Understanding the Urinary System Functions and Processes

1. Chapter 26: The Urinary System BIO 211 Lecture Instructor: Dr. Gollwitzer

2. Today in class we will discuss: • The interrelationship between the CVS and urinary system • The major functions of the urinary system • Excretion • Elimination • Homeostatic regulation • The basic principles of urine formation • Major functions of each portion of the nephron and collecting system • The 3 basic processes involved in urine formation • Glomerular filtration • Filtration pressures • Tubular reabsorption • Tubular secretion

3. CVS and Urinary System • CVS delivers nutrients (from digestive tract) and O2 (from lungs) to cells in peripheral tissues • CVS carries CO2 and waste products from peripheral tissues to sites of excretion • CO2 removed at lungs • Most physiological waste products removed by urinary system

4. Major Functions of Urinary System • Excretion • Elimination • Homeostatic regulation of: • Blood plasma volume • Solute concentration

5. Major Functions of Urinary System • Excretion • Removal of organic wastes (e.g., urea, uric acid, creatinine) from body fluids (= urine formation) • Performed by kidneys which act as filtering units • Elimination • Discharge of waste products into environment (urination) • Occurs when urinary bladder contracts and forces urine through urethra and out of body

6. Major Functions of Urinary System: Homeostatic Regulation • Regulation of blood volume (water balance) and BP • Adjusts volume of water lost in urine • Releases • Renin • Involved in production of angiotensin II that affects BP, thirst, and other hormones (ADH, aldosterone) that affect water retention by kidneys • Erythropoietin • Stimulates erythropoiesis in bone marrow, maintains RBC volume

7. Major Functions of Urinary System: Homeostatic Regulation • Regulation of plasma ion concentrations (electrolyte balance) • Controls amounts lost in urine (e.g., Na+, K+, Cl-) • Controls Ca2+ levels by synthesis of calcitriol • Reabsorption (conservation) of valuable nutrients • Recycles valuable nutrients • e.g., amino acids, glucose • Prevents excretion in urine

8. Major Functions of Urinary System: Homeostatic Regulation • Stabilization of blood pH (acid-base balance) • Controls loss of H+ and HCO3- in urine • Detoxification • Of poisons, e.g., drugs • Deamination • Removes NH2 (amino group) so amino acids can be metabolized

9. Basic Principles of Urine Formation • Urine = fluid containing: • Water • Ions • Soluble compounds • Goal of urine production • To maintain homeostasis • By regulating volume and composition of blood

10. Basic Principles of Urine Formation • Involves excretion of solutes (i.e., metabolic/organic waste products) • Urea • Most abundant • Produced by breakdown of amino acids • Creatinine • Generated in skeletal muscle by breakdown of creatine phosphate (CP, high energy compound that plays a role as energy source in muscle contraction) • Uric acid • Formed by recycling nitrogenous bases from RNA

11. Basic Principles of Urine Formation • Waste products dissolved in bloodstream can only be eliminated when dissolved in urine • Thus removal accompanied by unavoidable water loss • To avoid dehydration, kidneys concentrate filtrate (i.e., reabsorb water) produced by glomerular filtration

12. Functional Anatomy of Nephron and Collecting System Figure 26–6

13. 3 Processes Involved in Urine Formation • Glomerular filtration • Forces water and solutes out of blood in glomerulus into capsular space •  filtrate • Tubular reabsorption • Recovers useful materials from filtrate • Tubular secretion • Ejects waste products, toxins, and other undesirable solutes into tubules

14. Glomerular Filtration • Occurs in renal corpuscle • Hydrostatic pressure forces water and solutes: • Out of blood in glomerulus • Into capsular space  filtrate • Occurs solely on basis of size • Small solute molecules carried with filtrate

15. Glomerular Filtration • Involves passage across filtration membrane which is composed of 3 cellular units • Glomerular capillary endothelium • Lamina densa • Filtration slits

16. Glomerular Filtration • Glomerular capillary endothelium • Filtered through pores in fenestrated capillaries • Least selective filter • Pores too small for RBCs to pass through • Large enough for plasma proteins

17. Renal Corpuscle Figure 26–8

18. Glomerular Filtration • Lamina densa • Basement membrane of glomerular capillaries • More selective filter • Blocks passage of large proteins • Only small polypeptides, nutrients, and ions can cross

19. Figure 26–10, 7th edition

20. Glomerular Filtration • Filtration slits • Gaps between pedicels of podocytes (visceral epithelium around glomerulus) • Finest filter • No polypeptides pass through • Only nutrients, ions into capsular space • Thus, glomerular filtrate: • Does not contain plasma proteins or polypeptides • Does contain small organic molecules (e.g., nutrients) and ions in same concentration as in plasma

