WHAT IS BREATHING?
Basically, breathing is ventilation. Ventilation is the mechanical act of moving air
in and out of your lungs. Respiration is commonly confused with ventilation.
Respiration takes place at the cellular level when oxygen diffuses on to the red blood
cells and carbon dioxide diffuses into the lung to be exhaled. When you inhale
(breathe in), fresh air enters your lungs. The lungs take oxygen from the air and add
carbon dioxide to the air. When you exhale (breathe out), you force the air from your
lungs back into the environment. You do not, however, force all the air out of your lungs
when you exhale. A person takes in about 500 ml of air when he inhales normally and
exhales the same amount. After a normal exhale, the lungs will still contain about 2300
ml of air.
a. Oxygen. The oxygen diffused from the air by the lungs is absorbed by the
red blood cells in the blood and taken to all parts of the body. Diffusion is the
movement of molecules from an area of higher concentration (the air) to an area of
lower concentration (the blood cells). The body cells use the oxygen to change stored
energy in the form of sugars and fats into usable energy. In addition to producing
energy, the process produces certain waste products, including carbon dioxide.
b. Carbon Dioxide. Carbon dioxide (CO2) is a byproduct of cellular respiration
and is carried in the blood stream as carbonic acid from the cells to the lungs. When
the carbon dioxide reaches the lungs, it has a higher concentration than the air and it
diffuses out of the blood to be exhaled in to the environment.
WHAT CAUSES BREATHING TO OCCUR?
Ventilation is caused by two muscle systems--the diaphragm and the intercostal
muscles. When the diaphragm and the intercostal muscles contract (get shorter), they
make the chest cavity larger. The lungs then expand in order to fill up the space. When
the lungs expand, air from the outside environment rushes in through the mouth or nose
to fill up this extra space. When the muscles relax, the chest cavity returns to its normal
size. This action compresses the air in the lungs and forces in some of the air from the
lungs, through the windpipe, and out of the nose or mouth.
a. Diaphragm. The diaphragm is a large dome-shaped muscle that separates
the chest cavity from the abdominal cavity. When the diaphragm contracts, the muscle
flattens somewhat and "lowers the floor" of the chest cavity (figure 4-1). When the
muscle relaxes, it returns to its normal (dome) shape. The diaphragm is responsible for
most of the air movement during breathing. The diaphragm is a skeletal muscle that is
under involuntary control of the part of the brain that controls breathing.