Systemic and Pulmonary Blood Circulation

Pulmonary circulation carries blood between the heart and the lungs to pick up oxygen, and systemic circulation carries blood between the heart and the rest of the body to deliver it. In brief: blood flows in a figure-eight pattern through the heart twice per circuit. The path is: body → right atrium → right ventricle → lungs → left atrium → left ventricle → body. Arteries carry blood away from the heart, veins carry blood toward it, and capillaries connect them and exchange materials with tissues.

Understanding the route of blood and the vessel types is central to understanding how the body delivers oxygen and nutrients. This guide traces the two circuits, compares arteries, veins and capillaries, explains how structure fits function, works through a path-tracing example and corrects common mistakes.

Why is circulation double?

In a single circulation, such as that of a fish, blood passes through the heart once per circuit, and the pressure drops after it passes through the gills. In mammals and birds there are two circuits. After blood has passed through the lungs, it returns to the heart for a second push, which gives it enough pressure to reach all tissues quickly.

This design supports the high metabolic rate of warm-blooded animals, because it keeps oxygen-rich blood from mixing with oxygen-poor blood and delivers it at high pressure.

What is pulmonary circulation?

Pulmonary circulation is the loop between the heart and the lungs. Its job is to exchange gases.

1. Oxygen-poor blood leaves the right ventricle through the pulmonary artery. This is the only artery (except for the umbilical artery of a fetus) that carries oxygen-poor blood.
2. The pulmonary artery splits into two, one for each lung, and branches into smaller vessels, ending in capillaries around the alveoli.
3. In the capillaries, blood releases carbon dioxide and picks up oxygen.
4. Oxygen-rich blood returns to the left atrium through four pulmonary veins.

The pulmonary circuit is short and operates at lower pressure than the systemic circuit, because the lungs are delicate and the walls of the capillaries are very thin.

What is systemic circulation?

Systemic circulation is the loop from the heart to the organs and tissues of the body and back. Its job is to deliver oxygen and nutrients and remove wastes.

1. Oxygen-rich blood leaves the left ventricle through the aorta, the largest artery, which is about 2.5 centimeters wide near the heart.
2. The aorta branches into arteries that supply the head, arms, trunk and legs, and these divide into arterioles and capillaries within each organ.
3. In the capillaries, oxygen and nutrients pass into tissue fluid and then into cells, while carbon dioxide and wastes move into the blood.
4. Oxygen-poor blood flows from the capillaries into venules, then into veins, and returns to the right atrium through the superior vena cava (from the upper body) and the inferior vena cava (from the lower body).

Some organs have special arrangements. The hepatic portal vein carries blood from the intestine to the liver before it returns to the heart. The kidneys receive about 20 to 25 percent of cardiac output through the renal arteries, and blood from the kidneys returns through the renal veins. The coronary circulation supplies the heart muscle itself.

What are arteries, veins and capillaries?

Arteries

Arteries carry blood away from the heart under high pressure. They have thick, muscular, elastic walls that withstand pressure and stretch with each heartbeat, which can be felt as a pulse. The lumen (the central space) is narrow compared with the wall thickness. Smaller arteries are called arterioles, and their muscle controls blood flow into organs.

Veins

Veins carry blood back toward the heart at low pressure. They have thinner walls with less muscle, a wide lumen and valves that stop backflow. Blood is pushed through veins by the squeezing action of nearby skeletal muscles and by breathing movements. Smaller veins are called venules.

Capillaries

Capillaries are the smallest vessels, about 5 to 10 micrometers wide, so red blood cells pass through in single file. Their wall is a single layer of flat cells, which makes diffusion distances very short. Capillary networks are so extensive that almost every cell is within a few cell widths of one.

Blood vessels compared

Worked example: tracing a red blood cell

Trace one red blood cell from the moment it returns from the big toe, to a muscle cell in the arm, and back to the toe.

1. The cell travels in veins of the leg with oxygen-poor blood and enters the inferior vena cava.
2. It enters the right atrium, passes the tricuspid valve and is pumped by the right ventricle into the pulmonary artery.
3. In lung capillaries the cell loads oxygen and returns through a pulmonary vein to the left atrium.
4. It passes the mitral valve into the left ventricle and is pumped into the aorta.
5. It goes along the subclavian artery and brachial artery to the capillaries in an arm muscle and releases oxygen.
6. It returns through veins to the vena cava and the right atrium, completing the circuit.

In this trip the cell goes through the heart twice, passes through two capillary beds (lung and muscle) and travels through the right side before the left. To check an answer on any similar question, remember the sequence: right heart, lungs, left heart, body.

The circulation time for a typical red blood cell is about 60 seconds at rest, which is consistent with the cardiac output of about 5 liters per minute and a total blood volume of 5 liters.

How was circulation discovered?

William Harvey’s 1628 measurements showed that the heart pumps out more blood in an hour than the body could possibly make, so blood must circulate. He predicted that tiny vessels connect arteries and veins, but they were seen only after Malpighi used a microscope in 1661. Ibn al-Nafis, in the thirteenth century, had already described the passage of blood through the lungs, and Michael Servetus described it in the sixteenth century.

Why does it matter in medicine?

Many diseases involve vessels. High blood pressure strains arteries, and atherosclerosis narrows them with fatty deposits. Varicose veins form when valves fail. Blood pressure is measured with a cuff on the arm artery, and the pulse can be taken at the wrist, neck or ankle.

Medical imaging and catheters follow the pathways described here. Surgeons can bypass a blocked coronary artery with a vessel taken from the leg, and some babies are born with holes between heart chambers that mix blood from the two circuits.

Which mistakes are common?

Mistake 1: Defining arteries by oxygen content

Arteries are vessels that carry blood away from the heart. The pulmonary artery carries oxygen-poor blood.

Mistake 2: Saying veins always carry oxygen-poor blood

The pulmonary veins carry oxygen-rich blood to the heart.

Mistake 3: Forgetting that capillaries have no valves or thick walls

They are one cell thick to allow exchange. Valves are found in veins.

Mistake 4: Describing blood as passing through the heart once per circuit

In humans it passes twice, in a double circulation.