Our respiratory system consists of two lungs and numerous air passages which unite to form the two main bronchi. The latter open into the trachea, the main airway connecting the lungs to the outside atmosphere. The lungs are spongy structures consisting of innumerable air spaces, the walls of which contain blood vessels from and to the heart. The total mass of blood received by the heart from the body is pumped into the lungs where it circulates in the walls of the air spaces. It is here that the gaseous exchange occurs the oxygen of the atmospheric air is transferred to the blood and carbon di oxide of the blood to the air spaces. With each inhalation we take in oxygen rich air and with each exhalation we let out carbon di oxide rich air into the atmosphere. Plants and trees absorb this carbon dioxide as their food and release oxygen into the air.
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