Gas exchange in humans

Large surface area (alveoli), thin surface (one-cell thick alveolar walls), good blood supply (dense capillary network), and good ventilation with air (breathing mechanism).

Lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli, and associated capillaries.

Limewater turns milky or cloudy in the presence of carbon dioxide. This is a common test to detect the presence of carbon dioxide in expired air.

Expired air has less oxygen, more carbon dioxide, and more water vapor than inspired air. This occurs because oxygen is used in respiration and carbon dioxide is produced as a waste product, also some water is produced.

Both the rate and depth of breathing increase during physical activity. This is to supply more oxygen to the muscles and remove the increased carbon dioxide produced by respiration.

Internal and external intercostal muscles are located between the ribs. They play a role in raising and lowering the rib cage during ventilation.

Cartilage rings provide support to the trachea, preventing it from collapsing, especially during inhalation when pressure decreases. This ensures a clear airway for efficient gas exchange.

During inhalation, the diaphragm contracts and flattens, the external intercostal muscles contract, raising the ribs. These actions increase the volume of the thorax, decreasing pressure and drawing air into the lungs.

Inspired air is about 21% oxygen, 0.04% carbon dioxide, and variable water vapor. Expired air is about 16% oxygen, 4% carbon dioxide, and saturated with water vapor, due to respiration.

Increased carbon dioxide concentration is detected by the brain, which stimulates an increased rate and depth of breathing. This helps to remove excess carbon dioxide and restore blood pH balance.

Goblet cells secrete mucus, which traps pathogens and particles. Ciliated cells have tiny hairs (cilia) that sweep the mucus and trapped particles upwards, away from the lungs, to be swallowed or expelled.

During exercise, muscle cells respire more, producing more carbon dioxide. This increases the carbon dioxide concentration in the blood. The brain detects this increase and signals the body to increase the rate and depth of breathing.