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Organisation: the heart and blood vessels

AQA GCSE Biology, paper 1

1. Double circulation and the heart's structure

• Double circulatory system: blood passes through the heart twice for every full circuit of the body.
• Right side: pumps blood to the lungs to pick up oxygen.
• Left side: pumps oxygenated blood round the rest of the body.
• Heart: an organ made of muscle tissue, with four chambers.
• Atria receive blood; ventricles pump it out.
• Valves stop blood flowing backwards.
• Left ventricle wall: thicker than the right, so it can generate more force.

The two sides are separate pumps, so oxygenated and deoxygenated blood do not mix. The thicker left ventricle must push blood all the way round the body, which needs more force.

Ponte a prueba: Why does the left ventricle have a thicker wall than the right ventricle?

2. Heart vessels and heart rate

Vessel	Job	Blood carried
Vena cava	brings blood from the body into the right atrium	deoxygenated
Pulmonary artery	takes blood from the right ventricle to the lungs	deoxygenated
Pulmonary vein	brings blood from the lungs to the left atrium	oxygenated
Aorta	takes blood from the left ventricle to the body	oxygenated
Coronary arteries	supply the heart muscle itself	oxygenated

• Pacemaker: a group of cells in the right atrium that sets the natural resting heart rate.
• Artificial pacemaker: an electrical device that corrects an irregular heart rate.

The natural pacemaker controls the rhythm, so a device can take over when the rhythm is irregular.

Trampa de examen: in a 4-mark question, always name the vessel and the chamber, not just one.

Ponte a prueba: Which vessel carries blood from the right ventricle to the lungs, and what else must you name in a 4-mark answer?

3. Blood vessels and blood flow

Vessel	Direction	Wall and reason
Arteries	carry blood away from the heart	thick, muscular and elastic, because blood is at high pressure
Veins	carry blood back to the heart	thinner walls, valves, low pressure; valves stop backflow
Capillaries	exchange with tissues	walls one cell thick, so substances diffuse in and out quickly

• Rate of blood flow = volume of blood / time, e.g. in mL per minute.

The thick elastic artery walls cope with high pressure, while the one-cell-thick capillary walls keep the diffusion distance short.

Trampa de examen: do not mix up arteries and veins. Arteries go away from the heart.

Ponte a prueba: Explain why capillary walls are one cell thick, using the idea of diffusion.

Lista para el examen

• Name both the vessel and the chamber in a 4-mark question.
• Arteries carry blood away from the heart; veins carry it back.
• Link artery wall thickness to high pressure.
• Link capillary wall thickness to rapid diffusion.
• Explain the thicker left ventricle as pumping blood round the whole body.
• Know which vessel joins each heart chamber, not just its function.
• Give blood flow rate in units such as mL per minute.

Repaso en un minuto

Blood makes two circuits, so it passes through the heart twice for each full circuit of the body. The heart's four chambers and valves keep blood moving one way. Arteries carry blood away under high pressure, veins return it under low pressure, and capillaries exchange substances through thin walls. The natural pacemaker sets the heart rate, and an artificial pacemaker corrects it when irregular.

Respuestas de «Ponte a prueba»

1. The left ventricle pumps blood all the way round the body, so it needs more force, which a thicker muscular wall provides.
2. The pulmonary artery carries blood from the right ventricle to the lungs. In a 4-mark answer you must also name the chamber, the right ventricle.
3. Capillary walls are one cell thick so that substances such as oxygen diffuse in and out quickly, because the diffusion distance is very short.