Excretory system

ANATOMY

1. EXCRETORY SYSTEM

2. Anatomy Hilum (hilus) Renal artery and vein Cortex Medulla Pyramids and papillae Major, minor calyces Renal Pelvis Ureters

3. Nephron

4. G. membrane

5. Tubules

6. Introduction A. arteriole – carries blood to glomerulus. Glomerulus – filters protein free plasma into tubular component. E. arteriole – carries blood from glomerulus. Peritubular capillary – supply the renal tissue, involved in exchanges with fluid in T. lumen. JGA – produces substances involved in control of kidney function.

7. Cont.. Bowman's capsule – collect G. filtrate. Proximal Tubules – uncontrolled reabsorption and secretion of selected substances occur here. Loop of Henle – establishes an osmotic gradient in renal medulla. Important in kidney’s ability to produce urine of varying concentration.

8. Cont.. Distal tubule and Collecting duct – variable, controlled reabsorption of Na⁺ and H₂O and secretion of K⁺ and H⁺ occur here. Fluid leaving the collecting duct is urine, which enters the renal pelvis .

9. Function of Kidney 1) Role in Homeostasis. Excreting the end product of bodily metabolism. Maintaining water balance in the body. Maintaining the Electrolyte balance. Maintain the acid base balance by adjusting urinary output of H⁺ and HCO₃-. 2) Producing erythropoietin. 3) Regulation of blood pressure 4) Converting vitamin D into its active form.

10. Kidney Bean – shaped, Location – retroperitoneally on the posterior abdominal wall, one on each side of V. column at the level of T₁₂ to L₁ vertebra . Weight – 150 gm, 10 cm x 5cm x 2.5 cm. Rt. Kidney is slightly lower than left kidney Outer cortex and inner medulla. Medulla contains 10 to 15 pyramids, which terminate medially in renal papillae.

11. Cont.. Papillae projects into calyces, 10 to 15 minor calyces join to form 2 major calyces . Which come out through the pelvis of kidney to widened end of kidney . Renal hilus – blood vessels, lymphatic's and nerves enter into or exit from kidney via hilus. Renal pelvis – flattened, funnel – shaped expansion of superior end of ureter.

12. Microscopic structure of kidney Cortex and medulla of kidney are composed of nephron, blood vessels, lymphatic's and nerves. Nephron F unctional unit of kidney. Each kidney consists of 1.3 million nephron. Each nephron is capable of forming urine. Nephron consists of 2 major part – renal corpuscle and renal tubules . The kidney cannot regenerate new nephron. Therefore, with renal injury, disease, or normal aging, there is a gradual decrease in nephron number.

13. Renal corpuscle Glomerulus Each nephron contains a tuft of glomerular capillaries called the glomerulus, through which large amounts of fluid are filtered from the blood, Bowman's capsule Encloses the glomerulus. Formed of two layers - visceral layer and parietal layer.

14. Cont.. Space between visceral and parietal layer is called bowman's space or urinary space. Ultra structure of glomerular membrane It separates blood of glomerular capillaries from the fluid present in bowman's space . Also called filtration barrier. Layer of the membrane. Capillary endothelium - basement membrane - bowman's visceral epithelium,

15. Structure of glomerular membrane 1) It is thin membrane and made up of five layers Layer 1.- foot process of podocytes . The final part of the membrane is a layer of epithelial cells (podocytes) that encircle the outer surface of the capillaries. The foot processes are separated by gaps called slit pores 25nm through glomerular filtrate moves. The epithelial cells also have negative charges, provide additional restriction to filtration of plasma proteins.

16. Cont.. Layer 2.- Lamina rara externa- overlying foot processes of podocytes. Layer 3.- lamina densa- dense structural portion of basement membrane. Layer 4 - Lamina rara interna – provide bed for capillary endothelium. Layer 5.- Endothelial cell layer- it is fenestrated, contains pores with diameter 70 to 90nm. And freely permeable to water, small solutes and even to small proteins.

17. Cont.. 2. Glomerular capillaries. Each glomerulus contain 6 lobules and each of these consists of 3- 6 capillary loops . Anastomoses occur between the capillaries within 1 lobule . Arrangement of both arterioles within glomerulus allows the maintenance of much higher pressure 60 mmhg in G. capillary. High capillary pressure is well adapted for filtration.

18. Cont. 3. Major function of G. membrane is to produce an ultra filtrate. Filtrate contains all constituents of plasma except protein. Permits the free passage of neutral substances up to 4 nm in diameter and excludes those diameters greater than 8 nm. The total area of glomerular capillary endothelium across which filtration occurs in humans is about 0.8 m 2 .

19. Epithelium lining Renal Tubule Tubules are lining by cuboidal except thin segment these are flat or squamous type . Apical surface of cuboidal cells have microvilli , present in proximal tubule. Basolateral membrane of PCT cells, thick ascending segment cells and DCT cells contain mitochondria ( transport function ). Descending and ascending thin limb have poorly developed basolateral surface contain few mitochondria.

