Coronary circulation in animals
Coronary circulation in animals starts as coronary orifices in the aorta and supplies the blood to the heart muscles through right and left coronary arteries. These orifices are in free communication with aorta both during systole and diastole.
The main coronary artery branches into right and left coronary arteries. The left coronary artery is more important in dog, cat and ruminants, whereas in man, swine, horse and some primates the right coronary artery is more prominent.
The left coronary artery gives of the left circumflex branch which runs along the auriculo-ventricular groove and distributes about 80% of the total blood flow to the heart.
It supplies mainly the atria and left ventricle, A.V. node, inter ventricular septum. The right coronary artery nourishes the right ventricular muscles.
The venous system of the heart comprises the great cardiac veins, which empties through the coronary sinus drains the blood supplied by the left coronary artery into the right atrium of the heart, whereas the anterior cardiac veins collect the blood supplied by the right coronary artery and discharge their blood into the right atrium. Small amount of blood is drained through the besian veins directly into the right ventricle and other chambers of the heart.
About 4% of the cardiac output flows through the coronary arteries to meet the very high oxygen demand by the cardiac muscles under normal condition (10-15 ml of O2 is taken by cardiac muscle from each 100 ml of blood as against 5 ml/100 ml in skeletal muscles). This blood supply may be increased to 40% during heavy muscular exercise.
In the left coronary artery the blood flow is decreased during systole because of the high coronary resistance developed by the contraction of ventricular muscles. However, this gets reversed during ventricular diastole which shows increased coronary blood flow. On the other hand the right coronary artery shows increased blood flow during systole and lower during diastole.
If a coronary artery is gradually blocked by stenosis, there may be a progressive development of new blood vessels from the pre-existing capillaries, known as the collateral vessels.
Regulation of coronary circulation
Heart rate
Higher the heart rates, lesser the duration of diastole, and lowers the coronary flow.
Aortic blood pressure
Increase in aortic pressure causes increased coronary flow.
Peripheral resistance
More the peripheral resistance greater will be the aortic pressure and coronary flow increases.
Autonomic control
Both the sympathetic (ß – adrenergic receptors (vasodilators), α – receptors (vasoconstrictors) and parasympathetic vasodilators are the primary autonomic control on the coronary arteries. Stimulation of ß1 and ß2 adrenergic receptors results in vasodilatation in major parts of the large coronary arteries, whereas the vasoconstrictor tone by α adrenergic receptors regulates the resting in coronary blood flow. The overall effect of stimulation of sympathetic nerves is coronary vasodilatation. Vasodilatation by vagus is less important.
Norepinephrine and neuropeptide Y are the neurotransmitters of sympathetic nerves of which neuropeptide Y is strong vasoconstrictor particularly in coronary vasculature.
Acetyl choline, the neurotransmitters of the parasympathetic system acts on the cholinergic receptors of the endothelium, causes the release of endothelium derived relaxing factor (EDRF). The EDRF acts on the smooth muscles of the blood vessel causes vasodilatation.
METABOLIC CONTROL
The most important controlling factor of coronary circulation is oxygen demand. Lack of oxygen in tissue release adenosine, which is a potent vasodilator and other metabolic products like CO2, H+ and lactic acid also produce vasodilatation.
CORONARY REFLEXES
Naturally mediated coronary vasomotor reflex effects can be elicited from the carotid sinus and aortic body.
Increased venous inflow reflex stimulates vagus and causes reflex vasoconstriction in abdominal and thoracic organs to divert increased coronary flow by vasodilatation