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Renal system Dr. Zainab H.H Dept. of Physiology Lec.1,2 Objectives • List the functions of kidney • Describe the morphology of the typical nephron and its blood supply. • Describe the concept of clearance What is excretion? Throwing out of waste product is known as excretion. The organs through which excretion occurs. 1. Kidneys: Excrete water and water soluble waste products. 2. Lungs: Excrete carbon dioxide, water vapour and other volatile substances such as acetone. 3. Skin: Excretes water and salts mainly in the form of sweat. 4. Gastrointestinal tract: Excretes undigested food. The Role of the Kidneys in the Body Includes: • Regulation of the volume and composition of the ECF, by maintaining a balance between intake and output of water and electrolytes in the body. • Excretion and elimination of waste products of metabolism, such as the excretion of urea, creatinine and uric acids; as well as the excretion of various toxins such as drugs and food additives. • The kidneys act as endocrine glands producing hormones, such as “erythropoietin hormone” and renin. • Playing a dominant role in the long-term and short-term regulation of arterial blood pressure. • Kidneys along with the respiratory system contribute to acid-base regulation. • Finally, kidneys synthesize glucose from amino acids and other precursors Metabolic waste products excreted by kidneys are: • • • • Urea from protein. Uric acid from nucleic acid. Creatinine from muscle creatine. End products of haemoglobin breakdown. STRUCTURE OF NEPHRON • Nephron is a structural and functional unit of the kidney. • Each nephron is capable of forming urine. • There are two types of nephrons: • 1. Cortical nephrons. Glomeruli are present near the surface of the kidneys. – These nephrons constitute about 86% of total nephrons. – The main function of cortical nephrons is absorption of sodium. • 2. Juxtamedullary nephrons. Glomeruli lie at the junction of cortex and medulla of the kidney. – These constitute 14% of the nephrons. – The main role of juxtamedullary nephron is to increase concentration of medullary interstitial fluid. Two kidneys together have two millions nephrons. Nephron consists of two major parts: – Glomerulus. – A long renal tubule. 1. Glomerulus. It is made up of tuft of capillaries which connect afferent arteriole with an efferent arteriole. • Capillaries have single layer of endothelial cells attached to a basement membrane. • Bowman’s capsule encloses the glomerulus and is formed of two layers: – inner layer which covers the glomerular capillaries is called visceral layer, – outer layer is called parietal layer. Space between visceral and parietal layers is continued as the lumen of the tubular portion. – fluid-filled space, Bowman’s space, is formed within the capsule. – Blood of capillary and fluid of Bowman’s space are separated by the glomerular membrane. – From the Bowman’s capsule, tubule of the nephron extends, the lumen of which is continuous with the Bowman’s space. • 2. Renal tubule. It is mainly formed of three parts: (a) Proximal convoluted tubule. (b) Loop of Henle consisting of: – Thin segment :walls of descending limb and lower end of ascending limb are very thin. Therefore, they are termed thin segment. – Hair pin bend. – Thick ascending limb or segment • (c) Distal convoluted tubules. • open into initial arched collecting ducts called cortical collecting ducts present in renal cortex. – Seven to ten such ducts form straight collecting duct which passes into medulla forming medullary collecting ducts. • In the inner zone of medulla they form papillary ducts or ducts of Bellini. – These open into papilla of minor calyces. – Three or four minor calyces unite to form one major calyx. – The major calyces open into pelvis of ureter. – The pelvis is an expanded portion present in renal sinus and it continues as ureter BLOOD FLOW TO KIDNEYS • Rate of blood flow to kidneys is 1200 ml/min. • This is quite high as compared to their size. • State peculiarities of renal circulation. 1.Very high blood supply, about 21% of cardiac output. 2.Two sets of capillaries. – The glomerular capillaries. • These combine to form efferent arteriole which in turn breaks into peritubular capillary network around the tubules of cortical nephrons. – vasa recta which are loop-shaped vessels in juxtamedullary nephrons • the efferent arterioles continue as these loops dip into the medullary pyramids alongside the loops of Henle 3.Glomerular capillary bed has a high hydrostatic pressure because efferent arteriole is of a smaller diameter than afferent arteriole which offers considerable resistance to blood flow. 4. Peritubular capillary bed is a low pressure bed. 5. Only 1 to 2% of blood flows through vasa recta. The flow is very sluggish. 6. Renal blood flow shows remarkable constancy in face of blood pressure changes due to autoregulation • The kidneys regulate the hydrostatic pressure in both capillary beds (glomerular & peitubular ) , by adjusting resistance of the afferent and efferent arterioles. • High hydrostatic pressure in GC (60 mmHg) causes rapid fluid filtration; whereas a much lower pressure in the peritubular capillaries (13 mmHg) permits rapid fluid reabsorption. • The afferent arteriole is a short, straight branch of the interlobular artery. • The efferent arteriole, that drains the GC, has a relatively high resistance than the afferent arteriole. The Process of Urine Formation: • Urine formation begins with the • Filtration of plasma through the GC into the Bowman’s space. • As the filtered fluid flows through the remaining portions of the tubule, its composition is altered as a result of two main processes: 1. Tubular reabsorption. 2. Tubular secretion. and both processes will produce the final product, urine. Concept of Clearance: • The renal clearance of a substance is the volume of plasma that is completely cleared or cleaned of that substance by the kidney per unit of time. (usually expressed as mL/ minute). • However, renal clearance provides a useful way of quantifying renal excretory functions. • It can be used to quantify the rate at which blood flows through the kidneys, as well as to measure the basic kidney functions such as GFR. Concept of Clearance (continues): • Renal clearance of a substance (S) is calculated by dividing the urinary excretion rate of (S) (US × V•) by its plasma concentration (PS), as expressed below: US × V• CS = PS • Where: US = urine concentration of S. V• = urine flow rate/ minute = (0.9 mL/ minute). PS = plasma concentration of S. • Clearance of any substance depends on the behavior of the tubular cell towards that substance: • If a substance like inulin, which is freely filtered at the glomerulus level and is neither reabsorbed nor secreted by the renal tubule, then its clearance equals to GFR. • On the other hand, if a substance is reabsorbed by the renal tubule, its clearance is lower than the GFR. • Finally, if a substance that is in addition to filtration is secreted by the renal tubule, then its clearance is higher than the GFR. THANK YOU