The heart is divided into a left and right side. The left side of the heart forces the blood filled with oxygen from the lungs to the working muscles. The right side of the heart pumps blood filled with carbon dioxide from the working muscles to the lungs for elimination. During increased activity, the muscles need a greater amount of oxygen to function. The heart meets this need by delivering more oxygen to the working musc1es via increased cardiac output.
Cardiac output is the amount of blood pumped by the heart in each minute.
The cardiorespiratory system has a maximal capacity. The maximal amount of oxygen that this system can deliver and exchange is called maximal oxygen consumption. The maximal amount of oxygen an individual can consume can be measured within a laboratory setting. The capacity for oxygen consumption is a determinant factor in personal physical fitness. For example, an unfit individual consumes about half the amount of oxygen that a fit individual can consume.