1. 1 Chapter 23 - Urinary System Lecture 17 Martini’s Visual Anatomy and Physiology First Edition Martini  Ober

2. 2 Lecture Overview Introduction to the Urinary System Location and function of the kidneys Gross anatomy Histology Urine formation

3. 3 Functions of the Kidneys Make urine Regulate blood volume and blood pressure Regulate plasma concentrations of Na +, K +, Cl -, HCO 3 -, and other ions Help to stabilize blood pH Conserve valuable nutrients Assist the liver in detoxification and deamination

4. 4 Urinary System Kidneys receive about 1.2 L of blood per minute and filter nearly 180 L of fluid from the bloodstream every day! Figure from: Hole’s Human A&P, 12 th edition, 2010

5. 5 Location of Kidneys Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Located retroperitoneally from T12 to L3 Left kidney is slightly higher than right kidney Adrenal glands sit on the medial and superior part of kidneys Nephro(s) = kidney

6. 6 Location of Kidneys Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Helps maintain position of kidney

7. 7 Gross Anatomy of the Kidneys Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Renal capsule is tough, fibrous capsule (tunica fibrosa) Hilus is entry point for renal artery, vein, and nerve (mostly sympathetic fibers) Bases of renal pyramids face the cortex, apices face the renal pelvis and end at renal papillae [Pyel(o)-]

8. 8 Renal Blood Vessels Figures from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Nerves, mainly sympathetic postganglionic fibers of the ANS, follow arteries to nephrons

9. 9 The Nephron Nephrons are the structural and functional units of the kidney (80%) (20%) Figure from: Hole’s Human A&P, 12 th edition, 2010

10. 10 Blood Supply of the Nephron The capillary loop of the vasa recta is a type of capillary that is closely associated with the nephron loop of juxtamedullary nephrons Medulla Figure from: Hole’s Human A&P, 12 th edition, 2010

11. 11 Blood Flow Through Kidney and Nephron Know this! Figure from: Hole’s Human A&P, 12 th edition, 2010

12. 12 Renal Corpuscle (Glomerulus + Capsule) Notice that the efferent arteriole is smaller than the afferent arteriole This creates a high pressure (~55-60 mm Hg) in the glomerular capillary bed Filtrate in capsular space Figure from: Hole’s Human A&P, 12 th edition, 2010

13. 13 Visceral Glomerular Epithelium Material passing out of the blood must be small enough to fit through the filtration slits (slit pores) Figure from: Hole’s Human A&P, 12 th edition, 2010

14. 14 Renal Cortex and Renal Medulla Figure from: Hole’s Human A&P, 12 th edition, 2010

15. 15 Specialization of the Nephron 1. PCT – simple cuboidal with a brush border 2. Thin segment of the descending nephron loop - simple squamous epithelium 3. Thick ascending nephron loop - cuboidal/low columnar 4. DCT - simple cuboidal with no microvilli (specialized for secretion, not absorption) The order of the parts of the nephron is important to know (PCT) (DCT) Figure from: Hole’s Human A&P, 12 th edition, 2010

16. 16 Juxtaglomerular Apparatus Juxtaglomerular cells (JG) - modified smooth muscle cells in the wall of the afferent arteriole that contract (and secrete renin) Cells of the macula densa (MD) are osmoreceptors responding to solute concentration of filtrate MD + JG cells = juxtaglomerular apparatus Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

17. 17 Glomerular Filtrate and Urine Composition Glomerular filtrate is about the same composition as plasma: H 2 O, glucose, amino acids, urea, uric acid, creatine, creatinine, Na, Cl, K, HCO 3 -, PO 4 3-, SO 4 2-. But notice how different the composition of urine is. Additionally, note that protein is not normally present in urine. (1.8 L/day)

18. 18 Urine Formation Glomerular Filtration (GF) *Adds to volume of urine produced substances move from blood to glomerular capsule Tubular Reabsorption (TR) *Subtracts from volume of urine produced substances move from renal tubules into blood of peritubular capillaries glucose, water, urea, proteins, creatine amino, lactic, citric, and uric acids phosphate, sulfate, calcium, potassium, and sodium ions Tubular Secretion (TS) *Adds to volume of urine produced substances move from blood of peritubular capillaries into renal tubules drugs and ions, urea, uric acid, H +  Urine formation = GF + TS - TR About 125 ml/minute (180 L/day) of the total 1200 ml/min of blood that passes through the glomerulus becomes filtrate

19. 19 Overview of Reabsorption and Secretion Figure from: Hole’s Human A&P, 12 th edition, 2010

20. 20 Glomerular Filtration Glomerular filtrate is plasma that passes through 1) the fenestrae of the capillary endothelium, 2) the basement membrane around the endothelium, and 3) the filtration slits (slit pores) of the pedicels This is called the ‘filtration membrane’ Glomerular filtration is a mechanical process based primarily on molecule size Figure from: Hole’s Human A&P, 12 th edition, 2010

21. 21 Glomerular Filtration Rate (GFR) Net filtration pressure is normally positive, i.e., favors the movement of fluid out of the glomerular capillaries GFR = amount of filtrate produced each minute (~125 ml/min) Net Filtration Pressure = force favoring filtration – forces opposing filtration (*glomerular capillary ( capsular hydrostatic pressure hydrostatic pressure) + glomerular capillary osmotic pressure ) NFP = HP g – (HP c + OP g ) Figure from: Hole’s Human A&P, 12 th edition, 2010

