human physiology of underwater diving human physiology of underwater diving human physiology of underwater diving is the physiological influences of the underwater environment on the human diver, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. it, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. all of these may affect diver performance and safety. for physiologiacl influences of the underwater environment on air-breathing vertebrates, see physiology of underwater diving. immersion affects fluid balance, circulation and work of breathing. exposure to cold water can result in the harmful cold shock response, the helpful diving reflex and excessive loss of body heat. breath-hold duration is limited by oxygen reserves, the response to raised carbon dioxide levels, and the risk of hypoxic blackout, which has a high associated risk of drowning. large or sudden changes in ambient pressure have the potential for injury known as barotrauma. breathing under pressure involves several effects. metabolically inactive gases are absorbed by the tissues and may have narcotic or other undesirable effects, and must be released slowly to avoid the formation of bubbles during decompression. metabolically active gases have a greater effect in proportion to their concentration, which is proportional to their partial pressure, which for contaminants is increased in proportion to absolute ambient pressure. work of breathing is increased by increased density of the breathing gas, artifacts of the breathing apparatus, and hydrostatic pressure variations due to posture in the water. the underwater environment also affects sensory input, which can impact on safety and the ability to function effectively at depth. relevance in diver education and training some basic knowledge of anatomy and physiology are necessary for understanding the effects of diving on the human body, the mechanisms of reasonably foreseeable injuries that may be incurred during diving activities, and the response that may be necessary in the event of such injuries. the physiology of diving is part of entry level training for professional divers, but may vary for recreational divers, as some certification agencies provide the minimum needed for the specific certification. the scope and level of detail may vary between training providers and certification agencies, and is usually at roughly secondary school level of detail. some of the commonly required subtopics are listed here. aspects of basic physiology necessary for an adequate understanding of the effects of diving on the human body and the mechanism of diving injuries. metabolism – set of chemical reactions in organisms organs – collection of tissues with similar functions tissues – group of cells having similar appearance and performing the same function circulatory system – organ system for circulating blood in animals blood – organic fluid which transports nutrients throughout the organism heart – organ found inside most animals atrial septal defect, also known as patent foramen ovale – human heart defect present at birth perfusion – passage of fluid through the circulatory or lymphatic system to an organ or tissue pulmonary circulation – part of the circulatory system which carries blood from heart to lungs and back to the heart pulmonary shunt – bypass of the pulmonary capillaries by deoxygenated blood systemic circulation – portion of the cardiovascular system which transports oxygenated blood away from the heart musculo-skeletal system – organ system that gives humans the ability to move by using their muscular and skeletal systems nervous system – part of an animal that coordinates actions and senses central nervous system – brain and spinal cord ear – organ of hearing and balance respiration (physiology) – exchange of gases between environment and tissues blood–air barrier – membrane separating alveolar air from blood in lung capillaries breathing – process of moving air in and out of the lungs control of ventilation – breathing control dead space (physiology) – inhaled air not part of gas exchange gas exchange – process by which gases diffuse through a biological membrane hypercapnia, also known as co2 retention – abnormally high tissue carbon dioxide levels hypocapnia – state of reduced carbon dioxide in the blood hypoxia – medical condition of lack of oxygen in the tissues laryngospasm – involuntary contraction of the vocal folds restricting inhalation respiratory exchange ratio – ratio between the metabolic production of carbon dioxide and the uptake of oxygen respiratory quotient – ratio of carbon dioxide produced by the body to oxygen consumed by the body respiratory system – biological system in animals and plants for gas exchange aspects of basic physiology necessary for sufficient understanding of first aid techniques appropriate for commercial and some recreational diver certification. (mostly the same systems, but with more practical detail as may be necessary for first aid) first aid – emergency first response medical treatment cardiopulmonary resuscitation – emergency procedure for cardiac arrest expired air resuscitation – artificial ventilation using exhaled air from the rescuer oxygen administration – provision of oxtgen for therapeutic purposes immersion