Gaseous Exchange.

Every cell respires — it takes in oxygen and gives out carbon dioxide. Gaseous exchange is how an organism swaps these gases with its surroundings, fast enough to keep every cell supplied.

1 · A good gas-exchange surface

All efficient exchange surfaces share the same features — they are:

• Large in surface area (more room for diffusion),
• Thin (a short diffusion distance),
• Moist and permeable (gases dissolve and pass through),
• well supplied with blood (or air) to keep a steep concentration gradient.

The human alveoli and the leaf's air spaces are both built this way.

2 · The human respiratory system

Air travels: nose (warms, moistens, filters) → trachea (held open by C-shaped cartilage rings; cilia and mucus trap dust) → bronchi → bronchioles → alveoli (millions of tiny air sacs where exchange happens).

3 · Breathing (ventilation)

Breathing keeps fresh air moving in and out so the gradient stays steep. It is caused by changing the volume — and therefore the pressure — of the chest:

	Inspiration (breathe in)	Expiration (breathe out)
Diaphragm	Contracts & flattens	Relaxes & domes up
Ribs (intercostals)	Move up & out	Move down & in
Chest volume	Increases	Decreases
Pressure in lungs	Falls below outside	Rises above outside
Air	Rushes IN	Pushed OUT

4 · Exchange at the alveoli

Each alveolus is wrapped in blood capillaries. Oxygen diffuses from the alveolar air into the blood (and binds haemoglobin in red cells); carbon dioxide diffuses from the blood into the alveolus to be breathed out. That is why inhaled and exhaled air differ:

5 · Gas exchange in plants

Leaves exchange gases through tiny pores called stomata (each opened and closed by two guard cells); stems use lenticels. The net direction changes with the time of day:

• In daylight photosynthesis is faster than respiration, so the leaf takes in CO₂ and gives out O₂ overall.
• At night only respiration occurs, so the leaf takes in O₂ and gives out CO₂.