Blood circulation in the human body is a very complicated process that occurs within an intricate system. To compare to other creatures, insects and spiders have open circulatory systems in which blood is pumped forward by the heart and then flows through the whole body cavity, bathing all the internal organs in blood. Vertebrates, such as humans, have closed circulatory systems in which the blood remains within the circulatory network as it circulates. The human system that circulates blood is called the cardiovascular system and it is made up of the heart plus our arteries and veins. The distinction between arteries and veins is made by the direction of blood flow. Veins carry blood towards the heart from all over the body. Arteries carry blood away from the heart to reach every point in our body. This means blood is flowing in opposite directions at every point in our body: from your hands to you heart and your heart to your hands; from your heart to your brain and your brain to your heart; from your heart to your feet and your feet to your heart, etc. That´s a lot of different directions!

To make matters more complicated, we actually have a system of double circulation: we have a completely separate `pulmonary circuit´ to carry blood without oxygen to the lungs to get oxygen and then to take that oxygenated blood back to the heart to get pumped to the rest of the body, and a separate `systemic circuit´ that takes blood to and from every point in the body.

The typical adult body contains 5 liters of blood which can complete an entire circuit through the body in about one minute: the blood recirculates roughly 1500 times each day!