The Urinary System

1. Chapter 18 The Urinary System

2. The urinary system – consists of two kidneys, two ureters, the urinary bladder, and the urethra Function of the kidneys – to form urine to excrete waste products and to regulate the volume, electrolytes, and pH of the blood and tissue fluid. The other organs of the urinary system –

3. Kidneys Location: retroperitoneal on either side of the backbone, partially protected by – The renal fascia and adipose tissue – Hilus – an indentation on the medial side. The ureter emerges from the hilus, as does the renal vein; the renal artery enters it Question: Name another major organ with a hilus; does it have the same anatomic purpose?

4. Answer Each lung has a hilus, an indentation on the medial side. The anatomic purpose is the same. For the lung, the pulmonary artery and the primary bronchus enter at the hilus, and the pulmonary veins emerge.

5. Kidney – internal structure – a kidney has two layers of tissue and a medial cavity Renal cortex – the outer tissue layer, made of renal corpuscles and convoluted tubules. Renal medulla (pyramids) – the inner tissue layer, made of loops of Henle and collecting tubules – Renal pelvis – a cavity formed by the expanded end of the ureter within the kidney. The extensions are calyces:

6. The nephron – the functional unit of the kidney; consists of a renal corpuscle and a renal tubule Renal corpuscle – consists of a glomerulus surrounded by Bowman’s capsule. Glomerulus – a capillary network that extends from an afferent arteriole to an efferent arteriole.

7. The nephron (continued) Bowman’s capsule – the end of a renal tubule, encloses a glomerulus. Outer layer – not permeable. Inner layer – made of podocytes –

8. The nephron (continued) Renal tubule: Proximal convoluted tubule Loop of Henle Distal convoluted tubule Collecting tubule The collecting tubules unite – Peritubular capillaries – arise from the efferent arteriole, surround all parts of the renal tubule.

9. Blood vessels of the kidney (in sequence) Abdominal aorta Renal artery Interlobar arteries Arcuate arteries Interlobular arteries Afferent arterioles Glomeruli Efferent arterioles Peritubular capillaries Interlobular veins Arcuate veins Interlobar veins Renal vein Question: Into which vein do the two renal veins empty?

10. Answer The two renal veins empty into the inferior vena cava.

11. Blood vessels of the kidney (continued) Essential for the formation of urine from blood plasma are capillaries. The blood vessels of the kidney have two sets of capillaries. Questions: Name the two sets of capillaries. What are they sites for (that is, what happens in capillaries)?

12. Answers The two sets of capillaries are the glomeruli and the peritubular capillaries. These capillaries are the sites of exchanges between the blood and surrounding tissues.

13. Formation of urine The formation of urine involves three major processes: Glomerular filtration Tubular reabsorption Tubular secretion

14. Formation of urine (continued) Glomerular filtration – takes place from the glomerulus to Bowman’s capsule. Blood pressure in the glomerulus is relatively high: The podocyte layer of Bowman’s capsule is very permeable. The fluid in Bowman’s capsule is called renal filtrate. In the filtrate – Remaining in the blood –

15. Formation of urine (continued) Glomerular filtration is not selective in terms of usefulness – Glomerular filtration rate (GFR) – 100 to 125 mL per minute. Changes in blood flow through the kidney will change the GFR:

16. Formation of urine (continued) Tubular reabsorption – takes place from the filtrate in the renal tubules to the blood in the peritubular capillaries – useful materials in the renal filtrate are returned to the blood. Approximately 99% of the filtrate will be reabsorbed. Most reabsorption takes place in the proximal convoluted tubules. Question: The cells of the proximal convoluted tubules have a membrane modification that increases their surface area. Name this membrane modification.

17. Answer The cells of the proximal convoluted tubules have microvilli that increase the surface area of the cell membranes and make reabsorption more efficient.

