General Considerations
The main function of the lungs is (rapid) gas exchange. This is accomplished by a well-coordinated interaction of the lungs with the central nervous system, the diaphragm and chest wall musculature, and the circulatory system.
Gas exchange occurs in the alveolus where the thin laminar blood flow and inspired air are separated only by a thin tissue layer. Gas exchange takes 0.25 seconds or 1/3 of the total transit time of a red cell. The entire blood volume of the body passes through the lungs each minute in the resting state, that is 5 liters per minute. The total surface area of the lung is about 80 meters square, equivalent to the size of a tennis court.
Only about 10% of the lung is occupied by solid tissue, whereas the remainder is filled with air and blood. Supporting structures of the lung must be delicate to allow gas exchange, yet strong enough to maintain architectural integrity, that is sustain alveolar structure. The functional structure of the lung can be divided into (1) the conducting airways (dead air space), and (2) the gas exchange portions. The two plumbing systems are: airways for ventilation, and the circulatory system for perfusion. Both are under low pressure.
Total lung weight is about 300-400 gms. Upper and middle lobes are anterior, while the lower lobes are posterior. Development of each lobe results in division into 19 bronchopulmonary segments which are relatively constant and which often have pathophysiologic correlates, i.e. secondary tuberculosis is seen in the apical segments.
The Conducting Airways
From the trachea distally, the respiratory tree divides into paired branches of unequal length and diameter and is, therefore, described as having an arborizing branching pattern of irregular dichotomy.
The luminal diameter of a branch is related to the number of alveoli at the end of that branch (axial and lateral pathways). Since the longer airways with more branches and more alveoli usually have a wider lumen that allows greater airflow, newly inspired air reaches all of the alveoli throughout both lungs at the same time and in approximately the same amount, i.e. an even distribution of inspired air throughout all lobes in a given period of time. There are approximately 23 airway divisions to the level of the alveoli. The divisions include main bronchi, lobar bronchi, segmental bronchi (to designated bronchopulmonary segments), and so on to the smallest bronchioles which do not have alveoli and are lined completely by bronchial epithelium, the terminal bronchioles. Although the base airway diameter decreases with branching, the overall or total cross-sectional diameter increases tremendously so that peripheral airway resistance decreases.
In addition to gas exchange, the lungs and the other parts of the respiratory system have important jobs to do related to breathing. These include:
* Bringing all air to the proper
body temperature
* Moisturizing the inhaled air for necessary humidity
* Protecting the body from harmful substances by coughing, sneezing,
filtering or swallowing them, or by alerting the body through
the sense of smell
* Defending the lungs with:
1. cilia - microscopic
hairs along the air passages
2. phlegm (mucus or sputum) - a moving carpet of
phlegm collects dirt and germs inhaled into the lungs and moves
them out to be coughed up or swallowed
3. macrophages - scavenger cells in the lungs that
literally eat up dirt and germs invading the lungs
All Information above provided by: http://edcenter.med.cornell.edu/CUMC_PathNotes/Respiratory/Respiratory.html
(Volume Of The Human Lung)