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Anatomy of the Urinary System Article The kidney and urinary systems keep chemicals, such as potassium and sodium, and water in balance and remove a type of waste, called urea, from the blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys. ● Other important functions of the kidneys include blood pressure regulation and the production of erythropoietin , which controls red blood cell production in the bone marrow. Kidney and urinary system parts and their functions: ● Two kidneys - a pair of purplish-brown organs located below the ribs toward the middle of the back. Their function is to remove liquid waste from the blood in the form of urine; keep a stable balance of salts and other substances in the blood; and produce erythropoietin, a hormone that aids the formation of red blood cells. The kidneys remove urea from the blood through tiny filtering units called nephrons . Each nephron consists of a ball formed of small blood capillaries, called a glomerulus , and a small tube called a renal tubule . Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney. ● Two ureters - narrow tubes that carry urine from the kidneys to the bladder. Muscles in the ureter walls continually tighten and relax forcing urine downward, away from the kidneys. If urine backs up, or is allowed to stand still, a kidney infection can develop. About every 10 to 15 seconds, small amounts of urine are emptied into the bladder from the ureters. ● Bladder - a triangle-shaped, hollow organ located in the lower abdomen. It is held in place by ligaments that are attached to other organs and the pelvic bones. The bladder's walls relax and expand to store urine, and contract and flatten to empty urine through the urethra . The typical healthy adult bladder can store up to two cups of urine for two to five hours. ● Two sphincter muscles - circular muscles that help keep urine from leaking by closing tightly like a rubber band around the opening of the bladder. ● Nerves in the bladder - alert a person when it is time to urinate, or empty the bladder. ● Urethra - the tube that allows urine to pass outside the body. The brain signals the bladder muscles to tighten, which squeezes urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs. Kidney: The kidneys are bean-shaped organs that lie in the abdomen, retroperitoneal to the organs of digestion, around or just below the ribcage and close to the lumbar spine. The organ is about the size of a human fist and is surrounded by what is called Peri-nephric fat, and situated on the superior pole of each kidney is an adrenal gland. The kidneys receive their blood supply of 1.25 L/min (25% of the cardiac output) from the renal arteries which are fed by the abdominal aorta. This is important because the kidneys' main role is to filter water soluble waste products from the blood. The other attachment of the kidneys are at their functional endpoints the ureters, which lies more medial and runs down to the trigone of urinary bladder. The kidneys perform a number of tasks, such as: concentrating urine, regulating electrolytes, and maintaining acid-base homeostasis. The kidney excretes and re-absorbs electrolytes (e.g. sodium, potassium and calcium) under the influence of local and systemic hormones. pH balance is regulated by the excretion of bound acids and ammonium ions. In addition, they remove urea, a nitrogenous waste product from the metabolism of amino acids. The end point is a hyperosmolar solution carrying waste for storage in the bladder prior to urination. Humans produce about 2.9 liters of urine over 24 hours, although this amount may vary according to circumstances. Because the rate of filtration at the kidney is proportional to the glomerular filtration rate, which is in turn related to the blood flow through the kidney, changes in body fluid status can affect kidney function. Hormones exogenous and endogenous to the kidney alter the amount of blood flowing through the glomerulus. Some medications interfere directly 1

