Kidney, Kidney Bean..Which is. Which

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1 l AKA "The Urinary System" This is the system responsible for the removal of waste from blood. This waste is "cell waste" or "excretion" to distinguish it from "solid waste" or "feces". The principle organ of the Excretory System is, of course, the kidney. The kidney is shaped like, and named after, the kidney bean. Kidney, Kidney Bean..Which is Which? Which is Your kidneys constantly filter your blood to remove wastes that diffused into the Which? blood from cells. Many of the cells that produce these wastes are found in the liver. These wastes are referred to by the terms "urine", "waste #1", "liquid waste", and "cell waste". These wastes result from the metabolism or alteration of chemical compounds by living cells. Many of these wastes are based upon the element Nitrogen. The principal cell waste is urea, but the body also forms uric acid and ammonia (in smaller quantities.) In a nutshell, the three major functions of the Excretory System are: 1) 2) 3) Osmoregulation-control of tissue water balance. Elimination of excess salts. Elimination of wastes formed during the metabolism of amino acids, nitrogenous wastes. 1

2 The Major Organs of the Excretory System Skin It isn't often that we think of our skin as being an excretory system organ, but it is! Skin plays a small role in the excretion of metabolic wastes through perspiration. Perspiration serves as a means of elimination for the metabolic waste urea (just a small amount) and salt. Removing salt through the skin helps to make the body's water less saline - it is a form of osmoregulation. The role served by the skin in excretion is minimal, BUT it does play an important role in maintaining the body's water balance. The Lungs Waste gases are removed by the lungs, so by this simple measure they serve as part of our excretory system. Respired gases contain trace amounts of urea, but the contribution of the lungs in metabolic waste removal is very, very minor. The Kidneys The kidneys are reddish-brown organs surrounded by a smooth capsule (the tunic fibrosa). The kidneys lie on each side of the vertebral column in a depression high on the posterior wall of the abdominal cavity. The left kidney is usually slightly (1.5-2 cm.) higher than the right kidney. The kidneys are retroperitoneal - they are found outside of the peritoneum and lie against the deep muscles of the back. The kidney is held in place by connective tissue and is usually found embedded in a layer of fat. The medial side of the kidney is deeply concave. The lateral side of the kidney is convex. 2

3 The site where an artery and vein enter (leave) the kidney and the ureter enters is termed the hilum. An open space (called the renal sinus) houses the renal pelvis and calyces. Located within the renal sinus is the renal pelvis, which is a funnel-shaped sac that is really the superior end of the ureter. The pelvis is subdivided into two or three tubes (the major calyces, singular: calyx), which in turn are subdivided into 6-14 minor calyces. Hilum Major calyx Pelvis Minor calyces Renal Sinus Cortex Medulla Renal Pyramid The kidney includes two distinct regions: and inner medulla and an outer cortex. The medulla is composed of conical masses of tissue called renal pyramids. The tissue of the medulla looks striated because it consists of microscopic kidney tubules. The cortex, by comparison, appears granular (more like the liver!) Functions of the Kidney The main function of the kidney is to regulate the volume, composition, and ph of body fluids (mostly blood!). The principal source of these wastes is protein metabolism that occurs mainly in the liver. The kidneys also: (1) help control the rate of red blood cell formation by secreting the hormone erythropoietin. (2) help regulate blood pressure by secreting the enzyme renin. (3) activate vitamin D to help in the absorption of calcium ions. 3

4 The Kidney's Blood Supply The renal artery (4) carries blood to the kidney. The renal artery is a branch of the descending aorta. The renal artery branches down to a number of interlobar arteries (10). These, in turn, branch down to arcuate arteries (2), which lead to afferent arterioles (below), each leading to a nephron. Venous blood from the nephron is collected by arcuate veins (1), which unite to form the interlobar veins (9). The interlobar veins unite to form the renal vein (5). Newly-cleansed blood from the renal vein is directed upward through the inferior vena cava, and re-enters the heart through the right atrium. Inferior Vena Cava The Nephron Descending Aorta Efferent Arteriole Interlobular Artery Afferent Arteriole Arcuate Artery & Vein 4

