Chapter 24 – The Urinary System

1. Chapter 24 – The Urinary System

2. Urinary system functions • Regulates blood volume and composition • Removes nitrogenous wastes, toxins, excess ion removal

3. Kidney anatomy • Located in posterior abdominal cavity • Right kidney is lower than the left due to crowding by the liver • Medial surface has hilum • Depression that serves as entrance/exit for blood and lymphatic vessels, nerves, ureter

4. Kidney supportive layers • Renal capsule – deepest • Transparent membrane directly covering kidneys • Perirenal fat capsule • Fat the supports/protects kidneys • Renal ptosis – “dropping” of kidneys due to extreme loss of body fat • Can cause obstruction of ureters • Hydronephrosis – urine backup • Renal fascia – most superficial • Dense fibrosis connective tissue that anchors kidneys to surrounding structures

5. Kidney internal anatomy • Renal cortex – superficial • Light, granular appearance • Renal medulla – deep • Darker red-brown • Arranged in triangular renal pyramids • Base oriented toward cortex; apex/papilla points interiorly • Separated by renal columns • Extensions of cortex

6. Kidney internal anatomy cont • Minor calyces • Cuplike projections surrounding each papillae • Join together to form 2-3 major calyces • Collect urine and dump into renal pelvis • Smooth muscle in calyces and renal pelvis move contents via peristasis

7. Nephrons • Structural and functional units of kidneys • Over 1 million per kidney • Form urine • Transmitted via collecting ducts to renal pelvis

8. Nephron structure • Renal corpuscle • Glomerulus – ball of capillaries • Bowman’s capsule – blind end of renal tubule; surrounds glomerulus • Remainder of renal tubule • Simple epithelium • Proximal convoluted tubule – close to glomerulus • Loop of Henle • Descending limb and loops to ascending limb • Distal convoluted tubule • Empties into a collecting duct • One duct receives from multiple nephrons and empties into renal pelvis

10. Renal corpuscle • Capillaries are fenestrated (has pores) • Allows water and other small solutes to enter tubule • Filtrate • Bowman’s capsule layers • Parietal – simple squamous epithelium • Visceral • Podocytes – branching epithelial cells • Foot processes cling to basement membrane • Filtration slits – openings between foot processes • Filtrate enters

11. Capillary beds • Glomerulus • Afferent arteriole feeds into bed; efferent arteriole drains it • Diameter of afferent arteriole is larger than efferent • Causes blood pressure in bed to be much higher than other beds

12. Juxtaglomerular apparatus • Located where distal portion of tubule lies against afferent arteriole • Granular/juxtaglomerular cells • Surround arteriole • Granules of renin • Mechanoreceptors that detect blood pressure changes • Macula densa of loop of Henle • Chemoreceptors that detect sodium chloride concentration

13. Capillary beds cont • Peritubular capillaries • Arise from efferent arterioles • Have close association with renal tubules • Recapture water and other molecules • Empty into venules

14. Nephron classification • Cortical • 85% • Located mainly in the cortex, with just a small portion of the loop in the medulla • Juxtamedullary • Start at cortex/medulla border and go deep into medulla

15. Ureters • Carry urine from kidney to bladder • Smooth muscle – peristalsis • Lined with transitional epithelium • When bladder fills, increase in pressure compresses and closes ureters • Renal calculi – kidney stones • Excess calcium, magnesium, uric acid crystallizes and precipitates out • Large stones can block urine drainage • Can get trapped in ureter – muscle contractions on it cause severe pain

16. Urinary bladder • Trigone • Triangular shaped area formed by entrance of 2 ureters and urethra • Internal walls have rugae that disappear when distended • Micturition – urination • When ~200ml in bladder, stretch receptors send impulse to brain • Anuria <50ml per day • Extremely low blood pressure or kidney failure

17. Urethra • Carries urine from bladder to external environment • Changes from transitional epithelium → pseudostratified → stratified squamous • Sphincters • Internal urethral sphincter – involuntary • Opposite of most – contraction OPENS; relaxation closes • External urethral sphincter – voluntary • As urethra passes through urogenital diaphragm • Size in males is larger since it needs to travel through the penis

19. Kidney physiology • Forms ~180L of filtrate daily • Over 99% of which gets reabsorbed • 3 major processes • Glomerular filtration • Tubular reabsorption • Tubular secretion

20. Glomerular filtration • Passive process • Filtration membrane exceedingly permeable to water and small solutes • High pressure in glomerulus forces filtrate out • Water, glucose, amino acids, ions, nitrogenous wastes • Similar concentration to plasma concentration • Proteins and other large molecules prohibited from passage

21. Regulation of glomerular filtration rate (GFR) • Intrinsic – renal autoregulation • Myogenic mechanism • Tubuloglomerular feedback mechanism • Extrinsic – neural and hormonal • Rennin-angiotensin mechanism • Regulates GFR to regulate systemic blood pressure • Overrides intrinsic control in high stress or emergency • Shunts blood to vital organs

