Closed Glottis

 The human body can only go for a short amount of time without oxygen. In instances of drowning and near-drowning, the individual realizes that they may be drowning and becomes panicked, expending energy and rapidly consuming oxygen. Their body begins to accumulate carbon dioxide, which stimulates the body to feel the need to breathe. Eventually the individual will involuntarily draw breath. When water reaches the airway the first response is to cough or swallow, which results in more water being ingested. As water makes its way into the lower airways, the throat spasms, thing to close off the lungs. Eventually the individual loses consciousness, causing the throat to relax and allowing more water to flow into the lungs.