What We Know About Gas Exchange in Lungs

Oxygen & Carbon Dioxide
The body requires oxygen in order to produce energy for our cells to do work. Therefore, it is essential that we have an efficient system of obtaining oxygen from the air. Our bodies also produce a toxic gas called carbon dioxide that must be efficiently removed from the body to prevent cell damage.  When we inhale, we pick up oxygen from air.  When we exhale, we flush out carbon dioxide.

Each breath lasts only a few seconds (even less if we are running.)  Isn't it amazing that gas exchange occurs so quickly?  What is it about gases that lets them exchange so quickly?

The Structures of the Respiratory System
Here is a list of structures to become familiar with:

1. Mouth and Nose- these are the openings where respiratory gases enter and leave the body.

2. Trachea (windpipe)- this passage way connects the mouth and nose to the lungs.
3. Lungs- these are the balloon-like structures that temporarily hold air in the body.
4. Bronchial tube- the trachea breaks up into these smaller tubes to enter the right and left lungs.
5. Bronchioles- within the lungs the bronchi split into these even smaller tubes which attach to the alveoli.
6. Alveoli- these are the small sac-like structures where gas exchange occurs with the blood.

(Note: The above structures are listed in the order they are used during inspiration and in reverse order for expiration.)

Here is a detailed picture of the respiratory system:  (The structures we will discuss are highlighted.  Click here for a larger version)

Do you see how air moves into the lungs?

Why do you think the bronchi become so branched?

Why do you think there are veins and arteries at the alveoli?

What keeps food from going into the lungs?

Here are a few things that will help answer some of your questions:

•  Air enters your body through your mouth and nose. The nasal passage connects to the oral passage at the back of the mouth where a tube called the trachea connects the mouth and nose to the lungs. Dirt from the air is filtered before it ever reaches the lungs by the hairs and mucus in the nose. This hair and mucus trap some of the dirt and germs found in the air to protect the lungs from infection.

Why do we have nose hairs?  

• The trachea, or windpipe, is a tube that connects the nose and mouth to the lungs. You can see and feel your trachea on the front of your neck. The tube has C-shaped cartilaginous rings wrapped around it to prevent the tube from collapsing and blocking the air flow to and from the lungs. Air is the only substance designed to go down the trachea. 

Why do you think the trachea is only meant for air?  What happens if something blocks the trachea?

• A little flap covers the trachea each time you swallow to direct food and drinks to go down the esophagus, a tube that leads to the stomach. When food and drinks find their way into the trachea, you’ll know it immediately! You’ll begin coughing in order to push the food back out of the trachea to allow air to pass through uninhibited.

What happens when a person laughs so hard, that milk can come out of their nose?

• As the trachea approaches the lungs, it branches into two tubes. One tube leads to the right lobe of the lungs, and the other tube leads to the left lobe of the lungs. These tubes are known as the bronchial tubes. The bronchial tubes enter the lungs and begin branching into smaller and smaller tubes. 
• These smaller tubes are called bronchioles. The walls of the bronchioles contain smooth muscle that is used to dilate or constrict the lungs depending upon the body’s need for oxygen. The bronchioles continue the branching process until they reach small, thin sacs called alveoli. The function of the lungs depends primarily on these tiny structures.

Does this branching allow the tubes to fill the lungs more effectively?

Asthma is a disease in which dust, pollen, or other things to which you are allergic change the diameter of the bronchioles. What does this do?  Why?

What is the benefit of having such a large amount of surface for gas exchange to take place?

How about a look at a pair of lungs?

These are sheep lungs with a cut trachea at the top. Left: lateral view as seen from animal's right side. Right: ventral view, with heart removed.  Look closely at the end of the trachea in the picture on the right to see how it branches into the bronchi. Click here to see a larger version

Next we're going to discuss the alveoli in detail

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