Systemic Circuit: Definition, Blood Flow & Quiz

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Stephen Christensen

In this lesson you will explore a part of your circulatory system known as the systemic circuit. You will learn how it delivers blood to your tissues and then returns it to your heart. A quiz at the end of the lesson will test your knowledge.

We also recommend watching Anatomy of the Heart: Blood Flow and Parts and Total Peripheral Resistance & Blood Flow Regulation

Definition

The systemic circuit is that part of your circulatory system that carries blood away from your heart, delivers it to most of your organs and tissues, and returns it to your heart again. The systemic circuit is distinct from the pulmonary circuit, which only conducts blood between your heart and lungs.

Circulatory System Anatomy

A thorough understanding of the systemic circuit requires at least a basic understanding of your entire circulatory system. Your circulatory system consists of your heart, which is a muscular, four-chambered pump, and a series of closed tubes - your blood vessels - that carry blood away from your heart, deliver it to all of your tissues, and then return it to your heart.

The blood vessels that carry blood away from your heart are called arteries (a for away, a for artery). As they travel farther from your heart, your arteries divide into progressively smaller vessels called arterioles, which themselves divide into even smaller vessels called capillaries. It is within your capillaries that oxygen and nutrients are delivered to your tissues, and carbon dioxide and cellular waste products are picked up.

Once your capillaries have delivered oxygen to your tissues and gathered up the carbon dioxide from your cells, they join into progressively larger vessels called venules, which join into even larger veins as they course toward your heart. Eventually, your veins return the blood that has traveled throughout your body to your heart.

The circulatory system consists of your heart, arteries, arterioles, capillaries, venules, and veins.
Circulatory System

Separate Circuits

Although your circulatory system is one continuous, closed loop, it is convenient to divide it into two circuits that have distinctly different purposes. The pulmonary circuit carries deoxygenated blood away from your heart, delivers it to your lungs where it picks up oxygen, and returns oxygenated blood to your heart.

The pulmonary circuit carries blood between the heart and lungs and is distinct from the systemic circuit.
Pulmonary Circuit

The systemic circuit carries oxygenated blood away from your heart, takes it to the rest of your body where its oxygen is delivered, and returns deoxygenated blood to your heart. The four-chambered design of your heart is necessary for separating oxygenated blood from deoxygenated blood and for routing your blood back and forth between the pulmonary circuit and the systemic circuit.

Oxygenated blood returning from your lungs enters the left atrium of your heart and is quickly channeled through a one-way valve into your left ventricle. As your heart contracts, it forces the oxygenated blood through another one-way valve into your aorta, which is the largest artery in your body and the first artery in your systemic circuit.

When deoxygenated blood returns from the systemic circuit, it enters your heart's right atrium, flows through a one-way valve to the right ventricle, and gets pushed through another one-way valve to reenter the pulmonary circuit and head for your lungs.

Journey through the Systemic Circuit

With each contraction of your heart, oxygenated blood is pushed farther into your systemic circuit. Since oxygen must be carried to every organ of your body before it returns to your heart, your systemic circuit contains innumerable arteries, arterioles, capillaries, venules, and veins.

Just as your aorta leaves your heart, it sends two small branches - the coronary arteries - along the outer wall of the heart to supply oxygenated blood to the hard-working heart muscle. Just beyond the origins of the coronary arteries, as it arches over and downward to serve the lower half of your body, your aorta gives off large arterial branches to your arms and head. The first branch is the brachiocephalic artery. This large arterial trunk quickly divides into the right common carotid artery, which supplies the right side of your head and brain, and the right subclavian artery, which serves your right arm.

The second branch arising from the aortic arch is the left common carotid artery, which travels toward the left side of your head. The third and last branch arising from the aortic arch is the left subclavian artery, which heads for your left arm. The aorta then dives downward along your backbone. As it courses toward your abdomen, your aorta sends out small arterial branches that supply blood to your ribs, spinal cord, and the muscles of your chest wall.

The aortic arch. Note the emerging coronary arteries and the first major arterial branches that serve the head and upper extremities.
Aortic Arch

(If you place your fingers alongside your neck just below the angle of your jaw, you can feel the pulsations of your carotid artery - there's one on each side. Similarly, you can feel a pulse wherever a major artery lies close to the surface of your body, such as at your wrist or in your groin.)

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