The Urinary System

1. The Urinary System CHAPTER 26

2. Introduction • The functions of the urinary system include 1.eliminating organic waste products • regulating blood volume and pressure by adjusting the volume of water lost and releasing erythropoietin and rennin • regulating plasma concentrations of ions by controlling the quantities lost in urine and calcium by calcitrol. • helping stabilize blood pH by controlling loss of hydrogen and bicarbonate ions. • conserving nutrients. Eliminates urea and uric acid • assisting the liver in detoxification, and during starvation deaminating amino acids.

3. Organization of the Urinary System • The urinary system includes the kidneys, the ureters, the urinary bladder and the urethra. • The kidneys produce urine-a fluid that contains water, ions and soluble compounds. • Urine leaving the kidneys travels along the paired ureters to the urinary bladder for temp.storage. • During urination (micturition) urine is forced out of the body. This occurs when the contraction of the muscular urinary bladder forces urine through the urethra and out of the body.

4. The Kidneys • The kidneys are located on either side of the vertebral column between vertebrae T12 and L3. The left kidney extends superioirly slightly more than the right kidney. • the superior surface of each kidney is capped by adrenal gland. • Each kidney is protected by three layers of conn. Tissue: the renal capsule (collagen fibers), adipose capsule (adipose tissue), renal fascia (anchors the kidney to surrounding structures). • Each kidney hangs suspended by collagen fibers from the renal fascia and packed in as oft cushion of adipose tissue. • Floating kidney.

5. Kidneys • Each kidney has the shape of a kidney bean. It is about 10 cm in length and weighs 150 g. The hilus provides entry for the renal artery and renal nerves and exit for the renal vein and the ureter. • The renal capsule has outer and inner layers. • The renal cortex is the outer layer in contact with capsule. The renal medulla consists of 6-18 conical or triang.structures called pyramids. The base faces cortex and tip-the renal papilla.

6. Kidneys • Adjacent pyramids separated by renal columns. • A renal lobe consists of a renal pyramid, the overlying area of renal cortex, and adjacent tissues of the renal columns. • Urine production occurs in the renal lobes. Ducts within each renal papilla discharge urine into a cup-shaped drain called minor calyx. Four or five merge to form-major calyx and two or three of these-renal pelvis. • Urine production occurs in microscopic structures called nephrons present in the cortex of each lobe. • There are roughly 1.25 million nephrons in each kidney with a combined length of 85 miles.

7. The Nephron • The nephron is the basic functional unit of the kidney. • It consists of a renal corpuscle and renal tubule. • The renal tubule begins at the renal corpuscle. It includes a knot of intertwined capillaries called the glomerulus surrounded by Bowman’s capsule. Blood arrives at the glomerulus via the afferent arteriole and departs in the efferent arteriole. Filtrate is produced at the renal corpuscle and then enters the tubule. • The renal tubule is divided into proximal convoluted tubule, loop of Henle and distal convoluted tubule. • The nephron empties tubular fluid into the collecting system through a connecting tubule, a tributary of a collecting duct.

8. The Nephron • Nephrons are responsible for the production of filtrate, the reabsorption of organic nutrients, the reabsorption of water and ions, and the secretion into the tubular fluid of waste products missed by filtration. • Roughly 85% of the nephrons are cortical nephrons found in the cortex. These perform most of the reabsorptive and secretory functions of the kidneys. • The juxtamedullary nephrons are found in the medulla, with their loops of Henle extending deep into the renal pyramids. These are responsible for the ability to produce a concentrated urine.

9. The Nephron • The proximal convoluted tubule (PCT)-made of cuboidal cells with microvilli- actively reabsorbs nutrients, plasma proteins, and ions from the filtrate. • The loop of Henle-made of low cuboidal or squamous cells- includes a descending limb, and an ascending limb. Each limb contains a thin segment and a thick segment. • The ascending limb-made of cuboidal cells with very little microvilli- delivers fluid to the distal convoluted tubule (DCT) which actively secretes ions, toxins, and drugs and reabsorbs sodium ions from the tubular fluid.

10. The collecting system • The DCT opens into the collecting system. • This consists of connecting tubules, collecting ducts, papillary ducts. • Individual connecting tubules connect each nephron to a nearby collecting duct. Each collecting duct receives tubular fluid from many connecting tubules. • Several collecting ducts converge to empty into a larger papillary duct, which in turn empties into a minor calyx. • The epithelium lining the connecting tubule is cuboidal and changes to columnar in the collecting and pappilary ducts. • Besides transport, this adjusts comp. And final osm.conc. And vol. Of urine.

11. The Blood and Nerve Supply to the Kidneys • 1200 mL of blood flows thriough the kidneys each minute. • Each kidney receives from a renal artery. • The vasculature of the kidneys includes the segmental, inlobular, arcuate, and interlobular arteries and the interlobular, arcuate, inlobular and segmental veins. • Blood travels from the efferent arteriole to the peritubular capillaries and the vasa recta. • The renal nerves that innervate the kidneys and ureters are dominated by sympathetic postganglionic fibers. Functions are:regulation of glomerular blood flow and pressure, stimulation of rennin release, direct stimulation of water and sodium reabsorption.

