The heart as the hub
The heart is the hub between the two circulatory systems in the body. The heart is a so-called "hollow muscle" that is divided into a left and a right side separated from one another by the septum.
In the smaller pulmonary circulation, the deoxygenated blood is transported from the right ventricle through the pulmonary artery into the lungs, where it is enriched with oxygen before it flows along the pulmonary veins into the left atrium. From here, it is pumped through the left ventricle and the aorta into the large somatic circulation.
The widely branching somatic circulation supplies all the organs and regions of the body with oxygen-rich blood. The used blood (low in oxygen) flows back in the veins towards the heart against gravity, where it passes through the superior vena cava and back into the right atrium. From there, the cycle starts all over again.
The two circulatory systems are connected to each other and only function as long as they work in parallel. It is noteworthy that the pressures in the two circulatory systems differ. The fact that the pressure in the vessels of the somatic circulation is higher than in the pulmonary circulation is due to the differing blood volumes.
The heart is about 4-5 inches long and approximately 3.5 to 4.5 inches wide. It can hold about 20 to 40 ounces of blood and weighs on average three-quarters of a pound. The blood volume transported each time the heart muscle contracts at rest (stroke volume) amounts to approximately 2.5 ounces.