Cardiac muscle is one of the three main types of muscle in the human body. Due to its vital function, it requires a highly specialised circulation. This article will look at the coronary circulation, its adaptations, and clinical conditions related to cardiac circulation.
Cardiac muscle has its own dedicated circulatory system: the coronary blood vessels. The cardiac muscle constantly undergoes phases of contraction and relaxation to pump blood around the body. As myocardial oxygen demand increases, coronary blood flow to the cardiac muscle must also increase to meet requirements.
Anything that prevents blood from passing through the coronary vessels, for example, atherosclerosis, will result in ischaemia and a consequent myocardial infarction.
Perfusion of the coronary arteries occurs during diastole. The entry of blood into the coronary arteries occurs through the aortic sinuses, which are openings found behind the flaps of the aortic valve. As the heart relaxes during diastole, blood fills the valve pockets, allowing blood flow into the coronary arteries to supply the cardiac muscle. The coronary arteries also send branches into the myocardium, which are compressed during systole. Therefore, when the muscle relaxes, blood flow to the myocardium is increased as there is less compression on these vessels.
To help maintain a high basal rate of blood flow, there is a continuous nitrous oxide production, which is a potent vasodilator. Moreover, the accumulation of metabolites such as adenosine, K+ ions and H+ ions also results in vasodilation. This phenomenon is known as reactive hyperaemia, and it occurs in tissues around the whole body.
Clinical Relevance – Myocardial Infarction
If the blood supply to the cardiac muscle is obstructed, for example, by a thrombus, then oxygen to the cardiac muscle is restricted. Due to the high oxygen demand from this tissue, it cannot survive for very long without this supply of oxygen, resulting in ischemia. Prolonged periods of ischemia results in the death of healthy of cardiac myocytes (infarction). This causes the classic symptom of central crushing chest pain, which may also radiate to the left upper limb and jaw.
The diagnosis of a myocardial infarction involves using an electrocardiogram (ECG) or biomarkers in the blood, such as troponin. Treatment is usually Percutaneous Coronary Intervention (PCI) if it is possible within 90 minutes of the onset of pain. However, if PCI is not available, thrombolysis can be used.