GCSE Biology - The Lungs & Gas Exchange (2027/28 exams)

The Lungs and Gas Exchange

The video explains how air moves through the human gas exchange system, how oxygen and carbon dioxide are exchanged in the alveoli, what makes alveoli efficient, and how to calculate breathing rate.

Air’s route through the gas exchange system

Body cells need oxygen for respiration, which releases energy for functions such as movement and thinking. Respiration also produces carbon dioxide as a waste product.

Inhaled air enters through the nose or mouth, travels down the trachea (windpipe), and then passes into the two bronchi, with one leading to each lung. The bronchi branch into smaller bronchioles, which lead to tiny air sacs called alveoli. Small blood vessels called capillaries surround the alveoli.

Oxygen moves from the alveoli into the blood, enters red blood cells, and is carried by haemoglobin to body cells. Carbon dioxide travels in the opposite direction: from body cells into the blood, then into the alveoli, where it is exhaled through the airways.

Gas exchange by diffusion

Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration.

• Blood arriving at the lungs has relatively little oxygen because body cells have used it. Since the air in the alveoli has more oxygen, oxygen diffuses from the alveoli into the blood.
• Blood arriving at the lungs has a relatively high concentration of carbon dioxide, while the alveolar air has less. Carbon dioxide therefore diffuses from the blood into the alveoli and is exhaled.
• Blood carrying oxygen away from the lungs helps maintain a low oxygen concentration in the surrounding capillaries, allowing oxygen to continue diffusing into the blood.

Alveolar adaptations for gas exchange

Alveoli are efficient at gas exchange because they have:

• Walls one cell thick, providing a short distance for gases to diffuse.
• A large total surface area, because the lungs contain hundreds of millions of alveoli. This increases the overall rate of diffusion.
• Moist walls, allowing gases to dissolve and diffuse across more quickly.

Calculating breathing rate

Breathing rate is the number of breaths taken per minute.

Formula:

Breathing rate = number of breaths ÷ time in minutes

To calculate breathing rate:

1. Convert the time into minutes if it is given in seconds.
2. Divide the number of breaths by the time in minutes.
3. Give the answer in breaths per minute.

Example: A person takes 15 breaths in 30 seconds.

• Convert 30 seconds to minutes: 30 ÷ 60 = 0.5 minutes.
• Calculate the breathing rate: 15 ÷ 0.5 = 30 breaths per minute.

Breathing rate rises during vigorous exercise because muscles need more oxygen and produce more carbon dioxide, which must be removed more quickly.

Speakers and sources

• Cognito video narrator — unnamed; explains the gas exchange system and breathing-rate calculation.
• Cognito — identified in the video as the educational source and provider of related study materials.