There is not only one blood circulatory system in the human body, but two, which are connected: The systemic circulation provides organs, tissues and cells with blood so that they get oxygen and their other vital substances. The pulmonary circulation is where the fresh oxygen we breathe in enters the blood. At the same time, carbon dioxide is released from the blood.
Circulation starts when the heart relaxes between two heartbeats: blood flows from both atria (the upper two chambers of the heart) into the ventricles (the lower two chambers). The following phase is called ejection period, which is when both ventricles pump blood into the large arteries.
In the systemic circulation, the left ventricle pumps blood into the main artery (aorta). Oxygen-rich blood travels from the main artery to larger and smaller arteries into the capillary network. There the blood releases oxygen, nutrients and other important substances and takes on carbon dioxide and waste substances. The blood, which is now low in oxygen, is now collected in veins and travels to the right atrium and into the right ventricle.
Now pulmonary circulation start: The right ventricle pumps blood that carries little oxygen into the pulmonary artery, which branches off into smaller and smaller arteries and capillaries. The capillaries from a fine network around the pulmonary vesicles, grape-like air sacs at the end of the end of the airways. This is where carbon dioxide is released from the blood and fresh oxygen enters the bloodstream. When we breathe out, carbon dioxide leaves our body. Oxygen-rich blood travels through the pulmonary vein and the left atrium into the left ventricle. The net heart beat start a new cycle of systemic circulation.
