Baroreceptor: a sensory receptor that responds to pressure
Eupnea: normal relaxed breathing
Vasoconstriction: a decrease in diameter of blood vessels
As freedivers reach deeper and deeper depths a number of physiological questions have been raised concerning gas exchange, gas composition and the energetics of deep freediving. During a typical no-limits descent, gas exchange at the alveolar and tissue level may be influenced by a number of factors. Research has shown for example, that during immersion there is an increase in cardiac output and a re-distribution of venous blood volume and peripheral blood flow. As the freediver descends deeper, the partial pressures of gases in the lungs increase due to the increased compression. Because the freediver performs a breath-hold at a large lung volume with relaxed respiratory muscles there is an increase in intrathoracic pressure that results in a reduction of cardiac output and intrathoracic blood volume. Essentially, what this means is that in the descending freediver there is a translocation of body gas stores. The time-course of these gases changing places has implications for the freediver since oxygen is required not only for a successful ascent to the surface but also to maintain various metabolic functions during the dive. A redistribution of venous blood is important for example, since this blood is the largest component of the exchangeable tissue oxygen stores. One of the ways this blood is redistributed as the diver dives deeper is a vasoconstriction of the blood vessels, a mechanism that has been observed in animals: a reflex mechanism that causes few problems during a typical descent. During the ascent however, there are a different set of problems.
During ascent alveolar gases become greatly altered and their composition shortly before the surface is reached reveals not only the degree of hypoxia and hemoglobin desaturation that has occurred, but also to what degree motor and mental performances have been impaired. Freedivers are aware that excessive hyperventilation prior to a dive leads to typical signs of hypocapnia, namely tingling sensations in the extremities, dizziness, and occasionally spasms. They are also aware of mental confusion when carbon dioxide concentrations reach a high level. The gradually developing hypoxia during breath-hold is not easily recognized, and on occasion a brief state of euphoria may precede blackout and loss of consciousness due to lack of oxygen.
The effects of ascent. Two primary effects experienced by an ascending freediver are hypocapnia and alkalosis, both of which have an effect on the respiratory, cardiovascular and central nervous systems.
Respiratory System. The effect of hypocapnia on the respiratory system is primarily on the blood buffer system. Seventy percent of the carbon dioxide present in the blood is carried as a bicarbonate ion. The overall reaction for bicarbonate formation occurs as follows;
CO2 + H2O ??H2O CO3 ?? H+ + HCO3
The major influence determining the direction in which the reaction proceeds is the concentration, or partial pressure of carbon dioxide. When the carbon dioxide levels in the blood increase, the reaction proceeds to the right, toward the formation of greater hydrogen and bicarbonate ions. When the carbon dioxide level decreases, the reaction reverses toward the formation of carbon dioxide and water. When an freediver moves into increasingly into a state of hypocapnia, the excessive elimination of carbon dioxide causes a reduction in hydrogen ion concentration that is too rapid for the blood buffer system to replace. The pH is elevated and a respiratory alkalosis ensues.
Cardiovascular System. It is generally agreed that the onset of hypocapnia causes tachycardia, increased cardiac output and reduced systemic vascular resistance and mean arterial blood pressure. Other measurable effects include a vasoconstriction of cerebral blood vessels, vasodilation of systemic blood vessels and reduced blood flow. The combined effects of systemic vasodilation and cerebral vasoconstriction cause a restriction in blood flow to the brain. This shift increases the capacity of blood to on-load oxygen on the lung level but restricts off-loading at the tissue level. The combined effect of restricted blood flow and increased oxygen-binding results in stagnant hypoxia at the brain, which ultimately leads to unconsciousness (usually just below the surface).
Central Nervous System. As the freediver moves into an increasing state of hypocapnia the consequent elevated pH causes an increased sensitivity and irritability of neuromuscular tissue. This increase manifests itself by a tingling and numbness of the extremities and mouth, and possible muscular spasms. The hands and feet may exhibit carpopedal spasm, a fixation of the hand in which the fingers are flexed toward the wrist or a marked plantar flexion of the ankle. Muscle spasm usually occurs when the arterial carbon dioxide tension has been excessively reduced. In more severe hypocapnia, the whole body becomes stiff (a condition referred to as tetany) due to contraction of skeletal muscle.
Signs and Symptoms of Hypocapnia. The objective signs of hypocapnia most often observed in a freediver are:
(a)Muscle twitching and tightness. (b) Paleness. (c) Muscle spasms. (d) Rigidity. (e) Unconsciousness.
The subjective symptoms, i.e: those experienced by the freediver include:
(a) Dizziness. (b) Light-headedness. (c) Tingling. (d) Numbness. (e) Muscular incoordination. (f) Visual disturbance.