The Heart and Blood Vessels

The heart is an organ made mostly of muscle. Its job is to pump blood around the body. It has four chambers: two at the top called atria (singular: atrium) and two below called ventricles.

The atria receive blood arriving at the heart. The ventricles pump blood out. Blood always flows in one direction — atria fill first, then push blood down into the ventricles, which contract to force it out into the large vessels.

The heart is a double pump: the right side and left side work side by side at the same time, but push blood to two different destinations.

Four named blood vessels connect the heart to the rest of the body, plus the coronary arteries that feed the heart muscle itself. AQA only requires these names — you do not need to name the valves.

Vessel	Carries blood	Connected to	Oxygen level
Vena cava	Into the heart, from the body	Right atrium	Deoxygenated
Pulmonary artery	Out of the heart, to the lungs	Right ventricle	Deoxygenated
Pulmonary vein	Into the heart, from the lungs	Left atrium	Oxygenated
Aorta	Out of the heart, to the body	Left ventricle	Oxygenated

The pulmonary vessels are the exception to the usual rule: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, because they connect to the lungs rather than the body.

The heart muscle needs its own oxygen supply. The coronary arteries branch off the aorta and run across the surface of the heart, delivering oxygenated blood to the muscle. If a coronary artery becomes blocked, that section of muscle is starved of oxygen and can be damaged.

Valves sit between the atria and ventricles, and at the exits to the arteries. They snap shut to stop blood flowing backwards, keeping it moving in one direction only.

In a double circulatory system, blood passes through the heart twice for every full circuit of the body. There are two separate loops:

• Pulmonary circuit — heart → lungs → heart (picks up oxygen)
• Systemic circuit — heart → body → heart (delivers oxygen)

Worked example — follow one red blood cell. Start with a cell arriving from the body, low on oxygen:

Step by step:

1. Body (low oxygen)
2. Vena cava
3. Right atrium
4. Right ventricle
5. Pulmonary artery
6. Lungs (gains oxygen, loses CO₂)
7. Pulmonary vein
8. Left atrium
9. Left ventricle
10. Aorta
11. Body (delivers oxygen)

Steps 2–5 are the right side pumping deoxygenated blood to the lungs. Steps 7–10 are the left side pumping the now-oxygenated blood to the body. The cell went through the heart twice — once on each side.

A double circulatory system keeps oxygenated and deoxygenated blood completely separate, and lets the body return blood to the heart to be re-pressurised before the long journey to the tissues.

Blood loses pressure as it squeezes through the tiny vessels in the lungs. In a single circuit, that low-pressure blood would have to travel straight on to the rest of the body — slow and inefficient. The double circuit solves this:

Feature	Benefit
Blood returns to the heart after the lungs	It is pumped again at high pressure to the body
Oxygenated and deoxygenated blood kept apart	Blood reaching tissues is fully oxygenated
Left ventricle pumps separately to the body	Pressure can be high enough to reach every cell

This is why the left ventricle has a much thicker muscular wall than the right: it must generate enough pressure to push blood all the way around the whole body, whereas the right ventricle only pumps the short distance to the nearby lungs.

Higher blood flow means tissues receive oxygen and glucose faster, supporting a high rate of respiration. This is essential for active, warm-blooded animals like humans.

Heart muscle contracts on its own, but the resting heart rate is set by a group of cells in the right atrium that act as a natural pacemaker. These cells produce regular electrical impulses that spread across the heart and tell the muscle when to contract, keeping the beat steady and coordinated.

If this natural pacemaker stops working properly, the heartbeat can become too slow, too fast, or irregular. An artificial pacemaker can fix this:

An artificial pacemaker is a small electrical device, fitted under the skin, that sends regular electrical signals to the heart to correct an irregular heartbeat.

We can measure how hard the heart is working using cardiac output — the volume of blood pumped by a ventricle each minute:

$\text{cardiac output}=\text{stroke volume}\times \text{heart rate}$

Worked example. A person has a stroke volume (blood pushed out per beat) of 70 mL and a heart rate of 72 beats per minute:

$70\text{ mL}\times 72=5040\text{ mL/min}=5.04\text{ L/min}$

During exercise the heart rate rises, so cardiac output rises and the muscles receive oxygen faster.

The body has three types of blood vessel, each adapted to its job. The simple rule: arteries carry blood Away from the heart; veins carry blood back towards the heart.

Feature	Artery	Vein	Capillary
Direction	Away from heart	Towards heart	Connects arteries to veins
Wall	Thick, muscular, elastic	Thin	One cell thick
Lumen (hole)	Narrow	Wide	Very narrow
Valves?	No	Yes	No
Blood pressure	High	Low	Falling
Main function	Carry blood at high pressure	Return blood at low pressure	Exchange substances with cells

• Arteries have thick, elastic, muscular walls to withstand the high pressure of blood leaving the heart, and a narrow lumen to keep that pressure high.
• Veins have thin walls and a wide lumen because the blood is at low pressure. They contain valves to stop blood flowing backwards on its slow journey back to the heart.
• Capillaries are only one cell thick, so oxygen and glucose can diffuse out to cells and waste like CO₂ can diffuse in across a very short distance. Their walls are too thin to be felt as a pulse.

In the lungs, capillaries form a dense network around the alveoli (tiny air sacs), giving a huge, thin surface for gases to be exchanged between the air and the blood.

Exams often describe a vessel and ask you to name it and justify your answer. Match the structure to the function.

Worked example — vessel A. "Has a thick muscular wall, a narrow lumen, no valves, and carries blood at high pressure."

• Thick muscular wall + high pressure → it carries blood leaving the heart.
• No valves and a narrow lumen fit this too.
• Answer: an artery. Justification: thick muscular walls withstand the high pressure of blood being pumped away from the heart.

Worked example — vessel B. "Has a wall that is a single cell thick and connects small arteries to small veins."

• A one-cell-thick wall is the giveaway.
• It links arteries to veins and allows exchange.
• Answer: a capillary. Justification: a wall one cell thick gives a short diffusion distance, so oxygen and glucose pass to cells and CO₂ passes into the blood.

A vessel with thin walls, a wide lumen and valves would be a vein — the valves and wide lumen suit low-pressure blood returning to the heart.

Justify your answer by linking structure to function. "It is an artery" earns little; "it is an artery because its thick muscular wall withstands high pressure" is much stronger.

1. Saying the right ventricle has the thicker wall

It is the left ventricle that has the thicker, more muscular wall, because it pumps blood to the whole body. The right ventricle only pumps the short distance to the lungs, so its wall is thinner.

2. Mixing up the pulmonary vessels

The pulmonary artery carries deoxygenated blood (to the lungs) and the pulmonary vein carries oxygenated blood (from the lungs). This is the opposite of the usual artery/vein oxygen pattern — examiners test it often.

3. Confusing artery and vein direction

Arteries carry blood away from the heart (A for Away); veins carry blood back towards it. This is true regardless of whether the blood is oxygenated.

4. Forgetting why capillary walls are thin

A capillary wall is one cell thick so substances diffuse across a short distance. State the link to diffusion, not just "the wall is thin".

5. Describing the pacemaker as a muscle, not cells

The natural pacemaker is a group of cells in the right atrium that produce electrical impulses. An artificial pacemaker is a separate electrical device fitted to correct an irregular heart rate — do not confuse the two.

6. Saying blood passes through the heart once

In a double circulatory system blood passes through the heart twice per full circuit — once for the pulmonary (lung) loop and once for the systemic (body) loop.