The Heart and Circulatory System

The heart is the muscular organ at the centre of the circulatory system. In this lesson you will learn about the double circulatory system, the structure of the heart, and how blood flows through it.

The Circulatory System

The circulatory system transports substances around the body. It is made up of:

The heart — a muscular pump
Blood vessels — arteries, veins and capillaries
Blood — the transport medium

Humans have a closed circulatory system, meaning blood is always contained within blood vessels (it does not flow freely around body cavities).

Double Circulatory System

Humans have a double circulatory system. This means blood passes through the heart twice during one complete circuit of the body.

The two circuits are:

Circuit	Route	Purpose
Pulmonary circuit	Heart → lungs → heart	Blood picks up oxygen and releases carbon dioxide
Systemic circuit	Heart → body → heart	Blood delivers oxygen and nutrients to cells, and collects waste

Why a Double Circulatory System?

Because blood passes through the heart twice per circuit, it can be pumped at high pressure both to the lungs and to the rest of the body. This ensures:

Efficient gas exchange in the lungs — blood travels at moderate pressure so it has time to exchange gases in the delicate capillaries
Efficient delivery to body tissues — blood is boosted to high pressure again before being sent around the body

Exam tip: A common question asks you to explain the advantage of a double circulatory system. The key answer is: blood returns to the heart after visiting the lungs so it can be pumped at high pressure to the body, ensuring efficient delivery of oxygen and glucose to tissues.

Structure of the Heart

The heart is a muscular organ roughly the size of your fist, located slightly to the left of centre in the chest cavity. It is made of cardiac muscle, which contracts rhythmically without tiring.

The Four Chambers

Chamber	Position	Function
Right atrium (RA)	Top right	Receives deoxygenated blood from the body via the vena cava
Right ventricle (RV)	Bottom right	Pumps deoxygenated blood to the lungs via the pulmonary artery
Left atrium (LA)	Top left	Receives oxygenated blood from the lungs via the pulmonary vein
Left ventricle (LV)	Bottom left	Pumps oxygenated blood to the body via the aorta

Important note on heart diagrams: When you look at a diagram of the heart, the left side appears on the right of the diagram and vice versa. This is because diagrams show the heart as if you are looking at the patient — so their left is your right.

The Septum

The septum is a thick muscular wall that separates the left and right sides of the heart. It prevents oxygenated and deoxygenated blood from mixing.

Blood Flow Through the Heart

Follow this pathway carefully — it is a very common exam question:

Deoxygenated blood returns from the body through the vena cava (the largest vein)
Blood enters the right atrium
The right atrium contracts, pushing blood through the tricuspid valve into the right ventricle
The right ventricle contracts, pushing blood through the semilunar (pulmonary) valve into the pulmonary artery
The pulmonary artery carries deoxygenated blood to the lungs
In the lungs, blood picks up oxygen and releases carbon dioxide
Oxygenated blood returns to the heart via the pulmonary vein
Blood enters the left atrium
The left atrium contracts, pushing blood through the bicuspid (mitral) valve into the left ventricle
The left ventricle contracts, pushing blood through the semilunar (aortic) valve into the aorta
The aorta carries oxygenated blood to the rest of the body

graph TD
    A["Body"] -->|"deoxygenated blood via vena cava"| B["Right Atrium"]
    B -->|"through tricuspid valve"| C["Right Ventricle"]
    C -->|"through pulmonary valve"| D["Pulmonary Artery"]
    D --> E["Lungs - gas exchange"]
    E -->|"oxygenated blood"| F["Pulmonary Vein"]
    F --> G["Left Atrium"]
    G -->|"through bicuspid valve"| H["Left Ventricle"]
    H -->|"through aortic valve"| I["Aorta"]
    I --> A

    style B fill:#3498db,color:#fff
    style C fill:#3498db,color:#fff
    style D fill:#3498db,color:#fff
    style E fill:#2ecc71,color:#fff
    style G fill:#e74c3c,color:#fff
    style H fill:#e74c3c,color:#fff
    style I fill:#e74c3c,color:#fff

Exam tip: Remember — arteries carry blood Away from the heart and veins carry blood towards (into) the heart. The pulmonary artery is special because it carries deoxygenated blood, and the pulmonary vein is special because it carries oxygenated blood. They are named for the vessel type, not the blood they carry.

Valves in the Heart

Valves are essential structures that prevent the backflow of blood, ensuring it always flows in one direction.

Valve	Location	Function
Tricuspid valve	Between right atrium and right ventricle	Prevents backflow from RV to RA
Bicuspid (mitral) valve	Between left atrium and left ventricle	Prevents backflow from LV to LA
Semilunar valves	At the base of the pulmonary artery and aorta	Prevent backflow from arteries back into ventricles

The "lub-dub" sound of the heartbeat is caused by the valves closing.

Wall Thickness

The left ventricle has a much thicker muscular wall than the right ventricle.

Why? The left ventricle must pump blood at high pressure all the way around the body (systemic circuit). The right ventricle only needs to pump blood the short distance to the lungs (pulmonary circuit), so it requires less force.

Exam tip: If asked why the left ventricle wall is thicker, always link it to the distance blood must travel and the pressure needed. A common wrong answer is "because it pumps more blood" — both ventricles pump the same volume of blood per beat.

Coronary Arteries

The coronary arteries branch off the aorta and supply the heart muscle itself with oxygenated blood and glucose.

The heart is a muscle that contracts continuously, so it has a very high demand for oxygen and glucose for aerobic respiration
If a coronary artery becomes blocked (e.g. by a build-up of fatty deposits/plaque), the heart muscle is starved of oxygen
This can cause a heart attack (myocardial infarction) — the affected heart muscle cells die

The Pacemaker

The heart's natural rhythm is controlled by a group of specialised cells called the sinoatrial node (SAN), located in the wall of the right atrium.

The SAN generates electrical impulses that spread across the atria, causing them to contract
The impulse is then picked up by the atrioventricular node (AVN) which passes the signal to the ventricles, causing them to contract
This coordinated contraction ensures the atria empty before the ventricles contract

If the natural pacemaker malfunctions, an artificial pacemaker can be implanted — a small electrical device that sends regular electrical impulses to keep the heart beating at the correct rate.

Resting Heart Rate and Exercise

The resting heart rate for an average adult is about 60–100 beats per minute (bpm)
During exercise, the heart rate increases so that more oxygen and glucose can be delivered to muscles for respiration
The heart rate is also controlled by hormones (e.g. adrenaline increases heart rate) and by the nervous system

Summary

Key Term	Definition
Double circulatory system	Blood passes through the heart twice per circuit
Pulmonary circuit	Heart → lungs → heart
Systemic circuit	Heart → body → heart
Septum	Wall separating left and right sides of the heart
Coronary arteries	Blood vessels supplying the heart muscle with oxygen
SAN (pacemaker)	Group of cells in the right atrium that controls heart rate
Valves	Structures that prevent the backflow of blood

Exam tip: When describing blood flow, always name the specific blood vessels and chambers in the correct order. Simply writing "blood goes from the heart to the lungs" will not gain full marks — you need to say "blood travels from the right ventricle through the pulmonary artery to the lungs."