The Respiratory System

The respiratory system supplies the body with oxygen and removes carbon dioxide, the waste gas produced by cells. Air enters through the nose and mouth, where it is warmed, moistened and filtered, then passes through the pharynx and the larynx, which houses the vocal cords. From the larynx air travels down the trachea, or windpipe, which branches into two bronchi, one leading to each lung.

Inside the lungs the bronchi divide repeatedly into smaller tubes called bronchioles, ending in millions of tiny air sacs known as alveoli. The alveoli are surrounded by a dense network of capillaries, and their thin walls provide an enormous surface area for gas exchange. Here oxygen diffuses from the inhaled air into the blood while carbon dioxide diffuses from the blood into the air sacs to be exhaled. The oxygen is carried by red blood cells to tissues throughout the body.

Breathing is driven mainly by the diaphragm, a dome-shaped muscle below the lungs, together with the muscles between the ribs. When the diaphragm contracts and flattens, the chest cavity enlarges and air is drawn in; when it relaxes, the cavity shrinks and air is pushed out. This rhythm is regulated automatically by the brain, which adjusts the rate and depth of breathing in response to the level of carbon dioxide in the blood.

Frequently asked questions

Where does gas exchange take place in the lungs?

Gas exchange occurs in the alveoli, tiny air sacs surrounded by capillaries. Their thin walls let oxygen pass into the blood and carbon dioxide pass out to be exhaled.

What is the role of the diaphragm in breathing?

The diaphragm is the main breathing muscle. When it contracts and flattens, the chest expands and draws air in; when it relaxes, air is pushed back out.

What path does air take to reach the lungs?

Air travels from the nose or mouth through the pharynx and larynx, down the trachea, and into the bronchi and bronchioles before reaching the alveoli.