The principal bronchi are further divided into smaller and smaller bronchi. The alveoli are richly supplied with capillaries where the exchange of gases takes place between the red blood cells and the air. When the alveoli become inflammed due to an infection, a condition results which is referred to as pneumonia.
Pneumonia can be viral or bacterial in nature and can also result from yeast or fungus infections.
Stimulation from the sympathetic division of the autonomic nervous system causes these smooth muscles to relax, dilating the bronchial branches allowing more air to pass through the lungs. When danger is perceived, the sympathetic nervous system is stimulated to prepare the body for immediate action. When the parasympathetic division of the autonomic nervous system is stimulated, the smooth muscles of the bronchial branches contract, causing constriction of the bronchial branches allowing less air through the lungs. Normally the two divisions of the autonomic nervous system are in balance, but imbalances can occur.