The Human Body and Its Systems · Grade 8

Why does blood pass through the heart twice on every complete circuit?

Because there are two separate circuits, not one. In the pulmonary circuit blood leaves the heart for the lungs, collects oxygen and returns. In the systemic circuit it leaves again and travels to the whole body. The two circuits need very different pressures, so each one gets a ventricle of its own.

Learning objectives

Four chambers, two circuits

The heart has two atria on top and two ventricles below. The atria receive blood returning to the heart; the ventricles are the pumps that push it out. Valves between atrium and ventricle, and between ventricle and artery, stop it flowing backwards.

The right side receives oxygen-poor blood from the body and sends it to the lungs. The left side receives oxygen-rich blood from the lungs and sends it to the whole body. The two sides beat together but never mix.

A full circuit: body, right atrium, right ventricle, lungs, left atrium, left ventricle, body. The heart appears in it twice, and that is exactly what a double circulation means.

Why the left ventricle is thicker

The right ventricle sends blood to the lungs only — a short distance, and delicate tissue that high pressure would damage. The left ventricle sends blood as far as the foot and as far as the brain.

So the wall of the left ventricle is several times thicker. This has nothing to do with the quantity of blood, which is the same on both sides, and everything to do with the pressure it must generate. Structure follows function, again.

Three kinds of vessel

An artery carries blood away from the heart. The pressure in it is high, so its wall is thick, muscular and elastic — and that elasticity is what smooths the pulses and what you can feel as a pulse.

A vein carries blood back to the heart. The pressure is low, the wall is thinner, and it contains valves that stop blood falling back on its way upwards.

A capillary is the only vessel where anything actually happens. Its wall is one cell thick and it is so narrow that red blood cells pass almost in single file. There the oxygen and food cross into the cells and the waste crosses back.

Worked examples

  1. Follow one drop of blood from the leg until it returns to the leg.

    1. From the leg through veins to the right atrium
    2. From the right atrium to the right ventricle and on to the lungs
    3. From the lungs to the left atrium and the left ventricle
    4. From the left ventricle through the aorta and back to the leg

    Answer: Two passes through the heart, one complete circuit

  2. The pulmonary artery carries oxygen-poor blood. Does that contradict the definition of an artery?

    1. An artery is defined by direction, not by oxygen content
    2. The pulmonary artery carries blood from the heart to the lungs
    3. The direction fits the definition

    Answer: No contradiction. An artery is a vessel leaving the heart

  3. Why is a capillary so thin, when a thicker wall would be stronger?

    1. A capillary is where substances cross by diffusion
    2. Diffusion is only fast over a short distance
    3. A wall one cell thick is the shortest distance possible

    Answer: Because its job is not to withstand pressure but to let things across

Common mistakes

Identifying arteries by red blood and veins by blue
The distinction is direction: an artery leaves the heart, a vein returns to it. The pulmonary artery carries oxygen-poor blood and the pulmonary vein carries oxygen-rich blood.
Thinking the left ventricle is thicker because more blood passes through it
Exactly the same volume passes through both ventricles each beat. The difference is the pressure needed for the distance, not the volume.
Skipping the capillaries when describing the route
The capillaries are the only place where the blood does its job. Arteries and veins are plumbing; all the exchange happens in a capillary.

What to remember

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