Maintaining Water & Nitrogen Balance in the Body (AQA GCSE Biology)

• Maintaining water levels in the body is vital to prevent harmful changes occurring to cells of the body as a result of osmosis
• If body cells lose or gain too much water by osmosis they do not function efficiently:
  • Too much water in the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting)
  • Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death

• There are two sources of water in the body: water produced as a result of aerobic respiration and water in the diet
• The cytoplasm of all cells is largely composed of water, as is the blood plasma
• Water is lost from the body in the following ways:
  • Water leaves the body via the lungs during exhalation (breathing out)
  • Water, ions and urea are lost from the skin in sweat.

• However, the lungs and skin have no control over how much water, ion or urea is lost via exhalation or sweating
• Controlled loss of excess water, ions and urea is controlled by the kidneys when they filter the blood to produce urine

Organs of the human body involved in excretion of water, ions and urea

The excretory system table

Higher tier only

• The digestion of proteins from the diet results in excess amino acids which need to be excreted safely, as they cannot be stored by the body in the same way that excess glucose can
• Deamination is the process of breaking down excess protein and it predominantly occurs in the liver
• Enzymes in the liver split up amino acid molecules, with the part containing carbon turned into glycogen and the other part containing nitrogen (the amino part) turned into ammonia (this is why we say the amino acid has been deaminated)
• Ammonia is toxic to cells and so it is immediately converted to urea which can be transported around the body via the blood safely for excretion by the kidneys

Deamination of an amino acid results in the formation of urea from the amino part, which contains nitrogen

• The kidneys help to control the water content of the body and the concentrations of substances (such as sodium and potassium ions) dissolved in the fluids of the body
• The kidney contains highly branched capillary networks that form filters which contain pores with an average radium of about 3 nanometers
• When blood passes through the kidneys, the pressure it is under increases as it is pushed into the filters
• This high-pressure mass flow forces molecules that are small enough to pass through the pores out of the bloodstream – this is called filtration
• Substances forced out of the blood include glucose, urea and water with ions dissolved in it. The liquid formed is called filtrate

• Larger molecules (such as RBCs or proteins) are too big to pass out of the filter and so remain in the blood plasma
• The kidneys then selectively reabsorb substances needed by the body back into the bloodstream (this is an active process)
  • In a healthy kidney, this includes all of the glucose and some ions from the filtrate

• Anything not reabsorbed forms urine, which is then stored in the bladder until it is excreted
• Urea, formed from the deamination of amino acids in the liver, is not selectively reabsorbed
• The concentration of urea in the urine is far higher than that of the blood plasma
  • Reabsorption of water from the filtrate back into the bloodstream is why the concentration of urea in the filtrate is so much higher

The kidneys produce urine when they filter the blood

Higher tier only

• Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys
• The kidneys contain structures called tubules which filtrate passes through on its way to the bladder
• Water reabsorption occurs along these tubules; if the water content of the blood is too high then less water is reabsorbed, if it is too low then more water is reabsorbed

• The pituitary gland in the brain constantly releases a hormone called ADH; how much ADH is released depends on how much water the kidneys should reabsorb from the filtrate
• ADH, therefore, affects the permeability of the tubules to water
  • If the water content of the blood is too high, the pituitary gland releases less ADH which leads to less water being reabsorbed in the tubules of the kidney (the tubules become less permeable to water)
  • If the water content of the blood is too low and the blood is too concentrated, the pituitary gland releases more ADH which leads to more water being reabsorbed in the tubules of the kidney (the tubules become more permeable to water)

• The control of water reabsorption by the tubules is another example of negative feedback

In the control of water levels, the brain detects if the level is too high or low and modulates how much ADH the pituitary gland releases

• The kidneys might not work properly for several reasons, including accidents or disease
• Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body which will be fatal if not removed

People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis

• Dialysis is the usual treatment for someone with kidney failure
• Patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and salt balance of the blood
• Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then returned to a vein in the arm
• Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane – the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists
• Dialysis fluid contains:
  • A glucose concentration similar to a normal level in blood
  • A concentration of salts similar to a normal level in blood
  • No urea

Dialysis is an artificial method of filtering the blood to remove toxins and excess substances

• Kidney transplants are a better long term solution to kidney failure than dialysis; however, there are several disadvantages to kidney transplants, including:
• Donors won’t have the same antigens on cell surfaces so there will be some immune response to the new kidney (risk of rejection is reduced - but not removed – by ‘tissue typing’ the donor and the recipient first)
• This has to be suppressed by taking immunosuppressant drugs for the rest of their lives – these can have long term side effects and leave the patient vulnerable to infections
• There are not enough donors to cope with demand
• However, if a healthy, close matched kidney is available, then the benefits of a transplant over dialysis include:
  • The patient has much more freedom as they are not tied to having dialysis several times a week in one place
  • Their diets can be much less restrictive than they are when on dialysis
  • Use of dialysis machines is very expensive and so this cost is removed
  • A kidney transplant is a long term solution whereas dialysis will only work for a limited time