How does the heart pump blood around the body, and why do we have a double circulation?
Describe the structure of the human heart and the circulatory system, and explain how blood is pumped around the body in a double circulation
A scaffolded answer to the N(A)-Level Biology outcome on the human circulatory system. The structure of the heart, the three types of blood vessel, and how a double circulation carries blood to the lungs and the body.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
This outcome wants you to describe the main parts of the human heart and circulatory system, name the three types of blood vessel, and explain how blood is pumped in a double circulation. You should be able to label a simple diagram of the heart, trace the path of blood, and link the structure of each vessel to its job. The marks reward clear, ordered description and connecting structure to function.
The answer
The heart
The heart is a muscular pump made mostly of muscle that never gets tired. It has four chambers: two at the top called atria (one atrium each side) and two at the bottom called ventricles. The right side and the left side are completely separated by a wall, so oxygen-rich and oxygen-poor blood never mix.
- The atria receive blood coming back to the heart.
- The ventricles pump blood out of the heart. The left ventricle has the thickest, most muscular wall because it pumps blood all the way around the body.
- Valves inside the heart make sure blood flows one way only and cannot flow backwards.
The three blood vessels
Blood travels through three kinds of vessel:
- Arteries carry blood away from the heart. They have thick, muscular, elastic walls to cope with the high pressure of blood leaving the heart.
- Veins carry blood back to the heart. They have thinner walls and contain valves, because the blood is at low pressure and the valves stop it flowing backwards.
- Capillaries are tiny vessels with walls one cell thick. They reach every cell, and their thin walls let substances such as oxygen, glucose and waste pass between the blood and the cells.
Double circulation
Humans have a double circulation, which means the blood passes through the heart twice for each full trip around the body. There are two loops:
- The pulmonary circulation carries blood from the heart to the lungs and back. Here the blood picks up oxygen and gives up carbon dioxide.
- The systemic circulation carries blood from the heart to the rest of the body and back, delivering oxygen and glucose and collecting waste.
Because the blood is pumped again after it returns from the lungs, it travels around the body at high pressure, which keeps it moving quickly to all the cells.
Examples in context
Example 1. Why you can feel a pulse. A pulse is the surge of high-pressure blood pushed out each time the heart beats. You feel it where an artery runs close to the skin, such as the wrist or neck. You cannot feel a pulse in a vein because the blood there is at low, steady pressure.
Example 2. Why a blocked coronary artery is dangerous. The heart muscle has its own blood vessels, the coronary arteries, to supply it with oxygen. If one is blocked, part of the heart muscle is starved of oxygen and can be damaged, which is what happens in a heart attack. This shows that even the pump itself needs a blood supply.
Try this
Q1. Name the chamber of the heart that pumps blood to the whole body. [1 mark]
- Cue. The left ventricle.
Q2. State two differences between an artery and a vein. [2 marks]
- Cue. An artery has a thick, muscular wall and no valves; a vein has a thin wall and has valves to prevent backflow.
Q3. Explain why humans are said to have a double circulation. [2 marks]
- Cue. The blood passes through the heart twice for each complete trip: once to the lungs and once to the body, so it is pumped at high pressure to all cells.
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.
Original4 marksDescribe the path of one drop of blood as it travels from the right side of the heart, to the lungs, and back to the heart. Name the blood vessels involved.Show worked answer →
Blood leaves the right side of the heart from the right ventricle. It is pumped through the pulmonary artery to the lungs. In the lungs the blood picks up oxygen and loses carbon dioxide. The blood then returns to the heart through the pulmonary vein, entering the left side of the heart (the left atrium).
What markers reward: the correct order (right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium), the two named vessels, and the idea that gas exchange happens at the lungs. A common slip is saying the pulmonary artery carries oxygenated blood; here the artery carries deoxygenated blood to the lungs.
Original4 marksExplain two ways the structure of an artery is different from a vein, and link each difference to its job.Show worked answer →
Difference 1: an artery has a thick, muscular and elastic wall, while a vein has a much thinner wall. The thick wall lets the artery stretch and withstand the high pressure of blood being pumped straight from the heart.
Difference 2: a vein has valves along its length, while an artery does not. The valves stop blood flowing backwards, because blood in veins is at low pressure and could otherwise slip back.
What markers reward: two clear structural differences, each linked to function (thick wall for high pressure; valves to prevent backflow in low-pressure blood). Naming a feature with no explanation scores only half.
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