Breathing and Exchange of Gases

The One Thing

Breathing ventilates the lungs, while respiration uses oxygen in cells and releases carbon dioxide. The respiratory system maintains gas exchange through ventilation, diffusion across alveoli and blood transport, with breathing regulated mainly by carbon dioxide and hydrogen ion concentrations.

Reactions, Processes and Experiments

Key Terms

• Respiratory system: The organ system consisting of the nose, nasal passage, pharynx, larynx, trachea, bronchi, bronchioles, lungs and alveoli that carries out breathing and gas exchange.
• Inspiration: The process of taking air into the lungs; it occurs when the diaphragm contracts and the volume of the thoracic cavity increases.
• Expiration: The process of forcing air out of the lungs; during normal expiration, the diaphragm and external intercostal muscles relax.
• Alveoli: Tiny, thin-walled air sacs in the lungs surrounded by blood capillaries; they provide a large surface for gas exchange.
• Partial pressure: The pressure contributed by an individual gas in a mixture; differences in partial pressure drive diffusion.
• Diffusion: Movement of gases from a region of higher partial pressure to a region of lower partial pressure.
• Respiratory membrane: The thin barrier formed mainly by the alveolar epithelium, a fused basement membrane and capillary endothelium through which gases diffuse.
• Haemoglobin: An iron-containing respiratory pigment in red blood cells that transports most oxygen and some carbon dioxide.
• Oxyhaemoglobin: The reversible compound formed when oxygen combines with haemoglobin in the lungs.
• Carbaminohaemoglobin: The compound formed when carbon dioxide binds reversibly with haemoglobin.
• Bicarbonate transport: The major method of carbon dioxide transport, in which carbon dioxide is converted into bicarbonate ions in red blood cells.
• Tidal volume: The volume of air inspired or expired during a normal quiet breath.
• Inspiratory reserve volume: The additional volume of air that can be forcibly inhaled after a normal inspiration.
• Expiratory reserve volume: The additional volume of air that can be forcibly exhaled after a normal expiration.
• Residual volume: The volume of air remaining in the lungs after forceful expiration.
• Vital capacity: The maximum volume of air that can be exhaled after a maximum inspiration; it equals tidal volume plus inspiratory reserve volume plus expiratory reserve volume.
• Total lung capacity: The total volume of air present in the lungs after maximum inspiration; it equals vital capacity plus residual volume.
• Respiratory rhythm centre: A group of neurons in the medulla region of the brain that controls the basic rhythm of breathing.
• Pneumotaxic centre: A centre in the pons that helps regulate the duration and pattern of inspiration.
• Chemosensitive area: A region sensitive to changes in carbon dioxide and hydrogen ion concentration that helps regulate breathing rate.
• Asthma: A condition involving inflammation and narrowing of the airways, often causing difficulty in breathing and wheezing.
• Emphysema: A disease in which alveolar walls are damaged, reducing the respiratory surface and causing breathing difficulty; smoking is a major cause.
• Conducting part: The part of the respiratory system that transports, filters, warms and moistens air.
• Respiratory part: The part of the respiratory system, mainly the alveoli, that performs gas exchange.
• Pleura: A double-layered membrane covering the lungs. Its pleural fluid reduces friction and helps maintain contact between the lungs and thoracic wall.
• Oxygen-haemoglobin dissociation curve: A sigmoid curve showing the relationship between oxygen partial pressure and haemoglobin saturation, produced because haemoglobin binds oxygen cooperatively.
• Respiratory rate: The number of breaths taken per minute; the approximate normal rate of a healthy adult at rest is 12–16 breaths per minute.
• Vital capacity equation: Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume.
• Total lung capacity equation: Total lung capacity = vital capacity + residual volume.

Easily Confused

• Breathing and respiration: Breathing is the mechanical movement of air into and out of the lungs; respiration is the cellular use of oxygen and release of carbon dioxide.
• Inspiration and expiration: Inspiration brings air into the lungs through increased thoracic volume; expiration removes air through decreased thoracic volume.
• Normal expiration and forced expiration: Normal expiration involves relaxation of the diaphragm and external intercostal muscles; forced expiration additionally involves contraction of the internal intercostal and abdominal muscles.
• Oxygen transport and carbon dioxide transport: About 97% of oxygen is transported as oxyhaemoglobin, whereas carbon dioxide is transported mainly as bicarbonate ions, with smaller amounts as carbaminohaemoglobin and dissolved gas.
• Tidal volume and vital capacity: Tidal volume is the air moved during a normal quiet breath; vital capacity is the maximum air exhaled after maximum inspiration.
• Vital capacity and total lung capacity: Vital capacity excludes residual volume; total lung capacity includes it.
• Respiratory rhythm centre and pneumotaxic centre: The respiratory rhythm centre in the medulla establishes the basic breathing rhythm; the pneumotaxic centre in the pons regulates the duration and pattern of inspiration.
• Asthma and emphysema: Asthma involves airway inflammation and narrowing; emphysema involves damage to alveolar walls and loss of respiratory surface.
• Partial pressure and concentration: Partial pressure is the pressure contributed by an individual gas in a mixture; diffusion in the lungs is driven specifically by differences in partial pressure.

What Gets Asked

• State or sequence the pathway of air: Questions may require the order from nostrils through the nasal passage, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli. Omitting or rearranging structures costs marks.
• Explain inspiration and expiration: A full explanation must link muscle action, thoracic volume and intrapulmonary pressure. Stating only that the diaphragm moves is incomplete.
• Explain why alveoli are efficient exchange surfaces: Relevant features are their large number, thin walls, moist surface and rich blood supply. The role of partial-pressure gradients must also be recognised.
• Explain the direction of gas diffusion: Oxygen moves from alveoli to blood and carbon dioxide from blood to alveoli because of differences in their partial pressures. Reversing either direction loses marks.
• Describe gas transport in blood: The required proportions are approximately 97% of oxygen as oxyhaemoglobin and 3% dissolved in plasma; approximately 20–25% of carbon dioxide as carbaminohaemoglobin, 70% as bicarbonate ions and 7% dissolved in plasma.
• Calculate or identify lung volumes: Questions may use the equations Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume and Total lung capacity = vital capacity + residual volume. Residual volume must not be included in vital capacity.