This section is intended to provide the diver with some basic knowledge as to how the body reacts to physiological stresses imposed under water, and how to compensate for these stresses, and to physical limitations. The diver should study the text and become familiar with the terminology necessary to understand and describe any symptoms or physical dysfunctions experienced. Table 2-1 contains the definitions of commonly used diving medical terms.

2.1 CIRCULATION AND RESPIRATION

The activity of each cell of the body consists of a variety of delicate reactions which can only take place under well-defined chemical and physicochemical conditions. The chief function of the circulatory system is to maintain conditions around the cells that are optimum for their activity. The regulation of cardiac output and the distribution of the blood are central problems of the physiology of circulation.

Respiration is the process whereby an appropriate interchange of gases, oxygen and carbon dioxide, occurs between the tissues and the atmosphere. During respiration, air enters and leaves the lungs via the nose or mouth-the pharynx- the larynxthe trachea and the bronchial tubes. The bronchial tubes enter the lungs and divide and re-divide into a branching network-ending in the terminal air sacs and alveoli. The alveoli are surrounded by a thin membrane. The interchange of gases takes place across this membrane where the blood in the tiny pulmonary capillaries takes up oxygen and gives off carbon dioxide. This process is shown schematically in Figure 2-1.

Preliminary to a study of diving physiology, it is necessary to acquire a rudimentary grasp of

circulation and respiration and an acquaintance with certain problems associated with the aircontaining compartments of the body as they are affected by pressure changes experienced during diving.

2.1.1 Circulatory System

The heart is divided longitudinally into the right and left hearts, each consisting of two communicating chambers, the auricles and ventricles. Blood is pumped by the right ventricle into the pulmonary artery, through the pulmonary capillaries, and back to the left heart through the pulmonary veins. The left heart pumps the blood into the aortic artery which distributes it to the various bodily organs. This distribution is accomplished by a continual branching of arteries which become smaller and smaller until they become capillaries. The capillaries have a thin wall through which the interchange of substances between blood and tissue takes place. The blood from the capillaries flows into venules, then into the veins and is returned to the heart. In this way, carbon dioxide produced in the tissues is removed, transported to the lungs and discharged. This process is shown schematically in Figure 2-2.

During exercise, there is an increase in frequency and force of the heart beat as well as a constriction of the vessels of the skin, alimentary canal and quiescent muscle. Peripheral resistance is increased and arterial pressure rises. Blood is expelled from the spleen, liver, skin, and other organs which also augments the inflow, thus the output of the heart. This augmented output is distributed mainly to the organs where vessels are least constricted or actually dilated-namely, the brain, the heart, and any active muscles. Thus, the body responds to exercise by sending additional blood (oxygen) to those areas most actively involved.

2.1.2 Mechanism of External Respiration

The chest wall encloses a cavity, the volume of which is altered by rhythmic contraction and relaxation of muscles. This thoracic cavity contains the lungs which are connected with outside air through the bronchi, the trachea, and the upper respiratory passages (See Figure 2-1). When a change is made in the volume of the thoracic cavity, a decrease or increase in pressure occurs within the internal chambers and passages of the lungs. Air is thereby caused to flow into or out of the lungs through the respiratory passageways until the pressure everywhere in the lungs is equalized to the external pressure. This equilibrium is upset when the chest wall is again moved and the lungs assume a new volume. Respiratory ventilation takes place by rhythmic changes of this sort. It is affected by muscular action of the diaphragm and chest

wall under control of the nervous system which itself is responding to changes in blood oxygen and carbon dioxide levels. The normal respiratory rate at rest varies from about 12 to 16 times a minute. During and following heavy exertion, this rate may be increased severalfold.

In the normal resting position of the chest wall, that is, at the end of natural expiration, the lungs contain about 2.5 liters of air. Even when one voluntarily expels all the air possible, there still remains about 1.5 liters of residual air. The volume of air that is inspired and expired during rest is referred to as tidal air and averages about 0.5 liters per cycle. The additional volume (beyond the resting expiratory position of 2.5 liters) which can be taken in during a maximal inspiration, varies greatly from individual to individual but ranges from about 2 to 6 liters. The total breathable volume of air, called the vital capacity, depends upon the

Figure 2-2

The Circulatory System

size, development, and physical condition of the individual. Vital capacity is defined as the maximal volume that can be expired after maximal inspiration. A reduction in vital capacity limits the ability to respond adequately to a demand for increased ventilation during exercise. Because diving can require strenuous exercise, cardiovascular or respiratory disorders may seriously limit or prevent an individual from actively participating.

2.1.2.1 Pulmonary Ventilation

Air drawn into the lungs is distributed through smaller and smaller air passages until it reaches the honeycomb-like alveoli or air sacs through which the exchange of respiratory gases takes place (See Figure 2-1).

