The human body

How does the blood circulatory system work?

The blood circulatory system (cardiovascular system) delivers nutrients and oxygen to all cells in the body. It consists of the heart and the blood vessels running through the entire body. The arteries carry blood away from the heart; the veins carry it back to the heart. The system of blood vessels resembles a tree: The “trunk,” the main artery (aorta), branches into large arteries, which lead to smaller and smaller vessels. The smallest arteries end in a network of tiny vessels, the capillary network.

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 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.

Illustration: Pulmonary and systemic circulation
 
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) which then expand. The following phase is called ejection period, which is when both ventricles pump the blood into the large arteries.

In the systemic circulation, the left ventricle pumps oxygen-rich blood into the main artery (aorta). The 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 starts: 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 form a fine network around the pulmonary vesicles, grape-like air sacs at the end of the airways. This is where carbon dioxide is released from the blood into the air contained in the pulmonary vesicles 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 next heart beat starts a new cycle of systemic circulation.

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