11. Urinary System

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Information about 11. Urinary System
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Published on February 15, 2020

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Urinary System: Urinary System Kidney, Ureter , Bladder (KUB) General Training Module UNIT 08 RK Contents: Contents Anatomy of the Urinary Tract Overall Functions of the Kidney Gross Anatomy of the Kidney Histology of the Kidneys Formation of Urine Maintaining Homeostasis Urinary Outflow Tract Micturition Anatomy of Urinary System: Anatomy of Urinary System Bladder Position of the Organs of the Urinary System in the Male: Position of the Organs of the Urinary System in the Male Kidneys - where are they located? : Kidneys - where are they located? Posterior Abdominal Wall Anatomy & Main Functions: Anatomy & Main Functions Kidneys Blood is filtered by the kidneys to form urine. Urine is secreted in to the ureters Ureters Transport urine from the kidneys to the bladder Bladder Urine is temporarily collected and stored Urethra Urine stored in the bladder is excreted by urethra Kidneys: Kidneys They are bean shaped structures There are two – one on each side (Right & Left) Each one 11 cm long, 6 cm wide & 3 cm thick Lie embedded in the posterior abdominal wall on either side of the vertebral column, below the diaphragm. Right kidney located slightly lower than the left one (probably due to the liver on the right side) Functions of the Kidney: Functions of the Kidney Elimination of waste The kidney forms urine in which the waste products of protein metabolism are excreted. Maintains the internal environment of the body (homeostasis) by achieving: Water and electrolyte balance. Acid-base balance by excretion of H+ ions. Produces enzyme Renin Regulation of blood pressure Also produce the hormone erythropoetin involved in erythropoiesis (RBC formation) Kidneys – Gross Anatomy: Kidneys – Gross Anatomy Longitudinal Section - Kidney: Longitudinal Section - Kidney Longitudinal Section - Kidney: Longitudinal Section - Kidney Outer capsule Cortex Medulla & R enal pyramids Hilum through which vessels and the ureter pass Calyces: Lesser and greater Pelvis Ureter Blood Supply to the Kidneys: Blood Supply to the Kidneys Microscopy - Nephrons: Microscopy - Nephrons Nephron is the functional unit of the kidney. They are microscopic structures There are about a million nephrons in each kidney. Each has: Glomerular (Bowman’s) Capsule Proximal Convoluted tubule Loop of Henle Distal convoluted tubule Nephron: Nephron Each nephron is a tubule closed at one end while the other end opens to the collective tubule The closed end forms the cup-shaped Glomerular or Bowman’s Capsule It completely encloses a network of arteriolar capillary network – Glomerulus The rest of the tubule is divided into several parts Nephron: Nephron Closer to Glomerulus the proximal convoluted tubule Then the Loop of Henle , consist of: Descending limb The loop The ascending limb Distal Convoluted Tubule The Distal Convoluted tubule opens to Collecting tubules Nephron: Nephron Renal artery divides into smaller arteries and arterioles In the cortex an arteriole enters each of the Glomerular Capsule and divide into cluster of capillaries – this is the afferent arteriole The efferent arteriole leaves the capsule at the other end of the capillary network These again form another capillary network that supply oxygen and nutrients to the rest of the nephron Nephron: Nephron Veins that drain this capillary network converge to form the renal veins and leave each kidney to drain into the inferior vena cava Nephron: Nephron Nephron: Nephron From the renal artery Blood supply to Loop of Henle To renal vein Glomerulus: Glomerulus The walls of the glomerulus and the glomerular capsule contains a single layer of epithelial cells and are more permeable than those of the other capillaries. The caliber of afferent arterioles is larger to that of efferent arterioles. Therefore the blood pressure in the glomerulus is higher than in the other capillaries. These specialization helps the Glomerulus perform their function effectively Glomerulus: Glomerulus Formation of Urine: Formation of Urine There are 3 phases in urine formation: Filtration: Selective re-absorption: Secretion Formation of Urine: Formation of Urine Filtration: Occurs in the Glomerulus across a pressure gradient through semi-permeable wall Selective re-absorption: Occurs in the proximal and distal convoluted tubules, Loop of Henle and in the collecting tubules Secretion Occurs in the convoluted tubules Glomerular Filtration: Glomerular Filtration Due to the inequalities of the arteriolar caliber in the afferents and the efferents ( efferents are smaller) of the Tuft of Capillaries in the Glomerulus the pressure in the Glomerulus builds up. The pressure in the filtrate within the Glomerular Capsule is much less The wall of the Glomerulus and the Capsule is semi-permeable Therefore filtration takes place across a pressure gradient from the capillaries in the glomerulus to the filtrate in the capsule Glomerular Filtration: Glomerular Filtration Glomerular Capillary Glomerular Capsule Hydrostatic Pressure 70 mm Hg Hydrostatic Pressure 5 mm Hg Osmotic Pressure 30 mm Hg Net Filtration Pressure 35 mm Hg Glomerular Filtration: Glomerular Filtration In addition to the hydrostatic pressure of the filtrate, osmotic pressure of the glomerulus too act against the hydrostatic pressure of the glomerulus . However both these pressures are not enough