The Tracheal System

The tracheal system is a network of air-filled tubes that carries oxygen from the spiracles directly to every respiring cell of an insect.• Spiracles: openings on the body surface, controlled by valves, that lead into the tracheae.
• Tracheae: tubes about 1 mm in diameter, lined with cuticle and supported by spiral rings of chitin. The chitin keeps the tubes open when pressure changes, but it makes the tracheal walls impermeable, so little gas exchange takes place across them.
• Tracheoles: the tracheae branch repeatedly into tracheoles, less than 1 µm in diameter. Each tracheole is a single elongated cell with no chitin lining, so it is permeable to gases. Tracheoles push between and even into cells, especially muscle fibres, so the diffusion distance to mitochondria is very short.The ends of the tracheoles contain tracheal fluid. Oxygen dissolves in this fluid and diffuses into the surrounding cells.Gas movement depends on concentration gradients. Respiring cells use oxygen, so oxygen concentration at the tracheole ends is low, and oxygen diffuses from the air outside, through the spiracles, tracheae and tracheoles, down its gradient. Carbon dioxide produced by respiration diffuses out along the reverse gradient.The system provides a very large surface area for exchange, because there are huge numbers of tracheoles, and a short diffusion pathway, because they reach right into the tissues. Since the gases travel as air rather than in blood, they diffuse many thousands of times faster than they would through water.