EXCRETION Flashcards
EXCRETION
The removal from organisms of toxic materials and substances in excess of requirements
Deamination
The removal of the nitrogen-containing part of amino acids to form urea
excretion
A number of organs in the body are involved in excretion. These include: • the liver, which forms urea through the breakdown of excess amino acids (see Chapter 7); • the lungs, which remove carbon dioxide from the blood (see Chapter 11); • the kidneys, which filter blood, removing urea, excess water and salts.
urinary stem diagram
kidneys
The liquid produced by the filtration of blood in the kidneys is called urine – a solution of urea and mineral salts in water. The relative amount of water reabsorbed depends on the state of hydration of the body (how much water is in the blood). The volume and concentration of urine produced are affected by:
- Temperature: on a hot day, when we sweat more to cool down, the body needs to conserve water, so a small amount of concentrated urine is produced. On a cold day, little sweat is produced, so we tend to produce a larger volume of dilute urine.
- Exercise: more water is lost from the body because of increased sweating, so less is excreted via the kidneys. The release of more sweat helps to maintain the normal body temperature.
- Water intake: the more water taken in, the more urine will be produced and it will become more dilute. If you drink less and other conditions do not change (exercise or external temperature), urine production will decrease and its concentration will increase.
role of liver
Surplus amino acids in the bloodstream cannot be stored. They are removed by the liver and some are assimilated by converting them into proteins. Examples of these include plasma proteins (e.g. fibrinogen in the blood; see Chapter 9). The surplus amino acids are broken down into urea (which is the nitrogen-containing part of the amino acid) and a sugar residue (which can be respired to release energy). The breakdown of amino acids is called deamination. Urea is returned to the bloodstream (into the hepatic vein) and filtered out when it reaches the kidneys.
structure of kidney
FUNCTION OF KIDNEY
The kidneys receive blood from the aorta (the main artery) via renal arteries. In the cortex, the renal artery splits into millions of capillaries. Each capillary forms a knotted glomerulus, from which the blood is filtered under pressure. This forces all the small molecules and ions (such as glucose, urea, water and mineral salts) out of the capillary into a tubule. As the filtrate passes through the tubule, reabsorption takes place. Water is reabsorbed by osmosis, while glucose and mineral salts pass back into the blood by diffusion and active uptake (see Chapter 3). The reduction in water content leads to an increase in the concentration of urea. The remaining solution, called urine, passes down the collecting duct of the tubule, through the pelvis of the kidney into the ureter, then down into the bladder for removal through the urethra.
dialysis
Dialysis is a method of removing one or more components from a solution using the process of diffusion. The solution is separated from a bathing liquid by a partially permeable membrane (made of cellulose). The bathing liquid contains none of the components that need to be removed from the solution, so these pass from the solution, through the membrane, into the bathing solution by diffusion. The bathing solution needs to be changed regularly to maintain a concentration gradient. The principle of dialysis is used in a kidney machine, as shown in Figure 13.6.
A patient with kidney failure needs to have toxic chemicals removed from the blood to stay alive. Blood is removed from a vein in the arm, and is kept moving through dialysis tubing in the dialysis machine using a pump. The tubing is very long to provide a large surface area. The dialysis fluid has a composition similar to blood plasma, but with no urea or uric acid. Urea, uric acid and excess mineral salts are removed from the blood, by diffusion, into the dialysis fluid. The cleaned blood is then passed through a bubble trap to remove any air bubbles, before being returned to the patient’s vein. In summary, the process of dialysis corrects the salt balance of the blood, maintains the glucose concentration and removes urea.
Advantages of kidney transplants:
- The patient can return to a normal lifestyle – dialysis may require a lengthy session in hospital, three times a week, leaving the patient very tired after each session.
- A dialysis machine will be available for other patients to use.
- Dialysis machines are expensive to buy and maintain
disadvantages of kidney transplant
- Transplants require a suitable donor with a good tissue match. The donor may be a dead person or a close living relative who is prepared to donate a healthy kidney (humans 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.
- Transplantation is not accepted by some religions.
