regulating the water levels the chemical concentration of the body fluid compartments and pH or acidity of the blood It is important to understand that the kidneys do not produce water or electrolytes but only conserve them by reducing the amount removed from the body The elimination of waste or foreign substances is an important function of the kidneys This includes the removal of drugs food additives and vitamins that are excreted in the urineThe kidneys also act as an endocrine gland producing hormones or components of hormonal systems such as erythropoietin renin vitamin D and stanniocalcin Anatomy and Blood Supply of the KidneysThe kidneys are roughly the size of a fist They consist of an outer renal cortex a middle renal medulla and inner calyces that drain into a central renal pelvis The renal pelvis then drains into the ureter Located within the renal pyramids are the functional units of the kidneysthe nephrons Each nephron drains through a collecting duct into a calyx Blood flows to the kidneys through the renal artery This large artery branches into several interlobar arteries that in turn branch into arcuate arteries The blood in the arcuate arteries flows through the interlobular arteries not shown at right to supply the nephron which we will look at in more detail in a moment The blood supply to the nephron drains into the interlobular vein the arcuate vein the interlobar vein and then into the renal vein The NephronThe nephron is the functional unit of the kidneys There are over 1 million nephrons in each kidney whose function is to filter the blood reabsorb essential substances and excrete nonessential molecules and waste Each nephron is composed of a highly coiled hollow tube surrounded by a complex blood supply The glomerular capsule also called Bowmans capsule surrounds a very small highly permeable capillary bed called the glomerulus These structures are often collectively referred to as the renal corpuscle The tubular portion of the nephron consists of the following structures in order the proximal convoluted tubule a highly coiled portion of the nephron the descending and ascending limb of the loop of Henle the distal convoluted tubule and the collecting duct Blood Supply of the NephronThe blood supply of the nephron is very complex Blood from the renal artery eventually reaches the interlobular artery that drains into the afferent arteriole The afferent arteriole gives rise to the glomerulus where filtration takes place The blood from the glomerulus enters the efferent arteriole Blood then enters the peritubular capillaries a dense network of capillaries surrounding the tubes of the nephron which then drains into the interlobular vein and eventually back to the renal vein The Renal CorpuscleThe renal corpuscle is made up of the glomerular capsule also called Bowmans capsule and glomerulus This is the site where the blood is filtereda process called glomerular filtration The fluid that is filtered from the blood that enters the glomerular capsule or capsular space is called the filtrate Glomerular filtration as we will see is facilitated by a highly permeable capillary endothelium that is surrounded by podocytes The larger diameter afferent arteriole and smaller diameter efferent arteriole also enhance glomerular filtration Processes along the NephronImportant Terms The following terms are important for our understanding of the function of the nephron Filtration is the movement of fluid through the glomerular capillary due to hydrostatic pressuresThe filtrate is the solution created by filtration The filtrate is generally composed of water plus all the dissolved solutes in the blood except for large proteins that are too big to be filteredReabsorption is defined as the movement of a substance from the lumen of the nephron back into the bloodSecretion is the movement of a substance from the blood into the lumen of the nephronExcretion is the removal of a substance from the body
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