The Respiratory System / The Lungs

submitted by Dr. Gary Farr Last Updated August, 4, 2003

POST FIRST COMMENT!

Page: 2

Lung Functions

We normally have a respiration rate of approximately 16 breaths per minute, inhaling about 1 pint with each inspiration. The lungs hold about 1 gallon of air. During sleep we require about 2 gallons of air per minute and this can increase by 600% during exercise. During heavy exercise we need up to twenty times as much oxygen as during resting.

The lungs are somewhat elastic and expand and contract during breathing. The contraction and expansion of the diaphragm, elevation and depression of the ribs, and actions of the abdominal muscles, scalenes, and quadratus lumborum all play a role. During inspiration, the diaphragm moves downward and the lungs expand. This causes the atmospheric pressure in the lungs to decrease and air flows in. During expiration, the diaphragm moves up, the lungs contract, the atmospheric pressure increases, and air flows out.

Cilia are located along the air passages to and in the lungs. They help trap particles and they beat toward the pharynx to remove the particles and excessive mucus.

The lungs normally contain about 1 pint of blood but they can act as a blood reservoir carrying up to 3 pints (to release the extra when needed.

 Breathing

Although the words breathing and respiration are sometimes used interchangeably, they have distinct meanings. Breathing is the process of moving oxygen-rich air into and out of the lungs. Respiration refers to all of the processes involved in getting oxygen to tissues, including breathing, diffusion of oxygen from the lungs to the blood, transport by the blood, and diffusion from the blood to tissues. Respiration is essential for aerobic respiration, the process within cells in which nutrients and oxygen are used to build the energy molecule adenosine triphosphate (ATP). In aerobic respiration, body cells use oxygen to metabolize glucose, forming carbon dioxide as a waste product that is exhaled.

Because body cells are constantly using up oxygen and producing carbon dioxide, the lungs work continuously. An adult normally breathes from 14 to 20 times per minute, but vigorous exercise can raise the rate to 80 breaths per minute. A child's rate of breathing at rest is faster than an adult's at rest, and a newborn baby has a rate of about 40 breaths per minute. In general, smaller animals have faster breathing rates than larger animals. A rat, for example, breathes about 60 times per minute, while a horse breathes only about 12 times per minute.

The process of breathing is generally divided into two phases, inspiration and expiration. In inspiration, air is moved into the lungs. In expiration, air is forced out of the lungs. The lungs themselves have no muscle tissue. Their movements are controlled by the rib cage and the diaphragm. During inspiration the muscles around the rib cage contract, lifting the ribs upward and outward, and lowering the dome of the diaphragm until it forms a nearly flat sheet. As a result of these changes, the chest cavity expands. Because the lungs are attached to the chest cavity, they also expand. With the enlargement of the lungs, air pressure inside the lungs falls below the pressure of the air outside the body, creating a partial vacuum, and air from outside the body rushes into the lungs.

The amount of air normally taken into the lungs in a single breath during quiet breathing is called the tidal volume. In adults the tidal volume is equal to about 0.5 liters (about 1 pt). The lungs can hold about ten times this volume if they are filled to capacity. This maximum amount, called the vital capacity, is generally about 4.8 liters (about 1.3 gal) in an adult male, but varies from one individual to the next. Athletes, for example, can have a vital capacity of as much as 5.7 liters ( 1.5 gal). The vital capacity is reached only during strenuous exercise.

In expiration the muscles that lift the rib cage and lower the diaphragm relax. As a result, the rib cage and the diaphragm return to their original positions, and the lungs contract with them. With each contraction of the lungs the air inside them is forced out.

A person can alter the rate of breathing and can even stop breathing for a short time. But it is impossible to voluntarily stop breathing permanently because breathing, like the heartbeat, is an involuntary activity controlled by nerve centers in the brain stem, the lower part of the brain. These centers are connected with the muscles of the rib cage and diaphragm, and they increase or decrease the rate of breathing according to the needs of the body.

Aerobic Respiration

In the life-supporting process of aerobic respiration, oxygen from incoming air enters the blood; and carbon dioxide, a waste gas from the metabolism of food, is exhaled into the atmosphere. Air entering the lungs contains about 21 percent oxygen and 0.04 percent carbon dioxide. Air leaving the lungs contains about 14 percent oxygen and about 4.4 percent carbon dioxide. The composition of the air changes between inspiration and expiration, when the air is deep in the lung tissue.

The exchange of gases takes place when air reaches the alveoli. These small sacs are only one cell thick, and they are surrounded by blood capillaries that are also only one cell thick. Air diffuses through these cells into the capillary blood, which carries the oxygen-rich air to the heart to be distributed throughout the body. In the alveoli, at the same time, gaseous carbon dioxide diffuses from the blood into the lung and is expired.

Get Additional Help

Free Health Tests & Professional Consultations

Take this preliminary Free Test to see if your condition could respond to treatment.

Not sure on your treatment options? For a limited time you can schedule a Free Consultation to talk with a licensed doctor or clinician regarding your condition.

Use our NutritionLocator to find a doctor in your area.