GLOSSARY

Respiration and gas exchange refers to the whole process by which oxygen travels from the air to the mitochondria of cells and the CO₂ produced by metabolism returns to the air; in physiology, “respiration” is not limited to breathing in and out.

Main Stages

• Pulmonary ventilation: movement of air between the environment and the alveoli.
• External respiration: gas exchange between the alveoli and pulmonary capillary blood.
• Gas transport: carriage of O₂ and CO₂ in the blood, mainly by haemoglobin and as bicarbonate.
• Internal respiration: gas exchange between capillary blood and tissue cells.
• Cellular respiration: use of O₂ in the mitochondria to produce ATP, with production of CO₂.

Driving Force

Passive diffusion of gases from higher to lower partial pressure; the body has no active pump for moving O₂ or CO₂.

Gradients at Rest

• Oxygen: alveolus about 100 → mixed venous blood about 40 mmHg, a difference of about 60.
• CO₂: venous blood about 46 → alveolus about 40, a difference of only about 6, yet sufficient because of its high solubility.

Relevance to Sport

• During exercise: muscle oxygen consumption can rise 50-fold or more; all stages must increase in coordination.
• Limitations: in healthy people, oxygen delivery (cardiac output) is the main limiting factor; in lung patients and some elite athletes, pulmonary gas exchange may also be limiting.
• Practical example: a pulse oximeter shows oxygen saturation (SpO₂), normally 95–100 percent; a fall below 90 percent at altitude or in illness is significant.