Estimated level: Specialised English B2–C1

Scientific vocabulary: C1 or specialist academic vocabulary

Reading comprehension: Upper-intermediate to advanced

Biology knowledge: Upper-secondary level, potentially introductory university

Suitable audience: advanced ESL learners, or students studying biology in English

Imagine you are a tiny molecule of oxygen, floating in the air. As a person takes a deep breath, you are pulled into the nose and down through the windpipe. You travel through smaller and smaller tubes until you reach a bronchiole, one of the narrowest branches in the lungs. At the very end of this tube, you tumble into a tiny, stretchy air sac called an alveolus. This sac looks like a single grape in a large bunch. The walls of the alveolus are incredibly thin—so thin that you can see the blood vessels waiting on the other side.

Just outside the alveolus is a capillary, a blood vessel so narrow that cells must travel through it in single file. You press against the moist lining of the alveolar wall. Because there is a high concentration of oxygen in the air sac and a low concentration in the blood, you naturally drift across the membrane. This movement is effortless. Once you cross into the capillary, you encounter a red blood cell. You quickly bind to a protein called haemoglobin, which acts like a specialized seat on a bus. Now safely attached, you are swept away by the blood flow, ready to be delivered to the rest of the body.

The human respiratory system is designed for efficiency. The primary site of gas exchange is the alveolus (plural: alveoli). There are millions of these tiny sacs in your lungs, providing a massive surface area for gases to move. Surrounding each alveolus is a network of capillaries. These are the smallest blood vessels in the body, and their walls are only one cell thick. This short distance allows gases to move between the air and the blood almost instantly.

Blood travels to the lungs via the pulmonary artery. Unlike most arteries in the body, the pulmonary artery carries deoxygenated blood—blood that is low in oxygen and high in carbon dioxide. Once this blood reaches the capillaries surrounding the alveoli, gas exchange occurs. The newly oxygenated blood then travels back to the heart through the pulmonary vein. Inside the blood, the red blood cell is the primary vehicle for transport. These cells are packed with haemoglobin, which has a high affinity for oxygen, allowing the blood to carry much more oxygen than if it were just dissolved in the plasma.

Gas exchange is driven by a physical process called diffusion. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. In the lungs, we measure these concentrations as partial pressure, which is the pressure exerted by a single gas within a mixture. The unit used for this measurement is the kilopascal (kPa).

When you inhale, the air in the alveoli has a high partial pressure of oxygen. Meanwhile, the blood entering the capillaries from the pulmonary artery has a much lower partial pressure of oxygen because the body's tissues have used it up. This difference creates a "diffusion gradient." Oxygen naturally moves down this gradient, crossing from the air into the blood. At the same time, carbon dioxide—which is at a higher partial pressure in the blood than in the air—moves in the opposite direction, leaving the blood to be exhaled.

To understand exactly how gases move, we can look at the specific partial pressure values in the lungs. In the alveolar air, the partial pressure of oxygen (O2) is approximately 14.2 kPa, while the partial pressure of carbon dioxide (CO2) is 5.3 kPa.

As blood enters the capillaries from the pulmonary artery, it is "venous" blood that has returned from the body. Its oxygen levels are low, with a partial pressure of approximately 5.3 kPa, and its carbon dioxide levels are high, at about 6.1 kPa. Because the O2

O2

pressure is higher in the air (14.2 kPa) than in the blood (5.3 kPa), oxygen diffuses into the blood. Because the CO2 pressure is higher in the blood (6.1 kPa) than in the air (5.3 kPa), carbon dioxide diffuses out of the blood.

By the time the blood leaves the capillaries to enter the pulmonary vein, it has reached a state of equilibrium with the alveolar air. The blood leaving the capillaries now has an oxygen partial pressure of approximately 13.3 kPa and a carbon dioxide partial pressure of 5.3 kPa. This oxygen-rich blood is now ready to be pumped by the heart to the rest of the body's organs and muscles.

Gas always moves from an area of high partial pressure to an area of low partial pressure. This is known as moving down the concentration gradient.