Answer
Pulmonary ventilation is the movement of air between the lungs and the environment . Air moves into the
lung when pressure inside the lungs (Pip) is less than pressure outside the lungs (Pl). The amount of gas that flows into the lungs during ventilation is controlled by several factors, Among these are airway compliance (C), airway resistance (R), and alveolar surfactant -- affects compliance,
Compliance and Pulmonary Ventilation;
Lung compliance is the ease with which the lungs expand on inspiration. Total lung compliance is related to the elasticity of lung tissue, and the distensibility of the bony thorax--surfactant also affects the ease with which the lungs expand. If lung compliance is high, pulmonary ventilation is high; if lung compliance is low pulmonary ventilation is difficult. The higher the lung compliance the easier it is for the lungs to expand at a given transpulmonary pressure (Pl-Pip). Lung compliance is usually high but it my be negatively affected by lung damage due to infection(TB), pulmonary edema, bronchitis, rib injury, paralysis of intercostal muscles and/or deficiency in alveolar surfactant.
Work Step by Step
Airway resistance (R) is also an important factor affecting pulmonary ventilation. Normally, as airs flows through the conducting area of the lungs, it encounters resistance engendered by the friction of air against the walls of the airways --trachea, bronchi, bronchioles, medium bronchioles and terminal bronchioles. Flow of air through any kind of tubes (pipes) produces friction which results in resistance. Turbulent flow produces the greatest degree of friction and hence the highest resistance. The smaller the airway in diameter, the greater the resistance, theoretically. However, the large number of very small terminal airways produce less resistance than the total resistance of medium-sized bronchioles. The greatest amount of resistance in the lungs is encountered in the medium sized bronchioles.
Air flow is inversely related to resistance --but not directly. The halving of the diameter of a tube increases its resistance to flow by the power of four (4) Factors that affect bronchiolar diameter( constriction/dilation) are neurotransmitters --acetylcholine(ACH), epinephrine, and histamine. Histamine , released in allergic reactions, constricts bronchioles and increases resistance; epinephrine from the adrenal medulla, relaxes bronchioles and decreases resistance. People with asthma, COPD, and chronic bronchitis have narrowed airways, abnormally high airway resistance, and impaired pulmonary ventilation.
Alveolar surfactant affects both lung compliance and surface area of alveoli. The alveoli are covered with an aqueous fluid. The water in the fluid tends to reduce the the a;volar air bubble to the smallest size. This is the natural tendency of the molecules of water because the surface tension of water is high. This means that there is a natural physical force that tends to pull the water molecules together. This attraction between water molecules is stronger than the attraction between water molecules and air molecules. If this force/tendency of water were not counteracted (in normal respiration) the alveoli would collapse after each expiration, and would have to be opened up at each inspiration-- at great energy cost. Alveolar surfactant lowers the surface tension of the fluid covering the surfaces of alveoli and this inhibits the contraction of the alveoli. This saves energy on inspiration, facilitates lung expansion, and improves pulmonary ventilation.
Surfactant is secreted by Type II alveolar cells. In some premature infants not enough surfactant is produced and they develop a condition known as respiratory distress syndrome)RDS). in which breathing is difficult. Some of the signs and symptoms of this syndrome; are dyspnea, atelectasis and sepsis . This is a life threatening condition. Some treatment measures include surfactant therapy , oxygen therapy, breathing assistance, and corticosteroid therapy.
