Respiratory Pulmonary Ventilation

Transcription

1 Respiratory Pulmonary Ventilation Pulmonary Ventilation Pulmonary ventilation is the act of breathing and the first step in the respiratory process. Pulmonary ventilation brings in air with a new supply of oxygen and a very small amount of carbon dioxide from the atmosphere into the alveoli. This mixture then participates in external respiration, the exchange of oxygen and carbon dioxide between the alveoli and pulmonary capillary blood across the respiratory membrane. Internal respiration is the exchange of gasses between the tissues of the body and the blood, which provides oxygen for aerobic cellular respiration and removes carbon dioxide. Aerobic Cellular respiration refers to the intracellular use of oxygen and the generation of carbon dioxide waste through metabolic pathways. Pressure Breathing or ventilation, involves changes in pressure as a result of mechanical work. The physical movement of air into the lungs is a result of the production of differences in total pressure between the interior (alveolar pressure) and exterior (atmospheric pressure) of the respiratory zone. Atmospheric pressure at sea level is typically 1 atmosphere or 760 mmhg, and this value will be used as the reference atmospheric pressure here. One atmosphere is the air pressure that would push a column of the mercury up in a thin tube a distance of 760 mm. Mercury is used because it is a liquid at room temperature, and its density changes very little over pressure and temperature ranges. In the process of ventilation, air moves down pressure gradients going from an area of higher pressure to an area of lower pressure. Assuming a person is at sea level, if his or her intrapulmonary pressure, the pressure in the alveoli, is less than 1 atm, air enters the lungs and fills the alveoli. If his or her intrapulmonary pressure is greater than 1 atm, then air moves out of the lungs into the environment.

2 There is another pressure often discussed in the mechanics of ventilation, intrapleural (sometimes just pleural) pressure. This is the pressure within the pleural sac (space). Intrapleural pressure is maintained slightly less than the pressure in the alveoli. This pressure difference aids in keeping the lungs slightly inflated at all times and ensures that they do not collapse when exhaling. Inspiration and Expiration Gases have a property of distributing to fill whatever size and shape container they occupy. If a closed container is made larger (an increase in volume), the total number of gas molecules will stay the same, but they will redistribute to fill the larger space. In doing so, they decrease their concentration (remember that concentration is a property of mass and volume). This increase in volume but not number of molecules of

3 the gas leads to a corresponding decrease in the pressure exerted by that gas on the walls of the container. If the same closed container is made smaller, the concentration of gases increases( even though the actual number of gas molecules has not changed) and the pressure increases. This is an example of a physical law calledboyle s Law. The law is expressed mathematically as: P 1 V 1 = P 2 V 2 where P = pressure, V = volume, 1 is the initial pressure and volume, and 2 is the resulting pressure and volume. Boyle s Law states that, at a constant temperature, the pressure of a gas is inversely proportional with the volume of its container. Very simply, as volume goes up, pressure goes down, and as volume goes down, pressure goes up. These properties become important for ventilation because the passage of air into and out of the lungs is controlled by the size of the lungs container, the thorax. The difference between the example above and the lungs is that the lungs are not a closed system (unless you have a closed glottis), with the opening to the lungs maintained through the conducting zone. But over very short periods of time Boyle s law still applies. In this case, when we inhale, the movement of the diaphragm and the ribs increases the volume of the thorax. This immediately decreases the air pressure within the thorax, creating a pressure gradient between the atmosphere and the alveoli (the latter is called intrapulmonary pressure), and drawing air into the lungs. Air will enter until the atmospheric and intrapulmonary pressures are equal. However, the volume of the thorax remains larger than before the start of inhalation. To exhale, the thorax is allowed to decrease in volume (the ribs and diaphragm return to their original positions), increasing the intrapulmonary pressure and creating a gradient in the opposite direction resulting in the flow of air out to the atmosphere. Changing the size of the thoracic cavity would be impossible if the ribs were solidly attached to the sternum. If they were solidly attached, it would be like trying to move the sides of a birdcage.

