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GE 345: Week 3

Ventilation

Updated by Tracey 22 July 02

Air flow is controlled by changes in intrathoracic pressure. These changes are controlled through muscular contraction and elastic recoil.

Diaphragm moves up or down, shortening or lengthening the cavity. Diaphragm contracts during inspiration, increasing thoracic volume, and creating negative pressure, which in turn causes inflow of air into the lungs. Diaphragm relaxes during normal exhalation, and elastic recoil of surrounding structures creates higher pressure, prompting exhalation.
Intercostal muscles between the ribs raise the rib cage, moving the sternum forward, allowing inspiration.

Abdominal muscles contract during rapid expiration to provide extra force needed to move air quickly.

During normal respiration, almost all respiratory muscle contraction occurs during inspiration, whereas expiration is almost entirely passive. 3-5 percent of work energy expended by the body energizes pulmonary ventilation. During heavy exercise, energy requirements can increases 50-fold. Respiratory muscles "work" to:

expand the lung against its elastic forces, compliance/elastic work.
overcome viscosity of lung and chest wall structures, tissue resistance work.
overcome airway resistance during air movement into lungs, airway resistance work.

Lungs & Pressure Differentials

Lungs: elastic structures which float in the thoracic cavity surrounded by a thin layer of pleural fluid, which lubricates lung movement. Excess fluid is constantly drawn into lympatic channels, which create a slight suction between the lungs and chest cavity. Air and liquid tends to flow from areas of higher pressure to areas of lower pressure.

Pleural pressure: fluid pressure in space between lung and chest wall pleura. Tends to be slightly negative, which keeps the lungs open to resting levels. Rib cage expansion during inspiration increases the negative pressure, pulling the lung surfaces out

Alveolar Pressure: pressure inside the alveoli. With glottis open and no air flowing, pressure in the respiratory tree = atmospheric pressure. Aloveolar pressure falls slightly below atmospheric to cause inward airflow. During expiration, alveolar pressure rises slightly. About .5 liter of air is moved in and out of the airways with each breath.

Transpulmonary Pressure: Pleural pressure minus alveolar pressure.

Lung Compliance: extent to which the lungs expand for each unit increase in tanspulmonary pressure. Average for both lungs together is 200 ml/cm water pressure.

Surface Tension

Water molecules at the surface between the water and air have an extra strong attraction to each-other, causing the water surface to contract, and holding the mass together. Surface tension also works on the alveoli of the lungs, causing an elastic contractile force of the lungs, called surface tension elastic force.

Surfactant: a surface active agent, which spreads over a fluid's surface, reducing surface tension. In the lungs, it reduces the amount of transpulmonary pressure required to keep the lungs expanded. It is secreted by special cells and is a mix of phospholipids, proteins and ions. The phospholipid dipalmitoyl phosphatidylcholine has a hydrophilic portion which dissolves in the water lining the alveoli. The hydrophobic lipid portion is oriented toward the air, exposing a lipid hydrophobic surface to the air. This surface has about one sixth the surface tension of a pure water surface.

Thoracic Cage and Lung Expansibility

It takes almost double the pressure to inflate the complete pulmonary system (lungs and thoracic cage) as it does to inflate the lungs alone.

Alveolar Ventilation

The pulmonary ventilation system's primary function is to continually replenish the air in the gas exchange areas of the lungs, including the respiratory bronchioles, alveoli, and alveolar sacs and ducts. During normal respiration, tidal air fills the passageways down as far as the terminal bronchioles, with only a small portion of inspired air flowing to the alveoli. New air reaches alveoli via diffusion.

Dead Space Air is the gas that fills the respiratory passages but doesn't reach the gas exchange areas. Normal dead space volume is about 150 ml.

Alveolar ventilation rate is the volume of air entering the gas exchange areas per minute.

Alveolar Ventilation = Resp. Frequency x (Tidal Vol - Dead Space Vol)