Understanding the Urinary System Functions and Organs

1. Urinary and Excretory Systems Chapter 16

2. The Urinary System: Functions Carries out removal (excretion) of metabolic wastes • These include “nitrogenous wastes” • Urea • By-product of amino acid metabolism • Formed when ammonia (very toxic) combines with CO2 • Less toxic than ammonia • Creatinine • By-product of creatine phosphate breakdown (process that happens in working muscle and the brain). • Uric acid • By-product off breakdown of DNA • Build up of uric acid in joints causes gout.

3. The Urinary System: Functions cont’d. Maintenance of water-salt balance • Blood volume is associated with salt balance • Salts cause osmosis into the blood • The more water the higher the blood pressure • Kidneys play role in blood pressure regulation by controlling water and salt levels. • Regulates other ions such as potassium and calcium, too.

4. The Urinary System: Functions cont’d. Maintenance of acid-base balance • Excretion of hydrogen ions and reabsorption of bicarbonate • Urine pH is normally around 6 due to high acid diet

5. The Urinary System: Functions cont’d. Secretion of hormones • Renin • Helps to control sodium levels in the body • Erythropoetin • Stimulates red blood cell production • Activates vitamin D for calcium absorption

6. The Urinary System : Organs • Fig. 16.1

7. Urinary System : Organs Kidneys • Located in lumbar (“lower back”) region • Covered by tough capsule of tissue • Have a concave (“curved in”) side • Renal artery and vein are on this side Ureters • Carry urine from kidney to bladder • 3 layered wall • Mucosa (inner), smooth muscle (middle), connective tissue (outer). • Use peristalsis to move urine

8. Urinary System : Organs cont’d. Urinary bladder • Stores urine • Expandable • Contains 3 layers of muscle • 2 sphincters in bladder “neck” (narrow part) • Internal sphincter – made of smooth muscle • Involunary (you can’t control it) • External sphincter - made of skeletal muscle • Voluntary (you can control it)

9. Urinary Bladder (Male and Female)

10. Urinary System : Organs cont’d Urethra • Carries urine out of body • Very short in females • Higher risk of urinary tract infection (sorry! ) • Males • Involved with both the reproductive and urinary tracts

11. Urinary System : Micturition • AKA: urination • Stretch receptors in wall of bladder • Send nerve impulses when bladder fills to 250 ml • Spinal cord sends impulses back to the bladder • Bladder contracts • Micturition (urination) occurs • Urination reflex can be overridden by brain in adults and older children.

12. Urination • Fig. 16.2

13. Urinary System: Kidney • Divided into two main regions • an outer cortex and an inner medulla. • Both regions are packed with microscopic tubules called nephrons. • Nephrons are the functional units of the kidney • Approximately one million nephrons • Each composed of a system of tubules with its own blood supply

14. Excretory System: Kidney cont’d • Fig. 16.3

15. The Nephron • Nephron Blood Supply • Renal Artery = main blood vessel • From renal artery, afferent arteriole leads into sepcialized capillaries called the glomerulus • Blood leaves glomerulus by efferent arteriole to surrounding capillaries around the nephron (called the peritubular capillaries). • Blood then exits via renal vein

16. The Nephron cont’d • Nephron Anatomy • Bowman’s capsule • Cuplike structure • Allows passage of small molecules into the nephron • Proximal convoluted tubule (PCT for short) • Made of cells lined with microvilli • Increases surface area for absorption

17. The Nephron cont’d. • Nephron Anatomy cont’d • Loop of Henle • U-shaped tube • Loops of Henle can extend down into the medulla

18. The Nephron cont’d. • Nephron Anatomy cont’d • Distal convoluted tubule (DCT for short) • Lacks microvili • DCT’s of several nephrons enter one collecting duct • Glomerular capillaries and convoluted tubules lie within cortex of kidney

19. Fig. 16.4

20. Urinary system: Urine Formation • 3 main steps • Glomerular filtration • Tubular Reabsorption • Tubular Secretion

21. Urinary system: Urine Formation • Glomerular filtration • Blood enters glomerulus from afferent arteriole • Has a high blood pressure • Water and small molecules filtered into Bowman’s capsule • Water, Nitrogenous wastes and Salts (ions) are filtered • Blood Cells and Plasma Proteins stay in the blood • Glomerular filtrate (fluid that enters the glomerulus from the blood) • Composed of same substances as blood plasma minus the cells and large plasma proteins • We must reabsorb desirable substances into the blood, but keep wastes in the nephron.