22. Myogenic mechanism • When stretched, smooth muscle tends to contract • When blood pressure increases, smooth muscle of afferent arteriole contracts • Reduces pressure difference in the glomerulus • Otherwise, too much filtrate is formed • When blood pressure decreases, afferent arteriole dilates to keep pressure difference

23. Tubuloglomerular feedback mechanism • Controlled by macula densa cells in loop (NaCl levels) • When NaCl levels in filtrate are too high: • Afferent arteriole constricts – reduction of pressure difference • Filtrate travels more slowly through tubule, allowing for better reabsorption • When NaCl levels in filtrate are too low: • Afferent arteriole dilates – increases pressure difference • Filtrate travels quickly through tubule; NaCl doesn’t get reabsorbed (stays in filtrate)

24. Renin-angiotensin mechanism • Granular/juxtaglomerular cells release renin • Due to decline in blood pressure or decline in osmotic concentration of tubular fluid at macula dense • Angiotensinogen • Plasma protein produced by liver • In the presence of renin, converts to angiotensin I • Angiotensin I gets converted to angiotensin II by angiotensin converting enzyme (ACE) • Located in capillary endothelium, especially in lungs

25. Angiotensin II • Regulates blood pressure by: • Vascontriction of arterioles throughout body • Increases blood pressure • Stimulates reabsorption of NaCl • Causes water to be reabsorbed as well; increase in blood volume increases blood pressure • Stimulates hypothalamus • Produce ADH and stimulates thirst center • Increases blood volume • Decreases pressure in peritubular capillaries • Allows more fluid to enter • Causes contraction of glomerular mesangial cells • Located between glomerular capillaries • Decreases surface area for filtration

26. Tubular reabsorption • Most of filtrate needs to be reabsorbed and returned to bloodstream • Reabsorption of water and ions adjusted to regulate blood composition • Passive transport – no ATP required; active transport – ATP is required

27. Sodium reabsorption • Most abundant cation in filtrate • Active transport • Pumped into tubule cells; pumped out into interstitial fluid by sodium-potassium pump • From interstitial fluid, sodium enters peritubular capillaries

28. Reabsorption of water, ions, nutrients • Sodium ions in capillaries cause an electrical gradient that attracts anions (Cl- and HCO3-) • Sodium ions create an osmotic gradient that attracts water • Causes filtrate to become more concentated • Other solutes diffuse down concentration gradient • Secondary active transport • Glucose, amino acids, vitamins, cations • Sodium carrier protein can co-transport other solutes • Each solute has a different, specific carrier protein • Transport maximum • Finite number of carrier proteins for each solute • If saturated, excess will be excreted

30. Reabsorption cont • Fat-soluble substances • Do not require carrier proteins since they can travel directly through plasma membrane • Non-reabsorbed substances • No carriers for the specific molecule • Not fat-soluble • Too large to pass through tight junctions of tubular cells • Nitrogenous wastes from protein and nucleic acid metabolism • Urea, creatinine, uric acid

32. Tubular secretion • Secretes substances back into tubules: • Substances bound to proteins were too large to be filtered through glomerulus • Wastes that were reabsorbed due to passive transport • Excess K+ removal • Virtually all potassium is originally absorbed in the PCT • Blood pH homeostasis • If too low – H+ ions are secreted; bicarbonate ions are retained • If too high – bicarbonate ions remain in filtrate (don’t enter the bloodstream)

33. Formation of urine • Dilute • DCT and collecting ducts are impermeable to water • Water can not be reabsorbed • Concentrated • ADH causes aquaporins to be placed in cells of DCT and collecting ducts • Diuretics increase water output by: • Inhibition of ADH (alcohol) • Interferes with sodium absorption

35. Urine • Color • Pale to dark yellow due to pigment urochrome • Certain foods and drugs can change color • Pink – presence of red blood cells (infection or trauma) • Cloudy – presence of bacteria • pH • Usually acidic (~6) … can range from 4.5 – 8 • Acidity inhibits bacterial growth

36. Abnormal urinary constituents • Glycosuria – glucose • nonpathological – recent intake of excessive sugary foods • Pathological – diabetes mellitus • Proteinuria or albuminuria – protein/albumin • Nonpathological – pregnancy, high protein diet, excessive physical exertion • Pathological – severe hypertension, liver failure, renal disease, glomerulonephritis • Ketonuria – ketone bodies • Starvation, uncontrolled diabetes mellitus

37. Abnormal urinary constituents cont • Hemoglobinuria – hemoglobin • Transfusion reaction, hemolytic anemia, severe burns • Bilirubinuria - bile pigments • Liver disease, blockage of liver or gallbladder ducts • Hematuria – RBCs • Urinary tract bleeding – infection, trauma, stones, cancer • Pyruria - pus/WBCs • Infection