13) Excretion in humans

13) Excretion in humans


Excretion is the removal of the following substances:

  • toxic materials
  • waste products of metabolism
  • excess substances from organisms


Excess amino acids are deaminated in the liver to form glycogen and urea. The urea is removed from the tissues by the blood and expelled by the kidneys.


Excretory organs:

  • Liver – Breaks down excess amino acids and produces urea.
  • Lungs – Get rid of CO2 and H2O when breathing out.
  • Kidneys – Removes urea and other nitrogenous waste from the blood, and expel excess water, salts. Hormones and drugs.
  • Skin – Loses water, salt, urea but not an excretory organ.


The liver and its role in producing proteins:

  • Plays an important role in assimilation (absorbing) amino acids.
  • Removes amino acids from the plasma of the bloodstream and builds them up into proteins.
  • Proteins are long chains of amino acids, joined together by peptide bonds.


Deamination: is the removal of the nitrogen-containing part of amino acids to form urea.


The kidneys:

  • Urine is taken from the kidneys to the bladder by the ureters.
  • Urethra is the tube that carries urine out of the body.


The need for excretion:

  • Some of the compounds made in reactions in the body are potentially toxic if their concentrations build up.
  • CO2 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 acid 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.


Microscopic structure of the kidneys:

  • The kidney tissue consists of many capillaries and tiny tubes, called renal tubules, held together with connective tissue.
  • The cortex is the dark, outer region.
  • The medulla is the lighter, inner zone.
  • A renal artery carries blood to the kidney and a renal vein carries it away.
  • The ureter carries urine from the kidney to the bladder.
  • Where the ureter joins the kidney there is a space called the pelvis.


  • The renal artery divides up into a great many arterioles and capillaries, mostly in the cortex.
  • Each arteriole leads to a glomerulus. This is a capillary repeatedly divided and coiled, making a knot of vessels.
  • Each glomerulus is almost entirely surrounded by a cup-shaped organ called a renal capsule, which leads to a coiled renal tubule.
  • This tubule, after a series of coils and loops, joins a collecting duct, which passes through the medulla to open into the pelvis.
  • A nephron is a single glomerulus with its renal capsule, renal tubule and blood capillaries.


Function of the kidneys:

  • The blood pressure in a glomerulus causes part of the blood plasma to leak through the capillary walls.
  • The red blood cells and the plasma proteins are too big to pass out of the capillary, so the fluid that does filter through is plasma without the protein.
  • The fluid thus consists mainly of water with dissolved salts, glucose, urea and uric acid.
  • The process by which the fluid is filtered out of the body by the glomerulus is called ultrafiltration.
  • The filtrate from the glomerulus collects in the renal capsule and trickles down the renal tubule.
  • As it does so, the capillaries that surround the tubule absorb back into the blood these substances which the body needs.
  • First, all the glucose is reabsorbed, with much of the water.
  • Then some of the salts are taken back to keep the correct concentration in the blood.
  • The process of absorbing back the substances needed by the body is called selective reabsorption.
  • The molecules which are not selectively reabsorbed continue along the nephron tubule as urine .


The dialysis machine (artificial kidney):

Dialysis is a treatment that filters and purifies the blood using a machine

Dialysis fluid contains:

  • a glucose concentration similar to a normal level in the blood
  • a concentration of ions similar to that found in normal blood plasma
  • no urea


As the dialysis fluid has no urea in it, there is a large concentration gradient – meaning that urea moves across the partially permeable membrane, from the blood to the dialysis fluid, by diffusion.


As the dialysis fluid contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient exists.


As the dialysis fluid contains an ion concentration similar to the ideal blood plasma concentration, movement of ions across the membrane only occurs where there is an imbalance.

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