The human heart lies with its apex touching the chest-wall between the fifth and sixth ribs on the left side of the breast-bone. At every beat a sort of tap known as the " cardiac impulse," or " apex beat," may be felt by placing the finger at that point.

Events Occurring Within The Heart During A Cardiac Period

Let us commence just after the end of the ventricular systole. At this moment the semilunar valves at the orifices of the aorta and the pulmonary artery are closed so that no blood can flow back from those vessels. The whole heart, however, is soft and distensible, and yields readily to blood flowing into its auricles from the pulmonary veins and the hollow veins; this blood passes on through the open mitral and tricuspid valves, and fills up the dilating ventricles as well as the auricles. As the ventricles fill, back currents are set up along their walls, and carry up the flaps of the auriculo-ventricular valves, so that by the end of the pause they are nearly closed. At this moment the auricles contract; this contraction commences at and narrows the mouths of the veins so that blood cannot easily flow back from the auricles into them; the flabby and dilating ventricles oppose much less resistance, and so the general result is that the contracting auricles send blood mainly into the ventricles and hardly any back into the veins. The increased current into the ventricles produces a greater back current on the sides, which, as the auricles cease their contraction, and the filled ventricles become tense and press on the blood inside them, completely closes the auriculo-ven-tricular valves.

How during the systole? How is its bulk changed in systole?

Where does the apex of the heart touch the chest-wall ? What is the cardiac impulse ?

What is the position of the semilunar valves just after the end of a ventricular systole? What results from their closure? In what condition is the heart in general ? What parts of it does blood enter ? From what vessels? What cavities does this blood fill ? What happens as the ventricles till ? What is the position of the valves at the end of the pause ? Where does the auricular contraction commence ? What is the main result of the auricular contraction ?

The auricular contraction now ceases, and the ventricular begins. The blood in each ventricle is imprisoned between the auriculo-ventricular valves behind and the semilunar valves in front. The former cannot yield on account of the chordae tendineae fixed to their edges; the semilunar valves, on the other hand, can open outwards from the ventricle and let the blood pass on; but they are kept tightly shut by the pressure of the blood in the aorta and pulmonary artery, just as the lock-gates of a canal are by the pressure of the water on them. In order to open the canal-gates water is let in or out of the lock until it stands at the same level on each side of them; but they might be forced open without this by applying sufficient power to overcome the higher water pressure on one side. It is in this latter way that the semilunar valves are opened.

The contracting ventricle tightens its grip on the blood inside it. As it squeezes harder and harder, at last the pressure on the blood in it becomes greater than the pressure exerted on the other side of the valves by the blood in the arteries, the valves are forced open, and the blood begins to pass out; the ventricle continues to contract until it has obliterated its cavity and completely emptied itself. Then it commences to relax, and blood to flow back into it from the arteries. This back current, however, catches the pockets of the semilunar valves, drives them back, and closes the valve so as to form an impassable barrier, and so the blood which has been forced out of the ventricle is hindered from flowing directly back into it.

What is the consequence of the increased flow into the ventricles due to the auricular contraction?

What happens when the ventricle begins to contract? Why cannot the imprisoned blood escape back into the auricle? How are the semilunar valves kept closed? Illustrate. How might we force open the gates of a canal lock without bringing the water to the same level on each side?

How are the semilunar valves opened?