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Physiological response to water immersion
Immersion of the human body in water has several physiological effects, some of these are directly due to immersion, and are caused by the external hydrostatic pressure of the water providing support against the internal hydrostatic pressure of the blood. Other effects are due to water temperature and heat transfer.
Immersion in water affects the circulation, renal system and fluid balance, breathing, and thermal balance.
Immersion in cold water is causatively associated with drowning and hypothermia.
Plasma fluid losses due to immersion diuresis occur within a short period of immersion. Head-out immersion causes a blood shift from the limbs and into the thorax. The fluid shift is largely from the extravascular tissues and the increased atrial volume results in a compensatory diuresis. Plasma volume, stroke volume, and cardiac output remain higher than normal during immersion. The increased respiratory and cardiac workload causes increased blood flow to the cardiac and respiratory muscles. Stroke volume is not greatly affected by immersion or variation in ambient pressure, but bradycardia reduces the overall cardiac output, particularly due to the diving reflex in breath-hold diving.
Renal and water balance responses
In hydrated subjects immersion will cause diuresis and excretion of sodium and potassium. Diuresis is reduced in dehydrated subjects, and in trained athletes in comparison with sedentary subjects.
Snorkel breathing is limited to shallow depths just below the surface due to the effort required during inhalation to overcome the hydrostatic pressure on the chest. Hydrostatic pressure on the surface of the body due to head out immersion in water causes negative pressure breathing which shifts blood into the intrathoracic circulation.
Lung volume decreases in the upright position due to cranial displacement of the abdomen due to hydrostatic pressure, and resistance to air flow in the airways increases significantly because of the decrease in lung volume.
Hydrostatic pressure differences between the interior of the lung and the breathing gas delivery, increased breathing gas density due to ambient pressure, and increased flow resistance due to higher breathing rates may all cause increased work of breathing and fatigue of the respiratory muscles.
There appears to be a connection between pulmonary edema and increased pulmonary blood flow and pressure which results in capillary engorgement. This may occur during higher intensity exercise while immersed or submersed.
Facial immersion at the time of initiating breath-hold is a necessary factor for maximising the mammalian diving reflex in humans.
Thermal balance responses
The unprotected human body responds to cold water immersion in a progression from a stress situation to hypothermia and death, at a rate depending on time and water temperature. Hypothermia is not the major problem in the early stages of exposure as other stresses are more immediately life-threatening.
Cold shock response is the initial reaction to immersion in cold water. It generally starts with a gasp reflex in response to sudden and rapid chilling of the skin, and if the head is immersed there is a risk of inhaling water and drowning. This is followed by a reflexive hyperventilation, with a risk of panic and fainting if not controlled. Cold induced vasoconstriction causes the heart to work harder and the additional work can overload a weak heart, with a possible consequence of cardiac arrest. Cold incapacitation is the next stage, and generally occurs within 5 to 15 minutes in cold water. Blood flow to the extremities is reduced by vasoconstriction as the body attempts to reduce heat loss from the vital organs of the core. This accelerates the cooling of the periphery, and reduces the functionality of the muscles and nerves. The duration of exposure to produce hypothermia varies with health, body mass and water temperature. It generally takes in the order of 30 minutes for an unprotected person in water to become hypothermic.