What factors affect cardiac output?
1 Answer
Explanation:
Cardiac Output (CO) is a function of the Stroke Volume (SV) i.e. the volume of blood in millimetres that is ejected from the left ventricle during systole, multiplied by the Heart Rate (HR) in beats per minute (bpm).
This can be expressed as follows:
Therefore, the Cardiac Output is given in units of (ml/min)
The stroke volume is said to be ~70ml in an average sized male, so for the sake of providing a useful example we'll use that.
If an individual's heart rate is 80 bpm, and they have a stroke volume of 70ml per beat, then the cardiac output will be ~5600ml/min. In order to convert this value into litres, we simply divide it by 1000, so that we have ~5.6L/min.
You may see that this figure is fairly consistent with the blood volume of an average sized male, which lies somewhere between 5-6L; demonstrating how much blood the heart is capable of displacing in a single minute. The formula shows how the cardiac output is dependent on both the heart rate and the stroke volume, so any changes in either of these will alter the cardiac output.
If the stroke volume remains constant, we can say that an increase or decrease in the heart rate will alter the cardiac output. We experience this every time we engage in a physical activity. What happens when you sprint a hundred metres? What other situations cause your heart rate to vary?
If we change the value of the stroke volume, i.e. the ventricles become larger or smaller, then this will surely affect the cardiac output. The heart is a muscle, so when we cause it to work harder, what do you suppose happens to it? Did you ever hear anybody worrying about their high-blood pressure? What do you think may cause this?
