Nursing assignment Essay Example | Topics and Well Written Essays - 3000 words

Nursing assignment - Essay Example Diffusivity of a gas is a physical constant that is derived from calculations based on the solubility of the gas in the membrane and the molecular weight of the gas. The area available for diffusion and the thickness of the membrane is anatomy dependent. Thus the exchange of oxygen from the alveoli to the capillaries is mostly based on the pressure gradient across the alveolar-capillary membrane. The same law applies to the transport of carbon dioxide from the capillaries to the, but carbon dioxide is more soluble than oxygen in the membrane and so diffusion limitation is seldom an issue with carbon dioxide, the elimination of which is dependent heavily on ventilation. (Walley & Russell, 1999). Besides the difference between the partial pressures of the gases in alveoli and the pulmonary capillary bed another important determinant of gas exchange is the relationship between the pulmonary capillary perfusion (Q) and alveolar ventilation (V). A well ventilated alveolus needs to have an equally well perfused capillary, with the ratio V/Q ideally 1. A three compartment model of the lungs helps to understand the perfusion and ventilation relationship. Physiological dead spaces, where exchange of gases does not take place are areas of wasted ventilation, where V/Q is greater than 1. Perfectly matched areas of ventilation and perfusion have V/Q equal to 1. Areas that contribute to venous admixture, which is the mixing of non-oxygenated with oxygenated blood after passing through the lungs, where perfusion has been wasted, in the example of the right to left shunt, V/Q is less than 1. Even in normal lung function, there is a degree of variation in the perfusion and ventilation in different areas of the lung due to gravity, which requires the extra effort in forcing blood supply through the blood vessels above the heart in an erect position. Thus the lower portion of the lobes of the lung is better perfused