Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
By Matthew M. Green and Carlos Krumpach The purpose of the urinary system is to remove certain salts and nitrogenous wastes from the body, maintain normal concentrations of water and electrolytes within body fluids, regulate the pH and volume of body fluids, and help control red blood cell production and blood volume. The Urinary System consists of a pair of kidneys, a pair of tubular ureters, a saclike urinary bladder, and a tubular urethra A kidney is a reddish-brown, bean-shaped organ, with a smooth surface. (Adult kidneys are 12 centimeters long, 6 centimeters wide, and 3 centimeters thick.) They are enclosed in a strong, fibrous capsule. The kidneys lie on either side of the vertebral column in a depression high on the posterior wall of the abdominal cavity. The upper and lower borders of the kidneys are usually at the levels of the twelfth thoracic and third lumbar vertebrae. (The left kidney is 1.5-2.0 centimeters higher than the right kidney.) The kidneys are positioned retroperitoneally, or behind the parietal peritoneum and against the deep muscles of the back. (The kidneys are held in place by connective tissue and adipose tissue.) The kidneys later surface is convex, while its medial side is concave. The resulting medial depression leads to the renal sinus, a hallow chamber. The entrance to the renal sinus is called the hilium, this where blood vessels, nerves, lymphatic vessels and the ureter enter the kidney. The superior end of the ureter expands and forms a funnelshaped sac called the renal pelvis. The renal pelvis subdivides into two or three major calyces, which subdivide into several minor calyces. Renal papillae project into the renal sinus from its wall, and contain openings that lead into a minor calyx. The inside of the kidney is the renal medulla, which is composed of renal pyramids and appears striated. The renal cortex forms a shell around the medulla and dips in between renal pyramids, forming renal columns. Within in the renal cortex are nephrons, or the kidneys functional units. The kidneys help maintain homeostasis by regulating the composition, volume, and pH of extracellular fluid. They do this by forming, and excreting urine, a combination of metabolic wastes, (from the blood), water, and electrolytes. The kidneys also secrete the hormone erythropoietin, which helps control red blood cell production, they help activate vitamin D, and they help maintain blood volume and pressure by releasing the enzyme renin. Renal arteries supply blood to the kidneys. When a person is at rest these arteries usually carry 15-30% of the total cardiac out put into the kidneys. Renal arteries give off several branches called interlobar arteries, which pass between renal pyramids. Arcuate arteries at the junction between the medulla and cortex give rise to interlobular arteries. Afferent arterioles, the final branches of interlobular arteries, lead to nephrons. Venous blood returns in vessels that correspond to arterial pathways, which then lead to the renal vein. The renal vein leads the blood back to the inferior vena cava. Nephrons consist of a renal corpuscle and a renal tubule. Fluid flows through the tubules on its way out of the body. Renal corpuscles are made of tangled clusters of blood capillaries called glomerulus. Glomerular capillaries filter fluid, which is the first step in urine formation. The glomerular capsule, a thin-walled, saclike structure surrounds the glomerulus. The glomerular capsule, an expansion at the proximal end of a renal tubule, receives the fluid the glomerulus filters. The renal tubule leads away from the capsule and becomes tightly coiled, this coiled portion is the proximal convoluted tubule. This tubule dips towards the renal pelvis, and becomes the descending limb of the nephron loop. The tubule then curves back towards the renal corpuscle and forms the ascending limb of the nephron loop. This tubule returns to the region of the renal corpuscle, where it is tightly coiled and is called the distal convoluted tubule. These tubules merge in the renal cortex to form collecting ducts, which passes through the renal medulla and is joined by other tubules. The resulting tube empties into a minor calyx through an opening in a renal papilla. Blood Supply of a Nephron The capillaries that form a glomerulus all come from arterioles. Blood, after passing through the glomerular capillaries, enters an efferent arteriole, (that is smaller than the afferent vessel). The arteriole resists blood flow, which raises blood pressure in the glomular capillary. The efferent arteriole branches off into a freely interconnecting network of capillaries called the peritubular capillary system, that surrounds the renal tubule. After flowing through the peritubular capillary system, the blood rejoins from other branches and enters the venous system of the kidney. Juxtaglomerular Apparatus The structure of densely packed epithelial cells at the point of contact between the distal convoluted tubule and efferent and afferent arterioles makes up the macula densa. Within the walls of arterioles are juxtaglomerular cells, that with the cells of the macula densa form the juxtaglomerular apparatus. This apparatus controls the secretion of renin. Urine formation begins with glomerular filtration, which is the process of the glomerular capillaries filtrating plasma. Tubular reabsorption, which moves substances from tubular fluid back into the blood within the peritubular capillary, and tubular secretion, which is the reverse process of tubular absorption, also aid in forming urine. Urine formation begins with water and certain dissolved substances being filtered out. The resulting glomerular filtrate is sent into glomerular capsules. The hydrostatic pressure of blood and the osmotic pressure of plasma influence the pressure of glomerular capillaries, and capsules. The net filtration pressure is the net pressure for forcing substances out of the glomerulus. The net filtration pressure is responsible for the kidney’s filtration rate. The glomerular filtration rate is an example of a negative feedback mechanism. The filtration rate can also be regulated by sympathetic nervous system reflexes. Juxtaglomerular cells also control the rate by releasing renin, which reacts with the plasma protein angiotensinogen to form angiotensin I. A second enzyme called angiotensin converting enzyme, converts angiotensin I to angiotensin II. Angiotensin II is a plasma protein that helps maintain sodium balance, water balance, and blood pressure in the body. Tubular reabsorption is the process in which filtered substances are returned to the blood stream. Sodium and water can be reabsorbed multiple times making them more concentrated. Tubular secretion is when substances move from the bloods plasma in the peritubular capillary to fluid in the renal tubule. Aldosterone and antidiuretic hormone may stimulate the additional reabsorption of sodium and water. These changes in sodium and water excretion in response to these hormones are the final adjustments the kidney makes before urination. Urea is a by-product of amino acid catabolism, uric acid is a result of the metabolism of organic bases in nucleic acids. Both of these by-products are excreted in urine. Urine composition reflects the amount of water and solutes that the kidney must eliminate or retain. From the kidneys, urine passes through the renal pelvis, and a ureter conveys it to the urinary bladder and reference plate. The urethra then excretes urine to the outside. The ureters are 25 centimeters long, they begin as the renal pelvis, and connect to the urinary bladder. The urinary bladder is a hollow, distensible, muscular organ that stores urine and forces it out into the urethra. The detrusor muscle is a muscle that prevents the bladder from emptying until a certain pressure is reached. Micturition is the process that conveys urine from the urinary bladder to the outside of the body. Dysuria is a condition that causes painful urination. Nephrolithiasis, which is a kidney stone. Cystis, which is an inflammation of the urinary bladder. Butler, Jackie. Lewis, Ricki. Shier, David, ed. Hole’s Essentials of Human Anatomy and Physiology. New York: McGrawHill.2006. 454-471. Print.