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.
What Is a Systemic Circuit?
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 the arteries. To help you remember this, think ‘a’ for ‘away,’ like ‘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. So, don’t forget: arteries carry your blood away from your heart, and veins carry your blood back to your heart.
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 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 into 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 towards 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.Note that 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.
Pause the video now and see if you can find it!Once it enters your abdomen, your aorta gives off branches that supply blood to all of your internal organs, including your stomach, liver, gallbladder, pancreas, spleen, kidneys, and intestines. Just below the level of your belly button, your aorta divides into two vessels called the iliac arteries (the aorta looks like an upside-down Y at this point). The iliac arteries send off several branches to your pelvic organs prior to entering your legs as your femoral arteries.
Once oxygenated blood has made its way to all your tissues, it re-gathers in your veins and begins its journey back to your heart. In general, the larger arteries in your body are mirrored by similar-sized veins that carry blood in the opposite direction. Many, but not all, veins have the same names as their arterial counterparts.The veins returning deoxygenated blood from each side of your head are called jugular veins. These join the subclavian veins from your arms to become brachiocephalic veins, which themselves join to form the superior vena cava before it enters the right atrium of your heart.Similarly, the deoxygenated blood returning from your legs travels through the femoral veins to the iliac veins, which join to become the inferior vena cava.
The inferior vena cava then ascends alongside your aorta, gathering veins from your internal organs, until it (like the superior vena cava from above) enters your heart at the right atrium, thus completing the systemic circuit.As mentioned above, the deoxygenated blood in your right atrium is then channeled into your right ventricle, from which it is pushed into the pulmonary circuit to travel to your lungs.
The systemic circuit is the part of your circulatory system that carries oxygenated blood from your heart, delivers it to your body, and returns deoxygenated blood to your heart. The four-chambered design of your heart separates oxygenated blood from the deoxygenated blood and serves as a routing station between the pulmonary circuit and the systemic circuit.The aorta is the first artery in the systemic circuit, and it is the largest artery in your body. Multiple arterial branches arising from the aorta deliver oxygenated blood to your heart, head, upper extremities, chest wall, spinal cord, internal organs, and lower extremities.Arteries divide into smaller arterioles, which themselves divide into capillaries, where oxygen delivery is accomplished.
Capillaries rejoin to become venules, which then become veins. Veins from all areas of your body return deoxygenated blood to your heart via the inferior and superior vena cava, completing the systemic circuit.
Your goal at the end of the lesson should be to:
- Describe what the systemic circuit is
- Demonstrate an understanding of what the circulatory system consists of
- Explain the way that blood travels through the systemic circuit