The Respiratory System [Lesson 2]

Gas Exchange

Because the walls of both the alveoli and the capillaries are very thin, certain materials can pass through them easily. After air enters an alveolus, oxygen passes through the wall of the alveolus and then through the capillary wall into the blood. Carbon dioxide and water pass from the blood into the alveoli. This whole process is known as gas exchange.

How Gas Exchange Occurs

Imagine that you are a drop of blood beginning your journey through a capillary that wraps around an alveolus. When you begin that journey, you are carrying a lot of carbon dioxide and little oxygen. As you move through the capillary, oxygen gradually attaches to the hemoglobin in your red blood cells. At the same time, you are getting rid of carbon dioxide. At the end of your journey around the alveolus, you are rich in oxygen and poor in carbon dioxide.

Surface Areas for Gas Exchange

Your lungs can absorb a large amount of oxygen because of the large surface area of the alveoli. An adult's lungs contain about 300 million alveoli. If you opened the alveoli and spread them out on a flat surface, you would have a surface area of 70 square meters.
The huge surface area of the alveoli enables the lungs to absorb a large amount of oxygen. The lungs can, therefore, supply the oxygen that people need - even when they are performing strenuous activities. When you play a wind instrument or a fast-paced game of basketball, you have your aleoli to thank.
Your lungs are not the only organs that provide a large surface area in a relatively small space. Recall from the Lesson on The Digestive System the small intestine contains numerous, tiny villi that increase the surface available to absorb food molecules.

How You Breathe

In an average day, you may breathe more than 20,000 times. The rate at which you breathe depends on your body's need for oxygen. The more oxygen you need, the faster you breathe.

Muscles for Breathing

Breathing, like other body movements, is controlled by muscles. The lungs are surrounded by the ribs, which have muscles attached to them. At the base of the lungs is the diaphragm (DY uh fram), a large , dome-shaped muscle that plays an important role in breathing.

diaphragm - a large, dome-shaped muscle that plays an important role in breathing.

The Process of Breathing

When you breathe, the actions of your rib muscles and diaphragm expand or contract your chest. As a result, air flows in or out.
Here's what happens when you inhale, or breathe in. The rib muscles contract, lifting the chest wall upward and outward. At the same time, the diaphragm contracts and moves downward. The combined action of these muscles makes the chest cavity larger. The same amount of air now occupies a larger space, causing the pressure of the air inside your lungs to decrease. This change means that the pressure of air inside the chest cavity is lower than the pressure of the atmospheric pushing on the body. Because of this difference in air pressure, air rushes into your chest, in the same way that air is sucked into a vacuum cleaner.
When you exhale, or breathe out, the rib muscles and diaphragm relax. This reduces the size of the chest cavity. This decrease in size squeezes air out of the lungs, the way squeezing a container of ketchup pushes ketchup out of the opening.

Relating Breathing and Speaking

The air that moves out of your lungs as you breathe also helps you speak. The larynx (LAR ingks), or voice box, is located in the top part of the trachea, underneath the epiglottis. Place your fingers on your Adam's apple, which sticks out from the front of your neck. You can feel some of the cartilage that makes up the larynx. Two vocal cords, folds of connective tissue that produce your voice, stretch across the opening of the larynx.
If you've ever let air out of a balloon while stretching its neck, you've heard the squeaking sound that the air makes. The neck of the balloon is something like your vocal cords. The vocal cords have a slit-like opening between them. hen you speak, muscles make the vocal cords contract, narrowing the opening. Air from the lungs rushes through this opening. The movement of the vocal cords makes the air molecules vibrate, or move rapidly back and forth. This vibration creates a sound - your voice.
larynx - also known as the voice box; is located at the top part of the trachea, underneath the epiglottis.
vocal cords - folds of connective tissue that produce your voice and stretch across the opening of the larynx.

Lesson Complete
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