Every living thing requires energy for working of the body, machinery, its maintenance, repair, replacement, and biosynthesis. Energy is obtained by breaking down carbon based molecules in the process of respiration. Oxidation-reduction reactions are common chemical reactions involved in respiratory breakdown of molecules. Most organisms use oxygen obtained from outside for the process of respiration.
Human respiratory system consists of respiratory tract-nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli and the lungs.
The nose is the part of entry for the atmospheric air. Externally it bears two nostrils represented by a cartilaginous septum and internally it opens in to a pair of nasal chambers. Thehair lining the nostrils traps the entry of dust or any other large foreign particles into the system. Themucustrap the harmful organisms.
Pharynx is a common passage at the back of the mouth for air and food. Air enters the front tube called the trachea or wind pipe while food enters the hind tube called the esophagus. The opening of the wind pipe is guarded by a muscular flap called the epiglottis.
Larynx is located at the start of the trachea is a hollow cartilaginous structure called the voice box. It is more prominent visible in males at the front of the neck and is popularly called the Adam’s apple. Itconsist of two muscular folds called vocal cords. Vibrations in these cords produce sounds.
The trachea divides in the thoracic cavity into two bronchi-the left and right, which enter each lung and branch extensively into the bronchial tree. The smallest bronchial branches are called bronchioles. The bronchioles end in blind air sacs as alveoli. The clusters of alveoli form the lung tissue. Both lungs contain 300 to 400 microscopic alveoli which in a normal adult have a diameter of 0.2mm.
Incomplete rings of cartilage keep the trachea and bronchi distended and prevent their collapsing when the pressure inside them falls during inspiration. The cartilaginous rings also strengthen the walls of the trachea and bronchi. Bronchioles are without any cartilage.