Example: Oxygen diffusing into blood cells.

In the lungs, oxygen diffuses from the air in the alveoli, across two thin layers of cells, into the blood and into red blood cells, because the blood arriving at the lungs has less oxygen than the air; in the body's tissues the gradient is reversed and oxygen diffuses out to the cells that use it.

This lesson works through the best-known example of diffusion in the body step by step. It follows an oxygen molecule from the alveoli into a red blood cell, explains why it moves (the concentration gradient) and how breathing, blood flow and haemoglobin keep that gradient steep, and then follows the oxygen out of the blood into a working muscle. It ends with the factors that control the rate of diffusion, including a simple worked calculation of how lung disease and altitude slow oxygen uptake.

Example: Oxygen Diffusing into Blood Cells

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It needs no energy from the cell. Gas exchange in the lungs is the clearest example of diffusion in the human body, so it is worth following one oxygen molecule on its journey.

Worked example 1: from the air into a red blood cell

1. Breathing in brings fresh air, rich in oxygen, into millions of tiny air sacs called alveoli.
2. Each alveolus is wrapped in a network of capillaries. Blood arriving here has come back from the body, where cells have used much of its oxygen, so its oxygen concentration is low.
3. Oxygen dissolves in the thin layer of moisture lining the alveolus.
4. It diffuses across the alveolar wall and then the capillary wall, each only one flattened cell thick, into the blood plasma.
5. It diffuses into a red blood cell and binds to haemoglobin, forming oxyhaemoglobin.

Scientists measure the concentration of a gas as its partial pressure. In the alveoli, the partial pressure of oxygen is about 100 mmHg; in the blood arriving at the lungs it is about 40 mmHg. Oxygen therefore diffuses from the air into the blood until the blood leaving the lungs has almost the same oxygen level as the air in the alveoli.

Worked example 2: why the gradient stays steep

If oxygen simply flowed in until both sides were equal, diffusion would stop. Three things prevent that:

• Haemoglobin: oxygen that binds to haemoglobin no longer counts as free, dissolved oxygen, so the concentration of free oxygen inside the red blood cell stays low and more oxygen keeps diffusing in. Each haemoglobin molecule can carry up to four oxygen molecules, and each red blood cell contains millions of haemoglobin molecules.
• Blood flow: the heart keeps pumping oxygen-rich blood away and bringing oxygen-poor blood in.
• Breathing: each breath replaces some of the air in the alveoli, keeping its oxygen level high.

Worked example 3: out to the body's cells

In a working muscle, cells use oxygen in respiration all the time, so the oxygen concentration in and around them is low. When blood reaches the muscle's capillaries, the gradient is reversed: oxygen leaves haemoglobin, diffuses out of the red blood cell, across the capillary wall and into the muscle cells, where it is used in the mitochondria. Carbon dioxide made by respiration diffuses the opposite way, into the blood, and later out of the blood into the alveoli to be breathed out. The process is the same diffusion in each case; only the direction of the gradient changes.

Why the lungs are so good at this

A simple model of diffusion, known as Fick's law, says:

rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of the barrier

• Large surface area: the lungs contain hundreds of millions of alveoli, giving a total surface area of tens of square metres. Red blood cells add their own large surface area through their biconcave shape.
• Thin barrier: the oxygen crosses only two very thin layers of flattened cells. Capillaries are so narrow that red blood cells pass through in single file, pressed close to the wall.
• Steep concentration difference: maintained by breathing, blood flow and haemoglobin.

A worked calculation

Suppose oxygen normally diffuses into the blood at a rate R. If a lung disease doubles the thickness of the barrier, the rate becomes R ÷ 2, half the normal rate. If another disease destroys alveolar walls so that only 60% of the normal surface area remains, the rate falls to 0.6R. If both happened together, the rate would be 0.6 × 0.5 = 0.3, only 30% of normal. Altitude acts on the third factor: at high altitude the air holds less oxygen, so the concentration difference is smaller and oxygen diffuses into the blood more slowly, which is why climbers become breathless.