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

Water loss in the body

• Maintaining water levels in the body is vital to prevent harmful changes occurring to cells of the body as a result of osmosis

  • The cytoplasm of all cells is largely composed of water, as is the blood plasma

• 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

  • water in the diet

• Water is lost from the body in the following ways:

  • Water leaves the body via the lungs during exhalation

  • Water, ions and urea are lost from the skin in sweat

• The lungs and skin have no control over how much water, ions or urea is lost

• Loss of excess water, ions and urea is controlled by the kidneys when they filter the blood to produce urine

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Deamination

Higher Tier Only

• The digestion of proteins from the diet results in excess amino acids which need to be excreted safely

  • They cannot be stored by the body

• Excess amino acids are deaminated in the liver to form ammonia

• Ammonia is toxic to cells and so it is immediately converted to urea which is excreted by the kidneys

Structure & function of the kidney

• The kidneys help to control the water content of the body and the concentrations of substances (such as mineral ions) dissolved in the fluids of the body

• Kidneys filter the blood as follows:

  1. Blood flows into the kidneys at high pressure

  2. Small molecules are filtered out of the the blood into the kidney filtrate; these include glucose, urea, water and ions

     • Large molecules, such as proteins, are too large to pass through the filtration mechanism and remain in the blood

  3. The kidneys selectively reabsorb substances needed by the body

     • In a healthy kidney, this includes:

       • all of the glucose

       • some ions

       • some water

  4. Anything not reabsorbed forms urine, which is stored in the bladder until it is excreted

     • Urea is not selectively reabsorbed and so is excreted in the urine

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Interpreting information about kidney function

Control of water levels

Higher Tier Only

• The volume of water lost in urine is controlled in the kidneys

• This is achieved by regulating the volume of water that is reabsorbed by the kidney tubules before urine passes into the bladder

  • Kidney tubules are tiny tubes through which the filtrate flows inside the kidney

• When blood water level is low:

  • low blood water content is detected by the pituitary gland

  • the pituitary gland releases the hormone ADH

  • ADH travels in the blood to the kidneys where it increases the permeability of the kidney tubules to water

  • more water is reabsorbed into the blood

  • a smaller volume of concentrated urine is produced

• When blood water level is high:

  • high blood water content is detected by the pituitary gland

  • the pituitary gland releases less ADH

  • the permeability of the kidney tubules to water decreases

  • less water is reabsorbed into the blood

  • a higher volume of dilute urine is produced

• This is an example of negative feedback control

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Kidney failure

• 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

• Kidney failure can be treated by:

  • dialysis

  • kidney transplant

Dialysis

• 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

  1. Unfiltered blood is taken from an artery in the arm, and pumped into the dialysis machine

  2. Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane

     • 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

  3. Exchange of substances occurs between the blood and the fluid by diffusion

     • This exchange relies on concentration gradients

     • Urea diffuses from the blood (high concentration) into the dialysis fluid (no urea), down its concentration gradient

     • Protein molecules are too large to pass through the partially permeable membrane, so they stay in the blood

     • The fluid contains glucose and salts at normal blood levels, so useful amounts of these are not lost

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Kidney transplants

• Kidney transplants involve transferring a healthy kidney from a donor into the body of a recipient

• 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

  • A transplant is cheaper than dialysis in the long term (although there are still costs, such as lifelong immunosuppressant drugs)

  • A transplant can work for many years without regular hospital sessions, whereas dialysis has to be repeated several times a week, each session lasting hours (although a transplanted kidney may eventually need replacing)

  • A transplanted kidney keeps the concentrations of urea, water and ions in the blood constant (on dialysis, urea builds up between sessions), so there is less damage to body cells

  • The skin is not pierced repeatedly and the blood is not in contact with a machine, so there is less risk of infection and blood clots

• 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

  • Recipients must take immunosuppressant drugs for the rest of their lives, which can have long term side effects and leave the patient vulnerable to infections

  • There are not enough donors to cope with demand