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Urinary System

Learn about the functions of the urinary system, including regulating water and electrolyte balance, removing waste from the blood, and urine formation through processes such as glomerular filtration and tubular reabsorption. Discover how the nephron, the functional unit of the urinary system, plays a crucial role in these processes. Explore the factors that control the glomerular filtration rate and understand the mechanisms involved in urine formation.

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Urinary System

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  1. Urinary System

  2. Urinary System Functions • 1) Regulate water & electrolyte balance • 2) Regulate blood pH • 3) Remove waste from blood

  3. p 897

  4. p 900

  5. p 900

  6. p 899

  7. Nephron = functional unit of the urinary system Filtrate = in capsule Tubular Fluid = in PCT to DCT Urine = in collecting duct & beyond p 903

  8. Cortical Nephrons = about 85% • Most reabsorption & secretion • Juxtamedulary Nephrons = 15% • Produce concentrated urine p 901

  9. Urine Formation • 1) Glomerular Filtration • 2) Tubular Reabsorption • & Secretion • 3) Water Reabsorption • (Conservation) p 904

  10. Glomerular Filtration p 902

  11. Filtration (Endothelial-Capsular) Membrane • Fenestrated Endothelium • Basement Membrane • Filtration Slits • Prevents passage of • cells & most proteins • Plasma = 7% protein • Filtrate = 0.03% protein • Ca++, Fe++, T4 hormone • Bound to proteins p 905

  12. p 905

  13. Blood hydrostatic pressure (BHP) is much higher than normal capillaries • About 60 mm Hg vs 15 mmHg • Because • Afferent arteriole (inlet) • is larger than • efferent arteriole (outlet) p 905

  14. Capsular hydrostatic pressure (CP) • is about 18 mm Hg vs about 0 mm Hg • for interstitial fluid • Why? p 902

  15. Net Filtration Pressure (NFP)

  16. Glomerular Filtration Rate (GFR) = • amount of filtrate formed by BOTH kidneys per minute • Normal is 12.5 ml for each 1 mm Hg NFP for men & 10.5 ml for women • For NFP = 10 mm Hg • GFR = 125 ml/min • or 180 L/day

  17. What would happen if glomerular hydrostatic pressure dropped • to 50 mm Hg ? • What would be the effect on GFR?

  18. GFR Control • 1) Autoregulation = self regulation, no neural or endocrine • Myogenic Mechanism = smooth muscle contracts when stretched • How will smooth muscle of afferent arteriole react if BP decreases? If it increases? • What effect will dilation of afferent arteriole have on GFR? Effect of constriction?

  19. Juxtaglomerular Apparatus • Juxtaglomerular cells = • Smooth muscle in afferent arteriole, that secretes renin • Macula densa = receptor cells in wall of DCT • Mesangial cells = specialized contractile cells that may also ?? p 908

  20. GFR Control • 1) Autoregulation: Tubuloglomerular Feedback = Juxtaglomerular apparatus monitors fluid entering DCT and adjusts • GFR to maintain • homeostasis p 908

  21. Juxtaglomerular Apparatus • If GFR is too high macula densa detects the change & sends chemical signal to Juxtaglomerular cells, which constrict the afferent arteriole, and thereby reduce GFR p 908

  22. GFR Control • 2) Sympathetic Nervous System • In response to strenuous exercise the sympathetic N.S. redirects blood to heart, brain and skeletal muscle, and away from organs like kidneys by constricting the afferent arterioles • What effect will this have on GFR?

  23. GFR Control • 3) Renin-Angiotensin Mechanism • When BP drops, sympathetic N.S. triggers release of renin by juxtaglomerular cells • Renin converts a plasma protein, angiotensinogen into angiotensin I • Angiotensin-converting enzyme (ACE) from lungs & kidneys converts angiotensin I into angiotensin II, which is a hormone with many powerful effects

  24. Angiotensin II Effects • Systemic Vasoconstriction • Constricts afferent & efferent arterioles • Stimulates NaCl & water reabsorption by kidneys • Stimulates aldosterone release by adrenal cortex • Stimulates antidiuretic hormone (ADH) release • Stimulates thirst center in hypothalamus p 909

  25. Urine Formation • 1) Glomerular Filtration • 2) Tubular Reabsorption • & Secretion • 3) Water Reabsorption • (Conservation) p 904

  26. Proximal Convoluted Tubule (PCT) • Longest & most coiled of portion of tubule • Simple cuboidal epithelium with microvilli • Many mitochondria provide ATP for high amount of active transport • Tubular Reabsorption = movement of water and solutes from tubular fluid to blood • Tubular Secretion = movement of substances from blood into tubular fluid

