Gas Exchange in Humans

Gas exchange vs breathing vs respiration: three different things

Examiners deliberately test whether you can separate these:

• Breathing (ventilation): the muscular movements that move air in and out of the lungs.
• Gas exchange: the diffusion of oxygen into the blood and carbon dioxide out, across the alveoli.
• Respiration: the chemical reactions in cells that release energy from glucose.

Using 'breathing' for 'respiration' is one of the most reliably penalised slips in the whole subject. Read the command word and answer the process actually named.

Alveoli: adaptations linked to function

This is the bankable long-answer. State each feature WITH its consequence:

• Large total surface area (millions of alveoli). So more gas diffuses at once.
• Thin walls, one cell thick: so the diffusion distance is short and diffusion is fast.
• Good (rich) blood supply from surrounding capillaries. So a steep concentration gradient is maintained, keeping diffusion going.
• Moist lining: so gases dissolve before diffusing across.

The single most common cap on marks is listing the features without the 'so that' clause. 'Alveoli have a large surface area' = 1 mark; 'a large surface area so more oxygen can diffuse at once' = 2. Every adaptation must end in a diffusion consequence.

Inspired vs expired air, and the limewater/CO2 evidence

The reason for the differences is the paying point: oxygen is used and carbon dioxide produced by respiration in cells. Limewater turning from clear to cloudy/milky is the standard test for the higher CO2 in expired air; a question often asks you to design the comparison using two tubes (breathing in vs out through limewater).

The breathing mechanism: pressure logic students reverse

Breathing in (inspiration): the diaphragm contracts and flattens, the external intercostal muscles contract pulling the ribs up and out, so the volume of the thorax increases, the pressure inside decreases below atmospheric pressure, and air flows in. Breathing out reverses every step: muscles relax, volume decreases, pressure increases, air flows out. The trap is the pressure–volume relationship: larger volume means lower pressure (so air rushes in). Students routinely write it backwards. Anchor it: chest gets bigger → pressure drops → air comes in.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

• Alveolar wall: The thin wall of an alveolus, only one cell thick, through which gas exchange occurs between the air in the alveolus and the blood in surrounding capillaries. Its thinness provides a short diffusion distance for oxygen and carbon dioxide.
• Alveoli: The tiny air sacs in the lungs where gas exchange takes place. Oxygen diffuses into the blood and carbon dioxide diffuses out.
• Bronchiole: A small airway that branches from the bronchi inside the lungs. Bronchioles have smooth muscle in their walls but no cartilage, allowing them to constrict or dilate to control airflow. They end in clusters of alveoli.
• Bronchus: One of two large airways that branch from the trachea and enter the left and right lungs. The bronchi have walls supported by cartilage rings and are lined with ciliated cells and mucus-producing goblet cells. The plural is bronchi.
• Carbon monoxide: A toxic gas in tobacco smoke that binds irreversibly to haemoglobin, forming carboxyhaemoglobin. This reduces the oxygen-carrying capacity of the blood because the haemoglobin can no longer bind oxygen, forcing the heart to work harder.
• Diaphragm: A sheet of muscle beneath the lungs that plays a key role in ventilation. During inhalation, the diaphragm contracts and flattens, increasing the volume of the thorax and decreasing pressure so air is drawn into the lungs. During exhalation it relaxes and domes upwards.
• Intercostal muscle: Muscles found between the ribs that work together with the diaphragm to ventilate the lungs. External intercostal muscles contract during inhalation, pulling the ribs up and out. Internal intercostal muscles contract during forced exhalation, pulling the ribs down and in.
• Nicotine: An addictive chemical in tobacco smoke that stimulates the nervous system and causes the release of adrenaline, increasing heart rate and blood pressure. Nicotine is the substance responsible for tobacco addiction and makes it difficult to stop smoking.
• Tar: A sticky brown substance in tobacco smoke that accumulates in the lungs, damaging cilia on airway cells and increasing mucus production. Tar also contains carcinogens that can cause mutations in lung cells, leading to lung cancer.
• Trachea: The main airway (windpipe) leading from the throat to the bronchi. The trachea is supported by C-shaped rings of cartilage that prevent it from collapsing, and is lined with ciliated epithelial cells and goblet cells that produce mucus to trap dust and pathogens.
• Ventilation: The mechanical process of breathing that moves air in and out of the lungs. Inhalation draws fresh air (rich in oxygen) into the alveoli, and exhalation expels stale air (rich in carbon dioxide). Ventilation maintains concentration gradients for gas exchange.

Browse the full IGCSE Biology glossary when a related term needs checking.