5 The Structure of the Nephron Just as the alveolar sacs are the functional units of the lung, the nephrons are the chief functional units of the kidney. Each kidney holds about 1 million nephrons. The nephron is positioned so that the glomerulus, Bowman's capsule, proximal convoluted tubule and distal convoluted tubule are in the cortex. Kidney Cortex Kidney Medulla Note: You are viewing a pyramid! Each nephron is surrounded by a bed of capillaries. Exchange occurs between the fluids in the nephron and blood within the capillaries. "Wastes" exit the blood and travel into the nephron tubules - this begins to explain why the nephron and capillaries are so tightly interwoven. Examination of the diagram above reveals that there are two capillary beds that participate in filtration: (1) the glomerulus and (2) the peritubular capillaries that surround the convoluted tubules and the Loop of Henle. The glomerulus is surrounded by a sac called the Bowman's (Glomerular) capsule. It is here that we view the first interaction between the circulatory system (glomerulus) and the excretory system (Bowman's capsule). 5

6 Interactive Notes: In the diagram to the right, you should be able to identify AND LABEL: Glomerulus Bowman's Capsule Afferent Arteriole Efferent Arteriole Proximal Convoluted Tubule (PCT) Distal Convoluted Tubule (DCT) Loop of Henle Collecting Duct Peritubular Capillaries Urine Formation Urine Formation occurs as a result of three processes: 1. Force Filtration 2. Tubular Reabsorption 3. Tubular Secretion 1. Force Filtration The movement of material from blood into the Bowman's (Glomerular) capsule is the first step in urine formation. This movement of materials is called Force Filtration. During force filtration water, nutrient molecules, and waste molecules move from the glomerulus to the inside of the Bowman's capsule. The glomerular endothelium and the Bowman's capsule are extremely permeable - perhaps the most freely permeable tissue in the human body. The blood has been filtered because red blood cells and large molecules, such as plasma proteins, remain within the blood while small molecules, such as glucose, urea, and salt (mineral ions), leave the blood to enter the tubule. 6

7 The material which collects in the Bowman's (Glomerular) capsule is called filtrate, and it is very similar in form to tissue fluid elsewhere in the body. The body produces about 180 liters of filtrate each day! There are three reasons that there is so much filtrate: (1)There are 1 million nephrons per kidney. (2) The hydrostatic pressure within the glomerular capillaries is high (materials are PUSHED out!) (Higher quantities of water in the body result in higher hydrostatic pressures - the more we drink the more filtrate we produce.) (3)The walls of the glomerular capillaries are very porous (more porous than body capillaries!) Filtrate in the Bowman's capsule now begins its journey toward the collecting duct. 2. Tubular Reabsorption We have 180 liters of filtrate that must be reduced to the volume of urine produced by a typical human in an average day (1.5 liters). This means that a large amount of materials reabsorption must occur. The movement of materials from filtrate BACK into blood is termed reabsorption. Reabsorbed materials include: 99% of the water that was lost to filtrate. virtually all of the nutrients (glucose, amino acids, many essential ions Materials that are NOT reabsorbed: urea, uric acid, creatinine Most reabsorption occurs in the proximal convoluted tubule, but the distal convoluted tubule and collecting duct are also active and involved. The movement of materials during reabsorption is always in one direction: from the tubules back into the blood. Not only does the blood recover needed materials, but the filtrate becomes more concentrated. Amino acid movement occurs by active transport, most ions move by diffusion, and water moves by osmosis, following ions and nutrients. Reabsorption begins in the proximal convoluted tubule, where most ion and nutrient reabsorption occurs. A great deal (65%) of the water is reabsorbed here. Reabsorption continues through the Loop of Henle, and the filtrate becomes more and more concentrated, containing only 1% of its original volume when it reaches the distal convoluted tubule. Some reabsorption is done passively (for example, water passes by osmosis), but the reabsorption of most substances depends on active transport processes that use membrane carriers and are very selective. 7