22. 22 Afferent/Efferent Arterioles – Effect on GFR Afferent arteriole –Δ radius  GFR –  radius  GFR;  radius  GFR Efferent arteriole –Δ radius  1/GFR –  radius  GFR;  radius  GFR Innervated by sympathetic nerves

23. 23 Amounts of Glomerular Filtrate and Urine average amounts over a 24 hour period Glomerular Filtration Rate (GFR) is directly proportional to the net filtration pressure GFR  125 ml/min (180 L/day) This means that all of the plasma is filtered ~ 60x every day (How did we get this number?) Notice that urine output is only 0.6 – 2.5 L per day (an average of about 1.8 L, or about 1% of glomerular filtrate); 99% of filtrate is reabsorbed!! Figure from: Hole’s Human A&P, 12 th edition, 2010

24. 24 Glomerular Filtration Rate (GFR) Net filtration pressure, although normally positive, is relatively low (  10 mm Hg) Glomerular hydrostatic pressure is the blood pressure in the glomerular capillaries, and is usually higher than other capillary pressures Capsular hydrostatic pressure tends to push water and filtrate BACK into the capillaries Anything that alters the filtration pressures will alter GFR * Blood pressure is the most important factor altering the glomerular hydrostatic pressure (and NFP). A MAP fall of 10% will severely impair glomerular filtration; a fall of 15-20% will stop it.

25. 25 Summary of Factors Affecting GFR FactorEffect Vasoconstriction (↑ Sympathetic stimulation) Afferent arteriole  GFR Efferent arteriole ↑ GFR Vasodilation (  Sympathetic stimulation) Afferent arteriole↑ GFR Efferent arteriole  GFR Increased capillary hydrostatic pressure↑ GFR Increased colloid osmotic pressure  GFR Increased capsular hydrostatic pressure  GFR Know this table – it’s important!

26. 26 Regulation of GFR Autoregulation –Maintains GFR despite changes in local blood pressure and blood flow (between 90 – 180 mm Hg mean systemic pressure) –Myogenic mechanism – contraction of afferent arteriolar vascular smooth muscle when stretched (increased BP); relaxation occurs when BP declines –Tubuloglomerular mechanism – MD cells detect  flow rate and/or  osmolarity of filtrate in DCT -> JG cells contract -> afferent arteriole constricts ->  GFR

27. 27 Regulation of GFR Neural (Autonomic) Regulation –Mostly sympathetic postganglionic fibers = vasoconstriction of afferent arterioles  GFR (conserves water, redirects blood to other organs) –Stimulates juxtaglomerular apparatus to secrete renin –May override autoregulatory mechanism at afferent arteriole Hormonal Regulation –Renin-angiotensin system – stabilizes BP and ECF volume –Atrial Natriuretic Peptide (ANP) - ↑ GFR, ↑ fluid loss (dilates afferent arteriole, constricts efferent arteriole)

28. 28 Renin-Angiotensin System Renin is released by the juxtaglomerular apparatus due to: 1) Decline of BP (Renin  1/Pressure) 2) Juxtaglomerular stimulation by sympathethic NS 3) Decline in osmotic concentration of tubular fluid at macula densa ( Renin  1/[NaCl] ) Stabilizes systemic blood pressure and extracellular fluid volume (ACE) Figure from: Hole’s Human A&P, 12 th edition, 2010

29. 29 Review Kidneys –Are bean-shaped organs located retroperitoneally at level of T12 – L3 –Are enclosed by a tough, fibrous capsule –Consist of a cortex and a medulla Cortex – glomeruli Medulla – nephron loops, collecting ducts; renal pyramids, renal papillae –Renal sinus Major and minor calyces Renal blood vessels and nerves pass through the hilus and branch within the sinus

30. 30 Review Know this! Figure from: Hole’s Human A&P, 12 th edition, 2010

31. 31 Review Functions of the Kidneys –Making urine –Regulating blood volume and blood pressure –Regulating plasma concentrations of Na +, K +, Cl -, HCO 3 -, and other ions –Helping to stabilize blood pH –Conserving valuable nutrients –Assisting the liver in detoxification and deamination

32. 32 Review The functional unit of the kidney is the nephron –Glomerulus (filtration) Capillaries surrounded by podocytes Filter blood –PCT –Nephron Loop –DCT –Peritubular capillaries Surround proximal and distal tubules In juxtamedullary nephrons, forms the vasa recta –Collecting ducts (technically not part of nephron)

33. 33 Review Glomerular filtration –Glomerular filtration rate (GFR) Amount of filtrate produced each minute Directly proportional to net filtration pressure May be determined with creatinine or inulin tests Approximately 125 ml/min (180 L/day) –Factors affecting GFR Vasoconstriction / vasodilation Capillary hydrostatic pressure Capsular hydrostatic pressure Capillary osmotic pressure

34. 34 Review Glomerular filtration (cont’d) –Factors controlling GFR Autoregulation –Myogenic –Tubuloglomerular Hormonal –Renin-Angiotensin System –ANP Autonomic nervous system