1 / 38

Chapter 17.2

Chapter 17.2. Physiology of the Kidneys. 17-1. Chapter 17.2 Outline Renal Clearance Hormonal Effects Na + , K + , H + , and HCO 3 - Relationships Clinical Aspects. 17-2. Renal Clearance. 17-50. Renal Clearance.

avram-french
Download Presentation

Chapter 17.2

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 17.2 Physiology of the Kidneys 17-1

  2. Chapter 17.2 Outline • Renal Clearance • Hormonal Effects • Na+, K+, H+, and HCO3- Relationships • Clinical Aspects 17-2

  3. Renal Clearance 17-50

  4. Renal Clearance • Refers to ability of kidney to remove substances from blood and excrete them in urine • Occurs by filtration and by secretion • Secretion is opposite of reabsorption--substances from vasa recta are transported into tubule and excreted • Reabsorption decreases renal clearance; secretion increases clearance 17-51

  5. Renal Clearance • Excretion rate = (filtration rate + secretion rate) - reabsorption rate 17-52

  6. Secretion of Drugs • Many drugs, toxins, and metabolites are secreted by membrane transporters in the PCT • These transport organic anion and cation molecules • And determine the half-life of many therapeutic drugs • Many foreign molecules (xenobiotics) are eliminated by this system at a more rapid rate than by glomerular filtration 17-53

  7. Inulin Measurement of GFR • Inulin, a fructose polymer, is useful for measuring GFR because is neither reabsorbed or secreted • Rate at which a substance is filtered by the glomeruli can be calculated: • Quantity filtered = GFR x P • P = inulin concentration in plasma • Quantity excreted (mg/min) = V x U • V = rate of urine formation; U = inulin concentration in urine • Amount filtered = amount excreted GFR = V x U P 17-54

  8. Renal Clearance of Inulin 17-55

  9. Renal Plasma Clearance (RPC) • Is volume of plasma from which a substance is completely removed/min by excretion in urine • If substance is filtered but not reabsorbed then all filtered will be excreted RPC = GFR • If substance is filtered and reabsorbed then RPC < GFR • If substance is filtered but also secreted and excreted then RPC will be > GFR (=120 ml/ min) RPC = V x U P 17-56

  10. Clearance of Urea • Urea is freely filtered into glomerular capsule • Urea clearance calculations demonstrate how kidney handles a substance: RPC = V X U/P • V = 2ml/min; U = 7.5 mg/ml of urea; P = 0.2 mg/ml of urea • RPC = (2ml/min)(7.5mg/ml)/(0.2mg/ml) = 75ml/min • Urea clearance is 75 ml/min, compared to clearance of inulin (120 ml/min) • Thus 40-60% of filtered urea is always reabsorbed • Is passive process because of presence of carriers for facilitative diffusion of urea 17-57

  11. Measurement of Renal Blood Flow • Not all blood delivered to glomerulus is filtered into glomerular capsule • 20% is filtered; rest passes into efferent arteriole and back into circulation • Substances that aren't filtered can still be cleared by active transport (secretion) into tubules 17-58

  12. Total Renal Blood Flow Using PAH • PAH clearance is used to measure total renal blood flow • Normally averages 625 ml/min • It is totally cleared by a single pass through a nephron • So it must be both filtered and secreted • Filtration and secretion clear only molecules dissolved in plasma • To get total renal blood flow, amount of blood occupied by erythrocytes must be taken into account • 45% blood is RBCs; 55% is plasma •  total renal blood flow = PAH clearance • = 625/0.55 = 1.1L/min 0.55 17-59

  13. Total Renal Blood Flow Using PAH continued 17-60

  14. Glucose and Amino Acid Reabsorption • Filtered glucose and amino acids are normally 100% reabsorbed from filtrate • Occurs in PCT by carrier-mediated cotransport with Na+ • Transporter displays saturation if ligand concentration in filtrate is too high • Level needed to saturate carriers and achieve maximum transport rate is transport maximum (Tm) • Glucose and amino acid transporters don't saturate under normal conditions 17-61

  15. Glycosuria • Is presence of glucose in urine • Occurs when glucose > 180-200mg/100ml plasma (= renal plasma threshold) • Glucose is normally absent because plasma levels stay below this value • Hyperglycemia has to exceed renal plasma threshold • Diabetes mellitus occurs when hyperglycemia results in glycosuria 17-62

  16. Hormonal Effects 17-63

  17. Electrolyte Balance • Kidneys regulate levels of Na+, K+, H+, HCO3-, Cl-, and PO4-3 by matching excretion to ingestion • Control of plasma Na+ is important in regulation of blood volume and pressure • Control of plasma of K+ is important in proper function of cardiac and skeletal muscles 17-64

  18. Role of Aldosterone in Na+/K+Balance • 90% filtered Na+ and K+ reabsorbed before DCT • Remaining is variably reabsorbed in DCT and cortical CD according to bodily needs • Regulated by aldosterone (controls K+ secretion and Na+ reabsorption) • In the absence of aldosterone, 80% of remaining Na+ is reabsorbed in DCT and cortical CD • When aldosterone is high all remaining Na+ is reabsorbed 17-65

