Double Circulatory System

This lesson covers the double circulatory system — the pulmonary and systemic circuits — as required by the Edexcel GCSE PE specification (1PE0 — Topic 1). You need to understand why humans have a double circulatory system, the pathway of blood through each circuit, and how the two circuits work together to deliver oxygen to working muscles during exercise.

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What Is a Double Circulatory System?

Humans have a double circulatory system, which means blood passes through the heart twice during one complete circuit of the body. The two circuits are:

1. Pulmonary circuit — blood travels from the heart to the lungs and back to the heart.
2. Systemic circuit — blood travels from the heart to the body and back to the heart.

graph TD
    A["RIGHT SIDE<br>of the heart"] -->|Pulmonary artery<br>(deoxygenated)| B["LUNGS<br>(gas exchange)"]
    B -->|Pulmonary veins<br>(oxygenated)| C["LEFT SIDE<br>of the heart"]
    C -->|Aorta<br>(oxygenated)| D["BODY TISSUES<br>(muscles, organs)"]
    D -->|Vena cava<br>(deoxygenated)| A

    style A fill:#4a90d9,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#c0392b,color:#fff
    style D fill:#e67e22,color:#fff

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The Pulmonary Circuit

The pulmonary circuit carries blood between the heart and the lungs. Its purpose is gaseous exchange — loading blood with oxygen and removing carbon dioxide.

Pathway

1. Deoxygenated blood leaves the right ventricle through the pulmonary valve.
2. Blood travels via the pulmonary artery to the lungs.
3. At the lungs, blood passes through pulmonary capillaries surrounding the alveoli (tiny air sacs).
4. Gaseous exchange occurs: oxygen diffuses from the alveoli into the blood; carbon dioxide diffuses from the blood into the alveoli to be exhaled.
5. The now oxygenated blood returns to the left atrium of the heart via the pulmonary veins.

Feature	Pulmonary Circuit
Blood leaves	Right ventricle
Via	Pulmonary artery
Destination	Lungs
Purpose	Pick up O₂, release CO₂
Blood returns to	Left atrium
Via	Pulmonary veins
Blood changes from	Deoxygenated → oxygenated

Exam Tip: The pulmonary circuit is the shorter of the two circuits. This is why the right ventricle has a thinner wall — it only needs to generate enough pressure to push blood to the lungs, not around the entire body.

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The Systemic Circuit

The systemic circuit carries blood between the heart and the rest of the body. Its purpose is to deliver oxygen and nutrients to cells and collect carbon dioxide and waste products.

Pathway

1. Oxygenated blood leaves the left ventricle through the aortic valve into the aorta.
2. The aorta branches into smaller arteries, then arterioles, which carry blood to all tissues and organs.
3. Blood passes through capillaries in the body's tissues, where gas and nutrient exchange occurs.
4. Oxygen and nutrients diffuse from the blood into the cells. Carbon dioxide and waste products diffuse from the cells into the blood.
5. The now deoxygenated blood is collected by venules, then veins.
6. Blood returns to the right atrium of the heart via the superior and inferior vena cava.