1. Where do the coronary arteries originate?

Both the right and left coronary arteries arise directly from the ascending aorta at the aortic sinuses located just above the leaflets of the aortic valve (also known as the sinuses of Valsalva).

2. What is the rate of coronary flow at rest?

70–80 ml/min per 100 g of cardiac tissue. During exercise, this can increase to 300–400 ml/min per 100 g.

3. What percentage of the CO does the heart receive?

4–5%.

4. Given that there is a high myocardial oxygen demand at rest, what functional adaptations ensure that supply meets demand?

Note that the myocardial oxygen consumption is in the order of 8ml per 100 g of tissue. This is around 20 times that of skeletal muscle. Functional adaptations to ensure adequate oxygen delivery include:

• High capillary density: producing a very high surface area for oxygen delivery, and there is high blood flow per unit weight of myocardium

• High oxygen extraction ratio: the myocardium extracts around 70% of the oxygen that is delivered to it from the coronary flow. In contrast, the body average is only 25%

• Efficient metabolic hyperaemia: myocardial metabolites generated during situations of increased exercise and oxygen demand have a strong influence on control of blood flow

• During exercise, the increased oxygen demand is met predominantly through an increase in the rate of flow rather than an increase in the oxygen extraction ratio

5. Look at the graph below, showing the pattern of coronary flow during different phases of the cardiac cycle.