Air that cuts off circulation to a segment of the lung dead space

Lung tissue may be considered as a semi-fluid or gel-like substance confined within the chest cavity, and the weight of the tissue above compresses the tissue below such that the density of the lung increases as vertical height reduces. In addition to causing postural differences between ventilation of the left and right lungs, gravity also influences the distribution of ventilation within each lung. Table 8.1 Distribution of resting lung volume (FRC) and ventilation between the two lungs in humans Finally the concepts of dead space and shunt are presented.

Distribution of ventilation and perfusion are then considered in relation to one another. This chapter begins with consideration of the spatial and temporal distribution of ventilation, followed by similar treatment for the pulmonary circulation. Conversely, to take an extreme example, if ventilation were distributed entirely to one lung and perfusion to the other, there could be no gas exchange, although total ventilation and perfusion might each be normal. Gas exchange will clearly be optimal if ventilation and perfusion are distributed in the same proportion to one another throughout the lung. Thus both expired gas and arterial blood have tensions that differ from those in alveolar gas and end-pulmonary capillary blood. However, alveolar gas is mixed with dead space gas to give expired gas, and end-pulmonary capillary blood is mixed with shunted venous blood to give arterial blood. Under most circumstances equilibrium is obtained between alveolar gas and end-pulmonary capillary blood, the gas tensions in the two phases being almost identical. 8.1 In this functional representation of gas exchange in the lungs, the flow of gas and blood is considered as a continuous process with movement from left to right.

Air that cuts off circulation to a segment of the lung dead space