  19. K+ Secretion • Is only way K+ ends up in urine • Is directed by aldosterone and occurs in DCT and cortical CD • High K+ or low Na+ will increase aldosterone and K+ secretion 17-66

  20. Juxtaglomerular Apparatus (JGA) • Is specialized region in each nephron where afferent arteriole comes in contact with thick ascending limb LH 17-67

  21. Renin-Angiotensin-Aldosterone System • Is activated by release of renin from granular cells within afferent arteriole • Renin converts angiotensinogen to angiotensin I • Which is converted to Angio II by angiotensin-converting enzyme (ACE) in lungs • Angio II stimulates release of aldosterone 17-68

  22. Regulation of Renin Secretion • Inadequate intake of NaCl always causes decreased blood volume • Because lower osmolality inhibits ADH, causing less H2O reabsorption • Low blood volume and renal blood flow stimulate renin release • Via direct effects of BP on granular cells and by Symp activity initiated by arterial baroreceptor reflex (see Fig 14.26) 17-69

  23. 17-70

  24. Macula Densa • Is region of ascending limb in contact with afferent arteriole • Cells respond to levels of Na+ in filtrate • Inhibit renin secretion when Na+ levels are high • Causing less aldosterone secretion, more Na+ excretion 17-71

  25. 17-72

  26. Atrial Natriuretic Peptide (ANP) • Is produced by atria due to stretching of walls • Acts opposite to aldosterone • Stimulates salt and H2O excretion • Acts as an endogenous diuretic 17-73

  27. Na+, K+, H+, and HCO3- Relationships 17-74

  28. Na+, K+, and H+ Relationship • Na+ reabsorption in DCT and CD creates electrical gradient for H+ and K+ secretion • When extracellular H+ increases, H+ moves into cells causing K+ to diffuse out and vice versa • Hyperkalemia can cause acidosis • In severe acidosis, H+ is secreted at expense of K+ 17-75

  29. Renal Acid-Base Regulation • Kidneys help regulate blood pH by excreting H+ and/or reabsorbing HCO3- • Most H+ secretion occurs across walls of PCT in exchange for Na+ (Na+/H+ antiporter) • Normal urine is slightly acidic (pH = 5-7) because kidneys reabsorb almost all HCO3- and excrete H+ 17-76

  30. Reabsorption of HCO3- in PCT • Is indirect because apical membranes of PCT cells are impermeable to HCO3- 17-77

  31. Reabsorption of HCO3- in PCT continued • When urine is acidic, HCO3- combines with H+ to form H2CO3 (catalyzed by CA on apical membrane of PCT cells) • H2CO3 dissociates into CO2 + H2O • CO2 diffuses into PCT cell and forms H2CO3 (catalyzed by CA) • H2CO3 splits into HCO3- and H+ ; HCO3- diffuses into blood 17-78

  32. Urinary Buffers • Nephron cannot produce urine with pH < 4.5 • Excretes more H+ by buffering H+s with HPO4-2 or NH3 before excretion • Phosphate enters tubule during filtration • Ammonia produced in tubule by deaminating amino acids • Buffering reactions • HPO4-2 + H+ H2PO4- • NH3 + H+ NH4+ (ammonium ion) 17-79

  33. Clinical Aspects 17-80

  34. Diuretics • Are used to lower blood volume because of hypertension, congestive heart failure, or edema • Increase volume of urine by increasing proportion of glomerular filtrate that is excreted • Loop diuretics are most powerful; inhibit AT salt in thick ascending limb of LH • Thiazide diuretics inhibit NaCl reabsorption in 1st part of DCT • Carbonic anhydrase inhibitors prevent H2O reabsorption in PCT when HCOs- is reabsorbed • Osmotic diuretics increase osmotic pressure of filtrate 17-81

  35. Sites of Action of Clinical Diuretics 17-82

  36. Kidney Diseases • In acute renal failure, ability of kidneys to excrete wastes and regulate blood volume, pH, and electrolytes is impaired • Rise in blood creatinine and decrease in renal plasma clearance of creatinine • Can result from atherosclerosis, inflammation of tubules, kidney ischemia, or overuse of NSAIDs 17-83

  37. Kidney Diseases continued • Glomerulonephritis is inflammation of glomeruli • Autoimmune attack against glomerular capillary basement membranes • Causes leakage of protein into urine resulting in decreased colloid osmotic pressure and resulting edema 17-84

  38. Kidney Diseases continued • In renal insufficiency, nephrons have been destroyed as a result of a disease • Clinical manifestations include salt and H2O retention and uremia (high plasma urea levels) • Uremia is accompanied by high plasma H+ and K+ which can cause uremic coma • Treatment includes hemodialysis • Patient's blood is passed through a dialysis machine which separates molecules on basis of ability to diffuse through selectively permeable membrane • Urea and other wastes are removed 17-85

More Related