Blood Flow and the Double Circulatory System

This lesson covers the double circulatory system as required by the OCR GCSE PE specification (J587). You need to understand that humans have a double circulatory system consisting of the pulmonary circuit and the systemic circuit, and be able to describe the pathway of blood through both circuits.

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What Is a Double Circulatory System?

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

1. Pulmonary circuit — carries blood between the heart and the lungs.
2. Systemic circuit — carries blood between the heart and the rest of the body.

graph LR
    A["Heart<br>(right side)"] -->|"Pulmonary artery<br>(deoxygenated)"| B["LUNGS"]
    B -->|"Pulmonary veins<br>(oxygenated)"| C["Heart<br>(left side)"]
    C -->|"Aorta<br>(oxygenated)"| D["BODY<br>(muscles, organs)"]
    D -->|"Vena cava<br>(deoxygenated)"| A

    style A fill:#4a90d9,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#9b59b6,color:#fff

Why Is a Double Circulatory System Important?

The double circulatory system is more efficient than a single circulatory system because:

• Blood passes through the heart twice, meaning it is pumped at high pressure in both circuits.
• In the pulmonary circuit, the heart pumps blood to the lungs at lower pressure (to avoid damaging the delicate alveoli).
• In the systemic circuit, the heart pumps blood to the body at higher pressure (to ensure blood reaches every organ and muscle, including those far from the heart such as the toes).
• This means oxygenated blood is delivered to the muscles at high pressure and high speed, which is essential during exercise.

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The Pulmonary Circuit

The pulmonary circuit is the loop between the heart and the lungs. Its purpose is to exchange gases — picking up oxygen and releasing carbon dioxide.

Pathway

1. Deoxygenated blood enters the right atrium from the body via the vena cava.
2. Blood passes through the tricuspid valve into the right ventricle.
3. The right ventricle pumps blood through the pulmonary valve into the pulmonary artery.
4. The pulmonary artery carries deoxygenated blood to the lungs.
5. In the lungs, blood flows through tiny capillaries surrounding the alveoli (air sacs).
6. Gaseous exchange occurs: oxygen diffuses into the blood, and carbon dioxide diffuses out of the blood into the alveoli.
7. Oxygenated blood returns to the heart via the pulmonary veins, entering the left atrium.

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The Systemic Circuit

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

Pathway

1. Oxygenated blood enters the left atrium from the lungs via the pulmonary veins.
2. Blood passes through the bicuspid valve into the left ventricle.
3. The left ventricle pumps blood through the aortic valve into the aorta.
4. The aorta branches into smaller arteries, then arterioles, then capillaries that reach every tissue in the body.
5. At the capillaries, oxygen and nutrients diffuse from the blood into the body's cells, and carbon dioxide and waste products diffuse from the cells into the blood.
6. Deoxygenated blood is collected in venules, then veins, and returns to the heart via the vena cava, entering the right atrium.

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Complete Pathway of Blood

Here is the full pathway of blood through both circuits, starting from the vena cava: