Control of nitrogen and water levels

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This topic is for students studying the full biology course only

Water, ions and urea

If body cells lose or absorb too much water by osmosis, or if ion levels in the blood are too high or too low, they cannot function efficiently. The body also produces urea, a waste product from the breakdown of proteins that contains nitrogen, that needs to be safely removed.

The body loses these substances in various ways, including:

- - Through the lungs during exhalation.
  - From the skin in sweat, along with ions and urea.

There is no control over the water, ions, or urea lost through the lungs and skin.

Protein Digestion and Urea Formation (Higher Tier)

When proteins from food are digested, they release amino acids. Excess amino acids are processed by the liver, where they are deaminated to produce ammonia, a toxic substance. Ammonia is quickly converted into urea, which is then safely excreted by the kidneys.

The role of the kidneys

The kidneys act as natural filters, removing waste products and excess substances from the blood, including urea, some ions, and water, which are excreted in urine. In doing this, the kidneys help maintain a stable internal environment, a process crucial for homeostasis.

The kidneys produce urine by filtering blood. Almost all water, ions, glucose and waste urea is forced out of the blood, then useful substances (like glucose, some ions, and water) are selectively reabsorbed back into the bloodstream according to how much the body needs.

Regulation of water by ADH (Higher tier only)

The water level in the body is regulated by the hormone ADH (Anti-Diuretic Hormone):

When the blood becomes too concentrated, the pituitary gland releases ADH.
ADH increases the permeability of the kidney tubules, allowing more water to be reabsorbed into the blood.
This process is managed by negative feedback, ensuring the right water balance is restored.

Kidney failure and treatment

When the kidneys fail, waste substances build up in the blood, and the body loses its ability to control water and ion levels, which can be life-threatening. There are two main treatments:

Dialysis:
- A machine performs the function of the kidneys by filtering the blood to remove dissolved substances and wastes.
- Dialysis uses a special partially permeable membrane to filter out waste products while retaining necessary molecules like glucose.
- Dialysis must be done regularly, often 3-4 hours per session, several times a week. It can be an uncomfortable and time-consuming treatment.
Kidney Transplant:
- A kidney transplant replaces the damaged kidney with a healthy one from a donor.
- Transplants offer a long-term solution, but there is a risk of rejection. The immune system may attack the transplanted organ, so patients often need to take immunosuppressant drugs.

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