1. Chapter 16: Urinary System and Excretion

2. Urinary System Urinary Organs
The urinary system consists of the kidneys, ureters, urinary bladder, and urethra.
The bean-shaped kidneys are at the back of the abdominal wall beneath the peritoneum, protected by the lower rib cage.
The renal artery and renal vein along with ureters exit the kidney at the hilum.
The peritoneum is the lining of the abdominal cavity. Because of their location, the kidneys are somewhat prone to damage by blows to the back.

3. Two urethral sphincters control the release of urine.
The kidneys produce urine which is conducted by two muscular tubes called ureters to the urinary bladder where it is stored before being released through the urethra.
Two urethral sphincters control the release of urine.
In females, the urethra is 4 cm long; in males, the urethra is 20 cm long and conveys both urine and sperm during ejaculation.
Because the female urethra is short, it makes bacterial invasion of the urethra easier and consequently, females are more prone to urinary tract infections. The Health Focus (page 304) describes urinary tract infections.

4. The urinary system
Urine is found only within the kidneys, the ureters, the urinary bladder, and the urethra.

5. Urination
As the bladder fills with urine, sensory impulses travel to the spinal cord where motor nerve impulses return and cause the bladder to contract and sphincters to relax.
With maturation, the brain controls this reflex and delays urination, the release of urine, until a suitable time.

6. Urination
As the bladder fills with urine, sensory impulses go to the spinal cord and then to the brain. The brain can override the urge to urinate. When urination occurs, motor nerve impulses cause the bladder to contract and an internal sphincter to open. Nerve impulses also cause an external sphincter to open.

7. Functions of the Urinary System
Excretion refers to the elimination of metabolic wastes that were cell metabolites; this is the function of the urinary system.
Kidneys play a role in homeostasis of the blood by excreting metabolic wastes, and by maintaining the normal water-salt and acid-base balances of blood.
The metabolic wastes removed from the bloodstream by the kidneys are primarily those containing nitrogen.

8. Excretion of Metabolic Wastes
Kidneys excrete nitrogenous wastes, including urea, uric acid, and creatinine.
Urea is a by-product of amino acid metabolism.
The metabolic breakdown of creatine phosphate in muscles releases creatinine.
Uric acid is produced from breakdown of nucleotides.
Collection of uric acid in joints causes gout.

9. Maintenance of Water-Salt Balance
Kidneys maintain the water-salt balance of the body which, in turn, regulates blood pressure.
Salts, such as NaCl, in the blood cause osmosis into the blood; the more salts, the greater the blood volume and also blood pressure.
Kidneys also maintain correct levels of potassium, bicarbonate, and calcium ions in blood.

10. Maintenance of Acid-Base Balance
The kidneys regulate the acid-base balance of the blood.
Kidneys help keep the blood pH within normal limits by excreting hydrogen ions (H+) and reabsorbing bicarbonate ions (HCO3-) as needed.
Urine usually has a pH of 6 or lower because our diet often contains acidic foods.

11. Secretion of Hormones Kidneys secrete or activate several hormones:
They secrete the hormone erythropoietin to stimulate red blood cell production,
They activate vitamin D to the hormone calcitriol needed for calcium reabsorption during digestion, and
They release renin, a substance that leads to the secretion of aldosterone.
The kidneys release renin, a substance that leads to the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone promotes the reabsorption of sodium ions by the kidney.

12. Kidney Structure
The kidneys filter wastes from the blood, and thus the renal arteries branch extensively into smaller arteries and then arterioles inside each kidney.
Many venules unite to form small veins, which merge to become the renal vein.

13. Blood supply in a kidney
A longitudinal section of the kidney showing the blood supply. Note that the renal artery divides into smaller arteries, and these divide into arterioles. Venules join to form veins, which join to form the renal vein.

14. Microscopically, each contains over one million nephrons.
There are three regions to a kidney: an outer renal cortex, an inner renal medulla, and a central space called the renal pelvis.
Microscopically, each contains over one million nephrons.
The nephrons produce urine which flows into a collecting duct; several collecting ducts merge and drain urine into the renal pelvis.
A number of nephrons share the same collecting duct.