22. Reabsorption from nephrons • Table 16.1

23. Urinary system: Urine Formation cont’d. • Tubular reabsorption – the process of moving substances back from the nephron into the bloodstream. • 80% of filtrate reabsorbed in PCT (proximal convoluted tubule) • Both active and passive transport are involved • Sodium reabsorbed by active transport, causes chloride to follow passively, then water absorbed by osmosis

24. Urinary system: Urine Formationcont’d. • Reabsorbed particles: • Nutrients reabsorbed • Glucose- 100% up to maximum allowed by carriers • As reabsorbed levels reach 2 mg/ml plasma, excess lost in urine • Not enough carriers available to pick it up • Amino acids, water, and required salts • Substances NOT reabsorbed: • Some water, nitrogenous wastes, excess salts

25. Urinary system: Urine Formation cont’d. • Tubular secretion – pumping of substances directly into the PCT and DCT. • Hydrogen ions, potassium, creatinine, many drugs • Done by active transport from capillaries surrounding the PCT and DCT. • Final product of urine contains: • Filtered substances that have not been reabsorbed • Substances that have been actively secreted

26. Urine Formationcont’d. • Fig. 16.5

27. Kidney Functions: Water Reabsorption • Excretion of hypertonic urine depends on reabsorption of water from collecting ducts • Requires • #1. Reabsorption of salt • #2. Establishment of solute gradient • #3. Water reabsorbed by osmosis down gradient

28. Kidney Functions: Water Reabsorptioncont’d • #1. Reabsorption of salt • Usually 99% of sodium filtered into the nephron is reabsorbed into the blood • Most of this happens in the PCT and loop of Henle: • 67% by PCT, 25% by the ascending loop of Henle, • The rest (only 8%) is done by the DCT and collecting duct

29. Regulatory functions of the kidneys cont’d. • #2. Establishment of a solute gradient • Long loop of Henle has 2 parts • Descending limb and ascending limb • Salt diffuses out of lower part of ascending limb • Upper part of ascending limb actively transports more salt out • Creates high osmotic pressure (high solute concentration) in medulla tissue • Urea contributes to high solute concentration in medulla • Leaks from lower collecting duct • End result- concentration gradient favoring reabsorption of water

30. Kidney Functions: Water Reabsorptioncont’d • #3. Reabsorption of water • Water leaves DCT because of a concentration gradient • Water also leaves descending loop of Henle • As filtrate enters collecting duct it is isotonic to cells of renal cortex • Same concentration as plasma • As filtrate passes down collecting duct it again encounters high concentration gradient • Permeability of collecting duct is under hormonal control, so it won’t always let water flow out into the medulla.

31. Reabsorption of water at the loop of Henle and collecting duct • Fig. 16.7

32. Kidney Functions: Water Reabsorption cont’d • Diuretics • Increase flow of urine • Examples include: • Alcohol • Shuts off ADH • Dehydration causes hangover • Caffeine • Increases glomerular filtration rate • Decreases tubular reabsorption of sodium • Diuretic drugs • Many inhibit active transport of sodium at loop of Henle or DCT

33. Kidney Functions: Acid-Base Balance cont’d • Kidneys in pH regulation • Only kidneys can remove many acids and bases • Slower acting than respiratory system but more powerful • Reabsorb bicarbonate ions to raise acidity. • Excrete hydrogen ions to lower acidity. • In urine, ammonia and phosphate can absorb hydrogen ions to reduce acidity, as well.

34. Urinary System Disorders • Diabetes, hypertension, inherited conditions • All cause progressive kidney disease • Diabetes (mellitus) involves the loss of glucose (blood sugar) in the urine. • Glucose is normally reabsorbed. • Urethritis • inflammation of urethra • Cystitis • inflammation of bladder • Kidney stones • Causes include UTI’s, enlarged prostate, pH imbalance, ingestion of too much calcium • Pyelonephritis • infection of kidneys • Signs of kidney disease • Albumin (protein) in urine • Uremia – buildup of nitrogenous wastes in the blood. • Blood in urine (hematuria)

35. Urinary System : Disorders • Hemodialysis • Includes artificial kidney machine or continuous ambulatory peritoneal dialysis (CAPD) • Dialysis involves… • Diffusion of dissolved molecules through a membrane • Selective permeability • Blood is cleansed • pH is adjusted • Water and salt balance maintained • In CAPD the lining of the abdomen is the dialysis membrane • Replacing a kidney • 97% success if from a relative

36. An artificial kidney • Fig. 16.9