The cardiorespiratory system and its biological alter ego, the cardiopulmonary system, are the processes that utilize the same organs to perform distinct functions for human performance. The heart and its muscular power drive the approximate 5.8 qt (5.5 l) blood volume through the entire distribution network (the cardiovascular system). The heart operates in partnership with the lungs to ensure the efficient transport of blood to and from the heart to facilitate the exchange of oxygen and carbon dioxide (the cardiopulmonary system). The heart also works with the entire breathing mechanism for the operation of the lungs, to service the oxygen delivery and waste-air discharge (the cardiorespiratory system).
The respiratory system is an integrated series of organs and openings designed to deliver oxygen-rich air to the lungs. It is essential to athletic performance that the lungs and the airways be clear, unobstructed, and fully functional. The body has no alternate means of counteracting or compensating for a substandard respiratory system; athletic performance will suffer, as it is an irrefutable biological fact that the body's energy stores cannot be utilized without proper supplies of oxygen. Typical conditions that impair respiratory function in athletes are asthma (an often chronic inflammation of the lungs and its airways), the bronchial tubes that restrict the passage of air into the lungs, and the congestion of the lung with fluids caused by common colds and infections. Smoking and other forms of pollution will also inhibit proper lung function.
The respiratory system begins where the air is inhaled into the body, through either the nose or the mouth. All air ultimately passes through the throat and into the trachea (windpipe). The air then passes into the bronchial tunes (bronchi), which lead into each of the lung sacs. Tubes known as bronchioles flow from the bronchi, tapering into ends composed of air sacs. The air sacs are groupings of tiny, round organic structures called alveoli. It is the alveoli that are the actual functioning exchangers of oxygen and air, the point of contact in the lung where air physically enters and exits the human body. A single alveolus is one-cell thick, which permits ease of movement by oxygen and carbon dioxide molecules between the body and the alveolus. The alveolus is encircled with capillaries, the tiny blood vessels that are the means by which oxygen is passed from the alveolus into the body and through which waste carbon dioxide is passed out. The alveoli are so densely packed into the lungs that, if placed on a flat surface, they would form a surface of over 120 yd2 (100 m2) for the average adult.
From the capillaries that are the cardiovascular point of contact with the respiratory system, the blood is directed into the circulatory system, and ultimately passes through the pulmonary artery.
The act of inhaling and exhaling air occurs due to the function of the diaphragm, located below the lung cavities in the abdomen. The lungs have no muscle structure of their own. One function of athletic activity is a strengthening of the diaphragm.
In any sport in which athletic endurance is a component, training that will increase the amount of oxygen available to the athlete is essential to athletic improvement. Conversely, improvement of performance will be limited if the body cells cannot obtain enough oxygen to assist in energy production with available glycogen or glucose stores. When an athlete seeks to increase lung capacity, the training objective is defined as increasing the "VO2 max," a short-form expression for the maximum amount of oxygen that the athlete can usefully consume during a maximum-effort exercise: the V symbol represents the volume measured, the O2 is oxygen, and the max represents the maximum level. VO2 max is measured as the volume capacity per minute, or as a comparative measure in relation to the weight of the athlete.
Known as the training effect, exercise places demands on the body for increase energy to fuel muscle activity. Heart rate increases, in response to demands for oxygen transporting red blood cells to metabolize glucose stores. Respiratory rate increases to obtain more oxygen and to take away greater amounts of waste carbon dioxide. Intense exercise, on a regular basis, will increase the amount of red blood cells and corresponding oxygen uptake.
Lung size is primarily a function of genetics. The lungs have no muscle structure of their own. Increased physical training will improve the ability of the lungs to inhale and exhale air due to the strengthening of the diaphragm, the muscle that power the respiratory portion of the cardiorespiratory system.
The exercise training effect is not permanent, and it is reversible through a reduction or cessation of physical activity. If an athlete reduces the intensity of endurance training, or ceases workouts due to injury or voluntary inactivity, the VO2 max will decrease. Aging and the natural reduction in cell regrowth and reproduction also serves to reduce the VO2 max.
Studies have determined that for an average healthy young male, age 20-29, a typical VO2 max test result is between 39.9 qt (44 l) and 46.3 qt (51 l) per minute. Olympic marathon champion Frank Shorter had a test capacity of 64.4 qt (71 l) per minute; Tour de France champions Miguel Indurain and Greg Lemond had maximum oxygen uptakes of more than 79.9 qt (88 l) per minute.