Leaving Certificate Biology Practice Test

Excretion is the removal of metabolic wastes from the body. As cells carry out metabolism, they produce waste products that can be toxic if they build up. The excretory systems—kidneys, liver, lungs, and skin—work to dispose of these wastes: urea and other nitrogenous wastes from the breakdown of proteins, excess salts and water, and gases like carbon dioxide from respiration. Removing these wastes helps keep the internal environment stable, including proper fluid balance and pH. Oxygen is something the body uses to release energy, not a waste to be expelled. Hormones are signalling molecules released by glands in a controlled way, not wastes in the sense of excretory waste, though some can be eliminated in urine to regulate body chemistry. Nutrients are absorbed and used by the body rather than being wastes. So the term that describes what is removed by excretion is metabolic wastes.