Cardiac output is the amount of blood ejected by the heart in 1 minute. Cardiac output (Q) is the product of heart rate (HR) and stroke volume (SV). Cardiac output increases with aerobic training during maximal effort-it does not increase at rest or during sub maximal exercise. The reason that cardiac output does not change during rest or submaximal exercise is that the lowered resting heart rate compensates for the increase in stroke volume. What does change is the manner in which cardiac output is achieved. This is illustrated by the following example: an untrained 25 year old man has a resting heart rate of 72 bpm and a stroke volume of 70 ml of blood per beat. His cardiac output at rest is calculated as follows:
The same person after 2 years of aerobic training has the same cardiac output at rest, but it is achieved in the following way: resting heart rate has decreased to 55 bpm and stroke volume has increased to 92 ml of blood per beat.
The average cardiac output at rest is 4 to 6 liters of blood per minute. During maximal exertion, cardiac output reaches values of 18 to 20 liters of blood per minute for the average person but may reach as much as 40 liters per minute for large, well conditioned athletes. What an incredible piece of work by an organ that weighs less than a pound. To put this in perspective, imagine 40 1-liter cola bottles filled with blood. This is the amount that the hearts of some highly conditioned people can pump in 1 minute. Maximal cardiac output improves with training because of the increase in stroke volume. Maximal heart rate is essentially unaffected by training; therefore its influence on maximal cardiac output is relatively constant. However, maximal heart rate declines with age by about 1 bpm per year after age 20. Training cannot stop the decline, it can only slow the process somewhat. This is one of the factors that contributes to the decline in cardiac output and aerobic capacity as we grow older.
Blood Volume
Aerobic training increases total blood volume, plasma volume (the liquid portion of the blood), and the blood solids (the red blood cells, white blood cells, and blood platelets). The increase is greatest in plasma volume, so the blood becomes more liquid. The increase in the ratio of plasma volume to red blood cell volume is an adaptation to exercise that lowers the viscosity, or thickness and stickiness, of the blood. This change decreases the resistance to blood flow, allowing it to circulate more easily through the blood vessels.
Blood is automatically shunted by the body to areas of greatest need. At rest, a significant amount is sent to the digestive system and kidneys. During vigorous exercise, as much as 85% of the blood is sent to the working muscles, reducing the amount sent to the digestive and urinary systems.
Heart Volume
The muscles of the body respond to exercise by growing larger and stronger. As a muscular pump, the heart is no exception. The volume and weight of the heart increase with endurance training. Training that lowers the resting heart rate stimulates greater filling of the ventricles, whose muscle fibers respond to the increased pool of blood by stretching. This produces a recoil effect in the muscle fibers that results in a stronger contraction with more blood ejected per beat. Continued training causes the ventricles to enlarge and grow stronger, so the weight and the size of the heart increase. The hypertrophied (enlarged) heart is a normal response to endurance training that has no long term detrimental effects. In fact, although maintaining this effect for life is beneficial, several months of inactivity will reduce heart weight and size to pretraining levels. The atrophy (wasting away) associated with physical inactivity is inevitable.
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Aerobic Endurance Training