8 3. Tubular Secretion There are still waste materials in the blood when the peritubular capillaries reach the distal convoluted tubule. These substances include hydrogen and potassium ions and Creatinine. These substances are removed from the blood by an active transport process called tubular secretion and are transported INTO the filtrate. These substances are potentially toxic ions (K +, H + ), certain drugs (penicillin), molecules not produced by our bodies, and any wastes that mistakenly reentered blood during reabsorption (urea, uric acid, and creatinine). By the time filtrate reaches the collecting ducts, it is essentially urine - water, the nitrogen-based wastes urea, uric acid, creatinine and ammonia, some salt, any excess nutrient molecules,..) In 24 hours, the kidneys filter liters of blood plasma through their glomeruli into the tubules (1. force filtration), which process the filtrate by taking substances out of it (2. tubular reabsorption) and adding substances to it (3. tubular secretion). In the same 24 hours, only liters of urine are produced. Filtrate in the Bowman's capsule contains everything that blood plasma does (except plasma proteins), but by the time it reaches the collecting ducts, the filtrate has lost most of its water and just about all of its nutrients and necessary ions. What remains is called urine. Urine contains 95% water, nitrogenous wastes, and unneeded substances. Why Is Urine Yellow? Freshly voided urine is generally clear and pale to deep yellow. The normal yellow color is due to urochrome, a pigment that results from the body's destruction of hemoglobin. The more solute that there are in urine, the deeper yellow its color. 8

9 Urea and Uric Acid Urea and Uric Acid are wastes that are based upon the element Nitrogen. Nitrogen is an atom that has a tendency to form strong covalent bonds with organic molecules. The presence of nitrogen in a molecule usually indicates that the molecule was used for a building (rather than metabolic) purpose. Amino acids, for example, have a nitrogen side-group. If a nitrogen-containing molecule is to be used as a source of energy, its nitrogen must be stripped off. This process is called deamination and occurs in the liver. Once removed of its nitrogen, organic molecules are simple C,H,O compounds that are suitable for use as a source of energy. The nitrogen, however, removed in union with its two hydrogen atoms, quickly becomes ammonia (NH 3 ). The third hydrogen is easy to procure within a cell. Liver cells quickly convert the ammonia to urea by combining the ammonia with carbon dioxide. The greater the amount of protein in the diet, the higher the amount of urea in urine. Uric acid results from nucleic acid metabolism. Since nucleic acids are not metabolized in the quantity that proteins are, there is much less uric acid in urine than there is urea. Creatinine Creatinine is formed when creatine phosphate is broken down in muscle tissue. Creatinine is formed at a fairly stable rate. Approximately 2% of the body's creatine phosphate is converted to creatinine every day. Creatinine is removed from the bloodstream during tubular secretion, so its absence in blood is an indicator that the kidneys are healthy and functioning properly. 9

10 The Composition of Urine The fundamental role served by the Excretory System can be described as homeostasis (or blood homeostasis). Urine composition changes with diet and body activity. The greater the amount of protein in the diet, the higher the quality of urea, the more concentrated (brightly colored) the urine. The greater the amount of salt in the diet, the greater the volume of salt in the urine, the greater the quantity of urine produced and passed. The greater body activity, the less the volume of urine. Urine is usually about 95% water. Additionally, there is significant urea from the catabolism of amino acids. Uric acid is present from the catabolism of nucleic acids. Creatinine is present from the metabolism of creatine phosphate in muscle tissue. There might be amino acids in urine Salt ions are present in urine - sodium, chloride, potassium, calcium, phosphate, and sulfate. Excess water-soluble vitamins are found in urine. A small amount of CO 2 is present in urine. Very low counts of bacteria are found in fresh, warm, healthy urine. 10

