Some 300 to 500 million bubble-like sacs at the ends of the airway tree, the site of gas exchange
Alveoli — the air sacs of the lungs.
Anatomy · Organs & viscera
Alveolus
About 0.2 to 0.3 mm across
Thorax · respiratory system
An alveolus is one of the millions of tiny air sacs at the end of the airway, where oxygen passes into the blood and carbon dioxide passes out. Each is a thin-walled bubble about a quarter of a millimetre across, wrapped in a mesh of capillaries. Together the two lungs contain roughly 300 to 500 million alveoli, with a total surface area often quoted at around 70 square metres.
How to spot them
- ▸A cluster of tiny bubble-like sacs at the ends of the smallest airways, like a bunch of grapes
- ▸Each sac is wrapped in a dense net of capillaries, the meeting place of air and blood
- ▸The wall is only one cell thick: mostly flat type I cells, with scattered rounded type II cells
- ▸Small holes called pores of Kohn link neighbouring sacs
What to know
A barrier as thin as about 0.5 micrometres separates air on one side from blood on the other
The blood-air barrier of the alveolus.
Flat cells that cover about 95 per cent of the surface and are the thin route for oxygen
Type I pneumocytes lining the alveolus.
Rounded cells that make a detergent-like film that stops the sacs collapsing, missing in premature babies
Type II pneumocytes, which secrete surfactant.
Roving macrophages, sometimes called dust cells, patrol its surface and engulf inhaled particles
Alveolar macrophages.
Small holes in the walls that let air pass between neighbouring sacs and may allow bacteria to spread
The pores of Kohn in the alveoli.
Its walls are destroyed in the disease often caused by smoking, which leaves large air spaces and less surface area
Emphysema — destruction of the alveolar walls.
Key ideas
- Gas exchange is by simple diffusion: oxygen moves from air into blood and carbon dioxide from blood into air, down their concentration gradients across a barrier less than a micrometre thick.
- Surfactant lowers surface tension so that the alveoli, small bubbles, do not collapse when breathing out; premature babies lacking it develop respiratory distress syndrome.
Quick-fire facts
- •Adult lungs hold about 300 to 500 million alveoli; a figure of about 480 million is often quoted.
- •Each alveolus is roughly 200 to 300 micrometres across.
- •Surface area for gas exchange is often given as around 70 square metres, though newer estimates are higher; 'a tennis court' is an exaggeration.
- •Type I pneumocytes make up most of the surface; type II pneumocytes are more numerous but cover only a small area.
- •The blood-air barrier is about 0.2 to 0.6 micrometres thick and consists of alveolar cell, fused basement membranes and capillary endothelium.
- •Alveoli branch from alveolar ducts and alveolar sacs at the end of the respiratory bronchioles.
- •Alveolar macrophages are sometimes called dust cells; in heart failure they may become 'heart failure cells' full of haemosiderin.
- •Alveoli are formed mostly after birth; the number rises through the first years of childhood.
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