Pressure and Flow

Airflow is governed by the same principles of flow, pressure, and resistance as blood flow (see chapter 20). The pressure that drives respiration is atmospheric (barometric) pressure—the weight of the air above us. At sea level, a column of air as thick as the atmosphere (60 mi) and 1 in. square weighs 14.7 lb; it is thus said to exert a force of 14.7 pounds per square inch (psi). In standard international (SI) units, this is a column of air 100 km high exerting a force of 1.013 X 106 dynes/cm2. This pressure, called 1 atmosphere (1 atm), is enough to force a column of mer-

Chapter 22 The Respiratory System 851

Chapter 22 The Respiratory System 851

Spongy Texture The Lung
Figure 22.11 Tissue of the Lung. (a) Light micrograph; (fa) SEM micrograph. Note the spongy texture of the lung.

cury 760 mm up an evacuated tube; therefore, 1 atm = 760 mmHg. This is the average atmospheric pressure at sea level; it fluctuates from day to day and is lower at higher altitudes.

One way to change the pressure of a gas, and thus to make it flow, is to change the volume of its container. Boyle's law states that the pressure of a given quantity of gas is inversely proportional to its volume (assuming a constant temperature). If the lungs contain a quantity of gas and lung volume increases, their intrapulmonary pressure—the pressure within the alveoli—falls. If lung volume decreases, intrapulmonary pressure rises. (Compare this to the syringe analogy on p. 734.) To make air flow into the lungs, it is necessary only to lower the intra-pulmonary pressure below the atmospheric pressure. Raising the intrapulmonary pressure above the atmospheric pressure makes air flow out again. These changes are created as skeletal muscles of the thoracic and abdominal walls change the volume of the thoracic cavity.

Saladin: Anatomy & I 22. The Respiratory System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

852 Part Four Regulation and Maintenance

852 Part Four Regulation and Maintenance

Structure Alveolus
Figure 22.12 Pulmonary Alveoli. (a) Clusters of alveoli and their blood supply. (b) Structure of an alveolus.

What matters to flow is the difference between atmospheric pressure and intrapulmonary pressure. Since atmospheric pressures vary from one place and time to another, it is more useful for our discussion to refer to relative pressures. A relative pressure of —3 mmHg, for example, means 3 mmHg below atmospheric pressure; a relative pressure of +3 mmHg is 3 mmHg above atmospheric pressure. At an atmospheric pressure of 760 mmHg, these would represent absolute pressures of 757 and 763 mmHg, respectively.

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  • jared kelly
    What is the unity of atmospheric pressure?
    7 months ago

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