Why and How Do We Breathe?

HOW AND WHY DO WE BREATHE

REFERENCES:

I. Essay pp. 3-5

II. References p. 6

The diaphragm is a muscle in the chest cavity that contracts and relaxes according to signals received by the respiratory center in the brain. In doing so, the diaphragm changes the pressure in the cavity. When the diaphragm contracts, it increases the volume of the chest cavity and lowers the air pressure, thus, air rushes in. This is called inhalation. Exhalation is caused by the relaxation of the diaphragm, which decreases the volume in the chest cavity and increases the air pressure. This causes air to rush out.

Air passes the nostrils and enters the nasal passages where it is warmed, moistened and filtered, by the hairs, mucous, and blood vessels, which line the passages. Hairs collect dust and germs, mucous moistens the air, and the blood vessels warm the air.

The pharynx is the next structure the air comes in contact with, then larynx (voice box). Air passes the vocal chords and the epiglottis. From there its to the trachea, which branches out into the left and right bronchial tubes, which each branch out and become smaller and smaller. At the end, there is a tiny group of tubes called bronchioles.

Each bronchiole ends in a cluster of sacs called the alveoli. The alveoli contain respiratory surfaces (thin, moist, and loaded with blood vessels). Gas exchange occurs between the blood cells and the alveoli. Smoking brings in particles to the alveoli. These particles get in the way of gas exchange and reduce the amount of oxygen that the cells get.

There are actually four stages of gas exchange:

l. Breathing - air in and out of lungs

2. External respiration - gas exchange between blood and air from environment

in the alveoli

3. Transport of oxygen to cells and carbon dioxide away from the cells

4. Internal respiration - gas exchange between blood and internal cells

The point of breathing is to get rid of carbon dioxide and to oxygenate the blood flowing through the capillaries next to the alveoli. Oxygen has to get from the outside air to the alveoli to the capillary blood, carbon dioxide has to get from the capillary blood to the alveoli to the outside air. In other words we "exchange" oxygenated air for carbon dioxiginated air.

Gas exchange depends on diffusion of each gas from regions of its own high partial pressure to a region of its own low partial pressure. Blood in the pulmonary capillaries has been through the systemic circulation and much of a barrier to the gas's movement; but because there are always fluids and secretions present, the Po2 is reduced another few mmHg by the time it reaches the blood stream. The oxygenation of the blood depends upon the breathing. But at rest, we usually do not breathe very deeply and our lungs don't inflate fully. That means not all of the alveoli are actually ventilated. When we increase the depth of our inspirations and inflate those alveoli, we also increase the number of capillaries open to blood flow.

The pressure relationships in the thoracic cavity are illustrated by the atmospheric pressure defined as the pressure exerted by