The circulatory system is your body’s way of getting nutrients, oxygen, and other essential elements to its cells. It is a closed loop system with the heart as its pump and blood vessels as its distribution network.

The Circulatory System

It all starts with the heart. Your heart has two halves:

The left side, made up of the left atrium and ventricle, receives oxygen-rich blood from the lungs and distributes it out to the body through your arteries.

The right side, made up of the right atrium and ventricle, receives returning deoxygenated blood from the body brought back by your veins.

There are really two separate circulations in the circulatory system: systemic and pulmonary. Here’s how they work:

Systemic Circulation

The left ventricle pumps oxygen-rich blood to your body’s capillaries [the smallest of your blood vessels that actually connect your arteries (delivery vessels) to your veins (return vessels)]. Once they deliver their cargo of nutrients and oxygen, they return to the right atrium.

The Heart

Pulmonary Circulation

The now oxygen-depleted blood, loaded with carbon dioxide (CO₂) in the right atrium passes to the right ventricle which pumps it to the lungs, ditches its CO₂ ,and replenishes it with oxygen. It then returns to the left atrium where it passes to the left ventricle and starts the cycle all over again.

Blood Vessels

As oxygen-rich blood leaves the heart, it’s pumped through your largest artery, the aorta. From there it branches out the supply your head, internal organs, and limbs. As the vessels get further from the heart, they become smaller in diameter — from the aorta, to smaller arteries, to arterioles, to capillaries.

At their opposite ends, these same capillaries join with blood vessels that return the oxygen-depleted blood to the heart. As the diameter increases, they grow from capillaries, to venules, to veins. The two largest veins in your body are the superior and inferior vena cavae; they drain into the heart from the head and from below the heart.

The circulatory system — another of your resident engineering marvels.