The respiratory system in human beings consists of the following organs: nose, pharynx, trachea, bronchi and lungs.
Nose encloses a nasal cavity which opens to the outside through two nostrils. Nose has fine hair and a sticky liquid, mucus, produced by the inner lining of the nose, both of which serve to filter the air entering the nasal cavity. Nose performs the following functions:
1. Fine hair and mucus present in the nose prevent the entry of dust particles and germs into the respiratory system.
2. Mucus moistens the air in the nasal cavity.
3. Blood circulating in the nose warms the air.
From the nasal cavity air passes into the pharynx which has two openings in its lower part, one opening into the oesophagus and the other into the windpipe or trachea. The opening of the pharynx into the trachea is called glottis.
The upper part of trachea is a box-like chamber called larynx or voice box. Glottis is guarded by a flap-like structure called epiglottis which prevents food from entering into the windpipe.
Trachea or windpipe is a long cylindrical tube supported by cartilaginous rings (rings made of cartilage), which prevent it from collapsing.
Trachea branches braches into two tubes called bronchi, one of which enters each lung. Inside the lung, bronchi branch into fine breaches called bronchioles.
A pair of lungs are the organs of gaseous exchange in human beings. Finest branches of bronchioles end in tiny sacs called by a network of fine capillaries. Exchange of gases between the alveoli and the blood capillaries takes place across their thin walls. Thus, alveoli are the structures responsible for gaseous exchange. Oxygen present inside the alveoli diffuses across the walls of the alveoli and the capillaries into the blood in the capillaries. In a similar manner, carbon dioxide present inside the blood capillaries diffuses into the alveoli. The carbon dioxide thus collected is breathed out through the nose.
Blood carries the oxygen received from the lungs to different parts of the body. Blood contains red blood cells (RBC) which contain the red pigment hemoglobin. (Hemoglobin gives the blood its red color.) Haemoglobin combines with oxygen and carries it to all the cells. As the blood passes through the tissues in thin-walled capillaries, hemoglobin releases oxygen which diffuses into the cells. Oxygen reaches each and every cell of the body in this manner. Simultaneously carbon dioxide from the cells of the body diffuses out into the blood which carries it to the lungs for breathing out.
In addition to the organs mentioned above, some other body parts are also involved in the process of respiration. They are chest muscles, ribs and diaphragm. During breathing, the chest goes up and down repeatedly to let the air in and out. Repeated contraction and expansion of the chest cavity during breathing are called breathing movements. Breathing movement is of two types- inhalation and exhalation.
During inhalation, muscles and diaphragm contract and the ribs are pushed outward and upward. The diaphragm flattens and the chest cavity increases in volume. This reduces the air pressure inside the lungs. The atmospheric air gushes in through the nostrils to equalize the air pressure.
During exhalation, the chest muscles relax and ribs are pushed downward and backward. The diaphragm becomes dome-shaped. As a result, the volume of the chest cavity decreases. The pressure of air inside the lungs increases and air is thrown out through the nostrils.