The main function of the respiratory system is to provide oxygen, the most important energy source for
the body's cells. Inspired air (the air taken in when a person breathes) contains oxygen, and travels down the respiratory tree to the alveoli. The oxygen moves out of the alveoli and is sent into circulation throughout the body as part of the red blood cells. The oxygen in the inspired air is exchanged within the alveoli for the waste product of human metabolism, carbon dioxide. The air you breathe out contains the gas called carbon dioxide. This gas leaves the alveoli during expiration. To restate this exchange of gases simply, you breathe in oxygen, you breathe out carbon dioxide
Respiratory system defenses
The healthy human lung is sterile. There are normally no resident bacteria or viruses (unlike the upper respiratory system and parts of the gastrointestinal system, where bacteria dwell even in a healthy state). There are multiple safeguards along the path of the respiratory system. These are designed to keep serious, pathogenic organisms from invading, and leading to infection.
The first line of defense includes the hair in the nostrils, which serves as a filter for larger particles. The epiglottis is a trap door of sorts, designed to prevent food and other swallowed substances from entering the larynx and then trachea. Sneezing and coughing, both provoked by the presence of irritants within the respiratory system, help to clear such irritants from the respiratory tract.
Mucus produced by the respiratory system also serves to trap dust and infectious organisms. Tiny hair-like projections (cilia) from cells lining the respiratory tract beat constantly. They move debris trapped by mucus upwards and out of the respiratory tract. This mechanism of protection is referred to as the mucociliary escalator.
Cells lining the respiratory tract produce several types of immune substances that protect against various organisms. Other cells (called macrophages) along the respiratory tract actually ingest and kill invading organisms.
The organisms that cause pneumonia, then, are usually carefully kept from entering the lungs by virtue of these host defenses. However, when an individual encounters a large number of organisms at once, the usual defenses may be overwhelmed. Infection may happen either by inhaling contaminated air droplets, or by aspiration of organisms inhabiting the upper airways.
CONDITIONS PREDISPOSING TO PNEUMONIA. In addition to exposure to sufficient quantities of causative organisms, certain conditions may make an individual more likely to become ill with pneumonia. Certainly, the lack of normal anatomical structure could result in an increased risk of pneumonia. For example, there are certain inherited defects of cilia which result in less effective protection. Cigarette smoke, inhaled directly by a smoker or second-hand by an innocent bystander, interferes significantly with ciliary function, as well as inhibiting macrophage function.
Stroke, seizures, alcohol, and various drugs interfere with the function of the epiglottis. A weak epiglottis leads to a leaky seal on the trap door, with possible contamination by swallowed substances and/or regurgitated stomach contents. Alcohol and drugs also interfere with the normal cough reflex. This inteference further decreases the chance of clearing unwanted debris from the respiratory tract.
Viruses may interfere with ciliary function, allowing themselves or other microorganism invaders (such as bacteria) access to the lower respiratory tract. One of the most important viruses is HIV (Human Immunodeficiency Virus), the causative virus in AIDS (acquired immunodeficiency syndrome). In recent years this virus has resulted in a huge increase in the incidence of pneumonia. Because AIDS results in a general decreased effectiveness of many aspects of the host's immune system, a patient with AIDS is susceptible to all kinds of pneumonia. This includes some previously rare parasitic types that would be unable to cause illness in an individual possessing a normal immune system.
The elderly have a less effective mucociliary escalator, as well as changes in their immune system. This causes this age group to be more at risk for the development of pneumonia.
Various chronic conditions predispose a person to infection with pneumonia. These include asthma, cystic fibrosis, and neuromuscular diseases that may interfere with the seal of the epiglottis. Esophageal disorders may result in stomach contents passing upwards into the esophagus. This increases the risk of aspiration into the lungs of those stomach contents with their resident bacteria. Diabetes, sickle cell anemia, lymphoma, leukemia, and emphysema also predispose a person to pneumonia.
Genetic factors also appear to be involved in susceptibility to pneumonia. Certain changes in DNA appear to affect some patients' risk of developing such complications of pneumonia as septic shock.
Pneumonia is also one of the most frequent infectious complications of all types of surgery. Many drugs used during and after surgery may increase the risk of aspiration, impair the cough reflex, and cause a patient to underfill their lungs with air. Pain after surgery also discourages a patient from breathing deeply enough, and from coughing effectively.
Radiation treatment for breast cancer increases the risk of pneumonia in some patients by weakening lung tissue.
In addition, the use of mechanical ventilators to assist patients in breathing after surgery increases their risk of developing pneumonia. The mortality rate among ventilated patients who develop pneumonia is 46%.
Mai Tran, Rebecca J. Frey PhD, The Gale Group Inc., Gale, Detroit,