Respiration is the breakdown of nutrient compounds into energy. While respiration without the presence of oxygen leaves relatively intact large energy-containing molecules such as lactic acid and ethyl alcohol, respiration in the presence of oxygen make possible the complete oxidation of nutrient compounds into carbon dioxide and water. In fact, ninety percent of the energy yield from the respiratory breakdown of nutrients depends on the existence of aerobic conditions. As the human body requires large amounts of energy to carry out its life activities, humans, as do most organisms, depend on aerobic respiration. The respiratory system carries out the functions of oxygen procurement and carbon dioxide elimination that are so necessary in maintaining aerobic respiration.
In order to perform these two functions, the human respiratory system must meet the four basic needs common to all aerobic respiratory systems: 1) It must provide a large enough respiratory surface to take in enough oxygen for all the cells of an individual; 2) it must provide a method of transporting gases between the area of exchange with the outside environment and the internal cells; 3) it must protect the exchange surface from mechanical injury; and 4) it must keep the exchange surface moist so that gases can diffuse into solution and be absorbed by the body. In humans, the lungs are the organs which meet these needs. The two lungs are invaginated gas-exchange organs. Their highly subdivided inner surface has a surface area of over one hundred square meters (over fifty times the surface area of the body) and is highly
