General Adaptations for Gas Exchange

Surface Area and Volume: Adaptations for gas exchange can be linked to the surface area to volume ratio of the organism. In general, smaller organisms have larger surface area to volume ratios and do not require specialised exchange surfaces.
Diffusion Pathway: Exchange surfaces are generally very thin to reduce the diffusion distance for gases, speeding up the rate of diffusion.
Moisture: Gases must be in solution to pass across the exchange surface, so a moist environment is maintained.

Adaptations in Terrestrial Insects

Spiracles: Openings on the insect body, known as spiracles, allow gases to enter and exit the body. Insects can control the opening and closing of spiracles to conserve water.
Tracheae and Tracheoles: Internal network of tubes, the tracheae and the smaller tracheoles, penetrate throughout the body providing a short diffusion pathway for gases.
Air Sacs: Some insects have air sacs to pump air throughout the tracheal system, increasing the rate of gas exchange.

Gills: Gills are the primary gas exchange organ in fish, where oxygen is absorbed from water and carbon dioxide is expelled.
Lamellae: Layers of thin, flat plates called lamellae increase the surface area for gas exchange.
Counter Current Flow: Water and blood flow in opposite directions in the gills, maintaining a steep concentration gradient for efficient diffusion.

Lungs: In mammals, the lungs provide a large surface area for gas exchange. The branching bronchi and bronchioles lead to small air sacs called alveoli.
Alveoli: These are extremely small air sacs with thin, moist walls to aid diffusion. They are richly supplied with blood capillaries to maintain a steep diffusion gradient.
Ventilation and Circulatory System: Breathing (ventilation) refreshes the air in the alveoli, maintaining a steep concentration gradient for oxygen and carbon dioxide. The circulatory system rapidly transports oxygenated blood away and brings deoxygenated blood to the lungs.
Haemoglobin: Mammalian red blood cells contain the protein haemoglobin which binds oxygen, assisting transport and maintaining the concentration gradient.