THE URINARY SYSTEM

1. THE URINARY SYSTEM

2. THE URINARY SYSTEM • Every day the kidneys filter nearly 200 liters of fluid from the bloodstream, allowing toxins, metabolic wastes, and excess ions to leave the body in urine while returning needed substances to the blood • Although the lungs and skin also participate in excretion, the kidneys are the major excretory organs • As the kidneys perform these excretory functions, they simultaneously regulate the volume and chemical makeup of the blood, maintaining the proper balance between water and salts and between acids and bases • Other renal (kidney) functions: • Gluconeogenesis during prolonged fasting • Produces enzyme renin which helps regulate blood pressure and kidney function • Produces hormone erythropoietin which stimulates red blood cell production • Metabolizing vitamin D to its active form

3. Urinary System • Kidneys: urine forming • Urinary bladder: temporary storage reservoir for urine • Paired ureters: transport urine from kidneys to urinary bladder • Urethra: void urine to exterior

4. KIDNEY ANATOMY

5. Location and External Anatomy • The kidneys are bean-shaped organs that lie retroperitoneal (between the dorsal wall and the parietal peritoneum) in the superior lumbar region • From T12 to L3 • Receive some protection from the lower rib cage

6. POSTERIOR VIEW OF KIDNEYS

7. Kidney Anatomy • Right kidney is crowded by the liver and lies slightly lower than the left • Adult kidney is about 150 g (5 ounces) and its dimensions are 12 cm long, 6 cm wide, and 3 cm thick (size of a large bar of soap) • Lateral surface is convex • The medial surface is concave and has a cleft called the renal hilus that leads into a renal sinus • Ureters, renal blood vessels, nerves, and lymphatics all join the kidney at the hilus and occupy the sinus • Atop each kidney is an adrenal gland (endocrine gland) • Three layers of supportive tissue surround each kidney: • Renal capsule: a fibrous, transparent capsule that prevents infections in surrounding regions from spreading to the kidneys • Adipose capsule: a fatty mass that attaches the kidney to the posterior body wall and cushions the organ • Renal fascia: an outer layer of dense fibrous connective tissue that anchors the kidney and adrenal gland to surrounding structures

8. POSTERIOR VIEW OF KIDNEYS

9. HOMEOSTATIC IMBALANCE • Hydronephrosis: backup of urine from ureteral obstruction • Loss of fat resulting in the kidney dropping to a lower position (renal ptosis) resulting in kinked ureter • Can severely damage the kidney, leading to necrosis (tissue death) and renal failure

10. KIDNEY ANATOMY

11. Internal Anatomy • There are three distinct regions of the kidney: the cortex, the medulla, and the renal pelvis • Renal cortex: most superficial region • Granular appearance

12. Internal Anatomy • Renal medulla: • Cone-shaped tissue masses called medullary or renal pyramids • Base of pyramid faces the cortex/apex points internally • Each pyramid and its surrounding cortical tissue constitutes one of approximately eight lobes of a kidney

13. Internal Anatomy • Renal pelvis: flat, funnel-shaped tube • Continuous with the ureter leaving the hilus • Branching extensions of the pelvis form 2 or 3 major calyces (calyx), each of which subdivides to form several minor calyces, cup-shaped areas that enclose the papillae • Calyces collect urine, which drains continuously from the papillae, and empties into the renal pelvis-ureter-urinary bladder • Walls of calyces, pelvis, and ureter contain smooth muscle that contracts rhythmically to propel urine along its course by peristalsis

14. INTERNAL ANATOMY OF KIDNEY

15. HOMEOSTATIC IMBALANCE • Pyelitis: • Infection of the renal pelvis and calyces • Pyelonephritis: • Infections or inflammations that affect the entire kidney • Kidney infections in females are usually caused by fecal bacteria that spread from the anal region to the urinary tract • Infections (less often) can be the result of blood borne bacteria

16. Blood and Nerve Supply • Kidneys continuously cleanse the blood and adjust its composition • The renal arteries branch at right angles from the abdominal aorta: • Blood supply into and out of the kidneys progresses to the cortex through renal arteries to segmental, lobar, interlobar, arcuate, and interlobular arteries, and back to renal veins from interlobular, arcuate, and interlobular veins • Renal veins empty into the inferior vena cava • The renal plexus (autonomic nerve fibers and ganglia) regulates renal blood flow by adjusting the diameter of renal arterioles and influencing the urine-forming role of the nephrons • Offshoot of the celiac plexus

