A major function of the cardiovascular system is to deliver oxygen to the working tissue of the body to be used for metabolism. To accomplish this, the heart must regulate the amount of blood pumped per minute. As described in the "Physiology of Sport and Exercise," changes in the body's demand for oxygen, as experienced during exercise, will place a greater demand on the heart to deliver oxygen to the system. Maintaining the intricate balance between demand and delivery is the role of cardiac output.
Stroke Volume and Heart Rate
Stroke volume is the difference in the amount of blood in the left ventricle before and after a contraction. Normal resting stroke volume can range from 60 to 80ml per heartbeat; however, this can be influenced by a number of factors. The amount of blood returned to the heart between beats and the amount of pressure needed to expel the blood are two key variables that determine the overall stroke volume.
The heart rate is equivalent to the number of times the heart contracts in 1 minute. The resting heart rate for a healthy average adult ranges from 60 to 80 beats per minute.
Cardiac Output
Cardiac output is a factor of stroke volume and heart rate and is therefore defined as the amount of blood pumped by the heart in 1 minute. For example, an individual with a stroke volume of 70ml per beat and a heart rate of 70 beats per minute would be able to pump 4,900ml, or 4.9L, of blood throughout the system per minute. Cardiac output for an average adult is approximately 5L per minute. Interestingly, the average adult has roughly 5L of blood in his system which means roughly all of the blood would pass through the heart once every minute.
Variations
Cardiac output translates into oxygen delivery. In order for the body to work efficiently, cardiac output must increase and decrease in accordance with the physical demands placed on the body. As an individual increases her metabolic rate through exercise, for example, the heart must increase its cardiac output to deliver the necessary oxygen. To do so, the heart rate and stroke volume will both increase, thereby increasing the total amount of blood pumped per minute.
Chronic exercise elicits a decrease in resting heart rate and an increase in resting stroke volume. These changes, described by the American College of Sports Medicine as positive chronic adaptations, result in a more efficient cardiovascular system. Because exercise increases the size of the heart muscle and its ability to produce force, it is able to eject more blood per beat. Thus, the heart rate can decrease, the stroke volume can increase and the heart will continue to pump approximately 5L per minute.
Unfortunately, disease has an adverse affect in that it generally decreases the strength of the heart muscle, thereby lowering the amount of blood that can be pumped per beat and necessitating an increase in heart rate in order to keep up with the demands of the body.
References
- "The Physiology of Sport and Exercise, Fourth Edition"; Jack H. Wilmore, PhD; 2008
- "ACSMs Resource Manual for Guidelines for Exercise Testing and Prescription, Sixth Edition "; American College of Sports Medicine; 2010
Member Comments