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