17. INTERNAL ANATOMY OF KIDNEY

18. Nephrons • Nephrons are the structural and functional units of the kidneys that carry out processes that form urine • Each nephron consist of a renal corpuscle composed of a tuft of capillaries (the glomerulus), surrounded by a glomerular capsule (Bowman’s capsule): • The glomerular endothelium is fenestrated (penetrated by many pores), which makes these capillaries exceptionally porous: • Allows large amounts of solute-rich, virtually protein-free fluid to pass from the blood into the glomerular capsule (Bowman’s capsule) • This plasma-derived fluid or filtrate is the raw material that the renal tubules process to form urine

19. Nephrons • Glomerular capsule (Bowman’s capsule) membrane has filtration pores (slit pores) which allow the filtrate to enter the capsular space inside the glomerular capsule (Bowman’s capsule) • The renal tubule begins at the glomerular capsule (Bowman’s capsule) as the proximal convoluted tubule, continues through a hairpin loop, the loop of Henle, and turns into a distal convoluted tubule before emptying into a collecting duct • The meandering nature of the renal tubule increases its length and enhances its filtrate processing capabilities

20. Nephrons • The collecting ducts collect filtrate from many nephrons, and extend through the renal pyramid to the renal papilla, where they empty into a minor calyx

21. Nephrons • Walls of the proximal convoluted tubule (PCT) are formed by cuboidal epithelial cells with large mitochondria • Large exposed surfaces bear dense microvilli (brush border) which increases their surface area and capacity for reabsorbing water and solutes from the filtrate and secreting substances into it • The descending limb, called the thin segment, is a simple squamous epithelium that is freely permeable to water

22. Nephrons • The epithelial cells of the distal convoluted tubule (DCT) are thinner and almost entirely lack microvilli • May play a larger role in secreting solutes into the filtrate than in reabsorbing substances from it • Maintains a major role in maintaining the acid-base balance of the blood • Maintains the body’s water and Na+ balance

23. Nephrons • There are two types of nephrons: • Cortical nephrons: 85% • Located almost entirely within the cortex • Small parts of their loops of Henle dips into the outer medulla • Juxtamedullary nephrons: 15% • Located near the cortex-medulla junction • Play an important role in the kidney’s ability to produce concentrated urine • Loops of Henle deeply invade the medulla

24. LOCATION AND STRUCTURE OF NEPHRONS

25. COMPARISON OF THE TUBULAR AND VASCULAR ANATOMY OF CORTICAL AND JUXTAMEDULLARY NEPHRONS

26. NEPHRON

27. Nephron Capillary Bed • Every nephron is closely associated with two capillary beds: • Glomerulus capillaries: (a) • Capillaries run in parallel • Specialized for filtration • Produces filtrate • Differs from all other capillary beds in the body in that it is both fed and drained by arterioles (afferent and efferent arterioles) • Afferent arteriole has a larger diameter than the efferent • High-resistance vessels • Blood pressure in the glomerulus is extraordinarily high for a capillary bed and easily forces fluid and solutes out of the blood into the glomerular capsule (Bowman’s capsule) • Most of this filtrate (99%) is reabsorbed by the renal tubule cells and returned to the blood in the peritubular capillary beds

28. COMPARISON OF THE TUBULAR AND VASCULAR ANATOMY OF CORTICAL AND JUXTAMEDULLARY NEPHRONS

29. Nephron Capillary Bed • Peritubular capillaries: (b) • Arise from efferent arterioles draining the glomerulus • Cling closely to adjacent renal tubules and empty into nearby venules • Low-pressure, porous capillaries that readily absorb solutes and water from the tubules cells as these substances are reclaimed from the filtrate • Reclaims most of the filtrate

30. Nephron Capillary Bed • Note: (b) • Efferent arterioles serving the juxtamedullary nephrons tend not to break up into peritubular capillaries • They form bundles of long straight vessels called vasa recta that extend deep into the medulla paralleling the longest loops of Henle • The thin-walled vasa recta play an important role in forming concentrated urine