21. Filtration Pressures • Filtration pressure = balance between: • Hydrostatic (fluid) pressures • Glomerular hydrostatic pressure (GHP) in capillaries (50 mmg Hg) • Capsular hydrostatic pressure (CHP) (15 mm Hg) • Blood osmotic pressure (BOP) (25 mm Hg)

22. Filtration Pressures • Hydrostatic (fluid) pressures • Glomerular hydrostatic pressure (GHP) (50 mm Hg) • = BP in glomerular capillaries • Higher in glomerulus than in peripheral capillaries (35 mm Hg) • Because efferent arteriole smaller in diameter than afferent arteriole, need higher BP to force blood into it • Promotes filtration – pushes water and solutes out of plasma in capillaries into filtrate • Opposed by…

23. Filtration Pressures • Hydrostatic (fluid) pressures • Capsular hydrostatic pressure (CHP) (15 mm Hg) • Opposes filtration – pushes water and solutes out of filtrate into plasma in capillaries • Results from resistance to flow along nephron and conducting system that causes water to collect in Bowman’s capsule • More water in capsule  more pressure

24. Filtration Pressures • Blood osmotic pressure (BOP) (25 mm Hg) • Results from presence of suspended proteins in blood • Promotes return of water into glomerulus • Opposes filtration • Tends to draw water out of filtrate and into plasma

25. Figure 26–10, 7th edition

26. Summary of Filtration Pressures • Hydrostatic pressures • GHP (pushing out of glomerulus) = 50 mm Hg • CHP (pushing into glomerulus) = 15 mm Hg • Net = 35 mm Hg (pushing out of glomerulus) • Osmotic pressure • BOP (draws into glomerulus) = 25 mm Hg • Filtration pressure = 10 mm Hg • Difference between net hydrostatic pressure and blood osmotic pressure

27. Summary of Filtration Pressures • Problems that affect filtration pressure • Can seriously disrupt kidney function • Can cause a variety of clinical symptoms, e.g., • Drop in systolic pressure from 120 to < 110 mm Hg would eliminate filtration pressure (10 mm Hg)

28. Today in class we will discuss: • The 3 basic processes involved in urine formation • Glomerular filtration • Glomerular Filtration Rate • Renal Failure • Tubular reabsorption • PCT, Loop of Henle & Countercurrent Exchange,DCT • Collecting System • Tubular secretion • PCT, DCT and Collecting system • Urine • Compare/contrast to plasma • General characteristics • Hormone influence of volume and concentration • Voluntary & involuntary regulation of urination and the micturition reflex

29. Glomerular Filtration Rate (GFR) • Gomerular filtration • Vital first step essential to all other kidney functions • Must occur so: • Waste products excreted • pH controlled • Blood volume maintained • GFR = amount of filtrate kidneys produce per minute • Avg GFR = 125 mL/min or 50 gal/day (out of 480 gallons of blood flow/day) • 10% of fluid delivered by renal arteries enters capsular spaces • 99% of this reabsorbed so urinate only 0.5 gallons/day

30. Glomerular Filtration Rate (GFR) • Measured using creatinine clearance test (CCT) • Breakdown of CP in muscle  creatinine • Creatinine enters filtrate at glomerulus and is not reabsorbed so is excreted in urine • Can compare amount of creatinine in blood vs. in urine during 24 hour and estimate GFR • If glomerulus damaged, GFR will be altered (have more or less creatinine in urine than normal)

31. Glomerular Filtration Rate (GFR) • GFR depends on: • Adequate blood flow to glomerulus • Maintenance of normal filtration pressures • Affected by anything that reduces renal blood flow or BP, e.g., • Hypotension, hemorrhage, shock, dehydration • Decreased renal blood volume and/or BP  decreased filtration pressure  decreased GFR

32. Control of GFR • GFR increased by: • EPO (relatively minor) • Renin-angiotensin system • Natriuretic peptides (ANP and BNP)

33. Control of GFR • Decreased BP and/or blood volume  • Decreased O2  JGA  EPO  • Increased RBCs  • Increased O2 delivery • Increased blood volume  increased BP  • Increased filtration pressure • Increased GFR • Decreased renal blood flow  JGA  renin-angiotensin system  • Increased blood volume  increased BP  • Increased filtration pressure • Increased GFR

34. EPO and Renin Figure 18–19b

35. Renin-Angiotensin System • Renin (enzyme)  (prohormone) angiotensinogen  (hormone) angiotensin I (in liver) • Angiotensin I  angiotensin II (in lung capillaries) • Angiotensin II  increased blood volume and BP  increased GFR

