Breathing
Breathing provides the oxygen needed for the conversion of foodstuff to the energy used to support and maintain the various functions of life. The intake of oxygen is facilitated by the lungs.
Breathing is vital for life. Humans can live only a very short time without an infusion of oxygen. Without oxygen, irreversible alterations in the brain set in after only about 4-5 minutes. Reflecting this importance to life, breathing is an automatic function, and is controlled by nerve cells in the brain stem. Signals from the brain are sent down the spinal cord via the phrenic nerve to two sets of muscles that control the operation of the the diaphragm, a sheet-like muscle located at the bottom of the chest cavity, that moves down to force air into the lungs and up to force air out of the lungs.
Air can be breathed into the lungs through the nose and the mouth. When entering through the nose, air is warmed, moistened and cleaned on its travels through passages called sinuses. The air then passes through the throat, down the windpipe and into the lungs. As this is happening the chest expands downward and horizontally outwards. This allows the lungs to act like bellows, expanding to receive the incoming air. Once in the lungs, the air goes through a maze of smaller and smaller tubes--bronchi, secondary bronchi and bronchioles--until it reaches the tiny sacs called alveoli. The alveoli, 300 million in each lung, are the site where oxygen from the air enters the blood, and where carbon dioxide from the body enters the air, to be expelled from the lungs. These gases are able to diffuse across the thin walls of the alveoli.
The efficiency of breathing can be affected by several factors. As people age, and generally begin to assume a poorer, stooped posture, the expansion of the chest is more restricted. But even young people who adopt a poor posture can compromise their breathing efficiency. Gender also places a role; men generally breathe more fully and lower in their abdomen, whereas women tend to breathe higher in their thorax. Presence of a fetus pressing on the diaphragm and ribcage can lessen lung expansion in pregnant women. Respiratory infections or medical conditions such as asthma can also compromise breathing. Colds, flu and pneumonia can make inhalation more difficult and impair gas exchange in the lungs. Smoking has devastating effects on the breathing process. The toxic components of tobacco smoke can kill the many tiny hairlike cilia that line the airways, leading to the accumulation and stagnation of mucus, germs, and inhaled debris.
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