31. COMPARISON OF THE SOURCE AND PATTERN OF THE VASCULATURE OF CORTICAL AND JUXTAMEDULLARY NEPHRONS

32. Juxtaglomerular Apparatus • Each nephron has a region called: JGA: • The juxtaglomerular apparatus is a structural arrangement between the afferent arteriole and the distal convoluted tubule that forms juxtaglomerular cells and macula densa cells • In the arteriole walls are the granular juxtaglomerular (JG)cells—enlarged smooth muscle cells with prominent secretory granules containing renin • Act as mechanoreceptors that sense the blood pressure in the afferent arteriole • Macula densa: a group of tall, closely packed distal convoluted tubule (DCT) cells that lie adjacent to the JG cells • Chemoreceptors (osmoreceptors) that respond to changes in the solute content of the filtrate • Both cells (JG + macula densa) play important roles in regulating the rate of filtrate formation and systemic blood pressure

33. JUXTAGLOMERULAR APPARATUS OF A NEPHRON

34. Filtration Membrane • The filtration membrane lies between the blood and the interior of the glomerular capsule (Bowman’s capsule) • It is a porous membrane that allows free passage of water and solutes smaller than plasma proteins

35. FILTRATION MEMBRANE

36. FILTRATION MEMBRANE

37. Filtration Membrane • Three layers: • 1.Fenestrated endothelium of the glomerular capillaries: • Capillary pores allow passage of all plasma components but not blood cells

38. FILTRATION MEMBRANE

39. FILTRATION MEMBRANE

40. Filtration Membrane • 2.Intervening basement membrane composed of the fused basal laminae of the other layers: • Restricts all but the smaller proteins while permitting most other solutes to pass • Confers electrical selectivity on the membrane: • Negative glycoproteins repel anions (+) and hinder their passage into the tubule

41. FILTRATION MEMBRANE

42. FILTRATION MEMBRANE

43. Filtration Membrane • 3.Visceral membrane of the glomerular capsule made of podocytes: • Engulf and degrade macromolecules that are caught in the membrane

44. FILTRATION MEMBRANE

45. FILTRATION MEMBRANE

46. KIDNEY PHYSIOLOGY: MECHANISMS OF URINE FORMATION • The kidneys filter out your entire plasma volume more than 60 times each day consuming 20-25% of all oxygen used by the body at rest • Filtrate contains everything found in the blood plasma except proteins, but by the time filtrate has percolated into the collecting ducts, it has lost most of its water, nutrients, and ions • What remains is called urine • Contains mostly metabolic wastes and unneeded substances • Urine formation and the adjustment of blood composition involve three major processes: • 1.Glomerular filtration by the glomeruli • 2.Tubular reabsorption in the renal tubules • 3.Secretion in the renal tubules

47. Glomerular Filtration • Is a passive, nonselective process in which hydrostatic pressure forces fluids and solutes through the glomerular membrane • Does not consume metabolic energy • More efficient filter than other capillary beds because: • 1.Its filtration membrane has a large surface area and is thousands of times more permeable to water and solutes • 2.glomerular blood pressure is much higher than that in other capillary beds, resulting in a much higher net filtration pressure • Molecules smaller than 3nm in diameter—such as water, glucose, amino acids, and nitrogenous wastes—pass freely from the blood into the renal tubule • Larger molecules pass with greater difficulty, and those larger than 7-9 nm are generally barred from entering the tubule • Presence of proteins or blood cells in the urine usually indicates a problem with the filtration membrane

48. Single nephron

49. Net Filtration Pressure (NFP) • Glomerular hydrostatic pressure (HPg) (glomerular blood pressure) is the chief force pushing water and solutes out of the blood and across the filtration membrane • HPg is opposed by two forces that drive fluids back into glomerular capillaries (filtration-opposing forces) • 1.Colloid osmotic (oncotic) pressure of glomerular blood (OPg) • 2.Capsular hydrostatic pressure (HPC) exerted by fluids in the glomerular capsule (Bowman’s capsule) • NFP = HPg – (OPg + HPC) • = 55mmHg - ( 30mmHg + 15mmHg) • = 10mmHg

50. Glomerular Filtration Rate (GFR) • The glomerular filtration rate is the volume of filtrate formed each minute by all the 2 million glomeruli of the kidneys combined • Because the GFR is directly proportional to the NFP, any change in any of the pressures acting at the filtration membrane changes both the NFP and the GFR: • An increase in arterial (and glomerular) blood pressure in the kidneys increases the GFR, whereas dehydration (which causes an increase in glomerular osmotic pressure) inhibits filtrate formation