to overcome the high pressure in the glomerulus . Therefore water and a large number of small molecules pass through the semi-permeable membrane Glomerular Filtration: Glomerular Filtration The large molecules such as plasma proteins and cells in the blood do not pass through the membrane. Some of the small molecules and water are later reabsorbed and some others are actively and selectively secreted into urine Therefore though the 2 kidneys filter about 180 liters per day only about 1.5 liters are excreted as urine Composition of Filtrate : Composition of Filtrate Glomerular Filtrate (Pass through to filtrate from the blood) Remain in the glomerulus (does not pass through to filtrate) Water Red Blood cells Mineral salts White Blood cells Amino acids Platelets Ketoacids Blood Proteins (Plasma Proteins) Glucose Hormones Urea & Uric Acid Toxins Drugs Selective Re-absorption: Selective Re-absorption Not all the constituents of the glomerular filtrate appears in urine. That is because they are COMPLETELY reabsorbed when the glomerular filtrate pass through the tubules of the nephrons . This way those constituents that are required by the body are retained without excretion unless they are present in excess in the blood ( eg . Glucose) Selective Re-absorption: Selective Re-absorption This is because the kidneys have a maximum capacity to reabsorb each substance - known as transport maximum For example transport maximum for glucose is 160 mg/dl for which the normal blood level is 45 to 95 mg/dl. So if the blood glucose levels exceed 160 mg/dl only glucose starts to appear in urine Selective Re-absorption: Selective Re-absorption This capacity of the kidneys to reabsorb could be hormonally regulated for some substances: Calcitonin and Parathormone hormones together regulate the reabsorption of calcium and phosphate ADH or Anti Diuretic Hormone regulate water reabsorption Aldosterone regulates sodium reabsorption and potassium excretion Selective Re-absorption: Selective Re-absorption In addition substances required to maintain water, electrolyte and acid- base balance are reabsorbed as required by the body. The waste products are not reabsorbed or reabsorbed only slightly. (urea & uric acid) Substances that are not normal part of the blood are not reabsorbed at all. (Toxins and drugs) Secretion: Secretion If blood pass through the glomerulus too soon that the substances that are not required by the body including toxins and drugs could not be filtered to the glomerular filtrate they are actively secreted to the filtrate when passing through the convoluted tubules. Therefore the composition of the glomerular filtrate is altered both by subsequent selective reabsorbtion and secretion in renal tubules to finally form urine! Composition of Urine: Composition of Urine Water - 96% Urea - 02% Uric acid Creatinine Ammonia Sodium Potassium Chlorides Phosphates Sulphates Oxalates 02 % - In addition urine has the pigment urobilin , a bile pigment secreted to the gut and altered in the intestine and reabsorbed then excreted in the kidneys. - It gives urine its amber color. - It is acidic and has a specific gravity of 1020 to 1030 Maintaining Homeostasis: Maintaining Homeostasis Water Balance: Water Balance INTAKE BODY WATER OUTPUT Ingested and absorbed from the gastro -intestinal tract. The main constituent of the urine Saturated expired air As a part of the feces Through the skin as sweat A small amount produced by metabolic processes of the cells within the body Water Balance: Water Balance The amount of water lost in air and feces is fairly constant. The amount of sweat is dependent on thermoregulatory requirements of the body Therefore balance between fluid intake and output is maintained by the kidneys The minimum urinary output to excrete the waste products of metabolism is about 500ml/day Water Balance: Water Balance The amount of urine produced in excess of this minimum urine requirement is regulated by the hormone secreted from the posterior lobe of the pituitary ADH (Anti- Diurecti Hormone). ADH secretion is regulated by hypothalamus as a response to osmoreceptors in hypothalamus that are sensitive to osmotic pressure of blood Osmotic pressure is high  ADH secretion increased  water reabsorption increased  osmotic pressure decreases  feedback loop to reduce ADH secretion Water Balance: Water Balance This feedback system is overridden when there are excessive amounts of dissolved substances in blood that requires elimination. A common example is glucose. If there’s more than required in blood , glucose will be excreted in urine taking more water with it in the urine (despite of increased ADH). This causes excessive passage of urine which leads to frequent dehydration and therefore increased thirst and intake of fluid. Electrolytes Balance: Electrolytes Balance Electrolytes are those substances that exist in ionic form within the body: Na+, K+, H+, Mg2+, Ca2+, HCO3-, Cl - Na+ is the main extracellular cation (positively charged ion) K+ is the main intracellular cation In addition H+ levels are important as it maintains the acid- base balance of the internal environment. The blood concentrations of these are regulated by the kidneys in such a way they exist in a state of balance within the body keeping internal environment constant. Electrolytes Balance: Electrolytes Balance Sodium intake is through diet. There’s always more sodium in the food than body needs. The excess sodium is usually excreted either in the sweat or urine There are high levels of Na+ & K+ in gastric juices but under normal conditions they are reabsorbed by the intestine. However excessive loss could occur due to diarrhea – during which electrolytes may have to be replaced Amount of sweat varies and is regulated based on the body temperature requirements and can not be controlled to maintain electrolyte balance Therefore amount of sodium in the body is maintained mainly by the kidneys Electrolytes Balance: Electrolytes Balance Kidney reabsorb sodium in the tubules This is regulated by the adrenal cortex hormone aldosterone The renin-angiotensin system (each produced by the kidneys and liver respectively) regulates the secretion of aldosterone and therefore Na+ reabsorption . When sodium is absorbed in tubules K+ is excreted indirectly reducing intracellular potassium Water is absorbed along with Na+ and provides feedback meachanism for renin to reduce secretion from kidneys (increased blood flow) H+ Balance: H+ Balance In order to maintain normal blood pH the Kidneys secrete H+ in the tubules and are excreted in urine. They are excreted in combination with bi-carbonate as carbonic acid With ammonia as ammonium chloride With hydrogen phosphate as dihydrogen phosphate Therefore the normal pH of urine varies between 4.5 to 7.4 Urine outflow tract: Urine outflow tract Ureters: Ureters These are the two tubes that convey urine from the kidneys to the bladder About 25 to 30 cm long 3 mm in diameter It is continuous with the renal pelvis above Renal pelvis is funnel shaped – it is the receptacle that collects urine from the kidneys Ureters: Ureters Renal pelvis is formed by many branches at its end call calyces that collects urine from the renal medulla (to first lesser calyces and then greater calyces before passing into the renal pelvis) The walls of the calyces and pelvis are lined with transitional epithelium and has smooth muscles which are continuous in the bladder lining and wall. Intrinsic contractions could occur in the cells of the calyces with no external stimuli. These contractions occur in wave form and propel urine down the ureters to the bladder Bladder: Bladder Ureters open into the bladder obliquely preventing back flow of urine when the bladder contracts Bladder lies in the pelvic cavity and is the reservoir of urine. In the female it lies anterior to the uterus In the male anterior to the rectum At its lowest point starts the urethra The opening of which is controlled by the internal sphincter – a thickening of the smooth muscle which is controlled by the autonomic nervous sytem . Urethra: Urethra Extends from the bladder to the exterior Length differs from male to female In the female urethra is about 4 cm long and opens to the exterior, anterior to vagina The opening is guarded by the external sphincter (skeletal muscle) – which is under voluntary control It is lined either by a mucous membrane above and stratified squamous epithelium below Self-Study Questions: Self-Study Questions What is the main difference between the male and the female urethra when you consider their functions? Urethral Sphincters: Urethral Sphincters There are two   urethral sphincters: The Internal Urethral Sphincter The External Urethral Sphincter   They are two muscles used to control the exit of urine in the  urinary  bladder through the  urethra . The external  urethral sphincter  is made of skeletal muscle where as the internal urethral sphincter is made of smooth muscle Urethral Sphincters: Urethral Sphincters Urethral Sphincters: Urethral Sphincters Micturition (passing of urine): Micturition (passing of urine) The bladder is supplied by the sympathetic and parasympathetic nervous system of the Autonomic Nervous System Parasympathetic supply contracts the smooth muscles in the bladder wall Sympathetic supply contracts the internal sphincter which is closed under normal conditions Micturition (passing of urine): Micturition (passing of urine) When urine is being filled the bladder wall start stretching When there is about 500 ml of urine in the bladder the stretch receptors in the bladder are stimulated. They sends sensory signals to the central nervous system Sensory Cortex This causes desire to pass urine (micturition) and activates both the voluntary and involuntary responses At this stage voluntary inhibition of the activation of the autonomic responses could be achieved for a short while consciously controlling the desire to pass urine for a brief period of time. Micturition (passing of urine): Micturition (passing of urine) Parasympathetic nerve fibers are stimulated to contract bladder wall muscle and to inhibit and relax the internal sphincter smooth muscles. At the same time voluntary muscles of the external sphincter are relaxed consciously. They are supplied by the pudendal (somatic motor) nerve - along with contraction of abdominal muscles and increasing pelvic pressure . These actions initiate micturition If you hold urine voluntary controlling the desire to pass urine the external sphincter would be relaxed involuntarily for a small amount of urine to pass – this is very painful Micturition (passing of urine): Micturition (passing of urine) In the infant micturition is accomplished through a spinal reflex. That is sensation of a full bladder does not reach higher centers of the brain. Instead it activates the contraction of bladder wall, relaxation of internal sphincter and external sphincter in a reflex manner without the possibility of exerting any voluntary inhibition of passing urine. End of Unit 8: End of Unit 8

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