The heart and double circulation

Why do we need a transport system?

Every cell of your body needs food and oxygen, all the time. It also makes carbon dioxide and other wastes that must be taken away. In a tiny animal such as Amoeba, these materials simply diffuse in and out. In a large body, diffusion is far too slow. Oxygen would take years to diffuse from your lungs to your toes.

So large animals have a transport system. Blood carries the materials, the blood vessels are its pipes, and the heart is the pump that keeps it moving.

The human heart

The heart is a muscular organ about the size of your fist. It lies in the chest, between the two lungs.

It has four chambers:

• two upper chambers, the atria (one of them is an atrium), which receive blood;
• two lower chambers, the ventricles, which pump blood out of the heart.

A wall of muscle, the septum, keeps the right side apart from the left side. The right side holds blood with little oxygen. The left side holds blood rich in oxygen. Keeping the two apart means that the body always gets blood full of oxygen.

The ventricles have thicker walls than the atria, because they pump blood out of the heart. The left ventricle has the thickest wall of all, because it pushes blood around the whole body.

The path of the blood

Follow one drop of blood, starting from the body.

1. Blood with little oxygen comes back from the body through the large veins, the vena cava, into the right atrium.
2. The right atrium contracts, and the blood moves down into the right ventricle.
3. The right ventricle contracts and pumps the blood through the pulmonary artery to the lungs. There it gives up carbon dioxide and takes in oxygen.
4. Oxygen-rich blood returns through the pulmonary veins to the left atrium.
5. The left atrium contracts, and the blood moves down into the left ventricle.
6. The left ventricle contracts strongly and pumps the blood into the aorta, the largest artery. From there it goes to every part of the body.

Valves between the atria and the ventricles let blood flow only one way. When a ventricle contracts, the valve above it shuts, so the blood cannot flow back into the atrium.

Double circulation

In one complete round of the body, the blood passes through the heart twice: once on its way to the lungs, and once on its way to the body. This is called double circulation.

• The lung circuit takes blood from the right ventricle to the lungs and back to the left atrium.
• The body circuit takes blood from the left ventricle to the body and back to the right atrium.

Why is this useful? Birds and mammals keep their bodies warm, and they are very active. They need a lot of energy, so they need a steady, rich supply of oxygen. Double circulation sends fresh, oxygen-rich blood to the body under high pressure, and it never mixes with used blood.

Other animals do it differently.

• Fish have a heart with only two chambers, one atrium and one ventricle. The blood goes to the gills, takes in oxygen, and goes straight on to the body. It passes through the heart only once in each round, a single circulation.
• Amphibians and many reptiles have three chambers. Some oxygen-rich and oxygen-poor blood mixes. These animals take their body temperature from the surroundings, so they need less energy.

How much blood does the heart pump?

At rest, the heart of an adult beats about $72$ times a minute. Each beat of a ventricle pushes out about $70\ \text{mL}$ of blood. Multiply the two, and you get the blood pumped in each minute. Change the numbers below.

You should find that a resting heart pumps about $5\ \text{L}$ a minute. That is about all the blood in an adult's body. During hard exercise, both numbers rise, and the heart can pump four or five times as much.

Blood vessels

Three kinds of blood vessel carry the blood.

• Arteries carry blood away from the heart. The blood comes in bursts, under high pressure, so their walls are thick and elastic.
• Veins collect blood from the organs and bring it back to the heart. The pressure is low, so the walls are thinner. Valves inside them keep the blood flowing towards the heart.
• Capillaries are the smallest vessels. Their walls are only one cell thick. Food, oxygen and wastes pass between the blood and the cells through these walls.

Blood pressure

The force with which blood pushes against the wall of a vessel is called blood pressure. It is much greater in arteries than in veins.

• While a ventricle contracts, the pressure in the arteries is highest. This is the systolic pressure, normally about $120$ mm of mercury (mm of Hg).
• While the ventricle relaxes, the pressure is lowest. This is the diastolic pressure, normally about $80$ mm of Hg.

A doctor writes it as $120/80$ and measures it with an instrument called a sphygmomanometer. Pressure that stays too high is called hypertension. It can burst an artery and cause bleeding inside the body, so it must be treated.

Try it in Python

The program works out the blood pumped in a minute and in a day, and the beats in a day.

Practise with new numbers

Common mistakes

• "Arteries always carry oxygen-rich blood." The pulmonary artery carries blood with little oxygen, from the heart to the lungs. An artery is named by its direction: away from the heart.
• "Veins carry blood away from the heart." Veins always carry blood towards the heart.
• Mixing up left and right in a diagram. A diagram shows the heart as it faces you, so its right side is on your left.
• "The left ventricle is thick because it holds more blood." Both ventricles hold about the same amount. The left one is thicker because it pumps blood around the whole body.
• "Fish have a double circulation." In a fish, the blood passes through the heart only once in each round.

Remember these

• The heart has four chambers. The atria receive blood, and the ventricles pump it out.
• The right side carries blood with little oxygen, and the left side carries oxygen-rich blood. The septum keeps them apart.
• Body → right atrium → right ventricle → lungs → left atrium → left ventricle → body. Valves stop the blood from flowing back.
• In double circulation the blood passes through the heart twice in each round. Fish have a single circulation.
• Arteries carry blood away from the heart under high pressure. Veins bring it back and have valves. Capillaries exchange materials with the cells.
• Normal blood pressure is about $120/80$ mm of Hg, measured with a sphygmomanometer.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which chamber of the heart pumps blood to the lungs?

2. Name a vessel that carries blood with little oxygen but is still called an artery. Why is it called an artery?

3. Why do birds and mammals need to keep oxygen-rich and oxygen-poor blood apart?

4. A doctor reads a blood pressure of $130/85$. Which number is the systolic pressure, and when is it measured?

5. Why are the walls of capillaries only one cell thick?