12. Renal Physiology • Three organic waste products are:urea (21g), uric acid(480mg) and creatinine (1.8 g). • Urine formation involves filtration, reabsorption and secretion. • Filtration: glomerular filtration occurs as fluids move across the wall of the glomerulus, in response to the hydrostatic pressure in the capillaries-GHP. • This movement is opposed by the capsular hydrostatic pressure (CHP), and by blood colloid osmotic pressure (BCOP). • The NFP at the glomerulus is the difference between the GHP and CHP+BCOP.

13. Filtration • The glomerular filtration rate is the amount of filtrate produced in the kidneys each minute. • A drop in filtration pressures stimulates the juxtaglomerular apparatus (JGA) to release rennin and erythopoietin. • Sympathetic activation produces a powerful vasoconstriction of the afferent arterioles, decreasing the GFR and slowing the production of filtrate. It alters the GFR by changing the regional pattern of circulation and stimulates the release of rennin by JGA.

14. Reabsorption and Secretion • Four types of carrier-mediated transport are involved in modifying the filtrate. These are active transport, facilitated diffusion, cotransport and countertransport. • Facilikatated diffusion is important in the reabsorption of glucose and amino acids when their concentrations in the tubular fluid are relatively high. • Active transport mechanisms include for calcium, magnesium, chloride, iodide and iron. • Cotransport mechanisms are responsible for the reabsorption of glucose and other simple sugars, amino acids, lactic acid, phosphate, chloride ions and hydrogen ions from the tubular fluid. • Countertransport of-bicarbonate and hydrogen.

15. Reabsorption and Secretion • The saturation limit of a carrier protein is its transport maximum. This determines the renal threshold which is the plasma concentration at which various compounds will appear in the urine. • Glomerular filtration produces a filtrate with a composition similar to blood plasma but with fem plasma proteins. • The cells of the PCT normally reabsorb sodium and other ions, water, and almost all the organic nutrients that enter the filtrate. It also secretes various substances into the tubular fluid. • These include potassium, hydrogen and ammonium ions.

16. Reabsorption and Secretion • Water and ions are reclaimed from the tubular fluid by the loop of Henle. • A concentration gradient in the medullae encourages the osmotic flow of water out of the tubular fluid. • The countercurrent multiplication between the ascending and descending limbs of the loop of Henle helps create the osmotic gradient in the medulla. As water is lost by osmosis and the volume of tubular fluid decreases, the urea concentration rises. • The DCT performs final adjustments by actively secreting or absorbing materials. Sodium ions are actively absorbed in exchange for K or H ions.

17. The collecting system • The amount of water and solute loss in the collecting system is regulated in two ways: by aldosterone which controls sodium ion pumps and by ADH which controls the permeability of the DCT and collecting system to water. • The collecting system also has other reabsorptive and secretory functions which are important to the control of body fluid pH. • Important examples of solute reabsorption in the collecting system include: sodium, bicarbonate, urea. • Secretion of hydrogen and bicarbonate to control pH.

18. Composition of normal urine • More than 99% of the filtrate produced each day is reabsorbed before reaching the renal pelvis. • General characteristics: • pH=6.0 • S.G.=1.003-1.030 • osmolarity=855-1335mOsm/L • water-93-97% • volume=1200 mL/day • color-clear yello • odor-varies • bacterial content-sterile

19. Urine transport, storage and elimination • Urine production ends when the tubular fluid enters the renal pelvis. • Ureters: extend from the renal pelvis to the urinary bladder. Peristaltic contractions by smooth muscles move the urine. • Urinary bladder: this is stabilized by the middle and lateral umbilical ligaments. Internal features include the trigone, neck and the internal urethral sphincter. The mucosal lining contains prominent rugae. Contraction of the detrusor muscle compress the bladder and expels the urine into the urethra. • The urethra: ext.urethral sphincter under vol.control.

20. The Micturition Reflex • The process of urination is controlled by the micturition reflex, which is initiated by the stretch receptors in the bladder wall. The urge to urinate generally appears when the bladder contains about 200 mL of urine. • Voluntary urination involves coupling this reflex with the voluntary relaxation of the ext.urethral sphincter which allows opening of the int.urethral sphincter. • At the end of a normal micturition, less than 10 mL of urine remains in the bladder.

21. Aging and the urinary system • Age related changes include: • declining numbers of functional nephrons:drops by 30-40% • reduced GFR:thais results form decreased numbers of glomeruli. Damage to filtration apparatus, reduction in renal blood flow. • reduced sensitivity to ADH:distal portion of nephron and collecting system less responsive to ADH. More sodium ions lost in urine. • problems with the micturition reflex:sphincter muscles lose muscle tone. • Integration with other systems.