Excretion and the Formation of Urea

Why water balance matters to cells

Cells are surrounded by body fluids, including tissue fluid. Their cell membranes are partially permeable, so water can move between the cells and the surrounding fluid by osmosis. Osmosis is the net movement of water from a more dilute solution to a more concentrated solution through a partially permeable membrane.

The concentrations on the two sides of the membrane determine the direction of movement:

• If the surrounding body fluid becomes too concentrated, it contains relatively less water. Water moves out of cells by osmosis, so the cells shrink.
• If the surrounding body fluid becomes too dilute, it contains relatively more water. Water moves into cells by osmosis, so the cells swell.

A small amount of movement is normal, but cells that gain or lose too much water do not function efficiently. Their internal conditions change, disrupting the chemical reactions needed for life. Maintaining suitable concentrations of water and dissolved ions in body fluids therefore helps maintain suitable conditions inside every cell.

Water and waste continually leave the body

The body gains water from food and drink, but it also loses water continuously. Different substances leave by different routes:

Air becomes moist as it passes through the respiratory system, so breathing out removes water vapour. The body cannot adjust this loss to correct its water balance because breathing must continue for gas exchange.

Sweat contains water, dissolved ions and a small amount of urea. Sweating is important for cooling the body, but these losses are not adjusted according to the body's need to conserve water, ions or urea. In other words, sweat production may change to control temperature, but it is not a mechanism for controlling the concentrations of these substances.

The losses through the lungs and skin are therefore described as uncontrolled in the context of water, ion and urea balance. The kidneys provide the regulated route: they remove excess water and ions, along with urea, in urine. The detailed processes by which kidneys adjust urine are considered separately.

Excretion removes products that could become harmful

Excretion is the removal of metabolic waste from the body. Metabolic wastes are made by chemical reactions in cells. Urea is one such waste, so removing it in urine is excretion.

This is different from egestion, which is the removal of undigested material from the digestive system. Faeces are mainly material that was never absorbed into the body's cells, whereas urea was produced by metabolism inside the body.

Higher Tier: from excess amino acids to urea

Dietary proteins are digested into amino acids. Cells use amino acids to make the proteins needed for growth and repair, but excess amino acids cannot simply be stored as amino acids. Their nitrogen-containing part must be removed safely.

This processing happens in the liver. The liver removes the amino group from an excess amino acid in a process called deamination. This produces ammonia:

$$\text{excess amino acids} \longrightarrow \text{ammonia}$$

Ammonia is toxic, so allowing it to accumulate would damage cells. The liver therefore converts it immediately into urea, which is safer to transport for excretion:

$$\text{ammonia} \longrightarrow \text{urea}$$

The complete Higher Tier sequence is:

$$\text{dietary protein} \longrightarrow \text{amino acids} \longrightarrow \text{deamination in the liver} \longrightarrow \text{ammonia} \longrightarrow \text{urea}$$

Urea then travels dissolved in the blood to the kidneys. The kidneys remove it from the blood, and it leaves the body in urine. A smaller amount is also lost in sweat. This shows how the digestive system, liver, circulatory system and kidneys work together: digestion supplies amino acids, the liver converts their excess nitrogen into a safer waste, the blood transports that waste, and the kidneys excrete it.