ORGANIZATION OF THE CVS
The coronary vasculature supplies blood to the muscles of the heart (myocardium). These coronary arteries are the first branches (right & left coronary arteries) that arise from the aorta, and they run on the surface of the heart’s muscle. Branches from the arteries penetrate the muscle in order to supply blood to deeper myocardial tissues. When the heart contracts during systole, the small arteries that are embedded deeply within the heart musculature are also squeezed along with the heart. This mechanical compression of coronary arteries causes a brief period of occlusion of coronary blood flow during systole. Therefore, it should be established that the whole body receives oxygenated blood during systole except for the heart itself. The heart will be supplied with the oxygenated blood when the atria and ventricles are in a relaxed state which happens during diastole. At that time, the coronary vasculature is not compressed and the perfusion is at a maximum. The right heart muscles contract with lesser force compared to the left heart. Hence, a relatively small amount of perfusion is maintained in the right heart even during systole.
Even under normal resting conditions, the heart tissue extracts 80% of the total oxygen content from the blood that is supplied to it. This high extraction of oxygen is necessary to meet the demands of highly active muscles of the heart. The demand for oxygen is increased substantially when there is an increase in the heart rate, as happens during exercise. Compared to the cardiac muscle, the skeletal muscle extracts only 20% of oxygen from the blood at normal resting conditions. In other tissues, when oxygen demand increases, vasodilatory metabolites such as H+ ions and carbon dioxide cause a rightward shift of the oxygen dissociation curve. This allows greater dissociation of oxygen from the hemoglobin. However, this rightward shift of the hemoglobin oxygen dissociation curve is not valid for the blood in the coronary circulation. This is because the extraction of oxygen by heart tissue is already at maximum level even under normal resting conditions. Therefore, the increased demand for oxygen during exercise can only be compensated by increasing the blood flow rate of the coronary circulation.
The flow of coronary vasculature is under the influence of both intrinsic and extrinsic mechanisms. The extrinsic mechanisms include the sympathetic and parasympathetic nervous systems.
- The PANS input is relayed by the right and left vagus nerves. Right vagus nerve innervates the SAN, while the left vagus nerve innervates the AVN. The PANS regulates the blood flow indirectly by controlling the heart rate (chronotropy) and the force of contraction (inotropy).
- The SANS innervates the SAN &AVN, the myocardium and also the vascular smooth muscle of coronary vessels. Hence, SANS regulates the heart rate, force of contraction of ventricular myocardium and the flow into the coronary vasculature. However, the extrinsic mechanisms involved in vascular diameter changes are ineffective during physiological conditions. Although there is sympathetic stimulation on the vascular smooth muscles, the effects are negated by the constant release of nitric oxide from the endothelium. If this nitric oxide release is blocked by an atherosclerotic plaque, the unchecked sympathetic stimulation causes vasoconstriction leading to the pain of angina. Atherosclerosis decreases coronary perfusion by reducing the coronary artery lumen and also by blocking the release of NO from the coronary artery endothelium.
Under physiological conditions, the metabolism induced changes in vascular caliber are more prominent. The metabolites produced by the heart tissue responsible for vasodilation are:
- H+ ions
- K+ ions
- Bradykinin and Prostaglandins
Volume – Work Relationship
There is a direct relationship between the volume of blood entering the heart (preload) and the work done by the heart to pump it out. The pressure remains the same as there is no change in myocardial contractility or afterload. This can be observed when a person is exercising during which the volume of blood returning to the heart increases.
Pressure – Work Relationship
There is a direct relationship between the pressure developed in the peripheral vasculature and the work done by the heart against it. The demand for oxygen in pressure overload situations is higher than in volume overload. Patients with atherosclerosis show this kind of pressure – work relationship in their cardiovascular system.