Page 634 - The Veterinary Care of the Horse

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Figures 15.1a, b and c a) the respiratory system; b) the airways of the lung; c) the air sacs of the lung

Each bronchiole ends in a cluster of air sacs known as alveoli. The thin, elastic walls of
the alveoli are in close contact with a network of capillaries. Capillaries are tiny, thin-walled

blood vessels. Oxygen in the inhaled air diffuses from the alveoli into the blood. Here, it
combines with haemoglobin in the red blood cells and is transported to the tissues. Carbon
dioxide diffuses in the opposite direction and is exhaled.

During its passage from the nostrils to the lungs, air is warmed, moistened and filtered.

Inhaled dust and spores are trapped in a thin layer of mucus which lines the airways. The
mucus is continually moved towards the pharynx by tiny hair-like projections (called cilia)
on the epithelial cells. This is known as the mucociliary escalator. On reaching the pharynx,

the debris is swallowed.