Eq. 23.14 shows that there is an inverse relationship between PACO2 and VA, regardless of the amount of exhaled CO2. The lungs tightly regulate PaCO2 to remain constant at around 40 mm Hg in normal conditions.

Regional differences in ventilation and perfusion occur due to the effects of gravity. The ventilation/perfusion (V̇ /Q̇ ) ratio represents the ratio of ventilation to blood flow. In a normal lung, the overall V̇ /Q̇ ratio is approximately 0.8. When ventilation exceeds perfusion, the ratio is greater than 1 (V̇ /Q̇ > 1), and when perfusion exceeds ventilation, the ratio is less than 1 (V̇ /Q̇ < 1). At the apex of the lung, the V̇ /Q̇ ratio is high, indicating increased ventilation compared to minimal blood flow. Conversely, the V̇ /Q̇ ratio at the base of the lung is low. In individuals with normal lungs breathing room air, the AaDO2 is less than 15 mm Hg, with an upper limit of 25 mm Hg.

The pulmonary circulation is a unique low-pressure, low-resistance system. The lung can recruit new capillaries and dilate arterioles without increasing pressure, enabling adjustments during stress, such as exercise. Pulmonary vascular resistance, which is the change in pressure from the pulmonary artery (PPA) to the left atrium (PLA), divided by cardiac output (QT), is approximately 10 times less than in the systemic circulation.

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.

The development of hypercapnia can occur through two mechanisms: an increase in dead space ventilation and hypoventilation.

After studying this chapter, the student should be able to answer the following questions:
1. Explain the basic principles of gas diffusion and how they impact the absorption and expiration of O2 and CO2.
2. Compare and contrast the chemical transport mechanisms of O2 and CO2 in blood.
3. Describe the fundamental principles and clinical significance of the O2 and CO2 dissociation curves.
4. Discuss the chemical synthesis of H+ ions and their role in regulating acid-base balance.
5. Explain why the diffusion of O2 and CO2 is considered perfusion limited, while the diffusion of CO is considered diffusion limited.
6. Compare and contrast the factors that shift the oxyhemoglobin dissociation curve.
7. Discuss the clinical significance of the differences in the oxyhemoglobin and carboxyhemoglobin dissociation curves.
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.
Leff AR, Schumacker PT. Respiratory Physiology: Basics and Applications. Philadelphia: WB Saunders; 1993.
Lumb AB. Nunn’s Applied Respiratory Physiology. 8th ed. St. Louis: Elsevier; 2016.
Mead J, Macklem PT, vol eds. American Physiological Society Handbook of Physiology: The Respiratory System. Vol. 3. Mechanics. Bethesda, MD: American Physiological Society; 1986.
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.
Wasserman K, Beaver WL, Whipp BI. Gas exchange theory and the lactic acidosis (anaerobic) threshold. Circulation. 1990;81(1 suppl):1114-1130.
West JB. Ventilation/Blood Flow and Gas Exchange. 5th ed. New York: Blackwell Scientific; 1991.
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.