The body needs a continuous supply of oxygen. This invisible gas makes up about one-fifth of the air around us. Oxygen is used by cells to release energy from food. This process also releases waste carbon dioxide. The respiratory (breathing) system draws fresh air into the body, absorbs the vital oxygen from it into the blood, and then pushes out "stale" air containing carbon dioxide. The parts of the system where oxygen is absorbed are the lungs, which are two large, spongy organs, made up almost entirely of a latticework of tubes.

The respiratory system

Breathing muscles stretch the lungs to make them larger and suck in air. These muscles are the diaphragm below the lungs, and the intercostal muscles between the ribs. Fresh air passes in through the nose and mouth, down the pharynx (throat) and trachea (windpipe), into the lungs. The lung airways, called bronchi, divide many times and become thinner, ending in terminal bronchioles, narrower than human hairs.

Alveoli

Each terminal bronchiole ends in a grape-like cluster of air bags called alveoli. There are about 300 million alveoli in the two lungs, giving them a spongy texture. Besides fresh air, the lungs also receive low-oxygen blood from the heart along the pulmonary arteries. These divide and form networks of microscopic blood vessels (capillaries) around the alveoli. Oxygen from the air inside the alveoli passes easily through the extremely thin walls of the alveoli and capillaries into the blood. There oxygen binds to a substance called haemoglobin found inside red blood cells. This turns the blood bright red. This high-oxygen blood returns along pulmonary veins to the heart, from where it is pumped to all parts of the body.

Breathing in and out

During breathing in, the diaphragm flattens and the intercostal muscles move the ribs up and outwards. This increases the space inside the chest cavity so that air is sucked into the lungs. During breathing out, these breathing muscles relax and the space inside gets smaller, squeezing the lungs so that air is pushed out.

Throat

As well as a passage for food, the throat lets air reach the lungs and allows you to speak. A flap called the epiglottis prevents food slipping down the trachea by closing when you eat or drink (food or liquid which goes "down the wrong way" has to be coughed back up). At the top of the trachea is the larynx or voice box.

Speech

Air emerging from the lungs not only carries waste carbon dioxide. It has another use—speech. Located at the top of the trachea, the larynx (voice box) is made of pieces of cartilage. The two vocal cords are folded membranes that extend from the front to the back of the larynx. To speak, muscles pull the vocal cords together so that there is only a very narrow slit between them. Air rushing through the slit makes the cords vibrate. This produces sounds. Muscles stretch the vocal cords longer and tighter to make higher-pitched sounds. These sounds are shaped into clear words by movements of the mouth, cheeks, teeth, tongue and lips.