Cardiac Blood Flow
- Blood flow into the myocardium displays several unique features whose importance becomes apparent in discussions of ischemic heart disease. The myocardium extracts a fairly high, fixed percentage of oxygen from the blood it receives; consequently, changing cardiac oxygen demand can only be met by regulating the amount of blood flow through the heart (i.e. Cardiac Blood Flow).
|Coronary Blood Flow Hemodynamics|
- As discussed in hemodynamic integration the blood flow through a particular vessel is simply the ratio of the blood pressure gradient to the resistance of the vessel. Formally: Blood Flow = Blood Pressure Gradient/Resistance. Consequently, blood flow through coronary arteries is dependent on the pressure gradient across the arteries and their resistance.
- Coronary Blood Pressure Gradient
- For most tissues, the blood pressure gradient across the vasculature is greatest during systole when the arterial pressure is greatest. However, during systole the heart contracts, generating enormous pressures on the coronary vasculature which nearly ablates any blood pressure gradient across the coronary vessels. Consequently, the bulk of myocardial blood flow occurs during diastole when the heart is relaxed and thus the pressure in coronary veins is relatively low.
- The resistance to blood flow by coronary arteries follows the general pattern of resistance offered by any other vessels . Recall from the resistance page that the most important factor in determining a single vessel's resistance is its luminal radius. Like most arteries, the radius of the coronary arteries is subject to intense regulation by multiple mechanisms and forms the basis of coronary blood flow regulation, as discussed below.
|Coronary Blood Flow Regulation|
- The primary means of regulating coronary blood flow is through modulation of the luminal radius of coronary arteries. In general, regulation of coronary artery resistance is mediated by processes operating within the heart itself. Consequently, extrinsic neural processes do not have a large impact on coronary blood flow
- Local Metabolites
- Local metabolites generated by the myocardium probably impact coronary artery resistance the most. The nature of these metabolites and how they affect arterial resistance are discussed in Local Blood Flow Regulation - Intrinsic Mechanisms. These mechanisms guarantee that blood flow to the myocardium is precisely matched to the metabolic demands of the tissue.
- Nitric Oxide
- The highly diffusible gas Nitric Oxide exerts a profound affect on the luminal diameter of the coronary arteries. Regulated release of nitric oxide and the mechanisms by which it mediates arterial luminal diameter is discussed in Local Blood Flow Regulation - Nitric Oxide Mechanism.