How does blood travel around the human body, and why does it pass through the heart twice?
Describe the double circulatory system in humans and the path of blood around the body
A focused answer to the O-Level Biology outcome on the human circulation. The double circulatory system, the pulmonary and systemic circuits, the path of blood, and why a double circulation is an advantage.
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What this dot point is asking
SEAB wants you to describe the human circulatory system as a double circulation, to name the two circuits (pulmonary and systemic) and what they serve, to trace the path of blood through the heart and around the body, and to explain why having a double circulation is an advantage. Knowing oxygenated and deoxygenated blood is key.
The answer
A double circulation
In humans the blood passes through the heart twice for each complete journey around the body. This is called a double circulation, and it is made of two linked circuits.
The two circuits
Pulmonary circulation: carries blood between the heart and the lungs. Deoxygenated blood is pumped from the heart to the lungs, where it picks up oxygen and loses carbon dioxide, then returns to the heart.
Systemic circulation: carries blood between the heart and the rest of the body. Oxygenated blood is pumped from the heart to the body organs, where it delivers oxygen and food and collects carbon dioxide, then returns to the heart.
The path of blood
Following the blood around:
- Deoxygenated blood from the body enters the right atrium, then the right ventricle.
- The right ventricle pumps it to the lungs (pulmonary circulation), where it becomes oxygenated.
- Oxygenated blood returns to the left atrium, then the left ventricle.
- The left ventricle pumps it through the aorta to the body (systemic circulation), where it gives up its oxygen.
- The now deoxygenated blood returns to the right atrium, and the cycle repeats.
The right side of the heart handles deoxygenated blood (to the lungs); the left side handles oxygenated blood (to the body).
Why a double circulation is an advantage
Because the blood returns to the heart after the lungs, it can be pumped again at high pressure to the body. This makes the blood flow quickly, so oxygen and food are delivered to body cells fast and waste is removed efficiently. A single circulation would lose pressure at the lungs and deliver blood to the body more slowly.
Examples in context
Example 1. Why a fish is different. A fish has a single circulation: blood passes through the heart once, then to the gills, then straight to the body, losing pressure at the gills. The human double circulation keeps the pressure high, suiting an active, warm-blooded animal.
Example 2. Exercise and blood flow. During exercise, the heart pumps faster and harder, raising blood pressure and speeding the systemic circulation so working muscles get oxygen and glucose quickly. The double circulation makes this rapid delivery possible.
Try this
Q1. Explain what is meant by a double circulation. [2 marks]
- Cue. The blood passes through the heart twice for each complete circuit of the body.
Q2. Name the circuit that carries blood between the heart and the lungs. [1 mark]
- Cue. The pulmonary circulation.
Q3. State one advantage of a double circulation over a single one. [2 marks]
- Cue. Blood can be pumped to the body at high pressure after returning from the lungs, so it flows quickly and delivers oxygen and food faster.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksThe human circulatory system is described as a double circulation. (a) Explain what this means. (b) Name the two circuits and the part of the body each one serves. (c) State one advantage of a double circulation.Show worked answer →
(a) A double circulation means the blood passes through the heart twice for each complete circuit of the body.
(b) The pulmonary circulation carries blood between the heart and the lungs. The systemic circulation carries blood between the heart and the rest of the body.
(c) One advantage: the blood can be pumped to the body at high pressure (after being returned to the heart from the lungs), so it travels quickly and delivers oxygen and food to cells faster than a single circulation could.
Markers reward blood passing through the heart twice per circuit, the two named circuits with what they serve, and an advantage such as higher pressure and faster delivery.
Original4 marksDescribe the path of a red blood cell starting from the lungs, naming the heart chambers it passes through, until it reaches a body organ such as the leg.Show worked answer →
From the lungs the blood (now oxygenated) returns to the left atrium of the heart. It passes into the left ventricle, which pumps it out through the aorta. The aorta branches into arteries that carry the blood to body organs such as the leg.
Markers reward the correct order: lungs, left atrium, left ventricle, aorta, then arteries to the body. Naming the right side of the heart for oxygenated blood, or reversing atrium and ventricle, loses marks.
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