Figure 12a. The respiratory tract through which air enters the cardiopulmonary system.

The Respiratory System

Movement of air from the external environment into the lungs is accomplished by the action of two groups of muscles. The first is the diaphragm, a muscular wall that divides the trunk's body cavity into two parts, the chest and the abdomen. The second consists of the rib muscles (intercostal muscles). These muscles act together to change the size of the chest cavity. The rib muscles are attached to your ribs, which in turn encircle the lungs and chest cavity. Together, this system is often referred to as the rib cage.

The respiratory tract is the pathway for air to enter the body. It is one of only three main gateways that connect the "outside" of the body to the "inside" of the body (the other two are the digestive tract and the urinary tract). The process of respiration involves many interactions. Let us begin with inspiration (inhaling). Air is drawn into the lungs as a result of the combined expansion of the rib cage and the lowering of the diaphragm; in normal breathing it is lowered about 1 cm; in heavy breathing it can be lowered up to 10 cm. When the lungs are expanded in this state, atmospheric pressure, the pressure outside the body, is higher than the pressure in the lungs. Air flows from the higher to the lower pressure areas and into the lungs through a system of channels that begins with the oral cavity (mouth) and the nasal cavity (nose). The air flow then continues through the trachea (the windpipe) and into the bronchi which are two large tubes, one for each lung. Finally, stemming from the main bronchi are smaller bronchi and tiny bronchioles, much like branches and twigs stemming from a tree trunk.

Figure 12b. The exchange of gases occurs between the membranes ofthe alveoli and the surrounding capillaries. The red blood cells are thevehicles that carry the carbon dioxide to and oxygen away from thealveoli.

Arriving at the bronchioles, the air has yet to unload its cargo of oxygen. The start of this task is taken on by vast armies of tiny, expandable, thin-walled, clustering sacs called alveoli. There are estimated to be about 300 million of these small air sacs in an average-sized adult. The alveoli constitute the bulk of lung tissue; it is their substance that makes the lungs soft and spongy. When the lungs expand or contract, it is the alveoli that are expanding or contracting. It is from the alveoli that the blood receives its oxygen.

Every alveolus in your lungs is covered with capillaries (Figure 12b). Every single red blood cell (RBC) in your bloodstream flows through these pulmonary capillaries so they can pick up an oxygen molecule and give up carbon dioxide. Layers of capillary and alveolar cells lie in direct contact side by side with a double membrane, almost unimaginably thin, with air moving on one side and blood flowing past on the other. The oxygen is soaked into the blood via this virtually transparent wall and snatched up by the hemoglobin in the RBCs, where the iron in the hemoglobin locks the oxygen in a chemical embrace. Swept along in the bloodstream, the oxygen finally arrives at the body's waiting cells to unite with the body's fuel and free the energy in them to be used to enable our body to move and function.

The actual quantity of oxygen uptake per minute (the rate of oxygen taken in by the cells) during this process may vary from one minute to another, depending on the rate of breathing and the speed with which blood is being pumped through the arteries. This, in turn, depends on how much energy the body requires at the time. Someone snoozing in a hammock may absorb only half a pint of oxygen a minute, while a person running the mile to beat a world's record may soak up more than five quarts in the same period.

The lungs are not only responsible for the delivery of oxygen to the bloodstream. Simultaneously, they draw out of the blood the waste carbon dioxide produced by the utilization and breakdown of carbon compounds (fats and carbohydrates) that provide energy in the cells. The carbon dioxide is picked up from all the cells of the body and carried along by the RBC on its way to obtain oxygen from the lungs. Once inside the lungs, the carbon dioxide is brought alongside the alveolar membrane, where it seeps out of the bloodstream just as the oxygen seeps in. Although the two gases pass through the same membrane, they have absolutely nothing to do with each other. They are like total strangers boarding and leaving a train at a single station. From the lungs, the carbon dioxide makes its way out of the body along the same route the oxygen followed on its way in. This is done during the process of expiration (exhaling).

In expiration, the rib cage and the diaphragm relax and the lungs contract (become smaller). During this process, the air pressure inside the lungs is higher than the atmospheric pressure and air is forced out. However, the lungs never completely deflate; that is, some air always stays in the lungs. This volume of air that always stays in the lungs is called residual volume. The volume of air that moves in or out of the lungs in one normal breath is called the tidal volume. Definitions for other important respiratory measurements are found in Table 1.

Table I. Definitions of respiratory measurements.

Proper functioning of the cardiopulmonary system is essential for a human being to survive. In fact, the lungs work so closely with the heart and blood vessels that certain heart function measurements can actually be obtained from measurements of pulmonary function. That is, measurements that involve breathing can actually yield information about the blood flow through the heart. For instance, flow rates such as cardiac output can be obtained from pulmonary function measurements. This works because all the blood that flows into the lungs to be oxygenated equals the amount of blood that flows out of the heart. That is, the amount of blood flowing into the lung's capillary beds is equal to the amount of blood flowing out of the heart. Blood flow through the lungs is called pulmonary blood flow (PBF). Thus,

An important technique used to determine cardiac output involves using a sophisticated breath analyzer to analyze the composition of gases in the astronauts' breath. This is similar to the breath analyzer test that the police use to measure alcohol blood content in a person they suspect has been drinking and driving. We will go into this idea more completely later, when we discuss how this technique was used in space to determine cardiac output.

The cardiovascular system and the cardiopulmonary system work together to interact with every cell and organ in the body. The blood vessels serve as the communication line between all the body systems. Therefore, small changes in any of these body systems can have a "waterfall" effect that spreads and creates changes throughout the body. Every moment of our lives, hundreds of thousands of complex interactions take place in our bodies. Over millions of years, the human body has evolved in conjunction with the force of gravity. You have had a glimpse of the complexity of the body's systems down to the microscopic level of the capillaries and the alveoli. What do you suppose happens when an environmental constant like the force of gravity is removed?

Let's move on to our investigations into how space flight affects the heart, lungs, and blood vessels.