15. Gross anatomy of a kidney
This section of a kidney has the blood supply removed. The renal cortex, renal medulla, and renal pelvis are now evident. The ureter connects to the renal pelvis. The renal medulla consists of the renal pyramids. The enlargement shows the placement of nephrons.

16. Anatomy of a Nephron Each nephron has its own blood supply.
An afferent arteriole approaches the glomerular capsule and divides to become the glomerulus, a knot of capillaries.
The efferent arteriole leaves the capsule and branches into the peritubular capillary network.
Blood pressure is higher in the glomerulus because the efferent arteriole is narrower than the afferent arteriole.

17. Nephron anatomy
A nephron is made up of a glomerular capsule, the proximal convoluted tubule, the loop of the nephron, the distal convoluted tubule, and the collecting ducts. You can trace the path of blood about the nephron by following the arrows.

18. Parts of a Nephron
The closed end of the nephron is a cuplike glomerular capsule.
Spaces between podocytes of the glomerular capsule allow small molecules to enter the from the glomerulus via glomerular filtration.
The cuboidal epithelial cells of the proximal convoluted tubule have many mitochondria and microvilli to carry out active transport (following passive transport) from the tubule to blood.
Microvilli make up the inner brush border of the proximal convoluted tubule; these microvilli increase the surface area available for absorption.

19. Proximal convoluted tubule
The photomicrograph (left) shows that the cells lining the proximal convoluted tubule have a brushlike border composed of microvilli, which greatly increase the surface area exposed to the lumen. The peritubular capillary network surrounds the cells. The diagrammatic representation (right) shows that each cell has many mitochondria, which supply the energy needed for active transport, the process that moves molecules (green) from the lumen of the tubule to the peritubular capillary, as indicated by the arrows.

20. The descending loop of the nephron allows water to leave and the ascending portion extrudes salt.
The cuboidal epithelial cells of the distal convoluted tubule have numerous mitochondria but lack microvilli.
They carry out active transport from the blood to the tubule or tubular secretion.
Collecting ducts gather in the renal medulla and form the renal pyramids.
Each part of a nephron is anatomically suited to its specific function in urine formation.

21. Glomerular Filtration
Urine Formation
Glomerular Filtration
During glomerular filtration, small molecules including water, wastes, and nutrients are forced from the blood inside the glomerulus to the inside of the glomerular capsule.
Blood cells, platelets, and large proteins do not move across.
About 180 liters of water are filtered daily.
Urine formation is divided into three steps: glomerular filtration, tubular reabsorption, and tubular secretion.

22. Reabsorption from Nephrons
Substance
Amount Filtered
Amount Excreted
Reabsorp-tion (%)
Water, L
180
1.8
99.0
Sodium, g
630
3.2
99.5
Glucose, g
0.0
100.0
Urea, g 54
30.0
44.0
Amount filtered and amount excreted are amounts per day.

23. Tubular Reabsorption
During tubular reabsorption, certain nutrients, water and some urea moves from the proximal convoluted tubule into the blood of the peritubular capillary network.
Tubular reabsorption is a selective process because only molecules recognized by carrier molecules are actively reabsorbed.
The rate of this process is limited by the number of carriers.

24. Tubular Secretion
During tubular secretion, specific substances such as hydrogen ions, creatinine, and drugs such as penicillin move from the blood into the distal convoluted tubule.
In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, and substances that have undergone tubular secretion.

25. Steps in urine formation
The three main steps in urine formation are glomerular filtration, tubular reabsorption, and tubular secretion.
During glomerular filtration, water, salts, nutrient molecules, and waste molecules move from the glomerulus to the inside of the glomerular capsule. These small molecules are called the glomerular filtrate.
During tubular reabsorption, nutrient and salt molecules are actively reabsorbed from the proximal convoluted tubule into the peritubular capillary network, and water flows passively.
During tubular secretion, certain molecules are actively secreted from the peritubular capillary network into the distal convoluted tubule.
In the end, urine is composed of the substances within the collecting duct (water, salts, urea, uric acid, ammonia, and creatinine.

26. Maintaining Water-Salt Balance
The kidneys maintain the water-salt balance of the blood within normal limits.
By doing so, they also maintain blood volume and blood pressure.
Most of the water and salt (NaCl) present in the filtrate is reabsorbed across the wall of the proximal convoluted tubule.