4 The costal cartilage allows the ribs to move and expand or contract the chest wall. Inspiration Inspiration is the act of inhaling. As stated above, inspiration occurs by increasing the volume of the thorax. This active process involves the use of chest and neck muscles. Mostly movement of the diaphragm achieves resting inspiration. The relaxed shape of the diaphragm resembles a shallow dome with the apex pointing toward the lungs, similar to the shape of an open umbrella. When the diaphragm contracts, it tends to flatten out, expanding the volume of the thorax in an inferior direction. Consequently, the intrapleural and intrapulmonary pressures decrease below atmospheric pressure resulting in air being pulled through the conducting zone and into the lungs. The external intercostal muscles work in conjunction with the diaphragm. The normal orientation of the ribs is around the side of the thorax and angled inferiorly to the sternum. When the external intercostal muscles contract, the ribs are pulled up, also expanding the thoracic cavity in a horizontal direction. In adults, ventilation occurs about 12 times a minute and moves roughly 500 ml of air during each breath. A deep breath involves a greater shortening of the diaphragm and the external intercostal muscles plus additional contractions of other neck and chest muscles. The scalene muscles elevate the first two ribs. The sternocleidomastoid muscles elevate the sternum, and the pectoralis minor muscles help to elevate the third, fourth, and fifth ribs. Two physical conditions that interfere with inspiration are obesity and advanced pregnancy. In both cases, the abdominal organs are pushed against the diaphragm, restricting its downward movement and hindering the expansion of the thoracic cavity. Expiration Expiration is the act of exhaling. Resting expiration is a passive process. The muscles used during inspiration relax, and allow the chest wall and the diaphragm to move back to their original position, thus decreasing the volume of the thorax and forcing air from the lungs. The

5 compression of the chest wall also aids in moving blood and lymph through the vessels that drain the lungs. Expiration becomes an active process when a more forceful exhale is required. The internal intercostal muscles pull the ribs down, helping to compress the chest. The external and internal oblique and transverse abdominal muscles press on the abdominal organs, which move them up against the diaphragm and force the diaphragm higher than it would normally go on relaxation, further decreasing the volume of the thoracic cavity. Airway resistance, alveolar surface tension, and lung compliance One thing that works against the pressure gradient created by the expansion of the thorax is the resistance found within the conducting zone. Under normal conditions this resistance is quite low, and the airways have little trouble passing air between the atmosphere and the alveoli. The small amount of resistance found in the system stems mainly from the bronchioles, which are analogous to the arterioles of the vascular system. Both the bronchioles and the arterioles contribute a large portion of the resistance of their entire system, and both have smooth muscles in their walls that allow them to constrict or dilate. Under conditions of bronchiolar constriction (bronchoconstriction), resistance to airflow can increase dramatically. When this happens rapidly, it is classified as an acute asthma attack. The afflicted individual will need to generate much larger changes in intrapulmonary pressure to maintain a normal flow rate of air during breathing. Recall that if resistance increases an increased pressure gradient is necessary to maintain the same flow. To achieve this, the person uses the accessory muscles to breath, and will appear to be straining as he or she does so. Treatment usually involves an inhaler containing a bronchodilator. Resistance will also be affected if the airways become narrowed by mucus (chronic asthma) or aspirated substances. Along with the resistance of the airways, the compliance of the lung tissue also determines how much effort breathing requires. Compliance is a property that explains the relationship between a volume change and pressure change. Typically it takes a 2-3 cm of water change in pleural pressure to change volume in the lungs by 500 ml. This is a

6 compliance of about 200 ml/cm H2O. The compliance of the lungs is dependent on the elasticity of the connective tissues of the lung as well as the alveolar surface tension. Remember that a thin liquid layer containing water as well as other molecules lines the interior lung surface. Because of its polar nature, water exhibits cohesion, such that it takes some effort to separate water molecules (you may have experienced the force of this cohesion if you ever belly flopped into the water). This surface tension is actually enough to collapse the alveoli each time a person exhales. In order to prevent this from happening, the type II alveolar cells make a combination of lipids and proteins that serves as a surfactant in the alveoli. A surfactant is a chemical that acts like a detergent breaking the surface tension of the water lining the alveoli. This action allows the alveoli to remain open when exhaling. Compliance of the lung can be decreased either by fibrosis of the lung tissue creating a stiffening of the tissue (this can occur with conditions such as tuberculosis) or lack of surfactant (common in premature infants). Either way breathing becomes much more difficult, requiring more effort and sometimes mechanical ventilation.