Breathing is involuntary in that we cannot prevent the ventilation of the lungs. However, the lungs are not truly spontaneous and require inervation from the respiratory center of the brain to function properly. When CO 2 levels are high, the diaphragm is stimulated to contract. However, if CO 2 levels become too low, then smooth muscles in the blood vessels contract and blood is shunted away from the brain. This is what happens during hyperventilation.
Apnea is a condition in which the ventilaiton of the lungs is suspended for a short period of time.
Fish have gills , which allow them to extract oxygen from water using a unidirectional flow of water across the respiratory surface. The efficiency of fish gills at extracting oxygen is very high, much higher than that of terrestrial animals. Some fish need to swim continuously to survive. The tracheal system in insects is an elaborate network of tubes and passageways into the body tissues that allows direct delivery of oxygen. The tracheal system allows insects to achieve extrordinarily high rates of metabolism.