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FUNDAMENTALS

  1. The primary function of the respiratory (pulmonary) system is to maintain systemic arterial blood gas levels within normal range.

    1. The rates of O2 uptake and CO2 excretion at the lungs must match the rates of O2 use and CO2 production by cellular respiration.

  2. The main components of the respiratory system are the lungs, chest wall, and pulmonary blood vessels.

    1. Muscles of the chest wall power the movement of air into the lungs during inspiration.

    2. The distribution of pulmonary blood flow and ventilation must match for proper gas exchange.

  3. The levels of systemic O2 and CO2 are monitored by chemoreceptors, allowing the pulmonary system to respond to changes in cellular respiration.

  4. The blood-gas interface.

    1. Repeated dichotomous branching of the airways begins at the trachea and terminates in over 300 million closed air sacs called alveoli, with a collective surface area of 50–100 m2.

    2. Ventilation is the process whereby air enters the lungs and comes into contact with alveoli, which are the sites of gas exchange.

    3. The blood-gas interface is less than 1 μm thick and consists of the following four elements in series (Figure 5-1):

      1. A thin layer of surface liquid.

      2. Alveolar lining cells (type 1 pneumocytes).

      3. A thin layer of interstitial fluid.

      4. Pulmonary capillary endothelial cells.

  5. Gas laws (Table 5-1).

    1. According to Boyle's law, the volume of a gas varies inversely with its pressure at a constant temperature. For example, a gas can be compressed to a smaller volume at higher pressure.

      1. Boyle's law explains air flow into the lung on inspiration—lung volume is first increased when contraction of the inspiratory muscles expands the chest, which reduces the alveolar pressure below atmospheric pressure and draws air into the lung.

    2. Dalton's law states that each gas in a mixture of gases exerts a partial pressure that is proportional to its concentration (Figure 5-2).

      1. The term “gas tension” is used interchangeably with partial pressure.

      2. The sum of partial pressures equals the total pressure.

    3. Henry's law states that the volume of gas dissolved in a liquid is proportional to its partial pressure.

      1. In clinical medicine, partial pressures are reported for gases in blood as well as in air.

      2. Partial pressure in a liquid refers to gas molecules that are free in solution rather than bound to hemoglobin.

      3. At the alveolus, respiratory gases diffuse to equilibrium so that the partial pressures of O2 (PO2) and CO2 (PCO2) are the same in alveolar air and in end-pulmonary capillary blood plasma.

  6. Units and terminology used in respiratory physiology (Table 5-2).

    1. Gas pressures are traditionally reported based on the height of a column of liquid; for example, mm Hg (also called Torr) or cm H2O (1 mm Hg = 1.36 cm H2O).

      1. The Pascal is the international unit of pressure: 1 kPa (kilopascal) = 7.5 ...

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