What makes air enter the lungs when you breathe in?

The diaphragm contracts and moves down. The volume of the chest increases. The pressure inside the chest falls below the pressure outside. The pressure difference. Air is pushed in, not sucked in

Why is the wall of an alveolus only one cell thick?

The crossing happens by diffusion. The rate of diffusion falls as the distance grows. A thicker wall would slow the crossing. So the distance the oxygen has to cross is the smallest possible

Someone runs for ten minutes. Why does their breathing rate rise?

The muscles carry out more cellular respiration. So they use more oxygen and produce more carbon dioxide. The carbon dioxide concentration in the blood rises. To supply more oxygen and clear carbon dioxide faster

The Human Body and Its Systems · Grade 8

What is the difference between breathing, gas exchange and cellular respiration?

Three different things that share a word. Breathing is air moving in and out, driven by muscles. Gas exchange is oxygen crossing from an alveolus into the blood and carbon dioxide crossing the other way, and it is nothing but diffusion. Cellular respiration is the chemical reaction inside the cell that releases energy from oxygen and sugar.

Learning objectives

Describe how the diaphragm and the rib muscles change the volume of the chest and move air
Explain gas exchange in an alveolus as diffusion down a concentration gradient
Distinguish breathing, gas exchange and cellular respiration from one another
Explain how the structure of an alveolus suits the job it does

How air gets in

The diaphragm is a dome-shaped muscle below the lungs. When it contracts it drops and flattens, and the rib muscles lift the chest. The volume of the chest increases, the pressure inside falls below the air pressure outside, and air is pushed in.

In ordinary breathing out, the muscles simply relax. The chest returns to its smaller volume, the pressure rises, and air leaves. That is why breathing in takes effort and ordinary breathing out almost none.

Notice the direction of the reasoning: the lungs do not pull air in, and they contain no muscle at all. Change the volume, and the pressure does the rest.

What happens in an alveolus

At the end of each tiny airway sits a cluster of alveoli — minute sacs surrounded by blood capillaries. There are hundreds of millions of them, and their combined surface area is on the order of a tennis court.

In the air inside the alveolus the oxygen concentration is high; in the blood arriving from the body it is low. Oxygen moves down the concentration gradient into the blood, and carbon dioxide moves the other way for exactly the same reason.

This is pure diffusion: the body spends no energy on the crossing itself. What it does is keep the conditions favourable — a wall one cell thick, moisture, and a blood supply that constantly renews the gradient.

And what happens in the cell

The oxygen that reached the blood is delivered to the cells, and there, in the mitochondrion, it reacts with sugar. The products are carbon dioxide, water and energy the cell can use.

This is why the body breathes at all. Breathing and circulation are not the goal — they are the supply chain for this reaction, and everything in the previous lessons leads to it.

Worked examples

What makes air enter the lungs when you breathe in?
1. The diaphragm contracts and moves down
2. The volume of the chest increases
3. The pressure inside the chest falls below the pressure outside
Answer: The pressure difference. Air is pushed in, not sucked in
Why is the wall of an alveolus only one cell thick?
1. The crossing happens by diffusion
2. The rate of diffusion falls as the distance grows
3. A thicker wall would slow the crossing
Answer: So the distance the oxygen has to cross is the smallest possible
Someone runs for ten minutes. Why does their breathing rate rise?
1. The muscles carry out more cellular respiration
2. So they use more oxygen and produce more carbon dioxide
3. The carbon dioxide concentration in the blood rises
Answer: To supply more oxygen and clear carbon dioxide faster

Common mistakes

Saying respiration when you mean breathing: One moves air; the other is a chemical reaction in the mitochondrion. In an exam, a question about energy is almost always about the second.
Thinking the lungs pull air in: There is no muscle in a lung. The muscles around it change the volume, a pressure difference appears, and the air is pushed in by itself.
Assuming the body pumps oxygen into the blood: Gas exchange is diffusion and costs no energy. What the body does spend energy on is moving air and moving blood, which is what maintains the gradient.

What to remember

Breathing = moving air. Gas exchange = diffusion. Cellular respiration = a reaction in the cell.
Volume changes, pressure changes, air moves.
A thin wall and a large area are the two conditions for fast diffusion.
The blood is what keeps the concentration gradient alive.