Cardiac output is the amount of blood coming out of the heart every minute and is calculated using your heart rate. Cardiac output increases during exercise and decreases at rest. People who do consistent aerobic training also experience decreased cardiac output during low to moderate intensity exercise. This adaptation however, is reversible with a decrease in training.
Stroke Volume
Stroke volume is the amount of blood the heart can pump out in one contraction. The size of the cavity of the heart affects how much blood it can hold; a small cavity will hold less blood compared to a larger cavity. Continued use of steroids or increased abdominal fat can thicken the heart muscle and therefore decrease the size of the cavity. The arteries feeding the heart muscle must be clear of plaque to bring sufficient oxygen to the heart cells. When arteries feeding the heart are blocked, the heart is not able to contract properly to push out sufficient blood, resulting in chest pain or angina. Such factors decrease stroke volume and ultimately affect cardiac output.
Heart Rate
The number of times the heart contracts and relaxes per minute is measured in terms of heart rate per minute. The heart rate is based on the oxygen demands of the body systems. During exercise, there is an increased need for oxygen in the muscles, and the heart must pump faster to meet and maintain oxygen need. Cardiac output goes up with increased heart rate.
Blood Pressure and Total Peripheral Resistance
While the heart beats faster and pushes out a given amount of blood during exercise, there is also resistance to the blood within the walls of the arteries. Cardiac output is affected by how much resistance there is in the arteries. Plaque build-up in the arteries supplying the body and undilated blood vessels in the muscles increase blood pressure and peripheral resistance.
Venous Return
Oxygen-poor blood returns to the heart through the venous system. Faulty veins with damaged valves reduce the amount of blood returned to the heart. A reduction in blood flow back to the heart reduces the volume of blood to be pumped out. Aerobic exercise helps with venous return as the rhythmic contraction of exercising muscles causes them to act like pumps bringing blood from the muscles back to the heart.
Regular Aerobic Exercise
Regular aerobic exercise reduces resistance to blood flow, improves extraction of oxygen in the cells, improves the function of the heart, reduces resting heart rate and improves blood returned through the veins. Additionally, aerobic exercise increases the amount of energy producing enzymes, increases the number of capillaries and reduces plaque build-up. All these factors together enhance the function of the cardiovascular and muscular systems, reducing cardiac output at rest and during low to moderate intensity exercise.
References
- "Exercise Physiology, Energy, Nutrition &Human Performance"; William D. McArdle, Frank I. Katch and Victor L. Katch; 2007
- "Personal Trainer Manual"; American Council on Exercise; 1997
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