Respiration is a physical process in which living organisms take in oxygen from the surrounding medium and emit carbon dioxide. the term respiration is also used to refer to the liberation of energy, within a cell. from fuel molecules such as carbohydrates and fats. Carbon dioxide and water are the products of this process, which is sometimes called cellular respiration to distinguish it from the physical process of breathing.
Small organisms of the kingdoms Protista and Prokaryotae have no specialized respiratory mechanisms; instead, they rely on the diffusion of oxygen and carbon dioxide across a cell membrane. The concentration of oxygen in the organism is less than that of the surrounding air or water, and the concentration of carbon dioxide is greater. As a result, oxygen diffuses into the organism, and carbon dioxide diffuses out.
In aquatic lower animals that are more complex than sponges, a circulating medium carries the respiratory gases from outer tissues to cells that are distant from the sight of gas exchange.
These animals have gills, which auxiliary respiratory mechanisms keep a constant current of fresh water flowing. The gases are diffused through the epithelium of the gill, and the extended surface, produced by the branching of the gills, enables large quantities of blood to be oxygenated in a short time.
The respiratory and circulatory systems of air breathing animals have become adapted and modified for life in oxygen deficient environments. For example, people living in higher altitudes would have larger lungs than people living at lower altitudes. Humans respire through their lungs, as do most mammals and reptiles. Within the thorax, the lungs are held close to the body wall by atmospheric pressure. When the thorax expands, the lungs fill with air drawn through the upper respiratory passages. Relaxation of the muscles expanding...