The Urinary System 1 PDF

The Urinary System 1 Content: Overview of the urinary system Organs of the urinary system Location of the kidneys Functions of the kidneys Structure of the kidney Formation of urine – the nephron Composition of normal urine, abnormal urinary constituents Dehydration – formation of c...

The Urinary System 1 Content: Overview of the urinary system Organs of the urinary system Location of the kidneys Functions of the kidneys Structure of the kidney Formation of urine – the nephron Composition of normal urine, abnormal urinary constituents Dehydration – formation of concentrated urine (anti-diuretic hormone) The Urinary System Anatomy The urinary system consists of: Two kidneys Two ureters The bladder The urethra  Organs of the Urinary System Ureters Slender tubes that convey urine from the kidneys to the bladder. Ureters have walls of smooth muscle and propel urine into the bladder by peristalsis. Ureters enter obliquely through the posterior wall at the base of the bladder, this prevents backflow of urine into the ureters.  Organs of the Urinary System Urinary Bladder Smooth, collapsible, muscular sac that temporarily stores urine, and empties it periodically. It lies on the pelvic floor posterior to the pubic symphysis. Male bladder – lies anterior to the rectum, prostate gland surrounds the neck of the bladder. Female bladder – lies anterior to the vagina and uterus. Trigone – triangular area outlined by the openings for the ureters and the urethra (infections tend to persist in this region – eg thrush, cystitis and other  Organs of the Urinary System Urethra Muscular tube that drains urine from the bladder and conveys it out of the body. Sphincters keep the urethra closed when urine is not being passed: Internal urethral sphincter – involuntary sphincter (smooth muscle) External urethral sphincter – voluntary sphincter (skeletal muscle) Female Urinary Male Urinary System System  Functions of the Kidneys Functions of the kidneys fall into 3 categories: Excretory Regulatory Metabolic  Functions of the Kidneys Excretory: As blood flows through the kidneys the plasma is filtered producing about 180L of fluid daily, allowing metabolic wastes, excess water and ions to leave the body in urine. Only 1 to 2L of urine is excreted per day, the kidneys reclaim >99% of the substances filtered.  Functions of the Kidneys Regulatory: Regulates plasma fluid volume and electrolyte content. Maintains the balance between acids and bases (pH) in the blood. Regulates blood pressure. Regulates the production of red blood cells.  Functions of the Kidneys Metabolic: Activates the vitamin D produced by the skin Renal Renal Renal Renal Renal Renal Renal Renal  Renal Renal Test Yourself - Label the Kidney  Label the Kidney - Answers A. Interlobular blood vessels B. Renal artery C. Renal vein D. Ureter E. Renal pyramids F. Minor calyx G. Major calyx H. Renal pelvis I. Renal capsule J. Renal medulla K. Renal cortex L. Papillae  The Nephron The nephron is a tiny blood filtering structure which spans the cortex and the medulla. Nephrons filter waste products of metabolism and other substances that would be toxic to the body if they remained in the bloodstream. Each kidney has around 1 million nephrons. The Nephron Cortex Medulla Urine Formation Urine is made in the nephron in three stages: 1. Filtration (substances are filtered out of the blood into the Bowman’s capsule according to size). 2. Selective reabsorption (substances are reabsorbed back into the blood). 3. Secretion (substances are Filtration Filtration occurs in the glomerulus of the nephron. The glomerulus is a mesh of capillaries that act as a microscopic filter for the blood. The glomerulus sits in a capsule called the glomerular capsule or Bowman’s capsule. Together, the glomerulus and the Bowman’s capsule make up the renal corpuscle.  The Renal Corpuscle Efferent arteriole Proximal Convoluted Tubule Glomerulus Afferent arteriole Bowman’s Capsule Filtration Oxygenated blood comes into the glomerulus through the afferent arteriole under high hydrostatic pressure (pressure opposing osmotic pressure). This is because the afferent arteriole gets narrower as it gets nearer the glomerulus. Also, the afferent arteriole is wider than the efferent arteriole. This blood carries useful substances (such as glucose, amino acids, vitamins, electrolytes, hormones) and waste products (such as urea, uric acid, creatinine etc). These are small enough to passively filter out of the blood (no energy is required for Filtration Plasma containing dissolved water and substances smaller than proteins are pushed through the capillary membrane into the Bowman’s capsule (aka Glomerular capsule). This fluid is called the filtrate. Large cells, fats and proteins (red blood cells, white blood cells, plasma proteins) are left behind – too big to pass out of the capillaries. The volume of the filtrate produced by both kidneys each minute is called the glomuerular filtration rate (GFR). In a healthy adult the GFR is around  Glomerular Filtration Rate The glomerular filtration rate (GFR) is the rate at which the kidneys filter the blood. Your doctor can estimate your GFR by measuring creatinine in the blood. Creatinine is a metabolic waste product produced when your body uses the amino acid creatine for energy – it is not reabsorbed or secreted. Your doctor will measure the amount of creatinine in the blood, combined with information about your age, weight, gender and height to give an estimate of your GFR. Selective Reabsorption - Proximal Convoluted Tubule The nephron selectively reabsorbs substances that the body needs. These substances are moved from the tubules back into the peritubular capillary blood. Waste products are left in the filtrate. First, nutrients are reabsorbed from the proximal convoluted tubule (PCT) back into the bloodstream: All glucose (only the PCT has the transporters) All amino acids Electrolytes Most vitamins Selective Reabsorption - Descending Loop Glucose, ketones, nitrites and protein do of Henle not normally