Excretion

Excretion is the process by which living things get rid of waste from their bodies. This is very important for staying healthy because if waste builds up, it can harm the body. In animals with a backbone, like humans, this process mainly happens in the lungs, kidneys, and skin.

For example, humans get rid of urine, which is a kind of waste made in the kidneys, through a tube called the urethra. Tiny single-celled organisms just release their waste right through their outer surface. Mammals, including humans, also get rid of solid waste called feces through the anus.

When our cells use energy in a process called cellular respiration, they create waste products like carbon dioxide, water, salts, urea, and uric acid. Getting these wastes out of the body is what we call excretion, and it is done by special organs known as the excretory system.

Processes across various types of life

Plants

In green plants, oxygen is made during photosynthesis and leaves the plant through tiny openings called stomata, as well as through root cell walls and other paths. Some plants also release materials like resin, saps, and latex. These are pushed out by pressure inside the plant and by the way plant cells absorb water. This happens without needing extra energy. When leaves fall off from deciduous plants, this is also a way they get rid of waste.

Animals

Animals get rid of waste materials such as carbon dioxide, ammonia, urea, uric acid, guanine, and creatine. The liver and kidneys help remove these from the blood, and they leave the body in urine and feces.

Aquatic animals often release ammonia directly into the water because it mixes easily and there is plenty of water to dilute it. On land, animals change ammonia-like compounds into materials like urea, which are less harmful since there is less water around and ammonia can be toxic. This process helps keep the body safe.

Birds

Birds get rid of nitrogen waste as uric acid in a paste form. This uses more energy but helps save water and makes it easier to store in the egg. Many seabirds can also remove salt through special glands in their noses, releasing it through their beak.

Insects

In insects, tiny tubes called Malpighian tubules help remove waste. The waste moves into these tubes and then goes to the intestines, where it leaves the body together with other materials.

The material that leaves the body may be called ejecta.