The process of one cardiac cycle is very simple yet also very effective. It is through millions of years of evolution that it has come to the relatively perfected stage it is now. During the diastole phase the atria and ventricles are relaxed and the atrioventricular valves are open. De-oxygenated blood from the superior and inferior vena cavae flows into the right atrium. The open atrioventricular valves allow blood to pass through to the ventricles. The sino-atrial node fires it’s impulse triggering the atria to contract. The right atrium empties its contents into the right ventricle. The tricuspid valve prevents the blood from flowing back into the right atrium.
During the systole phase the right ventricle receives impulses from the Purkinje fibres and contracts. The atrioventricular valves close and the semi lunar valves open. The de-oxygenated blood is pumped into the pulmonary artery. The pulmonary valve prevents the blood from flowing back into the right ventricle. The pulmonary artery carries the blood to the lungs. There the blood picks up oxygen and is returned to the left atrium of the heart by the pulmonary veins.
In the next diastole period, the semi lunar valves close and the atrioventricular valves open. Blood from the pulmonary veins fills the left atrium. (Blood from the vena cava is also filling the right atrium.) The SA node fires again triggering the atria to contract. The left atrium empties its contents into the left ventricle. The mitral valve prevents the oxygenated blood from flowing back into the left atrium.
During the systole phase the atrioventricular valves close and the semi lunar valves open. The left ventricle receives impulses from the Purkinje fibres and contracts. Oxygenated blood is pumped into the aorta. The aortic valve prevents the oxygenated blood from flowing back into the left ventricle.
The aorta branches out to provide oxygenated blood to all parts of the body. The oxygen-depleted blood is returned to the heart via the vena cava.
Whilst it is true that cardiac muscle harbours unfatiguing myogenic properties, meaning that it will contract independently of nervous impulse, it does need regulation. And this is the job of the Sino-atrial node. It is the mammalian body’s pacemaker if you will, that, with the aid of both the sympathetic and parasympathetic (vagus) nerves, regulates the speed of impulse and therefore heart rate. The sympathetic nerve speeds up heart rate whilst the vagus nerve will slow it down. This is all directed from the medulla oblongata, located in the mid-brain adjacent to the pituitary gland.
The SA node is located in the wall of the right atrium or auricle, near the point where the inferior vena cava enters the heart. Nodal tissue contracts like muscle, but the result is an electrical impulse. Each time the SA contracts it releases a wave of excitation that travels through the wall of the heart. The impulses spread rapidly, causing the two atria contract simultaneously. At the bottom of the wall separating the two atria is another node, the atrioventricular (AV) node to be precise. When the wave of excitation reaches the AV node, it is temporarily halted, which ensures that the atria will contract before the ventricles do. After this delay, the signal to contract is the purkinje fibres and on through the walls of the ventricle.
Coronary heart disease arises when the blood supply to the heart muscle is reduced by obstruction in the coronary arteries. This could have a variety of causes. During stress when glucocorticoids, noradrenalin and adrenalin are released into the bloodstream, to aid the fight or flight response. Obviously stress in today’s society does not require this sort of response so it is just left to meander about the bloodstream furring up the vessels of the body. Further and perhaps more importantly, LDL cholesterol is a major factor in the furring of vessels. This blockage may then travel toward the heart.
An artificial pace maker may be needed if the SA node is damaged. It is a device that uses electrical impulses to regulate the heart rhythm of the patient. An artificial pacemaker will make the heart beat more regularly, ensuring that the patient will live on for many years after the operation. That is of course if they can keep away from electrical interference. On such culprit is the modern microwave.
Overall, the heart is a wonderful organ, one that cannot be underestimated in the importance of human survival.
