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Chapter 26

Chapter 26. The Urinary System. The Urinary System. Renal Anatomy. The kidneys are located between the peritoneum and the posterior wall of the abdomen (in the retroperitoneal space). They are partially protected by the eleventh and twelfth pairs of ribs.

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Chapter 26

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  1. Chapter 26 The Urinary System

  2. The Urinary System

  3. Renal Anatomy • The kidneysare located between the peritoneum and the posterior wall of the abdomen (in the retroperitoneal space). • They are partially protected by the eleventh and twelfth pairs of ribs. • Because of the position of the liver, the right kidney is slightly lower than the left.

  4. Transverse plane ANTERIOR Large intestine Abdominal aorta Stomach Pancreas Liver Renal artery and vein View RENAL HILUM Inferior vena cava Body of L2 Peritoneum LEFT KIDNEY RENAL FASCIA Layers ADIPOSE CAPSULE Spleen RENAL CAPSULE Rib RIGHT KIDNEY Quadratus lumborum muscle POSTERIOR (a) Inferior view of transverse section of abdomen (L2) Location & layers

  5. Renal Anatomy • A frontal section through the kidney reveals two distinct regions of internal anatomy, the cortex and medulla. • The main function of the cortex is filtration to form urine. • The main function of the medulla is to collect and excrete urine.

  6. PATH OF URINE DRAINAGE: Nephron Collecting duct Renal hilum Minor calyx Renal cortex Major calyx Renal artery Renal medulla Renal pelvis Renal vein Renal column Renal pyramid in renal medulla Renal papilla Ureter Renal capsule Urinary bladder Renal lobe (a) Anterior view of dissection of right kidney

  7. Renal Anatomy • The renal pyramids within the medulla contain the kidney’s tubules. The renal papilla is the location where the pyramids empty urine into cuplike structures called minor calyces. 2-3 minor calyces empty into a major calyx • From the major calyces, urine drains into • the renal pelvis and then out through the ureter

  8. Renal Blood Flow • The renal artery and renal vein pass into the substance of the kidney at the hilum. • The renal arteries are very large branches of the aorta, and up to a third of total cardiac output can pass through them to be filtered by the kidneys.

  9. Renal Blood Flow

  10. The Nephron • Nephrons are the structural and functional units that form urine 1 million in each kidney- consist of: • Renal corpuscleassociated with a • Renal tubule

  11. The Nephron: Renal Corpuscle • The Renal Corpuscle consists of two structures: • The glomerular capillaries • The Bowman’s capsule– a double-walled epithelial cup that surrounds the glomerular capillaries.

  12. Each nephron receives one afferent arteriole, which divides into a tangled, ball-shaped capillary network called the glomerulus. • The glomerular capillaries then reunite to form an efferent arteriole that carries blood out of the glomerulus.

  13. Bowman’s capsule consists of visceral and parietal layers. • The visceral layer is made of epithelial cells called podocytes. • The parietal layer of the glomerular capsule is a simple squamous epithelium and forms the outer wall of the capsule. • Fluid filtered from the glomerular capillaries enters Bowman’s space, (the space between the two layers of the glomerular capsule)

  14. Renal corpuscle (external view) Parietal layer of glomerular (Bowman’s) capsule Afferent arteriole Mesangial cell Juxtaglomerular cell Capsular space Macula densa Ascending limb of loop of Henle Proximal convoluted tubule Mesangial cell Efferent arteriole Podocyte of visceral layer of glomerular (Bowman’s) capsule Endothelium of glomerulus Pedicel (a) Renal corpuscle (internal view)

  15. Glomerular capsule: Glomerulus Parietal layer Podocytes of visceral layer of glomerular capsule Visceral layer Afferent arteriole Juxtaglomerular cell Capsular space Ascending limb of loop of Henle Macula densa cell Simple squamous epithelial cells Efferent arteriole Proximal convoluted tubule 1380x LM (b) Renal corpuscle

  16. The Nephron: Tubule • Filtered fluid passes into the renal tubule, which has three main sections: • 1. the proximal convoluted tubule (PCT) • 2.the loop of Henle; consists of: • the descending limb • the ascending limb • 3.the distal convoluted tubule (DCT)

  17. Microvilli Mitochondrion Lining epithelium Apical surface Proximal convoluted tubule (PCT) Loop of Henle: descending limb and thin ascending limb Loop of Henle: thick ascending limb Most of distal convoluted tubule (DCT) Last part of DCT and all of collecting duct (CD) Intercalated cell Principal cell

  18. The Nephron • The distal convoluted tubules of several nephrons empty into a single collecting duct. • Collecting ducts unite and converge into large papillary ducts which drain into the minor calyces.

