In order to understand the circulatory system, it is good to remember the following three things:

• The heart has two halves: right and left. There is no movement of blood between them.
• Types of blood vessels: arteries carry blood from the heart, and veins carry blood to the heart. The capillaries are small vessels between the cells and the larger blood vessels.
• During one heartbeat and the following resting period, all of the following things happen:

As the heart rests, the inferior vena cava and the superior vena cava carry carbon dioxide-rich blood from all over the body to the right atrium. At the same time, oxygen-rich blood is carried from the lungs to the left atrium. From the atria, blood flows immediately to the right and left ventricles.

Pulmonary arteries carry the blood into the lungs where carbon dioxide is replaced by oxygen. This takes place in the pulmonary capillaries, where carbon dioxide is removed from the blood and oxygen is bound to red blood cells. This oxidized blood returns to the heart via the pulmonary veins.

From the left ventricle, the blood, which is now under a high pressure, exits the heart via the aorta. The arteries that branch from the aorta carry the blood everywhere in the body. Nutrients and oxygen are released to all the cells of the body through the thin walls of capillaries.

The blood flow from the left ventricle of the heart to the right atrium is called systemic circulation. The circulation of blood in the lungs (right ventricle → lungs → left atrium) is called pulmonary circulation.

Image: A = left atrium, B = left ventricle, C = right atrium, D = right ventricle, E = lungs, F = organs.
1 = aorta (artery), 2 = artery, 3 = capillaries, 4 = vein, 5 = inferior vena cava, 6 = superior vena cava, 7 = pulmonary artery, 8 = pulmonary vein, 9 = lymphatic vessel and lymph node.

Cellular respiration uses the oxygen transported by the blood and produces toxic carbon dioxide. The inferior and superior vena cava bring oxygen-poor blood from the body into the right atrium.

Blood flows through your heart and lungs in four steps: 1. The blood moves into the ventricles. 2 Blood enters the ventricle. 3. Ventricle is full of blood. 4. The ventricular muscle cells contract and blood flows from the heart to the body.