The heart (fig 2.19)
The heart lies in the middle of the chest behind the breast bone and rests on the upper part of the diaphragm. It is conical in shape, about the size of a closed fist, and lies with two-thirds of its bulk to the left of the vertical middle line. One can feel it beating in the space between the fifth and sixth ribs on the left side of the breast bone, a little to the inside in line with the nipple.
The heart is enclosed in a sac called the pericardium, and between the heart and the sac is a thin layer of fluid; this allows heart movement with the least possible amount of friction.
The heart is divided into four chambers: two upper receiving chambers, the right and the left atria, and two lower expelling chambers, the right and left ventricles. The walls of the heart are formed of muscle, and the walls of the ventricles are much thicker that those of the atria.
A muscular partition completely divides the heart into a right half and left half, so that no blood can pass directly from one half of the heart to the other. Each side of the heart therefore has its atrium or receiving chamber, and its ventricle or expelling chamber, so that there are two distinct systems of blood circulation in the body.
The first is the circulation (already described), to and from the tissues of the body generally, nourishing them, supplying them with pure or arterial blood laden with oxygen, and bringing back to the heart impure or venous blood laden with carbon dioxide. This circulation system starts in the left ventricle, follows through the aorta, and brings the blood back by the large veins to the right atrium. It is called the general or systemic circulation (fig 2.15, 2.20).
Second, there is the circulation system that conveys the impure blood to the lungs. In the capillaries the blood gives off carbon dioxide to the air and takes up oxygen. The blood is then returned to the heart purified in the sense that it has rid itself of the impurity, carbon dioxide, and has regained its full load of oxygen. In this system of circulation the blood leaves the right ventricle by the pulmonary artery, and returns by the pulmonary veins to the left atrium. It is called pulmonary or respiratory circulation (fig 2.15).
These two circulation systems are quite distinct and are different in purpose, the systemic circulation nourishing the tissues, the respiratory circulation purifying the blood.
On the right side of the heart, then, we find the great veins bringing the impure blood to the right atrium, from which it passes to the right ventricle, and from this again, by the pulmonary artery to the lungs. The right side of the heart therefore contains only impure or venous blood.
On the left side of the heart we find the pulmonary veins bringing pure or arterial blood from the lungs to the left atrium and from there to the left ventricle, from which it passes to the arteries of the body generally. The left side of the heart therefore contains only pure or arterial blood.
If we follow the course of a drop of blood in the right atrium it must pass to the right ventricle and through the pulmonary circulation to the left atrium, then to the left ventricle and through some part of the systemic circulation, before it can come back to the right atrium again.
In summary, all the blood from the lungs passes to the left atrium and all the blood from the rest of the body passes to the right atrium.