• skin - perspiration
• lungs- removes CO2 and water
• liver - secretes bile pigments
• kidneys - produce urine

• urea
• creatinine
• uric acid

• To maintain salt-water balance
• To maintain pH
• Secretion of hormones
• Reabsorption of nutrients and convert vitamin D

• by-product of amino acid metabolism
• urea is less toxic than ammonia

• From breakdown of nucleotides
• Insoluble
• can form crystals in urine

• pressure/glomerular filtration
• selective reabsorption into bloodstream
• tubular secretion
• concentration by re-absorption of water

Nutrients and salt molecules are actively reabsorbed into the blood from the convoluted tubules. Water & ions flow passively into the blood from the convoluted tubes by osmosis.

When blood (pH decreases) becomes acidic the kidneys reabsorb HCO₃^- and excrete H⁺

When the blood gets too basic (pH increases) the kidneys excrete HCO₃^- and absorb H⁺.

• Secreted by the posterior pituitary gland
• Increased ADH increases the permeability of collecting duct to water so that more water is reabsorbed into the blood.
  • decreased amount of urine that is more concentrated
  • Blood pressure and volume rises
• Decreased ADH decreases permeability of collecting duct to water so that less water is reabsorbed into the blood.
  • Increased amount of urine
  • Blood pressure and volume decreases

• Produced by the adrenal cortex
• Causes reabsorption of Na⁺ and water into the blood and excretion of K⁺ in the distal convulated tubule.
• Increases blood volume and blood pressure.

1. When blood pressure & blood volume in kidneys gets too low for filtration kidneys secret renin.
2. Renin promotes the secretion of aldosterone by the adrenal glands and increases the blood pressure and blood volume.

1. Renin enzymatically changes angiotensinogen to angiotensin I.
2. Angiotensin I travels to the lungs where it is enzymatically converted to angiotensin II.
3. Angiotensin II is a vasoconstricotor, blood pressure rises. And it stimulates the adrenal cortex to produce aldosterone which raises blood pressure as well.

• Secreted by the atria of the heart when there is increased blood volume.
• Inhibits the secretion of renin and aldosterone and thus lowers the blood volume and pressure.
• Promotes excretion of Na⁺ which creates  an osmotic gradient that causes water to be reabsorbed as well à blood pressure falls.

• infectious agent usually e. coli
• urethritus: infections of the urethra
• cystitis: infections of the bladder
• pyelonephritis: infections of the kidneys themselves

• decrease in glomerular filtration with decreased urine output (kidney stones, glomerulo-nephritus)
• detected when albumin (blood cells) appear in the urine