Urinary Physiology PDF

Urinary physiology  Agenda Introduction Function of kidney Functional anatomy of kidney Basic renal processes Glomerular filtration Tubular reabsorption  The kidneys are organs specialized to filter the blood. As such, they make an important contribution to the removal of metabolic wa...

Urinary physiology  Agenda Introduction Function of kidney Functional anatomy of kidney Basic renal processes Glomerular filtration Tubular reabsorption  The kidneys are organs specialized to filter the blood. As such, they make an important contribution to the removal of metabolic waste products as well as to maintenance of fluid and electrolyte balance. Functions of the kidneys: 1- Regulation of extracellular fluid volume 2- Regulation of inorganic electrolyte concentration in extracellular fluid 3-Regulation of the osmolarity of extracellular fluid 4- Removal of metabolic waste products 5- Excretion of foreign compounds 6- Maintenance of acid-base balance 7- Hormone and enzyme production Functional anatomy of kidneys The kidneys lie outside the peritoneal cavity in the posterior abdominal wall, one on each side of the vertebral column, slightly above the waistline. These organs are divided into two regions: the inner renal medulla and the outer renal cortex. The functional unit of the kidney is the nephron. Approximately 1 million nephrons are in each kidney.  The nephron has two components: a. Vascular component. 1. Glomerulus (i.e., glomerular capillaries): located in the cortical region of the kidney.Water and solutes exit the vascular compartment through these capillaries to be processed by the tubular component of the nephron. 2. Afferent arterioles deliver blood to the glomerulus. 3. Efferent arterioles: The glomerular capillaries then join together to form these arterioles.The efferent arterioles then lead to a second set of capillaries, the peritubular capillaries. 4. Peritubular capillaries provide nourishment to the renal tissue, return the substances reabsorbed from the tubule to the vascular compartment and are closely associated with all portions of the renal tubules and wrap around them. They then join together to form venules and progressively larger veins that remove blood from the kidneys. b. Tubular component. 1. Upon leaving the glomerular capillaries, the filtrate enters the first portion of the tubule, Bowman’s capsule. 2. From Bowman’s capsule, the filtrate passes through the proximal tubule which is also located in the cortex of the kidney. 3. The next segment of the tubule is the Loop of Henle found in the medulla of the kidney. The descending limb penetrates into the medulla and the ascending limb returns toward the cortex. 4. From the Loop of Henle, the filtrate passes through the distal tubule in the cortex of the kidney. 5. Finally, up to distal tubules empty into a collecting duct that runs downward through the medulla. Any filtrate remaining within the tubule at the end of the collecting duct drains through the renal pelvis to the ureters and is excreted as urine.  Basic renal processes The nephron performs three basic renal processes: 1. Glomerular filtration: ▪ The first step in the formation of urine is glomerular filtration. ▪ Filtration is the movement of fluid and solutes from the glomerular capillaries into Bowman’s capsule. ▪ Filtration is a nonselective process, so everything in the plasma except for the plasma proteins is filtered. ▪ Approximately 20% of the plasma is filtered as it passes through the glomerulus. ▪ Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman's capsule per unit time. ▪ On average, this results in a glomerular filtration rate (GFR) of 125 ml/min or 180 L of filtrate per day.  2. Tubular Reabsorption: Reabsorption is the movement of filtered substances from the renal tubule into the peritubular capillaries for return to the vascular compartment. This process takes place throughout the tubule. Approximately 178.5 L of filtrate are reabsorbed, resulting in an average urine output of 1.5 L per day. -There are two types of tubular reabsorption: Passive tubular reabsorption: the movement of a given substance is from an area of high concentration to an area of low concentration by way of passive diffusion. Water is passively reabsorbed from the tubules back into the peritubular capillaries. Active tubular reabsorption: The movement of a given substance across the surface of the tubular epithelial cell requires energy. Substances that are actively reabsorbed from the tubule include glucose; amino acids; and Na+,PO4 -3and Ca++ions - The tubular reabsorption of sodium, chloride, and water: Reabsorption of Na+ ions is an active process Reabsorption of Cl–ions is a passive process; Cl– ions are reabsorbed according to the electrical gradient created by the reabsorption of Na+ions. Reabsorption of water is a passive process; water is reabsorbed according to the osmotic gradient created by reabsorption of Na+ ions. - Mechanism of Na+ reabsorption: Sodium ions leave the filtrate and enter the tubular epithelial cell by way of the following processes 1. Na+ -glucose, Na+ -amino acid, Na+ -phosphate, and Na+ -lactate symporter mechanisms; Na+ -H+antiporter mechanism: first half of the proximal tubule 2. Coupled with Cl–reabsorption by way of transcellular (through the epithelial cell) and paracellular (in between the epithelial cells) pathways: second half of the proximal tubule 3. Na+, K+, 2Cl-symporter mechanism: ascending limb of the Loop of Henle 4. Na+, Cl- symporter mechanism: distal tubule 5. Na+channels: distal tubule, collecting duct - Control of Na+ reabsorption: This reabsorption occurs regardless of the sodium content of the body. In order to make adjustments in the sodium load, the reabsorption of the filtered Na+ions from the distal tubule and collecting duct is physiologically controlled by two hormones: Aldosterone: Aldosterone released from the adrenal cortex promotes the reabsorption of sodium from the distal tubule and collecting duct. The mechanisms of action of aldosterone: 1. Formation of Na+channels in the luminal membrane of the tubularepithelial cells (facilitates passive diffusion of Na+ions into the cell). 2. Formation of Na+, K+-ATPase carrier molecules in the basolateral membrane of the tubular epithelial cells (promotes extrusion of Na+ions from the cells and their movement into plasma by way of peritubular capillaries; enhances the concentration gradient for passive diffusion through Na+channels in the luminal membrane( Atrial natriuretic peptide (ANP): Atrial natriuretic peptide released from myocardial cells in the atria of the heart inhibits the reabsorption of sodium from the collecting duct. The mechanisms of action of ANP: Inhibition of aldosterone secretion Inhibition of Na + channels in the luminal membrane of the tubular epithelial cells -Chloride reabsorption: Chloride ions are reabsorbed passively according to the electrical gradient established by the active reabsorption of sodium. Chloride ions move from the tubular lumen back into the plasma by two pathways: Transcellular; through the tubular epithelial cells Paracellular; in-between the tubular epithelial cells Most of the Cl-ions diffuse between the tubular epithelial cells. -Water reabsorption. ▪ Water is reabsorbed passively by way of osmosis from many regions of the tubule. ▪ The water enters the tubular epithelial cells through water channels, also referred to as aquaporins. ▪ These channels are always open in the early regions of the tubule. This reabsorption occurs regardless of the water content of the body: 1. As with sodium and chloride, 65% of the filtered water is reabsorbed from the proximal tubule. 2. An additional 15% of the filtered water is reabsorbed from the descending limb of the Loop of Henle. 3. In order to make adjustments in the water load, the reabsorption of the remaining 20% of the filtered water from the distal tubule and the collecting duct is physiologically controlled by antidiuretic hormone (ADH), also referred to as vasopressin.  Thank You Prof Dr/ Rania Ramadan Abdelaziz