BIO – 255 Anatomy & Physiology

1. BIO – 255 Anatomy & Physiology Chapter 25 – Urinary System

2. Urinary System • Functions? • Excretion – blood filtering • Gee Whiz! - each day the kidneys filter about 180 liters of plasma • About 25% of total cardiac output goes to the kidneys!!! • Renin and erythropoietin production • Vitamin D activation

3. Urinary System • Anatomy • Kidneys, Ureters, Urinary bladder, and Urethra – Figures 25.1, 25.2 and 25.3

4. Urinary System • What is the nephron? • There are about 1 million per kidney! • Figure 25.4 and 25.5

5. Urinary System • Juxtaglomerular apparatus – this is the site of renin production • The cells determine blood pressure by stretch of the afferent arteriole wall • Figure 25.6

6. Urinary System • Nephron Structure • Renal corpuscle = the Bowman’s capsule and the glomerulus • The inner layer of the Bowman’s capsule is composed of podocytes • Spaces between podocyte projections are called filtration slits • Figure 25.7

7. Urinary System • The glomerular capillary walls are lined with openings called fenestrae • The filtration membrane = capillary wall, basement membrane, and podocytes • Figure 25.7

8. Urine Production • Urine production occurs at the nephron • It occurs in three stages: • Filtration – fluid is moved from the blood into the Bowman’s capsule • This fluid is called filtrate • Reabsorption – movement of substances from the filtrate back into the blood • Secretion – active transport of wastes into the urine • Figure 25.8

9. Urine Production • Filtration • Total plasma volume is filtered ~60 times per day • The kidneys use about 20-25% of all oxygen in the body (at rest) but they only make up about 1% of total body weight • About 180 L of filtrate per day, but only about 1.8 L of urine (reabsorption is major)

10. Urine Production • Filtration Membrane • The filtration membrane is a barrier to large particles in the blood • Cells, most proteins, etc. • Fluid is pushed out of the capillary by the filtration pressure • Blood pressure in the glomerulus is extremely high • Figure 25.9

11. Urine Production • Filtrate is similar to plasma, but urine is not • The primary changes occur during: • Reabsorption – this occurs in the tube system of the nephron • Proximal tubule – most solutes and water are reabsorbed here • For most molecules this occurs by secondary active transport (cotransport) with Na+ ions • Na+ is actively transported out of the cell into the interstitial fluid • This generates the concentration gradient to power the cotransport mechanism • Figure 25.12

12. Urine Production • By the end of the proximal tubule, 65% of the filtrate volume has been reabsorbed

13. Urine Production • Loop of Henle • Descending limb extends into the medulla • the interstitial fluid has an extremely high solute concentration (1200 mOsm/kg) • This causes the movement of water out of the nephron • A few solutes enter the nephron • At the bottom of the loop, an additional 15% of filtrate has been removed • Figure 25.14

14. Urine Production • The ascending limb is impermeable to water, but additional solutes are removed • This causes a dilution of the filtrate (100 mOsm/kg) • Figure 25.14

15. Urine Production • Distal tubule and collecting duct • More ions are absorbed • Na+ and Aldosterone • Water movement through the distal tubule and collecting duct walls is under hormonal control • Antidiuretic Hormone (ADH) is the primary regulator – What does it do? • More later • Table 25.1

16. Urine Production • Some materials are not actively removed from the filtrate • Urea, uric acid, phosphates and other toxic substances become more concentrated

17. Urine Production • Secretion • Some substances are actively transported into the urine in the proximal and distal tubules • Drugs, urea, excess K+ • Blood pH regulation and H+ secretion • Figure 25.8

18. Urine Formation • CD

19. Urine Production • Regulation of Urine Concentration and Volume • Osmolality is a measure of how many solutes are dissolved in a fluid • Dilute urine (~65 mOsm/kg) • Concentrated urine (~1200 mOsm/kg) • Blood and most other body fluids (~300 mOsm/kg) • The composition of urine is manipulated to ensure this remains constant

20. Urine Production • The concentration of urine depends on maintaining a high solute concentration in the medulla • water and solutes move from the filtrate into the medullary interstitial fluid at the loops of Henle • The vasa recta are more permeable to water than solutes • Figures 25.13 and 25.14

21. Urine Production • Concentration of urine depends on ADH • ADH activates a second messenger system that leads to more active aquaporins in the distal tubule and collecting duct • This allows water to be removed from the filtrate and enter the blood • “anti”-diuretic • Figure 25.15 and from other text

22. Urine Production • Formation of dilute urine occurs in the absence of ADH • Aquaporin function is inhibited so water is retained in the filtrate • Figure 25.15

23. Reg. of Urine Concentration & Volume • What if a person could not produce enough ADH? • Diabetes insipidus (10-20 liters of urine per day) • Figure 25.15

24. Reg. of Urine Concentration & Volume • Renin-Angiotensin-Aldosterone Mechanism • When BP decreases, the kidneys secrete renin • Renin is secreted by the cells of the juxtaglomerular apparatus • Renin activates a plasma protein called angiotensin • Active angiotensin causes vasoconstriction and the release of aldosterone from the adrenal cortex

25. Reg. of Urine Concentration & Volume • Aldosterone increases Na+ and Cl- reabsorption by the kidney • What effect would this have on urine concentration and volume? • Angiotensin can also stimulate the release of ADH • Figure 25.15 and from other text

26. Reg. of Urine Concentration & Volume • Atrial Natriuretic Hormone (ANH) • Is secreted by the cells of the right atrium when they are stretched • ANH blocks the secretion of ADH and the reabsorption of Na+ • Effect?

27. Other Information • Diuretics (such as Lasix and Diuril) inhibit Na+ reabsorption. Effect? • Kidney stones occur when minerals crystallize in the renal pelvis • Ultrasound treatment • Risk factors are bacterial infections, urine retention, high blood Ca2+ , and alkaline urine.

28. Micturition (Urination) • There are smooth and skeletal muscle sphincters that close off the urethra • When about 200 ml of urine is in the bladder, the urge to urinate occurs • The skeletal muscle can over-ride the urge • After another 2-300 ml, another urge is felt • This can also be suppressed • When the volume reaches about 600 ml the urge is constant and eventually micturition occurs (whether you like it or not)