appear in healthy urine – only if blood levels are excessive. In the descending loop of Henle, more water is reabsorbed into the peritubular capillary blood by osmosis (around 20%). Now around 90% of the water has been reabsorbed from the filtrate into the bloodstream. This leaves a concentrated filtrate which sets up a concentration gradient to facilitate reabsorption of electrolytes in the ascending Selective Reabsorption - Ascending Loop of Henle In the ascending loop of Henle, electrolytes (sodium, potassium, chloride etc) are reabsorbed back into the peritubular capillary blood. These are in high concentration in the filtrate which makes it easier for them to be reabsorbed as there is a concentration gradient. In the glomerulus, the PCT, the ascending and descending loop of Henle, the process is always the same. The same substances are always filtered from the glomerulus and reabsorbed in the PCT and the loop of Henle of the nephron and always roughly the same amount. Reabsorption and Secretion - Distal Convoluted Tubule In the distal convoluted tubule (DCT), electrolytes are either reabsorbed into the peritubular capillary blood from the DCT or secreted from the peritubular capillary blood into the DCT, depending on the needs of the body. For instance, if the body has too much K+, K+ will be secreted into the DCT, not enough K+, K+ will be reabsorbed into the blood, if the blood is too acidic, H+ will be secreted into the DCT too basic, H+ will be reabsorbed into the blood. Acid/Base Balance - Distal Convoluted Tubule The kidney is the only organ that can get rid of H+ or other acids or bases such as bicarbonate. If there is an excess, it will be secreted out of the blood into the DCT. If there is a lack, it will be reabsorbed by the DCT to be excreted in the urine. Remember: Acids increase the amount of H+ in a solution Acid/Base Balance - Distal Convoluted Tubule If the blood is too acidic (acidosis), the kidneys can raise the pH by secreting H+ ions from the bloodstream into the DCT. Bicarbonate (HCO­3-)will be reabsorbed into the bloodstream (because it is a base which can neutralise acids by hiding it in H2O). pH of urine will decrease – become more acidic. What will happen if the blood is too basic? Acid/Base Balance - Distal Convoluted Tubule What will happen if the blood is too basic? If the blood is too basic (alkalosis), the kidneys can lower the pH by secreting bicarbonate ions (HCO­3-) from the bloodstream into the DCT. Hydrogen ions (H+) will be reabsorbed (because H+ can neutralise HCO­3- by hiding it in H2O and CO2). pH of urine will increase – become more Distal Convoluted Tubule Ammonia, some drugs and other substances that may be toxic to the body if they remain in the bloodstream are also secreted from the peritubuluar capillary blood into the DCT. The DCT will move the appropriate transporters to the membrane according to what substances need to be moved. This keeps the body in homeostasis. Collecting Duct The final volume of urine is decided in the collecting duct. The collecting duct reabsorbs extra water into the peritubular capillaries during times of dehydration. This will mean there is a low urine output and concentrated (dark) urine. Urine drains from the collecting duct into the renal papilla and into the minor calyx. From the 180L of filtrate that is produced each day, around 1.5L - 2L of urine per day is formed. Formation of urine is constant – even if you  Composition of Normal Urine Physical characteristics of normal urine: Transparency Colour Specific gravity Odour Chemical composition: 96% water 2% urea 2% remaining solutes  Abnormal Urine Constituents Substance Name of Condition Possible Cause Glucose Glycosuria Diabetes mellitus Nitrites Nitrituria Urinary tract infection Protein Proteinuria, albuminuria Heart failure, hypertension, kidney disease Ketone bodies Ketonuria Untreated diabetes mellitus, starvation, low carb keto diet Erythrocytes Hematuria Urinary tract bleed – could be due to kidney stones, infection or cancer Leukocytes Pyuria Urinary tract infection Kidney stones. The ureter is around 3mm to 4mm wide. Besides urine, most water is lost from the body in breathing out and normal perspiration. Formation of concentrated urine – anti-diuretic hormone (ADH) Reabsorption of the last 10% of water from the urine happens in the collecting ducts under the influence of anti-diuretic hormone ADH (not otherwise). ADH is released by the posterior pituitary gland and travels through the bloodstream to the kidney in response to a rise in plasma osmolarity (solute concentration rises). ADH increases the permeability of collecting ducts to water. More water moves out of the collecting ducts and returns to the bloodstream. Formation of concentrated urine – anti-diuretic hormone (ADH) Diuresis – the excretion of urine, especially when excessive (polyuria - you need to pass urine a lot). Diuretic – a substance (medications, coffee, tea, coke etc) that increase urine output. Commonly used to treat high blood pressure. Anti-diuretic – reduces urination by stimulating the collecting ducts to reabsorb more water. Formation of concentrated urine – anti-diuretic hormone (ADH) As the blood passes through the hypothalamus, osmoreceptors detect the solute concentration (osmolarity) in the plasma. If the osmolarity is too high (dehydration), water will move from cells to the plasma. This will cause the osmoreceptor cells to contract. This causes the hypothalamus to signal to the pituitary gland to release ADH. ADH travels through the bloodstream to the kidney, where it finds it’s target cells in the collecting duct Formation of concentrated urine – anti-diuretic hormone (ADH) ADH causes the walls of the collecting ducts to become much more permeable to water. This allows more water to move out of the collecting ducts into the peritubuluar capillaries by osmosis. Water loss from the body is minimised, there will be a small volume of concentrated urine. Plasma osmolarity decreases, osmoreceptors will not be triggered, ADH will not be released.