The rates at which oxygen may be supplied and

carbon dioxide removed from the lungs depend upon several factors: (1) the composition and volume of the air supplied through the respiratory passages, (2) the partial pressures of respiratory gases in the blood, and (3) the duration of exposure of a given volume of blood to alveolar air. In a normal individual in good physical condition other factors influencing respiratory exchange are not likely to be significant.

At rest, about 0.5 liters of oxygen are utilized by the tissues per minute. During exercise a maximal exchange of about 3.5 liters or more of oxygen per minute may take place. The flexibility of the ventilatory system is accomplished by increased movement of the chest, and by increased heart action propelling blood through the pulmonary capillaries, and by increased differences in partial pressures

of oxygen and carbon dioxide during exercise. Figure 2-3 depicts oxygen consumption as a function of work rate. Normally, despite wide differences in rates of gaseous exchange between resting and heavy exercise conditions, the blood leaving the lungs is almost completely saturated with oxygen.

2.1.2.2 Blood Transport of Oxygen and Carbon Dioxide

Blood can take up a much greater quantity of oxygen and carbon dioxide than can be carried in simple solution. Hemoglobin, which is the principal constituent in red blood cells and gives the red color to blood, has a chemical property of combining with oxygen and with carbon dioxide and carbon monoxide. The oxygen-carrying capacity of the blood is increased by virtue of its normal hemoglobin content by a factor of about 50. The reaction between oxygen and hemoglobin is governed primarily by the partial pressure of oxygen. At sea level where there is normally an oxygen partial pressure of 150 millimeters of mercury the alveolar hemoglobin becomes about 98 percent saturated in terms of its capacity to form oxy-hemoglobin. In the tissues where the partial pressure of oxygen is normally about 20 millimeters of mercury, more than half of this oxygen is given up by hemoglobin and made available to the tissues. It is apparent that persons lacking a sufficiency of hemoglobin, i.e., anemic persons, will be deficient in their capacity to carry oxygen. Hence, they will be less fit as divers.

Hemoglobin will also combine readily with carbon monoxide which may be present in contaminated air (See Paragraph 2.1.3.4).

The blood contains a small amount of carbon dioxide in simple solution, but a greater amount is found in chemical combinations such as carbonic acid, bicarbonate, or combinations thereof. All the forms of carbon dioxide tend toward chemical equilibrium with each other. Of some advantage with respect to the transport of respiratory gases is the fact that the taking up of oxygen by hemoglobin in the lung capillaries actually favors the unloading of carbon dioxide, at the same time, while the absorption of carbon dioxide into the blood in the tissues favors the release of oxygen.

2.1.2.3 Gas Exchange in the Tissues

The exchange of oxygen and carbon dioxide between the blood and body cells is in opposite

directions. Oxygen, being continually used up in the tissues, exists there at a lower partial pressure than in the blood. Carbon dioxide is produced inside the tissue cells, thereby making its partial pressure there higher than that of the blood reaching the tissues. Therefore, blood supplied by the arteries gives up oxygen and receives carbon dioxide during its transit through the tissue capillaries. The rate of exchange of these respiratory gases and the total amount of gas movement is dependent upon the respective partial pressure differences, since the exposure time of blood in the tissue capillaries is adequate for practically complete equilibration to be established. When tissues are more active the need for oxygen is greater. The increased oxygen is supplied not from an increase in the oxygen content of the arterial blood which is already approximately at maximum, but by a larger volume of blood flow through the tissues and by a more complete release of oxygen from a given volume of the blood. There can be a ninefold increase in the rate of oxygen supplied to active tissues.

2.1.2.4 Tissue Need for Oxygen

All living tissues need oxygen but tissues that are especially active during exertion, such as skeletal muscle, need correspondingly greater amounts of oxygen. The brain, however, is made up of tissue that has an extraordinarily high and nearly steady requirement for oxygen. Although the nervous system represents only about 2 percent of the body. weight, it requires about 20 percent of the total circulation and 20 percent of the total oxygen used by the body per minute at work or at rest. If the oxygen supply is precipitously and completely cut off and the lungs emptied by a full expiration, consciousness may be lost in about one-quarter of a minute, respiratory failure will occur within a minute, and irreparable damage to higher centers of the brain will occur within 3 to 5 minutes (See Paragraph 2.1.3.1).

2.1.2.5 Summary of Respiration Process

The process of respiration includes seven important phases (U.S. Navy Diving Manual 1973). 1. Breathing or ventilation of the lungs.

2. Exchange of gases between blood and air in the lungs.

3. The transportation of gases carried by the blood.

4. Exchange of gases between blood and body tissues.