The Kidneys

• Water can leave the human body in three ways: through the lungs when we exhale, through the skin when we sweat, and through the kidneys in urine
• Role of the kidney in maintaining the concentration of substances in the blood

  1. Filter out small molecules like urea, ions, and water from the blood

  2. Reabsorb some of the filtered glucose, ions, and water back into the blood

  3. Excrete excess urea, ions, and water as urine
• Osmosis


  Diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• The body needs to maintain a constant level of water in the blood
• If the blood becomes too dilute


  Water moves into cells by osmosis
• If the blood becomes too concentrated


  Water moves out of cells by osmosis
• The body cannot control water loss through the lungs or through sweating
• The body can control how much water is lost in urine
• Kidneys


  • Remove excess water, urea, and ions from the blood
  • Maintain the concentration of substances in the blood
• How the kidneys remove waste and adjust levels of substances in the blood

  1. Blood passes through capillaries in the kidney

  2. Small molecules like urea, ions, and water are filtered out of the blood

  3. Some of the filtered glucose, ions, and water are reabsorbed back into the blood

  4. Excess urea, ions, and water are excreted as urine
• The concentration of glucose in the blood does not change as it is all reabsorbed by the kidneys
• The concentration of ions in the blood decreases as the kidneys filter them out, but then reabsorb some back in
• The concentration of urea in the blood falls to virtually zero as the kidneys filter it all out and do not reabsorb it
• Deamination


  The process where the liver breaks down excess amino acids and produces ammonia
• Ammonia is a toxic chemical, so the liver converts it to urea which can be safely excreted by the kidneys