  27. PCT Sodium (Na+) reabsorption = 60 -70% of total through secondary active transport p 910

  28. PCT Chloride (Cl-) reabsorption = 60 -70% of total mostly by following sodium (electrical attraction) p 910

  29. PCT Bicarbonate (HCO3-) reabsorption? = most of total returned to blood by “slight of hand trick” & carbonic anhydrase p 910

  30. PCT Nutrient (glucose, amino acids) reabsorption = Normally 100% returned via cotransport with Na+ p 910

  31. PCT Nitrogenous waste reabsorption = About half of urea diffuses out of tubular fluid, uricacid diffuses out, NOT creatinine (too large) p 910

  32. PCT Water reabsorption = 60 -70% of total through osmosis, basically by following solutes, called obligatory water reabsorption, (no control/choice) p 910

  33. Uptake by Peritubular Capillaries • Water = Osmosis • Build up of interstitial H2O from PCT means greater hydrostatic pressure • Narrow efferent arteriole means low BHP in peritubular capillaries, about 8 mm Hg • Trapped proteins (not filtered with water) means higher colloidal osmotic pressure (COP) • Solutes = Solvent Drag • Water “drags” solutes into capillary

  34. Transport Maximum (Tm) = maximum rate of reabsorption that is reached when all carrier proteins are saturated • Transport maximum determines the renal threshold, or the plasma concentration at which a specific compound or ion will begin appearing in the urine • For example, renal threshold • for glucose is about 220 mg/dL • Untreated diabetes mellitus • may be 400 mg/dL p 106

  35. Nephron Loop • Thin, descending limb = water permeable • Reabsorbs about 15% of water, now about 80% of total • Thick, ascending limb = water impermeable • Reabsorbs about 25% of Na+ and Cl-, now about90% of total • So tubular fluid becomes more dilute • Leaves excess Na+ and Cl- in peritubular fluid (interstitial fluid around tubules)

  36. Osmolarity • Osmolarity = amount of dissolved particles in one liter of solution • Normal range of physiological osmolarity is measured in milliosmoles per liter (mOsm/L) • Blood plasma, interstitial fluid, and • intracellular fluid (cytosol) measure about300 mOsm/L

  37. Countercurrent Multiplier = establishes salinity gradient in renal medulla p 915

  38. Distal Convoluted Tubule (DCT) & Collecting Duct • Fluid arriving at DCT contains about 20% of water & 10% of salts (mostly NaCl) • Reabsorption of remaining water and salts is variable and under the control of hormones • This allows for the regulation of water and salt balances

  39. Aldosterone = “salt-retaining hormone” • Secreted by adrenal cortex, triggered by; • Low blood Na+ • Elevated K+ • Low BP  Renin  Angiotensin II • Principle cells with aldosterone receptors in • Ascending limb of nephron loop • Distal convoluted tubule • Collecting duct (cortical part only)

  40. Aldosterone binds nuclear receptors & activates transcription of gene for Na+/K+ pump • In 10 to 30 minutes effect of pumps at work in membrane is seen • Na+/K+ pump in tubular cell basal membrane • Moves Na+ out • = Na+ reabsorption • Moves K+ in • = K+ secretion

  41. p 939

  42. Atrial Natriuretic Peptide (ANP) • Secreted by atrial myocardium in response to high blood pressure • Dilates afferent & constricts efferent arterioles • Inhibits renin and aldosterone secretion • Inhibits ADH secretion and action of ADH on kidneys • Inhibits NaCl reabsorption by collecting ducts • Reduces blood volume • which reduces blood pressure

  43. Urine Formation • 1) Glomerular Filtration • 2) Tubular Reabsorption • & Secretion • 3) Water Reabsorption • (Conservation) p 904

  44. Antidiuretic Hormone (ADH) • Secreted by posterior pituitary, triggered by; • Dehydration & rising blood osmolarity • Causes more water to be reabsorbed in collecting duct, so less water lost in urine • Relies on high osmolarity in extracellular fluid of inner medulla • Total = 1200 mOsm/L • About 750 mOsm/L from NaCl • About 450 from urea (papillary region)

  45. ADH causes cells in collecting duct to synthesize aquaporins (water-channel proteins) & install them in plasma membrane • Resulting in more water being able to pass through the tubule wall • What determines the highest osmolarity possible for urine? p 915

  46. Renal Clearance • Renal Clearance = volume of blood plasma from which a particular waste is completely removed in one minute • Glomerular filtration of waste (125 ml/min) • PLUS amount added by tubular secreation • MINUS amount removed by tubular reabsorption • Renal Clearance • Inulin = 125 ml/min • Urea = 60 ml/min • Creatinine = 140 ml/min

  47. Conduction of Urine p 899

  48. Conduction of Urine p 921

  49. p 923

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