Eq. 23.14 ). There is an inverse relationship between the PACO2 and VA, regardless of the exhaled quantity of CO2. In normal lungs, PaCO2 is tightly regulated to remain constant at around 40 mm Hg.
5. Because of the effects of gravity, there are regional differences in ventilation and perfusion. The ventilation/perfusion (V̇ /Q̇ ) ratio is defined as the ratio of ventilation to blood flow. In a normal lung, the overall ventilation/perfusion ratio is approximately 0.8. When ventilation exceeds perfusion, the ventilation/perfusion ratio is greater than 1 (V̇ /Q̇ > 1), and when perfusion exceeds ventilation, the ventilation/perfusion ratio is less than 1 (V̇ /Q̇ < 1). The V̇ /Q̇ ratio at the apex of the lung is high (ventilation is increased in relation to very little blood flow), whereas the V̇ /Q̇ ratio at the base of the lung is low. In individuals with normal lungs who are breathing room air, the AaDO2 is less than 15 mm Hg; the upper limit of normal is 25 mm Hg.
CHAPTER 23 Ventilation, Perfusion, and Ventilation/Perfusion Relationships 6. The pulmonary circulation is a low-pressure, low-resistance system. Recruitment of new capillaries and dilation of arterioles without an increase in pressure are unique features of the lung and allow for adjustments during stress, as in the case of exercise. Pulmonary vascular resistance is the change in pressure from the pulmonary artery (PPA) to the left atrium (PLA), divided by cardiac output (QT). This resistance is about 10 times less than in the systemic circulation.
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7.
There are four categories of hypoxia (hypoxic hypoxia, anemic hypoxia, diffusion hypoxia, and histotoxic hypoxia) and six mechanisms of hypoxic hypoxia and hypoxemia: anatomical shunt, physiological shunt, decreased FiO2, V̇ /Q̇ mismatching, diffusion abnormalities, and hypoventilation.
8.
There are two mechanisms of the development of hypercapnia: increase in dead space ventilation and hypoventilation.
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Upon completion of this chapter, the student should be able to answer the following questions: 1.
Describe the basic gas diffusion principles and how they affect O2 and CO2 absorption and expiration.
2.
Compare and contrast the chemical transport mechanisms of O2 and CO2 in blood.
3.
Describe the basic principles and clinical significances of the O2 and CO2 dissociation curves.
4.
Describe the chemical synthesis of H+ ions and their role in the regulation of acid-base balance.
5.
Explain why the diffusion of O2 and that of CO2 are considered perfusion limited and that of CO is considered diffusion limited.
6.
Compare and contrast factors that shift the oxyhemoglobin dissociation curve.
7.
Explain the clinical significance of the differences in the oxyhemoglobin and carboxyhemoglobin dissociation curves.
8.