27. Reabsorption of Water
Salt passively diffuses out of the lower portion of the ascending limb of the loop; the upper thick portion actively extrudes salt into the tissue of the outer renal medulla.
Water is reabsorbed by osmosis from all parts of the tubule.
The ascending limb of loop of the nephron establishes an osmotic gradient that draws water from the descending limb of the nephron and the collecting duct.

28. The permeability of the collecting duct is under the control of antidiuretic hormone (ADH).
Diuresis is an increase in urine flow and antidiuresis is a decrease.
When ADH is present, more water is reabsorbed, blood volume and blood pressure rise, and there is a decreased amount of urine.
If there is insufficient water intake, the posterior pituitary releases ADH, causing more water to be reabsorbed with a decreased urine output.

29. Reabsorption of water
Salt (NaCl) diffuses and is actively transported out of the ascending limb of the loop of the nephron into the renal medulla; also, urea is believed to leak from the collecting duct and to enter the tissues of the renal medulla. This creates a hypertonic environment, which draws water out of the descending limb and the collecting duct. This water is returned to the cardiovascular system. (The thick black line means the ascending limb is impermeable to water.)

30. Reabsorption of Salt
Kidneys regulate salt balance by controlling excretion and reabsorption of ions.
Two hormones, aldosterone and atrial natriuretic hormone (ANH), control the kidneys’ reabsorption of sodium (Na).
When the juxtaglomerular apparatus detects low blood volume, it secretes renin that eventually results in the adrenal cortex releasing aldosterone that restores blood volume and pressure through reabsorption of sodium ions.
Renin is an enzyme that changes angiotensinogen from the liver into angiotensin I. Later, angiotensin I is converted to angiotension II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone.

31. Juxtaglomerular apparatus
This drawing shows that the afferent arteriole and the distal convoluted tubule usually lie next to each other. The juxtaglomerular apparatus occurs where they touch. The juxtaglomerular apparatus secretes renin, a substance that leads to the release of aldosterone by the adrenal cortex. Reabsorption of sodium ions and then water now occurs. Therefore, blood volume and blood pressure increase.

32. Reabsorption of salt increases blood volume and pressure because more water is also reabsorbed.
ANH is secreted by the atria of the heart when cardiac cells are stretched by increased blood volume.
ANH inhibits secretion of renin; the resulting excretion of sodium also causes excretion of water and blood volume drops.

33. Diuretics
Diuretics are chemicals that lower blood pressure by increasing urine output.
Alcohol inhibits secretion of ADH; dehydration after drinking may contribute to the effects of a hangover.
Caffeine increases the glomerular filtration rate and decreases tubular reabsorption of sodium.
Diuretic drugs inhibit active transport of Na+ so a decrease in water reabsorption follows.

34. Maintaining Acid-Base Balance
Kidneys rid the body of acidic and basic substances.
If the blood is acidic, hydrogen ions (H+) are excreted and bicarbonate ions (HCO3-) are reabsorbed.
If the blood is basic, H+ are not excreted and HCO3- are not reabsorbed.
Breathing also ties up H+ when carbon dioxide is exhaled.
Hydrogen ions combine with ammonia (NH3), thus ammonia provides a means for buffering hydrogen ions in urine.

35. Acid-base balance
In the kidneys, bicarbonate ions (HCO3-) are reabsorbed and hydrogen ions (H+) are excreted as needed to maintain the pH of the blood. Excess hydrogen ions are buffered, for example, by ammonia (NH3), which is produced in tubule cells by the deamination of amino acids.

36. Chapter Summary
The urinary system has organs specialized to produce, store, and rid the body of urine.
Kidneys excrete nitrogenous wastes and maintain the water-salt and the acid-base balance of the blood within normal limits.

37. Kidneys have a macroscopic anatomy and a microscopic anatomy.
Urine is produced by many microscopic tubules called nephrons.
Urine formation is a multistep process.
Kidneys are under hormonal control as they regulate the water-salt balance of blood.
Kidneys excrete hydrogen ions and reabsorb bicarbonate ions to regulate the pH of blood.