or indirectly with urine production. Diuretics achieve this by altering the amount of absorbed or excreted electrolytes or osmalites, which causes a diuresis. The Nephron: The nephron consists of a cup-shaped capsule containing capillaries and the glomerulus, and a long renal tube. Blood flows into the kidney through the renal artery, which branches into capillaries associated with the glomerulus. Arterial pressure causes water and solutes from the blood to filter into the capsule. Fluid flows through the proximal tubule, which include the loop of Henle, and then into the distal tubule. The distal tubule empties into a collecting duct. Fluids and solutes are returned to the capillaries that surround the nephron tubule. The nephron has three functions: 1. Glomerular filtration of water and solutes from the blood. 2. Tubular reabsorption of water and conserved molecules back into the blood. 3. Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule. Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component 16 times each day. In a 24 hour period nephrons produce 180 liters of filtrate, of which 178.5 liters are reabsorbed. The remaining 1.5 liters forms urine. Urine Production 1. Filtration in the glomerulus and nephron capsule. 2. Reabsorption in the proximal tubule. 3. Tubular secretion in the Loop of Henle. Components of The Nephron ● Glomerulus: mechanically filters blood ● Bowman's Capsule: mechanically filters blood ● Proximal Convoluted Tubule: Reabsorbs 75% of the water, salts, glucose, and amino acids ● Loop of Henle: Countercurrent exchange, which maintains the concentration gradient ● Distal Convoluted Tubule: Tubular secretion of H ions, potassium, and certain drugs. The nephron carries out nearly all of the kidney's functions. Most of these functions concern the reabsorption and secretion of various solutes such as ions (e.g., sodium), carbohydrates (e.g., glucose), and amino acids (e.g., glutamate). Properties of the cells that line the nephron change dramatically along its length; consequently, each segment of the nephron has highly specialized functions. The proximal tubule as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion. [5] Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries, including approximately two-thirds of the filtered salt and water and all filtered organic solutes (primarily glucose and amino acids). The loop of Henle, also called the nephron loop, is a U-shaped tube that extends from the proximal tubule. It consists of a descending limb and ascending limb. It begins in the cortex, receiving filtrate from the proximal convoluted tubule, extends into the medulla as the descending limb, and then returns to the cortex as the ascending limb to empty into the distal convoluted tubule. The primary role of the loop of Henle is to concentrate the salt in the interstitium, the tissue surrounding the loop. Considerable differences distinguish the descending and ascending limbs of the loop of Henle. The descending limb is permeable to water and noticeably less impermeable to salt, and thus only indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into the hypertonic interstitium of the renal medulla, water flows freely out of 2

the descending limb by osmosis until the tonicity of the filtrate and interstitium equilibrate. Longer descending limbs allow more time for water to flow out of the filtrate, so longer limbs make the filtrate more hypertonic than shorter limbs. Unlike the descending limb, the ascending limb of Henle's loop is impermeable to water, a critical feature of the countercurrent exchange mechanism employed by the loop. The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange. In passing through the ascending limb, the filtrate grows hypotonic since it has lost much of its sodium content. This hypotonic filtrate is passed to the distal convoluted tubule in the renal cortex. The distal convoluted tubule has a different structure and function to that of the proximal convoluted tubule. Cells lining the tubule have numerous mitochondria to produce enough energy (ATP) for active transport to take place. Much of the ion transport taking place in the distal convoluted tubule is regulated by the endocrine system. In the presence of parathyroid hormone, the distal convoluted tubule reabsorbs more calcium and excretes more phosphate. When aldosterone is present, more sodium is reabsorbed and more potassium excreted. Atrial natriuretic peptide causes the distal convoluted tubule to excrete more sodium. In addition, the tubule also secretes hydrogen and ammonium to regulate pH. After traveling the length of the distal convoluted tubule, only about 1% of water remains, and the remaining salt content is negligible. Collecting duct system Each distal convoluted tubule delivers its filtrate to a system of collecting ducts, the first segment of which is the collecting tubule. The collecting duct system begins in the renal cortex and extends deep into the medulla. As the urine travels down the collecting duct system, it passes by the medullary interstitium which has a high sodium concentration as a result of the loop of Henle's countercurrent multiplier system. Though the collecting duct is normally impermeable to water, it becomes permeable in the presence of antidiuretic hormone (ADH). ADH affects the function of aquaporins, resulting in the reabsorption of water molecules as it passes through the collecting duct. Aquaporins are membrane proteins that selectively conduct water molecules while preventing the passage of ions and other solutes. As much as three-quarters of the water from urine can be reabsorbed as it leaves the collecting duct by osmosis. Thus the levels of ADH determine whether urine will be concentrated or diluted. An increase in ADH is an indication of dehydration, while water sufficiency results in low ADH allowing for diluted urine. Lower portions of the collecting organ are also permeable to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high concentration (which is very important for the nephron). Urine leaves the medullary collecting ducts through the renal papillae, emptying into the renal calyces [ , the renal pelvis, and finally into the urinary bladder via the ureter. Because it has a different origin during the development of the urinary and reproductive organs than the rest of the nephron, the collecting duct is sometimes not considered a part of the nephron. Instead of originating from the metanephrogenic blastema, the collecting duct originates from the ureteric bud. What causes problems in the urinary system? Problems in the urinary system can be caused by aging, illness, or injury. As you get older, changes in the kidneys’ structure cause them to lose some of their ability to remove wastes from the blood. Also, the muscles in your ureters, bladder, and urethra tend to lose some of their strength. You may have more urinary infections because the bladder muscles do not tighten enough to empty your bladder completely. A decrease in strength of muscles of the sphincters and the pelvis can also cause incontinence, the unwanted leakage of urine. Illness or injury can also prevent the kidneys from filtering the blood completely or block the passage of urine. 3

