Top Endurance

Because endurance-based sport relies heavily on the oxidative phosphorylation energy system, endurance athletes need to understand the basic concepts of cardiopulmonary physiology. The term cardiopulmonary refers to the heart and lungs and how those vital organs work in synchrony to ensure that the blood is carrying oxygen and nutrients to the working skeletal muscles during exercise.

Cardiopulmonary Anatomy

The primary anatomical structures of the cardiopulmonary system are the lungs, heart, and skeletal muscles. We begin our anatomical “tour” in the lungs. Blood passes through the capillary beds of the lungs, where it unloads carbon dioxide (CO2) and picks up oxygen (O2). This oxygen-enriched blood travels from the lungs to the heart via the pulmonary vein. Oxygen-enriched blood initially enters the heart in the left atrium and then flows into the left ventricle. When the heart contracts, or beats, oxygen-enriched blood is ejected from the left ventricle and exits the heart via the aorta. The aorta ultimately branches into several smaller arteries that carry oxygen-enriched blood to the entire body.

Once the oxygen-enriched blood reaches, for example, the leg muscles during running, it unloads oxygen and picks up carbon dioxide. Blood exiting the exercising muscles is “oxygen reduced” and returns to the heart via the venous system. Oxygen-reduced blood is ultimately delivered to the heart via two large veins, the superior and inferior vena cava. The venae cavae deliver oxygen-reduced blood to the right atrium of the heart; the blood then flows into the right ventricle. When the heart contracts, oxygen-reduced blood is ejected by the right ventricle and travels via the pulmonary artery to the lungs.

We have now arrived back at the starting point of our tour of cardiopulmonary anatomy—that is, as the oxygen-reduced blood enters the capillary beds of the lungs, it will unload carbon dioxide and pick up oxygen and then exit the lungs as oxygen-enriched blood. This synchrony between the lungs, heart, and tissues is taking place constantly, whether the person is awake or asleep. The entire cardiopulmonary system works overtime during any endurance-based sporting activity, such as a triathlon.

Oxygen Transport

As mentioned earlier, endurance-based sports are heavily dependent on the oxidative phosphorylation energy system for ATP. In the previous section, we referred to oxygen transport in very general terms: oxygen-enriched and oxygen-reduced blood. In this section, we examine oxygen transport in more detail, focusing on the gas physics and physiology of oxygen transport.

The first thing to consider when learning about oxygen transport is how oxygen is carried around in the body. Though a very small percentage of oxygen travels through the body dissolved in the fluid portion of the blood, the primary way by which oxygen is transported through the body is via the red blood cells, also called erythrocytes. Figure 1.9 shows the shape of a typical red blood cell. Blood contains trillions of red blood cells. The portion of the blood containing red blood cells is referred to as the hematocrit (Hct) and is expressed as a percentage of volume of red blood cells relative to the total blood volume. Hematocrits for healthy individuals residing at low elevation range from 35 to 45 percent for women and 40 to 50 percent for men.





If we “broke open” a single red blood cell, we would find that it contains about 250 million molecules of hemoglobin (Hb). The hemoglobin molecule is what actually transports oxygen throughout the body. A single molecule of hemoglobin can transport 4 molecules of oxygen. Thus, a single red blood cell (and remember, you have trillions of red blood cells) has the capacity to transport 1 billion molecules of oxygen.

Now that you understand that red blood cells—or more specifically, the hemoglobin molecules contained in red blood cells—transport oxygen throughout the body, let’s look at how oxygen-reduced blood becomes oxygen-enriched blood in the lungs. The entire process of oxygen transport is regulated by changes in the partial pressure of oxygen (PO2) that take place from the moment we inhale air through our nose and mouth until the air reaches our body’s tissues and organs. PO2 decreases as inspired air moves from the nose and mouth to the lungs. The decrease is due to the process of diffusion wherein molecules move from an area of high concentration to an area of low concentration. Specifically, the PO2 of inspired air at sea level is approximately 159 mm Hg (millimeters of mercury), which drops to 105 mm Hg in the lungs.

As already noted, blood entering the lungs via the pulmonary arteries contains red blood cells that are relatively low in oxygen. The PO2 of this oxygen-reduced blood is approximately 40 mm Hg. This pressure difference, or pressure gradient, in the lungs (105 mm Hg) compared to the oxygen-reduced blood (40 mm Hg) favors the diffusion of oxygen from the lungs to the oxygen-reduced blood (see figure 1.10), where it binds to hemoglobin molecules. The diffusion of oxygen from the lungs to the blood takes only about 0.75 seconds and occurs across a very sheer membrane in the pulmonary capillaries that is approximately 1/10,000 the width of a Kleenex!




As a result of the diffusion of oxygen in the lungs, oxygen-enriched blood exits the lungs with a PO2 of 100 mm Hg. The oxygen-enriched blood is transported via the pulmonary veins to the left ventricle of the heart; the blood is then circulated throughout the body, as discussed earlier. When oxygen-enriched blood arrives at the capillary bed of a skeletal muscle, the pressure gradient favors the release of oxygen from hemoglobin to the skeletal muscle. The oxygen-enriched blood is at approximately 100 mm Hg, and the muscle is at about 30 mm Hg. The oxygen that is unloaded in the skeletal muscle can now be used by the mitochondria to produce ATP via the oxidative phosphorylation energy system. Finally, the blood exits the skeletal muscle’s capillary bed in an oxygen-reduced state with a PO2 of about 40 mm Hg. The blood returns to the right ventricle of the heart to repeat the process of oxygenation in the lungs and oxygen transport throughout the body.