  19. The Nephron • Nephrons can be sorted into two populations: cortical nephrons ( 85%) with short loops of Henle their blood supply is from peritubular capillaries that arise from efferent arterioles. juxtamedullary nephrons ( 15%): • nephrons with long loops of Henle enable the kidneys to create a concentration gradient in the renal medulla and to excrete concentrated urine. Their loops of Henle receive their blood supply from the vasa recta

  20. The Cortical Nephron

  21. Juxtamedullary Nephron

  22. Juxtaglomerular Apparatus (JGA) • JGA is located where the distal tubule lies near the afferent arteriole • JGA consists of: • 1.Juxtaglomerular (JG) cells)- in the wall of afferent arterioles: • Are enlarged, smooth muscle cells • Have secretory granules containing renin • Sensitive to blood pressure changes • 2.Macula densa • Tall, distal tubule cells • Lie adjacent to JG cells • Sensitive to NaCl concentration in filtrate

  23. Juxtaglomerular Apparatus (JGA) • The JGA helps regulate blood pressure within the kidneys.

  24. Kidney functions • Removes toxins, nitrogenous metabolic wastes ( urea, uric acid, creatinine) • Regulates blood volume & blood pressure (produces Renin) • Maintains electrolyte balance • Maintains acid base balance • Role in erythropoiesis (produces EPO)

  25. Mechanisms of Urine Formation • Urine formation involves three major processes: • 1.Glomerular filtration • 2.Tubular reabsorption • 3.Secretion

  26. Glomerular Filtration: Filtration Membrane • Composed of three layers • Fenestrated endothelium of the glomerular capillaries- blocks formed elements • Basement membrane between the two layers- blocks large proteins • Podocytes slit membranes • (visceral layer of the glomerular capsule)-blocks intermediate sized proteins .

  27. Glomerular Filtration • Glomerular filtration is the formation of a protein-free filtrate (ultrafiltrate) of plasma across the glomerular membrane. • Only a portion of the blood plasma delivered to the kidney via the renal artery is filtered. • Plasma which escapes filtration, along with its protein and cellular elements, exits the renal corpuscle via the efferent arterioles

  28. Glomerular Filtration A passive process, by which fluids and solutes are pushed through the filtration membrane • The kidneys produce 180L of filtrate/day • The glomerulus is more efficient filter than other capillary beds because: • Its filtration membrane has higher surface area & is more permeable • Glomerular blood pressure is higher- higher net filtration pressure • Blood cells & plasma proteins not filtered (occasional small proteins may leak out)

  29. Glomerular Filtration • Filtration is controlled by Starling forces. • Blood hydrostatic pressure (55mmHg) is the main force that “pushes” water and solutes through the filtration membrane -(this is the blood pressure in glomerular capillaries) • Capsular hydrostatic pressure (15 mmHg) is exerted against the filtration membrane by fluid in the capsular space (opposes filtration). • Colloidal osmotic pressure (30 mmHg) is the pressure of plasma proteins “pulling” on water (opposes filtration).

  30. Net Filtration Pressure • Net Filtration pressure = Blood Hydrostatic Pressure – Blood Osmotic Pressure – Capsular Hydrostatic Pressure • Net Filtration pressure = 55-30-15 = 10 mmHg

  31. Glomerular Filtration Rate (GFR) • The total volume of filtrate formed per minute by the kidneys- 120-125ml/min • GFR mainly determined by: • Net filtration pressure- any change in the 3 pressures influences NFP & therefore GFR • Changes in GFR normally result from changes in blood hydrostatic pressure (glomerular blood pressure)

  32. Regulation of GFR • Regulation of the GFR is critical to maintaining homeostasis and is regulated by local and systemic mechanisms: • Renal autoregulation occurs when the kidneys themselves regulate GFR. • Neural regulation occurs when the sympathetic nerve supply regulates renal blood flow and GFR. • Hormonal regulation involves angiotensin II and atrial natriuretic peptide (ANP).

  33. Renal Autoregulation of GFR • Under normal conditions, renal autoregulation maintains a constant GFR despite fluctuations in BP • Two local mechanisms in the kidney that control GFR (Renal autoregulation) • Myogenic mechanism • Tubuloglomerular feedback • Myogenic – VC of afferent arteriole in response to increase in BP, & VD in response to fall in BP, therefore maintaining GFR

  34. Renal Autoregulation of GFR • Tubuloglomerular feedback – macula densa cells of JGA respond to NaCl concentration in the filtrate • As GFR increases- less time for Na reabsorption- high NaCl concentration in filtrate • macula densa cells release VCs--- VC of afferent arteriole ----leading to fall in GFR

  35. Tubuloglomerular feedback

  36. Regulation of GFR • Neural regulation of GFR is possible because the renal blood vessels are supplied by sympathetic fibers that release norepinephrine causing vasoconstriction. • Decreases GFR • Sympathetic input to the kidneys is most important with extreme drops of B.P. (as occurs with hemorrhage).

