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OBJECTIVES

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After studying this chapter, you should be able to:

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  • Describe the manner in which O2 flows “downhill” from the lungs to the tissues and CO2 flows “downhill” from the tissues to the lungs.

  • List the important factors affecting the affinity of hemoglobin for O2 and the physiologic significance of each.

  • List the reactions that increase the amount of CO2 in the blood, and draw the CO2 dissociation curve for arterial and venous blood.

  • Define alkalosis and acidosis and list typical causes and compensatory responses to each.

  • Define hypoxia and describe differences in subtypes of hypoxia.

  • Describe the effects of hypercapnia and hypocapnia, and give examples of conditions that can cause them.

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INTRODUCTION

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The concentrations for O2 and CO2 (measured as partial pressures, or Po2 and Pco2) change within each region of the lung, allowing for these gases to flow “downhill” or from higher partial pressures to lower partial pressures. For example, Po2 is highest in the alveoli upon inspiration and lowest in the deoxygenated blood, whereas, Pco2 is exactly opposite. This allows for O2 to cross the alveoli and re-oxygenate the blood in the pulmonary vasculature, while CO2 leaves the bloodstream and enters the alveoli where it is expired. However, the amount of both of these gases transported to and from the tissues would be grossly inadequate if it were not for the fact that about 99% of the O2 that dissolves in the blood combines with the O2-carrying protein hemoglobin and that about 94.5% of the CO2 that dissolves enters into a series of reversible chemical reactions that convert it into other compounds. Thus, the presence of hemoglobin increases the O2-carrying capacity of the blood 70-fold, and the reactions of CO2 increase the blood CO2 content 17-fold. In this chapter, physiologic details that underlie O2 and CO2 movement under various conditions are discussed.

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OXYGEN TRANSPORT

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OXYGEN DELIVERY TO THE TISSUES

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Oxygen delivery, or by definition, the volume of oxygen delivered to the systemic vascular bed per minute, is the product of the cardiac output and the arterial oxygen concentration. The ability to deliver O2 in the body depends on both the respiratory and the cardiovascular systems. O2 delivery to a particular tissue depends on the amount of O2 entering the lungs, the adequacy of pulmonary gas exchange, the blood flow to the tissue, and the capacity of the blood to carry O2. Blood flow to an individual tissue depends on cardiac output and the degree of constriction of the vascular bed in the tissue. The amount of O2 in the blood is determined by the amount of dissolved O2, the amount of hemoglobin in the blood, and the ...

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