The Kidneys (pg. 42 & 43)

• Excretion is the removal of waste products:

  • Carbon dioxide is a waste product from the lungs.
  • Sweat is a waste product from the skin.
• The kidneys are part of the urinary system. They perform 3 main roles:

  1. Removal of urea from the blood. Urea is produced in the liver from excess amino acids.
  2. Adjustment of ion (salt) levels in the blood.
  3. Adjustment of water content in the blood.
• Each kidney contains thousands of nephrons, which are filtration units. When blood passes through them 3 processes take place:

  1. Ultrafiltration.
  2. Reabsorption.
  3. Release of wastes.
• Ultrafiltration:

  1. Blood from the renal artery flows through the glomerulus - a bundle of capillaries at the start of the nephron.
  2. A high pressure is built which squeezes water, urea, ions and glucose out of the blood and into the Bowman's capsule.
  3. The membranes between the blood vessels of the glomerulus and the Bowman's capsule act like filters, so big molecules like proteins and blood cells stay in the blood.
• The filtered liquid in the Bowman's capsule is known as the glomerular filtrate.
• Reabsorption:

  1. All the glucose is reabsorbed from the proximal convoluted tubule so that it can be used in respiration. The reabsorption of glucose involves the process of active transport against the concentration gradient.
  2. Sufficient ions are reabsorbed. Excess ions aren't.
  3. Sufficient water is reabsorbed from the collecting duct into the bloodstream by osmosis.
• Release of wastes:

  The remaining substances (including water, urea and ions) form urine. This continues out of the nephron, through the ureter and down to the bladder, where it is stored before being released via the urethra.
• Water is lost from the body in 3 main ways:

  1. Sweating.
  2. Breathing.
  3. Urinating.
• Osmoregulation: the body has to constantly balance the water coming in against the water going out.
• One way osmoregulation happens is by adjusting the water that is excreted by the kidneys in the urine. When the kidneys reabsorb more water, the urine has a smaller volume and is more concentrated.
• The volume of water reabsorbed in the kidney nephrons is controlled by a hormone called anti-diuretic hormone (ADH).
• ADH makes the collecting ducts of the nephrons more permeable so more water is reabsorbed back into the blood.
• The brain monitors the water content of the blood and instructs the pituitary gland to release ADH into the blood according to how much is needed.
• The whole process of osmoregulation is controlled by a mechanism called negative feedback. This means that if the water content gets too high or too low a mechanism will be triggered that brings it back to normal.