  37. Regulation of GFR • Two hormones contribute to regulation of GFR • Angiotensin II is a potent vasoconstrictor of both afferent and efferent arterioles (reduces GFR). • Atrial natriuretic peptide (ANP). • A sudden large increase in BP stretches the cardiac atria and releases ANP causes the afferent arterioles to dilate & glomerulus to relax, increasing the surface area for filtration. (increases GFR).

  38. Tubular Reabsorption • Tubular reabsorption is the process of returning important substances (“good stuff”) from the filtrate back into the renal blood vessels... and ultimately back into the body.

  39. Tubular Reabsorption • The “good stuff” is glucose, electrolytes, vitamins, water, amino acids, and any small proteins that might have escaped from the blood into the filtrate. • Ninety nine percent of the glomerular filtrate is reabsorbed (most of it before the end of the PCT)!

  40. Tubular Reabsorption

  41. Tubular Reabsorption • Reabsorption into the interstitium has two routes: • Paracellular reabsorption is a passive process that occurs between adjacent tubule Cells Transcellular reabsorption is movement through an individual cell.( must pass through apical & basal membranes)

  42. Tubular Reabsorption; transport mechanisms • Reabsorption of fluids, ions, and other substances occurs by activeand passive means including: • Diffusion • Osmosis (water)- from high water to low water ( from low osmolarity to high osmolarity) • Primary active transport- energy derived from ATP is used to “pump” a substance across a membrane • Secondary active transport-energy stored in an ion’s electrochemical gradient drives another substance across the membrane.

  43. Transport mechanisms • Reabsorption of water can be obligatory or facultative • Obligatory reabsorption of water occurs when it is obliged to follow solutes as they are reabsorbed Occurs in PCT( always permeable to water) • Facultative reabsorption (upto 10% of total water reabsorption) is variable water • reabsorption, adapted to specific needs. • It is regulated by of ADH on the principal cells of DCT & collecting ducts (without ADH they are not permeable to water)

  44. Reabsorption in the Proximal Convoluted Tubule • The majority of solute and water reabsorption from filtered fluid occurs in the PCT which reabsorbs, 65% Na+, water, K+, 80-90% of bicarbonate ions • Most absorptive processes involve Na+reabsorption • PCT Na+ reabsorption promotes reabsorption of; water, K+; Cl- and other ions • Normally, 100% of filtered glucose, amino acids,and other nutrients are reabsorbed in the PCT by Na+ symporters. ( co-transported with sodium)

  45. Reabsorption of glucose due to active pumping of Na+ • Na+ symporters help reabsorb materials from the tubular filtrate • Glucose is completely reabsorbed in the first half of the PCT • Sodium levels are kept low inside the cells due to Na+/K+ pump---- sodium moves in from tubular fluid down the gradient, co-transporting glucose Reabsorption of Nutrients

  46. Transport Maximum • A transport maximum (Tm) is maximum reabsorption rate, i.e. rate when all the carriers are saturated- in mg/min • When the carriers are saturated, excess of that substance is excreted in urine • In DM, due to hyperglycemia-when all carriers are saturated glucose starts appearing in urine--glucosuria

  47. Reabsorption of water, other ions, due to active pumping of Na+ • Water follows sodium by osmosis throughaquaporinsin PCT (obligatory water reabsorption) • Passive diffusion of anions Cl-, K+, Ca++, to maintain electroneutrality • Urea, fat-soluble substances- move down their gradient as filtrate becomes more concentrated due to water movement PCT

  48. PCT-Reabsorption of Bicarbonate, Na+ & H+ secretion • Na+/H+ antiporters reabsorb Na+ and secrete H+ • PCT cells produce the H+ & release bicarbonate ion to the peritubular capillaries • important buffering system • For every H+ secreted into the tubular fluid, one filtered bicarbonate eventually returns to the blood

  49. Reabsorption in the Loop of Henle • Water reabsorption: about 15% of the filtered water is reabsorbed in the descending limb, little or no water is reabsorbed in the ascending limb • Solute reabsorption occurs in ascending limb • Na+-K+-Cl- symporters reclaim Na+, Cl-, and K+ ions from the tubular lumen fluid in the thick part of ascending limb

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