39. The coronary circulation
The coronary circulation is the one which supplies the heart, especially the myocardium, with oxygenated and nutrient-rich arterial blood. The myocardium doesn’t receive any oxygen or nutrients from the blood which is pumped out by the heart, so it needs its own circulation.
The myocardium requires a lot of oxygen, as it is contracting all the time with large force. All tissues supplied by arteries can increase the amount of oxygen they extract from the blood if necessary. In other words, when other tissues require more oxygen, they can both increase the amount of oxygen they extract from the blood, and they can increase the blood flow themselves.
Myocardium is an exception, because it already extracts the maximum amount of oxygen from the blood during rest. So, for the myocardium to receive more oxygen it must receive more blood flow.
Structure:
The coronary circulation originates from the very beginning of the ascending aorta. The right coronary artery originates from the right aortic sinus while the left coronary artery originates from the left aortic sinus. These main coronaries then branch into smaller arteries which wrap around the heart.
Some arteries in the coronary circulation lie between the endocardium and the myocardium, so-called subendocardial arteries.
Changes in coronary flow during the cardiac cycle:
When the myocardium contracts during systole the subendocardial arteries are compressed, and so no blood flows through them. In other words, blood only flows through the subendocardial arteries during diastole. We say that coronary circulation is diastole dependent.
This causes a problem when the heart rate increases. As previously discussed, when the heart rate increases the diastolic time decreases. In other words, there is less coronary circulation. However, when the heart rate increases the myocardial oxygen demand also increases. This puts a “roof” on the amount the heart rate can increase.
Regulation of coronary blood flow:
The coronary blood flow is autoregulated between MAP of 50 mmHg to 150 mmHg. This means that the coronary blood flow is regulated so that it is constant so long as the mean arterial pressure is between 50 and 150 mmHg (normal ~100). The renal and cerebral circulations are also autoregulated.
Autoregulation to these organs is essential. If there were no autoregulation and the blood pressure were to suddenly drop, the blood flow to these organs would suddenly drop as well. For the case of the coronary and cerebral circulations, that could be fatal. As such, autoregulation protects against drops in blood flow which could be deadly.
The exact mechanism of the autoregulation isn’t well known, but we know of many molecules which influence the coronary blood flow:
- Vasodilators (increase coronary blood flow)
- H+ (local acidosis)
- K+
- Lactate
- Adenosine
- Hypoxia
- Hypercapnia
- Nitric oxide (NO)
- Vasoconstrictors (decrease coronary blood flow)
- Catecholamines
- Epinephrine
- Norepinephrine
- Vasopressin (ADH)
- Catecholamines
The heart regulates the amount of vasodilation or vasoconstriction of the coronary circulation based on the oxygen requirements of the myocardium. For example, when the oxygen demand increases more H+ and lactate will be produced, and there will be local hypoxia. These factors will stimulate vasodilation of the arteries, which increases blood flow.