The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The urinary system eliminates wastes, controls blood volume, regulates blood ion concentration and pH, and regulates red blood cell production.

Functions of the Urinary System

• The kidneys excrete waste products.

• The kidneys control blood volume by regulating the volume of urine produced.

• The kidneys help regulate the concentration of major ions in the body fluids.

• The kidneys help regulate pH of the body fluids.

• The kidneys regulate the concentration of erythrocytes in the blood.

• The kidneys participate, with the skin and liver, in vitamin D synthesis.

Urinary System

Kidneys

• Each kidney is behind the peritoneum and surrounded by a renal capsule and a renal fat pad.

• The kidney is divided into an outer cortex and an inner medulla.

• Each renal pyramid has a base that extends into the cortex; the tip extends into the medulla and is surrounded by a calyx.

• Calyces are extensions of the renal pelvis, which is the expanded end of the ureter within the renal sinus.

• The functional unit of the kidney is the nephron. The parts of a nephron are the renal corpuscle, the proximal tubule, the loop of Henle, and the distal tubule.

• The filtration membrane is formed by the glomerular capillaries, the basement membrane, and the podocytes of Bowman's capsule.

Arteries and Veins

• Renal arteries give rise to branches that lead to afferent arterioles.

• Afferent arterioles supply the glomeruli.

• Efferent arterioles carry blood from the glomeruli to the peritubular capillaries.

• Blood from the peritubular capillaries flows to the renal veins.

Ureters, Urinary Bladder, and Urethra

• Ureters carry urine from the renal pelvis to the urinary bladder.

• The urethra carries urine from the urinary bladder to the outside of the body.

• The ureters and urinary bladder are lined with transitional epithelium and have smooth muscle in their walls.

• The internal and external urinary sphincter muscles regulate the flow of urine through the urethra.

Urine Production

• Urine is produced by the processes of filtration, reabsorption, and secretion.

Filtration

• The renal filtrate passes from the glomerulus into Bowman's capsule and contains no blood cells and few blood proteins.

• Filtration pressure is responsible for filtrate formation.

Reabsorption

• About 99% of the filtrate volume is reabsorbed; 1% becomes urine.

• Proteins; amino acids; glucose; fructose; and sodium, potassium, calcium, bicarbonate, and chloride ions are among the substances reabsorbed.

• About 80% of the volume is reabsorbed in the proximal tubule and descending limb of the loop of Henle. About 19% is reabsorbed in the distal tubule and collecting duct.

Secretion

• Hydrogen ions, some by-products of metabolism, and some drugs are actively secreted into the nephron.

Regulation of Urine Concentration and Volume

Hormonal Mechanisms

• ADH is secreted from the posterior pituitary when the concentration of blood increases or when blood pressure decreases. ADH increases the permeability to water of the distal convoluted tubule and collecting duct. It increases water reabsorption by the kidney.

• Renin is secreted from the kidney when the blood pressure decreases or when the concentration of sodium ions decreases in the blood. Renin converts angiotensinogen to angiotensin I which is then converted to angiotensin II by angiotensin-converting enzyme. Angiotensin II stimulates aldosterone secretion, and aldosterone increases the rate of sodium chloride reabsorption from the nephron.

• Atrial natriuretic hormone, secreted from the right atrium in response to increases in blood pressure, acts on the kidney to increase sodium and water loss in the urine.

Effect of Sympathetic Innervation on Kidney Function

• Increased sympathetic activity decreases blood flow to the kidney, decreases filtrate formation, and decreases urine formation.

Urine Movement

• Increased volume in the urinary bladder stretches its wall and activates the micturition reflex.

• Parasympathetic impulses cause contraction of the urinary bladder and relaxation of the internal urinary sphincter. Reduced somatic action potentials cause relaxation of the external urinary sphincter.

• Higher brain centers control the micturition reflex. Stretch of the urinary bladder stimulates ascending neurons that carry impulses to the brain and inform the brain of the need to urinate.

Body Fluid Compartments

• Water and ions dissolved in it are distributed in the intracellular and extracellular fluid compartments.

• Approximately 60% of the total body water is found within cells.

• Approximately 40% of the total body water is found outside cells, mainly in interstitial fluid, plasma of blood, and lymph.

Composition of the Fluid in the Body Fluid Compartments

• Intracellular fluid contains more potassium, calcium, magnesium, phosphate, sulfate ions, and protein than extracellular fluid.

• Extracellular fluid contains more sodium, chloride, and bicarbonate ions than intracellular fluid.

Exchange Between Body Fluid Compartments

• Water moves between compartments continually in response to hydrostatic pressure differences and osmotic differences between the compartments.

Regulation of Extracellular Fluid Composition

• The total amount of water and electrolytes in the body does not change unless the person is growing, gaining weight, or losing weight.

Thirst

• The sensation of thirst increases if extracellular fluid becomes more concentrated or if blood pressure decreases.

Ions

• Sodium ions are one of the dominant extracellular ions. Aldosterone increases sodium reabsorption from the filtrate. ADH increases water reabsorption from the nephron, and atrial natriuretic hormone increases sodium loss in the urine.

• Aldosterone increases potassium secretion in the urine. Increased blood levels of potassium stimulate, and decreased blood levels of potassium inhibit, aldosterone secretion.

• Parathyroid hormone secreted from the parathyroid glands increases extracellular calcium levels by causing bone resorption and increased calcium uptake in the kidney and small intestine. Parathyroid hormone increases vitamin D synthesis. Calcitonin, secreted by the thyroid gland, inhibits bone resorption and lowers blood calcium levels when they are too high.

• When phosphate and sulfate levels in the filtrate are low, nearly all phosphate and sulfate ions are reabsorbed. When levels are high, excess is lost in the urine.

Regulation of Acid-Base Balance

Buffers

• Three principal classes of buffers in the circulatory system resist changes in the pH: protein, phosphate, and bicarbonate buffers.

Respiratory System

• The respiratory system regulates pH. It responds rapidly. An increased respiratory rate raises the pH because the rate of carbon dioxide elimination is increased, and a reduced respiratory rate reduces the pH because the rate of carbon dioxide elimination is reduced.

Kidneys

• The kidneys excrete hydrogen ions in response to a decreasing blood pH, and they reabsorb hydrogen ions in response to an increasing blood pH.

Acidosis and Alkalosis

• Acidosis occurs when the pH of the blood falls below 7.35. The two major types are respiratory acidosis and metabolic acidosis.

• Alkalosis occurs when the pH of the blood increases above 7.45. The two major types are respiratory alkalosis and metabolic alkalosis.