11 What DOESN'T Urine Contain? Red blood cells, cellulose, copious mucous, lignin, fat-soluble vitamins, bile and bilirubin, glucose (normally), hemoglobin, E. coli bacteria How The Kidney Regulates The Body's Water Balance The reabsorption of water FROM FILTRATE BACK INTO THE BLOOD is under the control of a hormone called ADH (anti-diuretic hormone) that is released by the pituitary gland. ADH increases the permeability of the distal convoluted tubule and collecting duct so that more water can be reabsorbed. When ADH is present, more water is reabsorbed, and a decreased amount of urine results (also increasing or decreasing the amount of water in blood {affecting blood volume}). If an individual drinks an excess amount of water: 1. Does ADH production begin or stop? 2. Is more or less water excreted? 3. Is the urine dilute or concentrated? 4. Does blood volume increase, decrease, or is it maintained at a normal level? Kidney Illness Urinalysis - reveals if there are any abnormal substances in the urine. The most likely of these substances are: 1. Sugar - a symptom of diabetes. 2. Bacteria - a symptom of renal disease. Can affect: A. Urethra - bacterial infection is called urethritis. B. Bladder - cystitis C. Kidneys - nephritis 3. Plasma proteins - symptoms of kidney edema. As a result, fluid collects in body tissues, resulting in swelling and bloating. Most apparent in the abdomen. 4. Urea in the blood - symptom of uremia. Evidence of kidney failure. If death results, it comes from heart failure due to an imbalance of ions in the blood, particularly potassium ions. (Not because of nitrogen wastes accumulating in the blood.) Most people can live a normal life with one kidney. Kidney stones-result from uric acid precipitation in the kidney. 11

12 The Movement and Storage of Urine After forming in nephrons and collecting in collecting ducts, urine collects in the minor and major calyces of the kidney. From there is passes through the renal pelvis, into a ureter, and into the urinary bladder. The urethra delivers urine to the world as we know it. The Ureters The ureters are tubes about 25 cm. long. The ureters join the urinary bladder from underneath. The ureters are lined with smooth muscle. Muscular peristaltic waves, originating in the renal pelvis, move the urine through the length of the ureter. The greater the quantity of urine produced, the higher the frequency of contraction. (1 per 5 seconds to 1 per 5 minutes.) When a peristaltic wave reaches the bladder, it causes a jet of urine to spritz into the bladder. The entry site is covered by a flap of mucous membrane so urine doesn't "back up". The interior of the ureter is shaped like a "star". The Urinary Bladder The urinary bladder is a hollow, distensible, muscular organ. It is located within the pelvic cavity. In a guy, the bladder lies posteriorly against the rectum. In a girl, the bladder touches the anterior walls of the uterus and vagina. When the bladder is empty, its inner wall forms many folds. When the bladder fills, the folds disappear. As the bladder fills, the superior surface expands upward in a dome. It can extend upward to the small intestine. The musculature of the bladder is smooth. The muscle fibers are interlaced in all directions to cause a general tightening of the bladder walls during micturation (urination). 12

13 The circular muscle that prevents the bladder from emptying is a skeletal muscle named the external urethral sphincter, which is located about 3 cm. from the bladder along the length of the urethra. The bladder may hold as much as 600 ml of urine. The desire to urinate kicks in at about 150 ml. When a volume of 300 ml. is reached, a sense of urgency is felt. The battle is lost when the strength of contraction of the smooth muscle wall (which increases as the bladder fills) overcomes the restrictive contraction of the external urethral sphincter. The Urethra The urethra is a tube that conveys urine from the bladder, briefly through the earth's atmosphere, and onto some form of Earth surface. It is lined with mucous membrane and contains a thick layer of longitudinal smooth muscle. In a female, the urethra is only about 4 cm. long and is not used as a passageway for gametes. In a male, the urethra passes through the prostate gland (more during the next unit - The Male Reproductive System!) and its path is lengthier and more tortured (many hairpin turns!). The male urethra is also used as a conduit for the passage of reproductive cells. 13

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