20. Cont.. Lateral surface of cells form 2 type of tight junction. Leaky tight junction Permit H₂O and solutes to diffuse across them. Present in proximal tubule. Tight junction Do not permit H₂O, solutes to diffuse across them easily. Present in Distal tubule.

21. Renal Tubule Filtered fluid is converted into urine on its way to the pelvis of the kidney.

22. Proximal Convoluted Tubule P.C.T. is about 15 mm long and 55 m in diameter. Single layer of cells with curved outline and brush border formed by microvilli. Tubule cells are rich in mitochondria, they are responsible for active transport of 80% of Na⁺ filtered, out of the tubular fluid into the peritubular capillary blood.

23. Cont.. Tubular cells are united at the apex by tight junction while between base of the cells have extensions into extracellular space called lateral intercellular space.

24. Loop of Henle The descending portion of the loop and the proximal portion of the ascending limb are made up of thin, permeable cells. Thick portion of the ascending limb is formed by low cuboidal epithelium, containing many mitochondria. In humans, only 15% of the nephron have long loops.

25. Cortical nephron Short loops of Henle juxtamedullary nephron long loops of Henle extending down into the medullary pyramids.

26. Cont.. The thick end of the ascending limb of the loop of Henle reaches the glomerulus of the nephron from which the tubule arose and nestles between its afferent and efferent arterioles. Specialized cells at the end form the macula densa, which is close to the efferent and particularly the afferent arteriole

27. Cont.. The macula, the neighboring lacis cells, and the renin-secreting juxtaglomerular cells in the afferent arteriole form the JGA.

28. Cont.. Descending thin limb – H₂O passive reabsorption through tight junction. Ascending limb – impermeable to H₂O. Early part of DCT – impermeable to H₂O, active reabsorption of NACL. Later part of DCT and CT – H⁺ secretion, Na⁺ reabsorption ( 8% ), and K⁺ secretion and H₂O reabsorption ( 10 to 15% ).

29. Distal convoluted tubule D.C.T. starts at the macula densa , is about 5 mm long. Low cuboidal epithelium is lower than that of the proximal tubule, and although a few microvilli are present, There is no distinct brush border . Macula densa maintain the NACL concentration in DCT.

30. Collecting Ducts DCT join to form collecting ducts , 20 mm long and lined by clear cuboidal epithelium. Pass through the renal cortex and medulla to empty into the pelvis of the kidney at the apexes of the medullary pyramids. The collecting ducts epithelium consist of 2 types of cells principal cells (P cells) and intercalated cells (I cells).

31. Cont.. Principal cell Increase the permeability of CT to water in the presence of ADH by increasing pore size through Aquaporine-2. Intercalated cells Secrete acids, help transport of bicarbonate and are responsible for acidic urine. The total length of the nephron, including the collecting ducts, ranges from 45 to 65 mm

32. Juxta Glomerular Apparatus Combination of specialized tubular and vascular cells located at where afferent and efferent arterioles enter and leave glomerulus JGA composed of 3 types of cells. Juxtaglomerular cells. JG cells Macula Densa cells Mesangial cells of Lacis cell.

33. Juxtaglomerular cells Specialized myoepithelial cells ( modified vascular smooth muscle cells ). Located in the media of afferent arteriole. Have well developed G. apparatus , and E. reticulum, abundant mitochondria and ribosomes. They synthesize, store and release a P. enzyme called Renin stored in granules of JG cells.

34. Cont.. It acts as baroreceptor and respond to change in Transmural Pressure Gradient between afferent arteriole and interstitium. Innervated by sympathetic nerve fiber and release Renin in response to sympathetic discharge. These vascular volume receptor monitor Renal Perfusion Pressure and stimulated by hypovolemia or ↓ed Renal perfusion pressure

35. Macula Densa Cells Specialized Renal Tubular Epithelial cells. Located at site where thick segment of ascending limb of LOH continued as DCT. Cells are in direct contact with Mesangial cells , in close contact with JG cells and adjoining with both A and E arterioles. Cells have prominent nuclei and function as Chemoreceptor.

36. Cont.. Stimulated by ↓ed Na⁺ conc. (NACL) load causing ↑ed renin release . They are not innervated. They are not well adapted for reabsorption They do not show signs of secretory activity. Cells maintain the NACL concentration in DCT.

37. Mesangial Cells Supporting cells of JGA and found between capillary loops. They are contact with both JG cells and Macula Densa cells They are contractile a nd play role in regulation of glomerular filtration. They show granulation to secrete renin in conditions of extreme hyperactivity. Also secrete verious substances and take up immune complexes.

38. JGA regulate Renin secretion into blood stream Factors inhibit renin secretion Inhibition of JG cells by Stretch due to ↑ in afferent arterial pressure. Angiotensin ii - arteriole constrictor. Vasopressin ( ADH ) - strong vasoconstrictor, affect H₂O reabsorption in R. tubule. After severe hemorrhage its concentration rises to high level and shows vasoconstrictor effect. ↑ Na⁺ and CL⁻ reabsorption across macula densa.

