7.4 The Respiratory System

Learning Objectives

• Identify the parts of the respiratory system.
• Identify the main function of the respiratory system.
• Describe how breathing works.
• Outline how the respiratory system and the cardiovascular system work together.
• Identify how breathing and cellular respiration are connected.

Introduction

Do you remember how uncomfortable you felt the last time you had a cold or a cough?

You usually do not think about your respiratory system or how it works until there is a problem with it. You breathe mostly without thinking about it. Every cell in your body depends on your respiratory system. In this section you will find out how your respiratory systems helps to keep every cell in your body alive.

Guided Learning

Parts of the Respiratory System

Your respiratory system is made up of the tissues and organs that allow oxygen to enter the body and carbon dioxide to leave your body. Organs in your respiratory system include:

• Nose
• Mouth
• Trachea
• Lungs
• Diaphragm

These structures are shown in Figure below.

^[1]

The respiratory system. Air moves in through the nose and mouth and down the trachea, which is a long, straight tube in the chest.

Structures of the Respiratory System

Figure above shows many of the structures of the respiratory system. Each of the parts has a specific job. The parts of the respiratory system include the following:

• The diaphragm is a sheet of muscle that spreads across the bottom of the rib cage. When the diaphragm contracts, the chest volume gets larger and the lungs take in air. When the diaphragm relaxes, the chest volume gets smaller and air is pushed out of the lungs.
• The nose and nasal cavity filter, warm, and moisten the air you breathe. The nose hairs and mucus produced by the cells in the nose catch particles in the air and keep them from entering the lungs. When particles in the air reach the lungs, what do you think happens?
• The trachea, or windpipe, is a long tube that leads down to the lungs, where it divides into the right and left bronchi. The bronchi branch out into smaller bronchioles in each lung.
• A flap of tissue called the epiglottis closes over the trachea when food is swallowed to prevent choking or inhaling food.
• The bronchioles lead to the alveoli. Alveoli are the little sacs at the end of the bronchioles. They look like little bunches of grapes, as shown in Figure below. Oxygen is exchanged for carbon dioxide in the alveoli. Gas exchange is the name we give to the process that allows oxygen to enter the blood and carbon dioxide to move out of the blood; the two gases are "exchanged."

^[2]

The alveoli are the tiny grape-like structures in the lungs and the sites of gas exchange.

How we Breathe

Most of the time, you breathe without thinking about it. Breathing is mostly an involuntary action that is controlled by a part of your brain that also controls your heart beat. If you swim, do yoga, or sing, you know you can also control your breathing. Taking air into the body through the nose and mouth is called inhalation. Pushing air out of the body through the nose or mouth is called exhalation. The man in Figure below is exhaling before he surfaces from the pool water.

^[3]

Being able to control breathing is important for many activities, such as swimming. The man in the photograph is exhaling before he surfaces from the water.

How do lungs allow air in? As mentioned above, air moves into and out of the lungs by the movement of muscles. The diaphragm and rib muscles contract and relax to move air into and out of the lungs. During inhalation, the diaphragm contracts and moves downward. The rib muscles contract and cause the ribs to move outward. This causes the chest volume to increase. Because the chest volume is larger, the air pressure inside the lungs is lower than the air pressure outside. This difference in air pressures causes air to be sucked into the lungs. When the diaphragm and rib muscles relax, air is pushed out of the lungs. Exhalation is similar to letting the air out of a balloon.

The walls of the alveoli are very thin and allow gases to enter into them. The alveoli are lined with capillaries. These capillaries are shown in Figure below. Oxygen moves from the alveoli to the blood in the capillaries that surround the alveoli. At the same time, carbon dioxide moves in the opposite direction, from capillary blood to the alveoli.

^[4]

The bronchi and alveoli. During respiration, oxygen gets pulled into the lungs and enters the blood by passing across the thin alveoli membranes and into the capillaries.

Breathing and Respiration

When you breath in, oxygen is drawn in through the mouth and down into the lungs. The oxygen then passes across the thin lining of the capillaries and into the blood, through the process of diffusion. The oxygen molecules are carried to the body cells by the blood. Carbon dioxide from the body cells is carried by the blood to the lungs where it is released into the air. The process of getting oxygen into the body and releasing carbon dioxide is called respiration. (Figure below).

Sometimes breathing is called respiration, but there is much more to respiration than just breathing. Remember, cellular respiration occurs in the mitochondria!

The Journey of a Breath of Air

Breathing is only part of the process of bringing oxygen to where it is needed in the body. After oxygen enters the lungs, what happens?

1. The oxygen enters the bloodstream from the alveoli. Then, the oxygen-rich blood returns to the heart.
2. Oxygen-rich blood is then pumped through the aorta.
3. From the aorta, oxygen-rich blood travels to the smaller arteries and finally to the capillaries.
4. The oxygen molecules move out of the capillaries and into the body cells.
5. While oxygen moves from the capillaries and into body cells, carbon dioxide moves from the cells into the capillaries.

^[5]

Gas exchange is the movement of oxygen into the blood and carbon dioxide out of the blood.

Breathing and Cellular Respiration

The oxygen that arrives at the cells from the lungs is used by the cells to help release the energy stored in molecules of sugar. Cellular respiration is the process of breaking down glucose to release energy (see the Cell Functions chapter). The waste products of cellular respiration include carbon dioxide and water. The carbon dioxide molecules move out of the cells and into the capillaries that surround the cells. As explained above, the carbon dioxide is removed from the body by the lungs.

Summary

• Your respiratory system is made up of the tissues and organs that allow oxygen to enter and carbon dioxide to leave your body.
• Respiratory system organs include your nose, mouth, larynx, pharynx, lungs, and diaphragm.
• During inhalation, the diaphragm contracts and moves downward, and brings air into the lungs. During exhalation, the diaphragm and rib muscles relax and air is pushed out of the lungs.
• Oxygen enters the lungs, then passes through the alveoli and into the blood. The oxygen is carried around the body in blood vessels.
• Carbon dioxide, a waste gas, moves into the blood capillaries and is brought to the lungs to be released into the air during exhalation.
• The oxygen that arrives from the lungs is used by the cells during cellular respiration to release the energy stored in molecules of sugar.

Vocabulary

alveoli        

Tiny air sacs found in the lungs where oxygen and carbon dioxide are exchanged through diffusion.

bronchi

One of the two tubes that connect the lungs with the trachea; branches out into smaller bronchioles in each lung.

diaphragm

A muscle attached to the lower ribs which allows the lungs to expand and fill with air.

epiglottis

A flap of tissue that closes over the trachea when food is swallowed to prevent choking or inhaling food.

exhalation

The process of pushing air out of the body through the nose or mouth.

gas exchange

The process that allows oxygen to enter the blood and carbon dioxide to leave the blood.

inhalation

The process of taking air into the body through the nose and mouth.

respiration

The process of getting oxygen into the body and releasing carbon dioxide (not to be confused with cellular respiration).

respiratory system

An organ system made up of the tissues and organs that allow oxygen to enter the body and carbon dioxide to leave your body.

trachea

The windpipe; a long tube that leads down to the lungs, where it divides into the right and left bronchi.