1. What is the definition of the CO?

This is defined as the product of the heart rate and the stroke volume.

2. Give a normal resting value for the CO.

5–6 Lmin−1.

3. How do the outputs of the two ventricles compare to one another?

In the normal state, the outputs of both ventricles are essentially equal – both being 5–6 Lmin−1.

4. Which factors influence the stroke volume?

• Preload

• Afterload

• Myocardial contractility

• Heart rate

5. What determines the preload?

This is determined by the venous return to the heart. The amount of venous return itself depends on the difference between the systemic filling pressure (driving blood back to the heart) and the central venous pressure (CVP) (working against the venous return).

6. Define the afterload. What is this analogous to, in simple terms?

This is the ventricular wall tension that has to be generated in order to eject blood out of the ventricle. It is analogous to the arterial pressure. An increase in the CO causes a rise in the arterial pressure (afterload). This in turn, has a negative feedback effect on the output. Since more energy is consumed generating a high enough pressure to overcome a high arterial pressure, the resulting stroke volume is less – reducing the output in subsequent beats.

7. What happens to the Frank-Starling curve of the heart when the myocardial contractility is increased?

There is an upwards shift of the curve, so that the stroke volume is higher for any given end diastolic volume.