Urinary System Vocabulary 1 4 Vocabulary Terms Nephron glomerulus Bowman’s (glomerular) capsule renal corpuscle efferent arteriole Proximal convoluted tubule distal convoluted tubule loop of Henle collecting tubule Word Definition Memory Devise 1. Nephron 2. Glomerulus 3. Bowman's capsule 4. Renal Corpuscle 5. Efferent Arteriole 6. Proximal Convoluted Tubule 7. Distal convoluted Tubule 8. Loop of Henle 9. Collecting Tubule 10. _________________ 1. Produces urine in the process of removing waste and excess substances from blood 2. A cluster of nerve endings where waste products are filtered from the blood 3. A cup-like sac in the beginning of a nephron that functions in the first step in the filtration of blood to form urine. 4. Consists of a glomerulus, and a Bowman’s capsule; filters blood in the nephron. 5. Blood vessels that are part of the urinary tract of organisms 6. Reabsorbs filtrate in accordance with the needs of homeostasis. 7. Connects to the collecting duct system that fine-tunes salt and water reabsorption 8. A long u-shaped portion of the tubule that conducts urine within each nephron and aids in the recovery of water and sodium chloride from urine. 9. Any of the long narrow tubes that concentrate and transports urine from the nephrons to larger ducts that connect with the renal calyces. Nephron Bowman’s Capsule Efferent Arteriole Loop of henle

Anatomy of the Kidney Part I Article I. Kidneys ● Kidneys – main excretory organ; maintains homeostasis by removing harmful substances from the blood & regulating water balance ● Nephrons – filtering units of the kidneys; each kidney has millions of nephrons ● Urinary System - consists of: kidneys, ureters, urethra, and urinary bladder II. Kidney Structure ● 3 Parts: o Renal cortex – outer, rounded portion of the kidney o Renal medulla – located next to renal cortex; cone-shaped regions o Renal sinus - cavity next to renal medulla; ureter, nerves, blood and lymphatic vessels enter the kidney on the concave surface ● Renal Artery – delivers blood to the kidney for filtration; veins leave kidneys with “clean” blood III. Nephron Structure ● Nephrons – individual filtering units of the kidney; consists of: o Filtering body , renal corpuscle , and a urine-collecting and concentrating tube → renal tube ● Renal corpuscle → consists of two structures: the glomerulus and Bowman’s (glomerular) capsule o Glomerulus: dense ball of capillaries; blood enters under high pressure; substances small enough to pass through capillaries enter Bowman’s capsule o Bowman’s (Glomerular) Capsule : encircle glomerulus & collect material → filtrate forced out of glomerular capillaries ● Renal Tube : consists of three parts: o First section → proximal convoluted tubule (PCT) → large surface area supports functions of reabsorption & secretion o Middle → Loop of Henley 5 Collecting Tubule

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