Page 301 - Biology_F5

Biology for Advanced Level Secondary Schools

Alveolus The alveoli are thin, moist and are well
Task 6.2 supplied with a dense capillary networks,
hence they are highly adapted for efficient
Search from internet sources the gas exchange. In the process of gas
simulations of the gas exchange in alveoli. exchange, oxygen gas from the alveoli
Note down what you have observed. diffuses quickly across the alveolar wall
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through the air-blood barrier into the blood
The alveolus is a tiny air-filled sac of capillaries and carried to the heart through
the mammalian lungs. Gas exchange pulmonary vein. On the other hand, the
takes place in the alveolus, which is a carbon dioxide from the body tissues
functional unit of the lungs. The estimated enters the heart and carried from the heart
total amount of the alveoli in the average to the lungs by the pulmonary artery. It
human is about 700 million that has a diffuses through the blood capillary to the
total surface area of over 80 square metres alveoli and is then exhaled (Figure 6.2).
which facilitate the rate of gas exchange.

Trachea with rings Alveoli Bronchioles
of cartilage
Alveoli
Aorta
Right pulmonary Blood capillary
networks
arteries
Superior Carbondioxide
venacava leaves blood
Right capillaries and
pulmonary enters to the
vein
alveolus

Oxygen leaves the
alveolus and enters to
Right lung the blood capillaries
Pulmonary Pulmonary
Left lung artery vein

Figure 6.2: Structure of the mammalian lungs showing the association between alveoli and blood
vessels during gas exchange
Structurally, the alveoli possess connective the energy required to breathe in and inflate
tissues with elastic collagen fibres that allow the lungs. It also kills bacteria trapped in
them to expand and recoil during breathing. the incoming air through the nostrils. In
Alveoli have special epithelia cells in their addition, it increases the rate of gas exchange
walls that secrete a detergent-like chemical in mammals. They also have macrophages,
known as surfactant into the alveolar space. which are protective cells that remove debris
The surfactant lowers the surface tension of and microbes by phagocytosis (Figure 6.3).
the fluid inside the alveoli, hence reducing

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