Drowning is suffocation and death resulting from the lungs being filled with fluids or other substances. The term generally refers to death from submersion in water, but in certain illnesses fluid can fill the lungs because of the ineffective action of the heart. This edema fluid can cause drowning as readily as does water introduced into the lungs from outside. Normally the lungs are filled with air, to which blood is exposed in a multitude of tiny sacs called alveoli. The oxygen-carrying pigment hemoglobin, contained in red blood cells, picks up oxygen from the air and distributes it to the tissues of the body. When water or any other fluid or solid enters the lungs, however, it covers the alveoli and prevents diffusion of oxygen, so that the body tissues are starved and die. Because brain cells are the most active cells in the body, the brain is the organ most vulnerable to the loss of oxygen; thus one of the first steps in the drowning process is the loss of consciousness. If resuscitation is not applied promptly (see cardiopulmonary resuscitation), the brain will completely lose function. Application of resuscitation 20 to 30 minutes after drowning can result in resumption of heart function without recovery of the brain.
Under certain circumstances, however, a person can be immersed for 30 minutes or more before rescue and still regain complete consciousness following resuscitation. This is thought to be due to what is called the diver's reflex, in which a sudden decrease in the blood's oxygen content causes the body to slow down metabolism in order to conserve oxygen and to shunt most of the available oxygenated blood to the brain. The diver's reflex is strongest in infants and decreases with maturation but has been observed in older persons in cold-weather conditions.