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Urinary System
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  1. Urinary System Dr. Michael P. Gillespie

  2. Major Components • 2 kidneys. • 2 ureters. • 1 urinary bladder. • 1 urethra.

  3. Functions Of Kidneys • Excrete wastes in urine. • Regulate blood volume. • Regulate blood composition. • Regulate blood pressure. • Synthesize glucose. • Release erythropoietin. • Participate in vitamin D synthesis.

  4. Functions Of Urinary System Continued • Ureters transport urine from the kidneys to the urinary bladder. • Urinary bladder stores urine. • Urethra discharges urine from the body.

  5. Kidneys • The paired kidneys are reddish, kidney-bean-shaped organs. • They are located above the waist between the peritoneum and the posterior wall of the abdomen. • They are retroperitoneal. • The right kidney is slightly lower than the left due to the presence of the liver.

  6. External Anatomy Of The Kidneys • A typical kidney is 10-12 cm long, 5-7 cm wide and 3 cm thick. • The concave medial border of the kidneys faces the vertebral column. • The renal hilus is a deep vertical fissure through which the ureter, blood vessels, lymphatic vessels, and nerves pass.

  7. External Anatomy Of The Kidneys • Layers of tissue around the kidneys. • Renal capsule – deep layer. Smooth transparent sheet of dense irregular connective tissue. Maintains the shape of the kidney. • Adipose capsule – mass of fatty tissue. Protects the kidney from trauma and holds it in place. • Renal fascia – superficial layer. Anchors the kidney to the surrounding structures and to the abdominal wall.

  8. Nephroptosis • Nephroptosis or floating kidney is an inferior displacement or dropping of the kidney. • It occurs when the kidney slips from its normal position.

  9. Nephroptosis • This happens when it is not securely held in place by adjacent organs or its covering of fat. • It occurs most often in very thin people who have a deficient adipose capsule or renal fascia. • The ureter may kink and block urine flow.

  10. Internal Anatomy Of The Kidneys • Renal cortex – superficial layer. • Renal medulla – inner region. • 8-18 cone-shaped renal pyramids which taper to a renal papilla. • The renal columns are portions of the cortex that extend between the pyramids. • A renal lobe consists of a renal pyramid, overlying renal cortex, and one-half of each adjacent renal column.

  11. Internal Anatomy Of The Kidneys • Together, the renal cortex and renal pyramids constitute the parenchyma (functional portion) of the kidney. • The nephrons (functional units) of the kidney are within the parenchyma. • Urine formed by the nephrons drains into the papillary ducts, which drain into the minor and eventually major calyces.

  12. Internal Anatomy Of The Kidneys • Each kidney has 8 to 18 minor calyces and 2 to 3 major calyces. • The major calyces drain urine into a large cavity called the renal pelvis. • The hilus expands into the renal sinus (a cavity within the kidney).

  13. Blood & Nerve Supply Of The Kidneys • The kidneys have abundant blood vessels. • The kidneys remove wastes from the blood and regulate its volume and ionic composition. • Right and left renal arteries supple the kidneys. • They branch into segmental arteries, which branch into interlobar arteries.

  14. Blood & Nerve Supply Of The Kidneys • The interlobar arteries arch between the renal medulla and cortex and are referred to as arcuate arteries here. • Afferent arterioles branches come off the interlobar arteries and one supplies each nephron. • The afferent arteriole divides into a ball of capillaries called a glomerulus.

  15. Blood & Nerve Supply Of The Kidneys • The glomerular capillaries reunite to form the efferent arteriole. • The efferent arteriole divides to form the peritubular capillaries. • These reunite to form peritubular venules, then interlobar veins and eventually the renal vein.

  16. Blood & Nerve Supply Of The Kidneys • Most renal nerves originate in the celiac ganglion and pass through the renal plexus. • Renal nerves are part of the sympathetic division of the autonomic nervous system. • They regulate blood flow through the kidney causing vasodilation and vasoconstriction.

  17. Nephron • Nephrons are the functional units of the kidneys. • Two main parts: • Renal corpuscle – where blood plasma is filtered. • Renal tubule – into which the filtered fluid passes.

  18. Renal Corpuscle • Two components: • Glomerulus – a capillary network. • Glomerular (bowman’s) capsule – a double walled epithelial cup that surrounds the glomerular capillaries.

  19. Renal Tubule • 3 main sections: • Proximal convoluted tubule. • Loop of Henle (nephron loop). • Distal convoluted tubule.

  20. Nephron Continued • The distal convoluted tubules of several nephrons empty into a single collecting duct. • Collecting ducts then unite and converge into papillary ducts, which drain into minor calyces. • 1 kidney has approximately 1 million nephrons.

  21. Loop Of Henle • The loop of Henle connects the proximal and distal convoluted tubules. • It consists of a descending limb and an ascending limb.