39. Cont.. Factors stimulate Renin secretion Stimulation of JG cells by Increase in sympathetic activity due to hypovolemia, hypotension. Increase in circulating catecholamine's Prostaglandins – vasodilator Sodium depletion, diuretics, congestive cardiac failure.

40. Renin Angiotensin System Operate during Regulation of blood pressure Regulation of extracellular fluid volume. Renin secretion and Angiotensin formation Renin secretion stimulated by decrease in BP Renin catalyzes the conversion of angiotensinogen ( plasma ) to Angiotensin I. Angiotensin I converted into Angiotensin II by Angiotensin converting enzyme.

41. Cont ACE present in endothelium of blood vessels entire the body especially in lung and kidney . Angiotensin persist in blood until inactivated by multiple blood and tissue enzymes called angiotensinase.

42. Action of Angiotensin II Vasoconstriction – arteriole constriction increase both systolic and diastolic BP. Decrease in salt and water excretion by kidney By direct action on the kidney Angiotensin II constrict E. arteriole cause diminishes blood flow through P. capillaries allowing rapid osmotic reabsorption from tubule.

43. Cont.. Angiotensin II directly stimulate epithelial cells of tubule to increase reabsorption of sodium and water. By stimulating secretion of aldosterone. Angiotensin II stimulate adrenal gland to secrete aldosterone cause increase salt and water reabsorption by epithelial cell. Stimulation of thirst. Angiotensin II is powerful stimulator for thrust lead to increase water intake and increase blood volume.

44. Renal Blood Supply Blood flow to the two kidneys is normally about 22 % of the cardiac output, or 1100 ml/min. The renal artery enters the kidney through the hilum and branches to form the interlobar arteries, arcuate arteries, interlobular arteries ( radial arteries) and afferent arterioles. Lead to glomerular capillaries, where large amounts of fluid and solutes are filtered to begin urine formation.

45. The distal ends of the capillaries of each glomerulus coalesce to form the efferent arteriole, leads to the peritubular capillaries, that surrounds the renal tubules. The renal circulation has two capillary beds, the glomerular and peritubular capillaries, arranged in series and separated by the efferent arterioles, Which help regulate the hydrostatic pressure in both sets of capillaries.

46. Cont.. High hydrostatic pressure in the glomerular capillaries (about60 mm Hg) causes rapid fluid filtration. Lower hydrostatic pressure in the peritubular capillaries (about 13 mm Hg) permits rapid fluid reabsorption. By adjusting the resistance of the Afferent and Efferent arterioles, the kidneys can regulate the hydrostatic pressure in both the glomerular and peritubular capillaries.

47. Cont.. Change in glomerular filtration, tubular reabsorption in response to body homeostatic demands. The peritubular capillaries empty into the vessels of the venous system, which run parallel to the arteriolar vessels. To form the interlobular vein, arcuate vein, interlobar vein, and renal vein and leaves the kidney beside the renal artery and ureter.

48. Cont.. For the cortical nephron, - the entire tubular system is surrounded by an extensive network of peritubular capillaries . For the juxtamedullary néphrons, - long efférent artérioles extend from the glomeruli down into the outer medulla . Then divide into specialized peritubular capillaries called vasa recta that extend downward into the medulla, lying side by side with the loops of Henle.

49. Cont.. The vasa recta return toward the cortex and empty into the cortical veins . This specialized network of capillaries in the medulla plays an essential role in the formation of a concentrated urine. Total surface of renal capillaries is approx = total surface area of tubules, both being about 12 m 2 . The volume of blood in the renal capillaries at any given time is 30 to 40 mL.

50. Factors affecting blood supply to kidney Catecholamine's ( NE ) - ↓ RBF – constriction Dopamine - ↑ RBF – vasodilatation. Angiotensin II - ↓ RBF Low concentration cause constrict E arterioles. High concentration cause constrict both arterioles. Prostaglandins - ↑ RBF – vasodilatation. Exercise - ↓ RBF – release of NE and Angiotensin II by increases sympathetic activity.

51. Cont.. Haemorrhage - ↓ RBF – NE and Angiotensin II Change of posture - ↓ RBF Increase in renal arteriole BP – ↑ RBF decrease in sympathetic tone lead to arteriole dilation. Acetylcholine - ↑ RBF by producing vasodilatation. High protein diet - ↑RBF- by increasing glomerular capillary pressure.

52. Characteristics of renal blood flow Under basal condition RBF is 1.2 to 1.3L/M ie . (300 – 400 ml/ 100gm /M ). RBF is very high compare to other body organs example – - Coronary blood flow – 60-80ml/100gm/m - Brain blood flow – 55ml/100gm/m - Skeletal muscle blood flow – 3-4ml/100gm/m High RBF produce high GFR for excretion of metabolic waste products.