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Excretion in Humans

  • Deamination is the removal of the nitrogen-containing part of amino acids to form urea.
  • Urea is formed in the liver from excess amino acids
  • The kidneys excrete urea and excess amino acids
  • Carbon-dioxide is excreted through the lungs

The Role of the Liver in the Assimilation of Amino Acids by Converting them to Proteins

  • As well as being an excretory organ, the liver plays a very important role in the assimilation of amino acids.
  • Assimilation means the absorption of substances, which are then built into other compounds in the organism
  • The liver removes amino acids from the plasma of the bloodstream and builds them up into proteins.
  • These include plasma proteins such as fibrinogen

Factors that Affect the Volume and Concentration of Urine Produced

Water Intake, Temperature, and Exercise

  • The kidneys adjust the amount of urine that they produce according to the needs of the body.
  • If a person has lost lots of water by sweating during exercise, the kidney produces a small volume of concentrated urine
  • If the body contains too much water, the kidney produces a large volume of dilute urine
The Need for Excretion
  • Some of the compounds made in reactions in the body are potentially toxic (poisonous) if their concentrations build up
  • Carbon-dioxide dissolves in fluids, such as tissue fluid and blood plasma, to form carbonic acid
  • This increase in acidity can affect the actions of enzymes and can be fatal
  • Ammonia is made in the liver when excess amino acids are broken down
  • However, ammonia is very alkaline and toxic.
  • It is converted to urea which is much less poisonous, making it a safe way of excreting excess nitrogen
The Kidney

The Structure of the Kidney (Photo Credit: LumenLearning.com)

The Structure of a Kidney Tubule(Photo Credit: TopperLearning.com)


  • The main force responsible for moving substances by filtration through the glomerular capillary wall is the hydrostatic pressure of the blood
  • The high pressure forces small molecules such as water, glucose, amino acids, sodium chloride, and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman’s capsule into the nephron
  • The ultrafiltrate is collected in the urinary space of the Bowman’s capsule and then flows downstream, in turn the proximal tubule, the loop of Henle, the distal convoluted tubules, and a series of collecting ducts where tubular activity alters its composition and volume

Tubular Reabsorption

  • Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood
  • Most of the reabsorption occurs in the proximal convoluted tubule, where cells possess microvilli with carrier proteins


  • The dialysis membrane is partially permeable so minerals, salts, ions, urea move by diffusion from a high concentration to a low concentration gradient
  • Water moves by osmosis from a high water potential to a lower water potential across the membrane
  • Proteins and blood cells are too large to cross the membrane
  • Glucose is not removed by dialysate
  • Dialysate contains glucose, so glucose diffuses until the blood is at a correct concentration
  • Fresh dialysate maintains a concentration gradient

The Use of Dialysis in Kidney Machines

Photo Credit: Biology.IGSCE.Weebly.com

  • It provides liquid conditions to allow substances transfer
  • It maintains the concentration gradient so only urea and a little salt will diffuse out while not allowing glucose and other valuable substances to be diffused out

The Advantages and Disadvantages of Kidney Transplants Compared with Dialysis


  • The patient can return to a normal lifestyle
  • Dialysis may require a lengthy session in a hospital, three times a week, leaving the patient very tired after each session
  • The dialysis machine will be available for other patients to use


  • Transplants require a suitable donor with a good tissue match. The donor may be from a just dead person, or from a close living relative who is prepared to donate a healthy kidney (we can survive with one kidney)
  • The operation is very expensive
  • There is a risk of rejection of the donated kidney
  • Immunosuppressive drugs have to be used
  • Transplants are not accepted by some religions