  22. Types Of Nephrons • About 80-85% of the nephrons are cortical nephrons. • They have short loops of Henle and the renal corpuscles lie mainly in the renal cortex. • They extend only slightly into the medulla.

  23. Types Of Nephrons • The other 15-20% of the nephrons are juxtamedullary nephrons. • They have long loops of Henle and extend into the deepest regions of the medulla. • These allow the kidneys to excrete either very dilute or concentrated urine.

  24. Histology Of The Nephron & Collecting Duct • A single layer of epithelial cells forms the entire wall of the glomerular capsule, renal tubule, and ducts.

  25. Glomerular Capsule • The glomerular (bowman’s) capsule consists of visceral and parietal layers with a capsular (bowman’s) space in between. • Visceral layer – modified simple squamous epithelial cells called podocytes. • Parietal layer – simple squamous epithelium. • Fluid filtered from the glomerular capillaries enters the capsular space.

  26. Renal Tubule & Collecting Duct • Proximal convoluted tubule cells are simple cuboidal epithelial cells with a brush border of microvilli. • The descending limb and the first part of the ascending limb are composed of simple squamous epithelium. • The thick ascending limb is composed of simple cuboidal epithelium to low columnar epithelium.

  27. Number Of Nephrons • The number of nephrons is constant from birth. • Growth in kidney size is due to growth in size of the nephrons, not increase in number. • Signs of kidney dysfunction do not usually become apparent until function declines to less than 25% of normal.

  28. Number Of Nephrons • The remaining functional nephrons adapt to form a larger than normal load. • Surgical removal of one kidney stimulates hypertrophy of the other kidney. • One kidney can eventually filter blood at a rate of 80% of two normal kidneys.

  29. Functions Of Nephrons & Collecting Ducts • Glomerular filtration – water and most solutes move across the wall of glomerular capillaries into the glomerular capsule and into the renal tubule. • Tubular reabsorption – tubule cells reabsorb about 99% of the water and many useful solutes into the peritubular capillaries. • Tubular secretion – the tubule cells secrete wastes, drugs, and excess ions into the fluid as it moves through the tubule and collecting duct.

  30. Glomerular Filtration • Glomerular filtrate – the fluid that enters the capsular space. • Filtration fraction – the fraction of blood plasma in the afferent arterioles of the kidneys that becomes filtrate (typically 16-20%).

  31. Filtration Membrane • The endothelial cells of the glomerular capillaries and the podocytes, which encircle the capillaries, form a leaky barrier known as the filtration membrane.

  32. Filtration Membrane • Fenestrations (pores) in the glomerular epithelial cells cause them to be quite leaky.

  33. Filtration Membrane • Filtration slits are spaces between the pedicels (footlike processes from the podocytes), which allow passage of molecules smaller than 6-7 nm. • Water, glucose, vitamins, amino acids, very small plasma proteins, ammonia, urea, and ions can pass through. • Albumin is too large to easily pass through the slits. • Filtration utilizes pressure to drive fluids and solutes through a membrane.

  34. Factors That Affect Filtration • Glomerular capillaries present a large surface area for filtration because they are long and extensive. • The filtration membrane is thin and porous. Glomerular capillaries are about 50 times leakier than other capillaries due to fenestrations. • Glomerular capillary blood pressure is high due to a small diameter of the efferent arteriole resulting in backflow of blood.

  35. Net Filtration Pressure • 3 main pressures determine the level of glomerular filtration. • Glomerular blood hydrostatic pressure (GBHP) – promotes filtration. 55 mmHg. • Capsular hydrostatic pressure (CHP) – opposes filtration. Hydrostatic pressure exerted by fluid already in the capsular space (back pressure). 15 mmHg. • Blood colloid osmotic pressure (BCOP) – opposes filtration. Plasma proteins (albumin, fibrinogen, globulins) draw fluid into capillaries. 30 mmHg.

  36. Net Filtration Pressure • Net filtration pressure (NFP) = GBHP – CHP – BCOP. • NFP = 55mmhg – 15mmhg – 30mmhg = 10mmhg.

  37. Loss Of Plasma Proteins • Kidney disease can cause damage to glomerular capillaries allowing them to be permeable to plasma proteins. • As plasma proteins filter out, the osmotic pressure of the blood decreases, allowing water to be drawn from the blood plasma into interstitial tissues. This results in edema.

  38. Glomerular Filtration Rate • The amount of filtrate formed in all the renal corpuscles of both kidneys each minute is the glomerular filtration rate (GFR). • If the GFR is too high, substances may pass too quickly through the tubules that they are not reabsorbed. • If the GFR is too low, nearly all the filtrate may be reabsorbed resulting in inadequate excretion.

  39. Reabsorption • Primary and secondary active transport mechanisms are utilized to “pump” a substance across a membrane. • Obligatory water reabsorption occurs due to the solute reabsorption and corresponding osmotic pressure created. Water follows the solutes.