36. Primary Effects of Angiotensin II • Stimulates constriction of efferent arterioles  increased glomerular pressure • Directly stimulates reabsorption of Na+ and H2O in DCT  increased blood volume and BP • Stimulates adrenal cortex  aldosterone  reabsorption of Na+ (and H2O)  increased blood volume and BP • Stimulates posterior pituitary  ADH  reabsorption of H2O  increased blood volume and BP • Stimulates thirst  increased blood volume and BP • Stimulates vasoconstriction of arterioles

37. Renin-Angiotensin System: Response to Reduction in GFR Figure 26–11-0

38. Control of GFR • Increased blood volume or BP  stretched cardiac muscle cells  natriuretic peptides • ANP = atrial NP • BNP = brain NP (produced by ventricles) • Natriuretic peptides • Increase GFR • Decrease blood volume and BP • Via 2 mechanisms

39. Natriuretic Peptides Increase GFR • Act opposite to angiotensin II • Increase Na+ and H2O loss • Inhibit renin release • Inhibit secretion of aldosterone and ADH • Suppress thirst • Prevent increased BP by angiotensin II and NE • Increase glomerular pressures • Dilate afferent arterioles • Constrict efferent arterioles • Also increase tubular reabsorption of Na+ • Decreases blood volume and BP

40. Renal Failure • When filtration (GFR) slows, urine production decreases • Symptoms appear because water, ions, and metabolic wastes retained rather than excreted • Almost all systems affected: fluid balance, pH, muscular contraction, neural function, digestive function, metabolism • Leads to: • Hypertension (due to blood “backing up”) • Anemia due to lack of erythropoietin production • CNS problems (sleepiness, seizures, delirium, coma, death)

41. Renal Failure • Acute renal failure • From exposure to toxic drugs, renal ischemia, urinary obstruction, trauma • Develops quickly, but usually temporary • With supportive treatment can survive • Chronic renal failure • Condition deteriorates gradually • Cannot be reversed • Dialysis or kidney transplant may prolong life

42. Reabsorption and Secretion • Occur in all segments of renal tubules • Relative importance changes from segment to segment

43. Tubular Reabsorption • Molecules move from filtrate  across tubular epithelium into peritubular interstitial fluid and blood • Water, valuable solutes (e.g., nutrients, proteins, amino acids, glucose) • Occurs through diffusion, osmosis (H2O), active transport by carrier proteins • Occurs primarily along PCT (also along renal tubule and collecting system)

44. Tubular Secretion • Molecules move from peritubular fluid into tubular fluid • Lowers plasma concentration of undesirable materials • Necessary because filtration does not force all solutes out of plasma • Primary method of excretion for many drugs • Occurs primarily at PCT and DCT

45. Reabsorption and Secretion: PCT • Primarily reabsorption • 60-70% of filtrate • Includes: • Organic nutrients (99-100%), e.g., glucose, amino acids, proteins, lipids, vitamins • Water (60-70%) • Ions (60-70%), e.g., Na+, Cl-; also K+, Ca2+, HCO3- • Reabsorbed materials enter peritubular fluid and capillaries • Secretion • H+, NH4+, creatinine, drugs, toxins

46. Reabsorption: Loop of Henle • Reabsorption • Na+, Cl- • Water • Accomplished by countercurrent exchange • Refers to exchange by tubular fluids moving in opposite directions • Fluid in descending limb flows toward renal pelvis • Fluid in ascending limb flows toward cortex

47. Countercurrent Exchange • Occurs because of different permeabilities of segments of LOH • Descending limb (thin) • Permeable to water • Relatively impermeable to solutes • Ascending limb (thick) • Relatively impermeable to water and solutes • Has active transport mechanisms • Pump Na+ and Cl- from tubular fluid into peritubular fluid

48. Countercurrent Exchange • Na+ and Cl- pumped out of thick ascending limb into peritubular fluid • Increases osmotic concentration in peritubular fluid around thin descending limb • Results in osmotic flow of H2O out of thin descending limb into peritubular fluid  increased solute concentration in thin descending limb • Arrival of concentrated solution in thick ascending limb increases transport of Na+ and Cl- into peritubular fluid

49. Overview of Urine Formation Figure 26–16

50. Reabsorption and Secretion: DCT • Reabsorption (by vasa recta) • Na+ (under influence of aldosterone), Cl- • Ca2+(under influence of PTH and calcitriol) • H2O (under influence of ADH) • Secretion • K+ (in exchange for Na+), H+ • NH4+ (from deamination; produces lactic acid, ketone bodies  acidosis) • Creatinine, drugs, toxins