Whenever I have the chance of opening my eyes underwater without the benefits of wearing some form of eyewear, say goggles or diving masks, the first question that inadvertably materialise inside my head is, more often than not, “what am I going to do to prevent myself from being drowned” rather than that “will I drown”, which incidentally is a thought that I do not really relish much.
Fortunately, each human being has self preservation instincts which are genetically programmed into the body at birth. These instincts serve to protect the human body automatically without conscious effort and intervention in times of peril. In cases where a human body, particularly the face, is unwittingly subjected to a medium foreign to the natural environment, say a large body of water, the mammalian diving reflex kicks in.
As the name suggests, the mammalian diving reflex is is found in all mammals, but especially in marine mammals like whales and seals. In the case of the human being, this reflex puts the human body into a sort of energy saving mode whose aim is to maximize the time that the human body can be spent under water. For marine mammals, this diving reflex is obviously rather important since they live the greater part of their lives in frigid water itself, especially the whales. For mammels like human beings, the reflex is presumably triggered so that the person could indulge in swimming extensive distances for the purpose of exercise, or perhaps to do something useful in an effort to prevent himself from drowning after suffering shock from his inability to climatise fast enough with the foreign environment he is in because he just has to open his eyes under water.
The mammalian diving reflex is generally larger in cold water than in warm water, and includes three factors:
1. Bradycardia, a reduction in the heart rate of up to 50% in human beings;
2. Peripheral vasoconstriction, the restriction of the blood flow to the extremities to increase the blood and oxygen supply to the vital organs, especially the brain;
3. Blood shift, the shifting of blood to the thoracic cavity, which refers to the chest between the diaphragm and the neck to avoid the collapsing of the lungs under higher pressure during deeper dives.
In addition to the above, the same reflex also prevents an unconscious person from actually breathing underwater. As a result of the above, both a conscious and an unconscious person can survive longer without oxygen under water than in a comparable situation on dry land. In this case, the consious person will then have some time to ponder over the question “what am I going to do to prevent myself from being drowned”. As for the unconscious person, he will just have to find some way to wake up himself, otherwise, he should be pondering over the question “will I drown”.