Breathing in humans
Monacia Williams, Contributor

Hello again. Did you have a good week? Those of you who are about to do exams, I hope that your studies are going well and that you are feeling a little less apprehensive about the papers that you are going to sit in a few weeks.

This week, we will look at breathing in humans. Breathing is the process by which we take air into our lungs and send it out. Why do we need to do this? We do this to get oxygen in to the body and to remove carbon dioxide from it.

These processes require the use of specialised structures within the body. We will now take a look at the route that the air takes as it goes in and out.

Air enters the body through the nose or mouth and passes down through the pharynx into the larynx. The nasal cavity is, however, better suited for the passage of air. It has special adaptations for cleaning and warming the air that it takes in. It has:

• some small thin bones - turbinal bones - These are covered with a thin layer of cells which produce a liquid containing water and mucus, which evaporates and moistens the air in the cavity.
• cells with tiny, hair-like projections - cilia. The cilia move continuously to trap dust and bacteria.

From here, it goes to the trachea. The trachea also contains cells with cilia which trap bacteria and dust particles in mucus. At the top of the trachea is a piece of cartilage - the epiglottis. The epiglottis closes the trachea when food is being swallowed to prevent food from going into the trachea, which will cause you to choke. Have you ever been told not to talk with food in your mouth? Have you ever, done this? What happened. You nearly choked, didn't you?

The voice box

At the top of the trachea is the larynx or the voice box. For sounds to come out of your mouth, air has to pass through the larynx. In order to allow air to enter, the epiglottis has to be open, and if this is open, and there is food in your mouth, the food can slip down into your throat and into the trachea. This causes choking in order to dislodge the food!

The trachea branches into the left and the right bronchus. The bronchi branch into smaller branches called bronchioles. The trachea, bronchi and bronchioles are kept open for the passage of air by 'C' rings of cartilage. The bronchioles end in tiny air sacs called alveoli. The walls of the alveoli are the sites of gaseous exchange. These walls are surrounded by capillaries, the smallest of the blood vessels.

The lungs are spongy and are surrounded by a moist membrane called the pleural membrane. All of this is found in the thoracic cavity, that region of your body that is enclosed by your rib cage. The ribs protect the lungs and are controlled by the muscles, which are found between them. The muscles are known as the intercostal muscles. There is one layer on the inside - this is the internal intercostal and one layer on the outside. This is the external intercostal.

The base of the thoracic cavity is formed by the diaphragm, which is also made of muscle and elastic. The intercostal and diaphragm muscles contract and relax in order to change the volume of the lungs in order to get air into and out of the lungs.

Look at the following table to see the changes that occur.

Breathing in (Inspiration) Breathing out (expiration)

External intercostal muscles contract. Contraction raises the ribs upwards and outwards. Internal intercostal muscles contract, while the externals are relaxing. This lowers the ribs downwards and inwards.

The diaphragm muscle contracts, the diaphragm moves downwards and flattens. The diaphragm muscle relaxes and the diaphragm bulges upwards.

These movements increase the volume of your thorax. These movements decrease the volume of your thorax.

The pressure in the thorax is lowered below atmospheric pressure, causing the lungs to expand. The pressure inside the thorax increases and this squeezes the lungs.

Air rushes into the lungs through the nose and mouth. Air is pushed out of the lungs, passing out through the trachea and the mouth or nose and out of the body.

Go ahead, inhale while feeling your ribs and exhale while doing the same. Remember that you can do this to jog your memory during the exam!

An adult can take in up to five litres of air in his deepest breath, while at rest about half a litre of air is breathed in and out. The greatest amount of air that anyone can breathe out is called the vital capacity, while the volume of air that you breathe when you are resting is called the tidal volume. The vital capacities of individuals are not all the same. They are influenced by the individual's sex, age and level of activity.

Have you ever noticed what happens to your breathing when you exercise? It increases, doesn't it? Do you also notice that the intensity of the breathing is affected by the level of the exercise? That's right, the intensity increases as the exercise becomes more intense. Get a partner and measure your breathing rate while you exercise, collect the information and try your hand at plotting a graph with the results!

Next week we will look at gas exchange surfaces. See you then!

Monacia Williams teaches at Glenmuir High School.