Excretion in Humans

• Urea


  Formed in the liver as a result of an excess of amino acids. Amino acids cannot be stored in the body, so excess amino acids are removed from the body in a process called de-amination. De-amination takes place in the liver and involves the removal of the nitrogen-containing section of the amino acids, forming urea. Urea is then filtered from the blood by the kidney and excreted from the body as urine.
• Carbon dioxide

  A waste product produced in cells during respiration. Carbon dioxide exits cells and is dissolved in the blood, where it is carried to the lungs to be excreted.
• Excess water and salt


  Filtered from the blood by the kidney and excreted as urine.
• Kidney


  • Filters waste and excess substances from the blood to be excreted from the body as urine. It is important for these substances to be filtered out of the blood, as a build-up of toxins could cause harm to the organism.
• Water intake, temperature and exercise

  Affect the volume of urine produced
• Renal system


  1. Blood enters the kidney through the renal artery and exits through the renal vein

  2. Cortex - the outer region of the kidney, containing nephrons

  3. Nephron - the functional unit of the kidney, containing the glomerulus where blood is filtered

  4. Tubule - glucose, most water and some salts are reabsorbed into the blood, while urea is not reabsorbed

  5. Medulla - the inner region of the kidney

  6. Ureter - tubes that transport urine from the kidneys to the bladder
• Dialysis


  When the kidneys are not working, a dialysis machine is used to filter the blood. Blood is taken from the arm and travels through the dialysis machine, where it flows past a dialysis fluid containing glucose and ions. The blood and fluid are separated by a partially permeable membrane, allowing diffusion between the blood and fluid. Urea exits the blood into the dialysis fluid by diffusion, as the fluid contains no urea. The levels of glucose and ions in the dialysis fluid are similar to that of the blood, so there is usually no net diffusion of ions or glucose across the membrane, unless the blood is lacking in ions.
• Dialysis


  • Must be used regularly, typically 3 times a week for hours at a time, making it difficult to work and carry out a normal lifestyle. Lowers quality of life.
• Kidney transplant


  An alternative to constant dialysis. Although it comes with the risks associated with major surgery and the risk of rejection, a successful kidney transplant can raise the quality of life of the patient and patients are not required to undergo hours of dialysis. Most humans are born with two kidneys, although they can survive with one, thus kidney transplants are from family members, as there is less chance of rejection.
• Why is urea formed?


  Amino acids can’t be stored in the body