18. Formation of urine (continued) Mechanisms of reabsorption: Active transport – Passive transport – Osmosis – Pinocytosis –

19. Formation of urine (continued) Tubular secretion – takes place from the blood in the peritubular capillaries to the filtrate in the renal tubules – unwanted materials can be excreted in urine. Creatinine – Metabolites of medications – H+ ions –

20. Formation of urine (continued) Hormones that affect reabsorption (see Table 18–1): Aldosterone – Atrial natriuretic peptide – Antidiuretic hormone – Parathyroid hormone –

21. The kidneys and acid–base balance – the kidneys have the greatest capacity to compensate for pH changes If body fluids are becoming too acidic – The kidneys will excrete: The kidneys will retain: If body fluids are becoming too alkaline – The kidneys will excrete: The kidneys will retain:

22. Other functions of the kidneys Secretion of renin by juxtaglomerular cells. Stimulus: blood pressure decreases. Renin begins the renin-angiotensin mechanism (see Table 18–2) Angiotensin II – Secretion of erythropoietin Stimulus: hypoxia Question: What is the function of erythropoietin?

23. Answer Erythropoietin increases the rate of red blood cell production in the red bone marrow. Question: What vitamin is activated by the kidneys?

24. Answer Vitamin D is converted from inactive forms to the active form in the kidneys.

25. Elimination of urine – the function of the ureters, urinary bladder, and urethra Ureters – each extends from the hilus of a kidney to the lower, posterior side of the urinary bladder. Urine is kept moving toward the bladder by waves of peristalsis –

26. Elimination of urine (continued) Urinary bladder – a reservoir for temporary storage of urine; below the peritoneum, behind the pubic bones. In women: In men: Mucosa – transitional epithelium – Rugae – Trigone –

27. Elimination of urine (continued) Urinary bladder – detrusor muscle – a spherical muscle that forms the wall of the bladder. Contraction – Internal urethral sphincter – formed by detrusor muscle fibers around the opening of the urethra. Contraction – Relaxation –

28. Elimination of urine (continued) Urethra – takes urine from the bladder to the exterior In women: 1 to 1.5 inches in length In men: 7 to 8 inches in length: Prostatic urethra Membranous urethra Cavernous urethra External urethral sphincter – the skeletal muscle of the pelvic floor:

29. Elimination of urine – urination reflex Stimulus: stretching of the detrusor muscle – Sensory impulses ? spinal cord Motor impulses (parasympathetic) from spinal cord ? detrusor muscle Effectors: Detrusor muscle – Internal urethral sphincter – External urethral sphincter –

30. Characteristics of urine (see Table 18–3) Amount per 24 hours: 1 to 2 liters Factors that cause oliguria – Factors that cause polyuria – Color – the typical yellow color is called “straw” or “amber.”

31. Characteristics of urine (continued) Specific gravity: a measure of the dissolved materials in urine – 1.010 to 1.025. The specific gravity of distilled water is 1.000. The higher the specific gravity – The lower the specific gravity – Specific gravity is an indicator of the concentrating ability of the kidneys –

32. Characteristics of urine (continued) pH – the pH range of urine is 4.6 to 8.0, with an average pH of 6.0. A vegetarian diet – A high-protein diet – Composition of urine – 95% water, 5% salts and waste products.

33. Characteristics of urine (continued) Nitrogenous wastes excreted in urine: Urea – Creatinine – Uric acid –

34. Wrap-Up Question Name the part or aspect of the urinary system described. 1. Form the inner layer of Bowman’s capsule 2. Hormone that increases excretion of K+ ions 3. Increase surface area in proximal convoluted tubule 4. Vessels around the renal tubule 5. Secrete renin when BP decreases 6. Capillaries where renal filtration takes place 7. Reservoir for urine 8. Water is reabsorbed

35. Answers 1. Form the inner layer of Bowman’s capsule – podocytes 2. Hormone that increases excretion of K+ ions – aldosterone 3. Increase surface area in proximal convoluted tubule – microvilli 4. Vessels around the renal tubule – peritubular capillaries 5. Secrete renin when BP decreases – juxtaglomerular cells 6. Capillaries where renal filtration takes place – glomeruli 7. Reservoir for urine – urinary bladder 8. Water is reabsorbed – osmosis