title: Physiologic requirements to perform work.
abstract: In order to perform, exercising muscle must be provided with an appropriate blood flow, and the composition of the incoming blood must be kept within acceptable limits. The circulatory system maintains flow by increasing cardiac output and reducing supply to many beds (such as the splanchnic circulation), while the lung regulates arterial O2 and CO2 levels by adjusting alveolar ventilation to metabolic needs. In the process, the function of many organs is temporarily sacrificed or depressed, which means that: (1) a hierarchy of functions has to be established, and (2) some compromises among the numerous needs of the exercising muscle must be made. As a result of these readjustments, the heart and lungs of the normal subject are able to provide all the support that the exercising muscle requires. It is the peripheral machinery that fails first, as demonstrated by the switch to anaerobic metabolism while blood pressure is actually increased, and O2 and CO2 levels are maintained. Thus, if the exertion level of a patient is limited by his central circulation or by his respiration, there is a definite decrement in the functional capacity of that system.
mesh_terms: D002245:Carbon Dioxide / Q000097:blood; D002302:Cardiac Output; D002320:Cardiovascular Physiological Phenomena; D005080:Exercise Test; D006801:Humans; D009132:Muscles / Q000098:blood supply / Q000502:physiology; D010100:Oxygen / Q000097:blood; D005082:Physical Exertion; D012119:Respiration
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen