The Renal System PDF

The Renal System The Renal System is part of the Urinary System: kidneys, ureters, urinary bladder and urethra. Kidneys filter the blood but return most of the water and solutes back to the body. Waste (urine) flows from each kidney, dow...

The Renal System The Renal System is part of the Urinary System: kidneys, ureters, urinary bladder and urethra. Kidneys filter the blood but return most of the water and solutes back to the body. Waste (urine) flows from each kidney, down its ureter to the bladder. Passed to the exterior via the urethra. 7 Anatomy of the kidney The kidney is bean shaped and the size of a fist (11cm long) It is surrounded by the renal capsule (fibrous CT) It is retroperitoneal (lies behind the peritoneum on the posterior abdominal wall on either side of the vertebral column) The right kidney is lower than the left kidney. 8  Internal anatomy of the kidney Regions of the kidney: Cortex – outer portion Medulla – inner portion consisting of 8-18 cone-shaped structures called pyramids separated by renal columns The renal hilus – small area on the medial side: entrance/exit for renal artery and vein, nerves, lymphatic vessels and ureter Renal sinus is the drainage system fills the cavity and contains cuplike structures called minor and major calyx Renal pelvis is the enlargement of the ureter 9  The nephron It is the structural and functional unit of the kidney. There are two types of nephrons: Cortical (85%) and juxtamedullary (15%) 1. Renal corpuscle - plasma filtration Glomerulus Bowman's capsule 2. Renal tubules - reabsorption and secretion Proximal convoluted tubule (PCT) Loop of Henle (LoH) Distal convoluted tubule (DCT) 3. Collecting ducts and papillary ducts drain urine to the renal pelvis 10  Renal corpuscle Glomerulus or glomerular capillaries arise from afferent arteriole before emptying into efferent arteriole Bowman's capsule surrounds the capsular space containing the glomerulus and has two layers: parietal layer made of simple squamous cells visceral layer made of podocytes which cover glomerular capillaries 11  Renal tubules: Proximal Convoluted Tubule Reabsorption of water (65%) and solutes (glucose, amino acids, Na+, K+) 12 Renal tubules: Loop of Henle Sets up salt gradient in medulla a) Short in cortical nephrons b) Long in juxtamedullary nephrons c) Made of two limbs: 1. Descending (simple squamous epithelium) is permeable to water and moderately permeable to Na+ , urea and other solutes 2. Ascending where Cl - are actively reabsorbed followed passively by Na+ a) Thin (simple squamous epithelium) b) Thick (simple cuboidal epithelium) d) Surrounded by vasa recta (only in juxtamedullary nephrons) 13 Cortical vs justamedullary nephron 14 Renal tubules: Distal Convoluted Tubule Lies in the cortex Made of simple cuboidal epithelium with a few microvilli and many mitochondria Impermeable to water Reabsorption of water (15%) controlled by ADH Reabsorption of Na+ controlled by aldosterone Secretion of K+ , H+, drugs. 15 Collecting duct Receives filtrate from several nephrons Found within the cortex and medulla Empties into papillary ducts Made of simple cuboidal epithelium Site for reabsorption of Na+ controlled by aldosterone Site for reabsorption of water controlled by ADH 16  Kidney functions 1. Excretion: Filters blood (plasma) to remove wastes and foreign substances 2. Regulation of blood volume and blood pressure 3. Release of erythropoietin (stimulates hemopoiesis) 4. Release of calcitriol (vitamin D) (helps calcium reabsorption in kidneys) 17  Excretion: urine formation 1. Glomerular filtration within glomerulus 2. Tubular reabsorption mainly within PCT 3. Tubular secretion mainly within DCT 18  The glomerulus: adaptation for filtration Passive, selective process (determined by size and charge) in which pressure forces blood through the filtration (glomerular) membrane is called filtration. 1. Very long 2. Presence of fenestrae and filtrations slits 3. Thin wall (< 0.1m) 4. Smaller diameter of the efferent arteriole compare to the afferent arteriole 19 Filtration membrane components 1. Fenestrated endothelium (glomerulus) stops all cells and platelets 2. Basement membrane or basal lamina stops large plasma proteins 3. Podocyte cell layer (filtration slits) stops medium-sized proteins 20  Peff = GCP - (BCOP + CHP) This pressure is sufficient to produce an appropriate glomerular filtrate rate 21 Glomerular Filtration rate (GFR) is the amount of filtrate formed in all renal corpuscles of both kidneys/minute. Average GFR = 125 ml/min = 180 L/day Homeostasis requires GFR be constant a) If GFR is too high useful substances are lost due to the speed of fluid passage through nephron b) If GFR is too low waste products may not be removed from the body GFR is maintained constant by intrinsic mechanisms collectively called autoregulation via the juxtaglomerular apparatus, and extrinsic mechanisms via the autonomic nervous system and hormones 22  Justaglomerular apparatus (JGA) The Juxtaglomerular apparatus consists of two types of cells: a. Juxtaglomerular cells are modified muscle cells around the afferent arteriole which act as stretch receptors and monitor the blood pressure causing vasoconstriction or vasodilation of the afferent arteriole b. Macula densa cells are specialised cells of the distal convoluted tubule which detect changes in filtrate flow and send a signal to the juxtaglomerular cells to constrict or dilate 23  Autoregulation of GFR Intrinsic mechanisms in the kidney are able to monitor GFR (via macula densa cells) and adjust blood flow accordingly: Autoregulation maintains GFR constant when MAP is between 80 and 180mmHg and protects the kidneys from bp fluctuations In case of very low blood pressure (hemorrhage or dehydration) the sympathetic nervous system can override the process of autoregulation reducing the amount of blood to kidneys 24  Movement of solutes from filtrate to blood Nephron reabsorbs 99.5% of the filtrate It is a "DISCRIMINATING PROCESS": a) Na+ reabsorbed by active transport b) Glucose, amino acids and most cations reabsorbed via cotransport (symport) c) Water follows by osmosis PCT with their microvilli and mitochondria do most of work 25 Sodium reabsorption: PCT (67%), LoH (25%), DCT and CD (8%) DCT and CD controlled by aldosterone Important for regulation of extra cellular fluid volume Water reabsorption: PCT (65%), LoH (15%), DCT and CD (20%) Controlled by Na+ movement and ADH 26  Movement of solutes from the blood into the filtrate Most active in DCT Helps control blood pH (secretion of H+) Helps eliminate certain substances very quickly, e.g. NH4+ , creatinine, some drugs, K+. 27  Micturition Reflex External urethral sphincter is made of skeletal muscle 28