The circulatory system looks like a maze on a textbook diagram, with arrows pointing in every direction. It is actually two simple loops sharing one pump. The pulmonary circuit sends blood between the heart and the lungs to pick up oxygen. The systemic circuit sends blood between the heart and the rest of the body to deliver that oxygen. Follow one red blood cell through both loops and the whole circulatory system organizes itself.
One Pump, Two Circuits
The heart is a four-chamber pump arranged as two side-by-side pumps. The right side receives oxygen-poor blood from the body and pushes it to the lungs. The left side receives oxygen-rich blood from the lungs and pushes it to the body. The two halves beat at the same time but feed different circuits.
Each side has an atrium on top (receiving chamber) and a ventricle below (pumping chamber). Blood flow through the heart is always atrium → ventricle → great artery, never the other way, because the valves between them only open in one direction. The path is: right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta → body → vena cavae → right atrium.
This is also why "artery" does not always mean "oxygenated." Pulmonary arteries carry deoxygenated blood (they go away from the heart, which is the actual definition of an artery). Pulmonary veins carry oxygenated blood. The definitions are about direction, not oxygen content — a detail exams love to test.
The Three Vessel Types
Every blood vessel falls into one of three classes, distinguished by structure and function.
- Arteries carry blood away from the heart. They have thick muscular walls and a layer of elastic tissue so they can withstand the high pressure leaving the ventricles and recoil between beats, smoothing out the flow.
- Capillaries are the exchange vessels. They are only one endothelial cell thick — about 5–10 μm in diameter, narrower than a red blood cell itself — so oxygen, nutrients, and waste can diffuse across their walls. Almost everything the circulatory system actually delivers, it delivers across a capillary wall.
- Veins carry blood back to the heart. They have thinner walls than arteries and a much larger lumen. Because pressure in veins is low, many of them — especially in the legs — contain one-way valves that prevent backflow, and they rely on the squeezing action of skeletal muscle to push blood upward.
Pressure falls dramatically across the system. Blood leaves the left ventricle at about 120/80 mmHg, drops to roughly 35 mmHg entering capillaries, and arrives back at the right atrium at near zero. The pulmonary circuit runs at much lower pressures — roughly 25/8 mmHg — because the lungs sit right next to the heart and offer less resistance.
The Pulmonary Circuit: Heart to Lungs and Back
The pulmonary circuit is short. Deoxygenated blood, returning from the body, enters the right atrium from the superior and inferior vena cavae. It drops through the tricuspid valve into the right ventricle, which contracts and pushes it through the pulmonary semilunar valve into the pulmonary trunk. The trunk splits into the left and right pulmonary arteries, one to each lung.
Inside the lungs the arteries branch down to pulmonary capillaries that wrap each alveolus. Here oxygen diffuses in and carbon dioxide diffuses out. The newly oxygenated blood collects into four pulmonary veins that empty into the left atrium. That completes the pulmonary loop.
The Systemic Circuit: Heart to Everywhere Else
From the left atrium, blood drops through the bicuspid (mitral) valve into the left ventricle, the strongest chamber. The left ventricle contracts and pushes blood through the aortic semilunar valve into the aorta, the body's largest artery.
The aorta gives off branches to every region — coronary arteries to the heart muscle itself, the brachiocephalic and carotid arteries to the brain and arms, and (after arching down through the chest and abdomen) renal, mesenteric, and iliac arteries that supply the kidneys, gut, and legs. Each branch divides repeatedly into arterioles and then into capillary beds, where oxygen and nutrients are exchanged for carbon dioxide and waste.
Blood returning from the body collects into venules, then veins, and finally into the two vena cavae — the superior vena cava drains the head, neck, and arms; the inferior vena cava drains everything below the diaphragm. Both empty into the right atrium, and the cycle starts again.
One Red Blood Cell, Foot to Foot
Trace a single red blood cell and the whole system comes together. Start in a capillary in your left big toe. The red blood cell has just dropped off oxygen and picked up carbon dioxide. It drains into a venule, into the femoral vein, into the inferior vena cava, into the right atrium → right ventricle → pulmonary artery → lung capillary. There it dumps CO₂ and reloads with O₂. It flows through a pulmonary vein into the left atrium → left ventricle → aorta. From the aorta it could go anywhere — say back down through the abdominal aorta, the iliac artery, the femoral artery, into smaller and smaller arteries, finally into a toe capillary again. Two circuits, one pump, one cell — and the whole journey takes about a minute at rest.
Getting Help
The mechanical events that move the blood — atrial systole, ventricular systole, the heart sounds — are the subject of the cardiac cycle explained. For more cardiovascular walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
The circulatory system explained as two loops is straightforward: the pulmonary circuit oxygenates blood at the lungs, and the systemic circuit delivers that oxygen to the body. Arteries leave the heart, veins return to it, capillaries do the actual exchange. Follow one red blood cell through both loops and every label on the diagram has a place.
