Excretion of waste matter
The process of removing waste matter from the blood and from the body is called excretion.
Carbon dioxide is eliminated by the lungs into the air through breathing. The lungs also give off a considerable quantity of water as moisture in the breath. The lungs thus serve the dual purpose of taking up oxygen from the air into the blood, and of eliminating carbon dioxide from the blood to the air. This is the essential purpose of respiration or breathing.
Water, along with a small quantity of other waste substances, is eliminated by the skin in the form of sweat. The kidneys withdraw from one to one-and-a-half litres of water daily from the blood which contains dissolved mineral salts and several other important forms of waste matter (the most important being urea). These form the urine. The urine passes down from the kidneys to the bladder, from where it is expelled (fig 2.14).
Other waste matter is eliminated by the liver in the bile, and this together with the unabsorbed and undigested portions of the food is expelled from the body through evacuation of the bowels, as faeces or stools. The faeces also contain countless numbers of germs or bacteria, as well as a variety of viruses and yeast.