Gas Exchange in Humans

The One Thing

Human gas exchange depends on ventilated alveoli combining a large, moist surface, thin walls, and a rich blood supply. These adaptations maintain concentration gradients so oxygen diffuses into the blood and carbon dioxide diffuses out of it.

Reactions, Processes and Experiments

Key Terms

• Gas exchange: The movement of oxygen into the blood and carbon dioxide out of the blood, mainly by diffusion.
• Diffusion: The net movement of particles from a region of higher concentration to a region of lower concentration.
• Respiratory system: The organs and structures involved in breathing and gas exchange, including the airways, lungs, and breathing muscles.
• Trachea: The windpipe that carries air from the larynx toward the bronchi.
• Bronchi: Two large tubes that carry air from the trachea into the left and right lungs.
• Bronchioles: Smaller branching airways that distribute air throughout the lungs and lead to alveoli.
• Alveoli: Tiny air sacs where oxygen and carbon dioxide are exchanged between air and blood.
• Ventilation: The movement of air into and out of the lungs, commonly called breathing.
• Inhalation: The movement of air into the lungs when the volume of the thorax increases and pressure inside the lungs decreases.
• Exhalation: The movement of air out of the lungs when the volume of the thorax decreases and pressure inside the lungs increases.
• Diaphragm: A sheet of muscle below the lungs that contracts and flattens during inhalation and relaxes during normal exhalation.
• Intercostal muscles: Muscles between the ribs that move the ribcage during breathing.
• Pleural membranes: Membranes surrounding the lungs that help reduce friction as the lungs move during breathing.
• Concentration gradient: The difference in concentration between two regions that drives diffusion.
• Ciliated cells: Cells lining parts of the airways that move mucus toward the throat using tiny hair-like structures called cilia.
• Goblet cells: Cells that produce mucus, which traps dust particles and microorganisms in the airways.
• Cartilage: Strong, flexible tissue in the walls of the trachea and bronchi that prevents these airways from collapsing.
• Surface area: The total area available for gas exchange; the alveoli have a large surface area because there are very many small air sacs.
• Diffusion distance: The distance particles must cross during diffusion; alveolar walls and capillary walls are each approximately one cell thick.
• Tidal volume: The volume of air moved into or out of the lungs during one normal breath.
• Breathing rate: The number of breaths taken per minute.
• Minute ventilation: The volume of air moved per minute, calculated using: minute ventilation = breathing rate × tidal volume.
• Aerobic respiration: The process that uses oxygen and releases energy from glucose: glucose + oxygen → carbon dioxide + water.
• Haemoglobin: A substance in the blood that binds carbon monoxide; carbon monoxide binding reduces the blood's ability to transport oxygen.

Easily Confused

• Ventilation and gas exchange: Ventilation is the movement of air into and out of the lungs, whereas gas exchange is the diffusion of oxygen and carbon dioxide between alveoli and blood.
• Inhalation and exhalation: Inhalation involves increased thorax volume and decreased lung pressure; exhalation involves decreased thorax volume and increased lung pressure.
• Bronchi and bronchioles: Bronchi are the two large tubes branching from the trachea, whereas bronchioles are smaller branches that lead to alveoli.
• Goblet cells and ciliated cells: Goblet cells produce mucus, whereas ciliated cells move mucus toward the throat.
• Alveolar walls and capillary walls: Both are approximately one cell thick and together provide a short diffusion distance, but alveolar walls separate air from the alveolus and capillary walls separate blood from the capillary.
• Oxygen diffusion and carbon dioxide diffusion: Oxygen moves from alveoli into blood, whereas carbon dioxide moves from blood into alveoli.
• Breathing rate and tidal volume: Breathing rate is the number of breaths per minute, whereas tidal volume is the volume moved in one normal breath.
• Minute ventilation and tidal volume: Minute ventilation is calculated as breathing rate × tidal volume; tidal volume is only the volume moved per normal breath.
• Emphysema and lung cancer: Both can result from smoking, but emphysema involves damage to alveolar walls and reduced gas-exchange surface area, whereas lung cancer is a disease caused by abnormal cell growth.
• Mucus and cilia: Mucus traps dust particles and microorganisms, whereas cilia move the mucus toward the throat.

What Gets Asked

• Explain how the alveoli are adapted for efficient gas exchange. Marks depend on stating the large surface area, walls approximately one cell thick, moist lining, dense capillary network, and ventilation.
• Describe the pathway of air through the respiratory system: nose or mouth, trachea, bronchi, bronchioles, alveoli. A common error is omitting the bronchioles or placing the structures in the wrong order.
• Explain inhalation and normal exhalation using diaphragm movement, external intercostal muscle movement, rib movement, thorax volume, and lung pressure. Marks are lost when volume and pressure changes are reversed.
• Explain why oxygen and carbon dioxide diffuse in opposite directions. The required distinction is that oxygen concentration is higher in alveolar air, while carbon dioxide concentration is higher in the blood.
• Calculate or use minute ventilation with minute ventilation = breathing rate × tidal volume. The relevant quantities must not be confused: breathing rate is breaths per minute, while tidal volume is the volume per normal breath.
• Explain the effects of smoking and carbon monoxide. Full answers distinguish damage to cilia, increased mucus production, damage to alveolar walls, reduced gas-exchange surface area, diseases such as emphysema and lung cancer, and carbon monoxide binding strongly to haemoglobin.