Cardiorespiratory Factor

The cardiorespiratory system is the combination of the respiratory and cardiovascular systems, which work together to transport oxygen to the cells and to support metabolism by delivering the nutrients that fuel the neuromuscular and neuroendocrine systems. During exercise the demand for energy climbs, and meeting that demand requires oxygen to be delivered in the right volume. How much is needed depends on the intensity, duration and type of activity.

To match this demand, the respiratory side — pulmonary ventilation, external respiration and internal respiration — works as one. Pulmonary ventilation is simply the mechanical movement of air into and out of the lungs through breathing. External respiration is the exchange of oxygen and carbon dioxide between the air inside the alveoli and the blood in the surrounding capillaries. Internal respiration is the matching exchange of these same gases between the blood and the body's tissues and cells. All three must step up together during exercise — if any one of them lags, the extra oxygen the working muscles are calling for cannot actually reach them. The cardiovascular response to exercise is directly proportional to how much oxygen the skeletal muscles demand. As activity rises, several parameters increase together: maximal oxygen consumption (VO2 max), blood pressure, blood volume, oxygen diffusion and extraction, and blood flow through the muscles and arteries.

A key cardiovascular measure here is stroke volume — the volume of blood pumped out of the left ventricle during each systolic contraction of the heart. Stroke volume combines with heart rate to give cardiac output (Cardiac Output = Heart Rate x Stroke Volume), the total volume of blood the heart pumps out each minute. A resting adult has a cardiac output of roughly 5 litres per minute; during maximal effort this can climb to about 20 litres per minute in a sedentary person, and beyond 40 litres per minute in a well-trained endurance athlete — a difference driven mainly by a much larger stroke volume per beat, not just a faster heart rate. This is why stroke volume is such a vital indicator of how effectively the cardiovascular system is meeting the body's demand for oxygen during activity, and why it is a favourite CBSE Class 12 Physical Education examination point.

VO2 max — the maximum rate at which the body can take in, transport and use oxygen during intense exercise — is widely regarded as the single best laboratory measure of cardiorespiratory fitness. It is influenced by genetics, which sets a natural ceiling that varies from person to person, but it can be substantially raised through regular aerobic training, which increases stroke volume, blood volume, capillary density, and the muscles' own ability to extract oxygen from the blood that reaches them. In endurance disciplines such as long-distance running, cycling, swimming and football, the cardiorespiratory factor is usually the real ceiling on performance — two athletes with identical muscle-fibre composition and technique will still differ in endurance mainly because of differences in their VO2 max, stroke volume and cardiac output. …