The_Functional_Role_of_the_Urinary_System

The urinary system is made up of the following structures: • 2 kidneys, which secrete urine • 2 ureters, transports urine from the kidneys to the bladder • The urinary bladder: stores urine temporarily • The urethra: expels urine from the bladder to the exterior It is also the main excretory system in the body as it produces urine, ensuring that the body gets rid of unwanted and potentially harmful substances. Filtering blood plays a vital part in maintaining homeostasis of water and electrolyte concentrations within the body. The primary role of the kidneys is to maintain a healthy balance of water, minerals and electrolytes.The kidneys form urine, which consists of consists of 96% water, 2% urea and 2% (Uric acid, Creatinine, Ammonia, Sodium Potassium, Chlorides, Phosphates, Sulphates and Oxalates). Foods such as poultry, eggs and beans, produce urea when the proteins are broken down and transferred through the blood stream to the kidneys. Waste products are produced constantly due to the metabolism of nutrients in the cells. The kidneys help maintain a normal blood pressure, by the secretion of the hormone renin, stimulating the release of the hormone, Erythropoietin (EPO), which increases the production of red blood cells and, a type of Vitamin D called Calcitriol, which aids the absorption of calcium in the small intestines. The hormones produced also control blood pH levels: pH 7.4 is the norm for blood. Urine is formed in the nephrons of the kidney. Each kidney contains approximately a million filtration units. The nephron is comprised of: • The proximal convoluted tubule • The loop of Henle • The distal convoluted tubule • Collecting Duct Nephrons are connected to collecting ducts, which open into the renal pelvis. Clusters of capillaries, or glomeruli, provide the nephron’s blood supply. The blood is supplied at a high pressure, compressing the nephron so that water, amino acids, salt, glucose and waste products are expelled. The glomerular capsule and glomerulus walls consist of a layer of flattened epithelial cells, which are more permeable than other capillaries. A single layer of highly specialised cells forms the rest of the nephron and collecting tubule. The nerve supply of the kidney is made up of sympathetic and parasympathetic nerves, which permits control of renal blood vessel diameter and renal blood flow independently of autoregulation. Urine is formed in three stages:  Filtration  Selective reabsorption  Secretion Simple filtration occurs in through the semi permeable walls in the glomerulus and the glomerular capsule. Water and small molecules are able to pass through, although some are reabsorbed later on. The difference between the blood pressure in the glomerulus and the pressure of the filtrate in the glomerular capsule assist filtration. Almost all of the filtrate is reabsorbed with less that 1.5 litres excreted as urine, the volume and concentration difference is due to selective reabsorption of some constituents of the filtrate, and tubular secretion of others. Selective Reabsorption is the process by which the composition and volume of the glomerular filtrate change during the journey through the convoluted tubules, medullary loop and collecting tubule. Certain constituents ae needed by the body to maintain the balance of electrolytes and fluid, so this is reabsorbed into the body. The renal threshold of some substances varies according to the body’s need for them at a specific time. Reabsorption can also be regulated by certain hormones. Secretion occurs when blood flows through the glomerulus. Foreign materials and substances, such as drugs, which are not required, may not be removed through filtration as it hasn’t spent the required time in the glomerulus for this to occur. These are then removed by the action of secretion into the convoluted tubules and excreted from the body in the urine, tubular secretion of hydrogen ions is vital in maintaining homeostasis of the blood pH. The colour of urine is produced by a bile pigment named urobilin; a bile pigment altered in the intestine, reabsorbed then excreted by the kidneys.Urine leaves the kidneys for the bladder via the ureters. The uterters are two tubes, continuous with the renal pelvis with a 3mm diameter and are roughly 25 – 30 cm in length. They open into the posterior wall of the bladder. The bladder is a resevoir for urine. The shape, size and position vary with the amount of fluid it contains. When empty it is in rugae, lies in folds and lies within the lesser pelvis. When distended with urine it expands upwards and forwards into the abdominal cavity. When full with urine its appearance is smooth but. A moderately full bladder is approximately 12.5 cm long and holds about 500 ml of urine; however, it could hold twice that amount. When full to capacity the bladder becomes firm and pear-shaped and lies just above the pubic symphysis, the release of urine is voluntary when convenient. When there is enough urine stored in the bladder, the need to urinate will occur. The bladder wall contracts and a sphincter muscle at the base of the bladder will relax. The release of urine can be controlled voluntarily by another sphincter muscle found at the bottom of the urethra. The urethra leaves the bladder at its base. Bibliography Boyle M., Senior K., (2002) Human Biology. (2nd ed) Hammersmith, London: HarperCollins Publishers Ltd. Marieb E N, (2009) Essentials of Human Anatomy and Physiology. (9th ed). San Francisco, CA: Pearson/Benjamin Cummings. Seeley et al (2007) Anatomy and Physiology (8th Revised edition) McGraw Hill Higher Education, UK. Watson, R (2005) Anatomy and Physiology for Nurses (12th Ed). Edinburgh: Elsevier Waugh, A., Grant, A., (2006) Ross and Wilson: Anatomy and Physiology in Health and Illness. (10th Ed). Edinburgh: Churchill Livingstone.