Читать книгу Manual of Equine Anesthesia and Analgesia - Группа авторов - Страница 195

 Intrapleural pressure is approximately −5 cm H2O at rest, resulting in a transpulmonary pressure of 5 cm H2O.

 Inspiration is characterized by expansion of the chest, (due to contraction of the diaphragm and the external intercostal muscles) which results in a decrease in intrapleural pressure and an increase in the transpulmonary pressure gradient. As a result, gas moves from the atmosphere into the respiratory passages.

  Gas flow ceases at the end of inspiration, as there is no longer a gradient between the atmospheric and alveolar pressure.

 Expiration is characterized by relaxation of the inspiratory muscles, elastic recoil of the lung, and contraction of the internal intercostal and abdominal muscles. These processes result in an increase in the transpulmonary pressure, and movement of gas from the lung to the ambient atmosphere.

 In horses, both inspiration and expiration have a biphasic pattern. The first phase of inspiration is passive, from a forced and active contraction of abdominal muscles at the end of expiration that forces the respiratory system below its equilibrium. The second part of inspiration is active through contraction of the diaphragm and other inspiratory muscles. The first part of expiration is passive through recoil of the respiratory system down to its equilibrium position, but then is active so that the next inspiration begins with relaxation of the abdomen.

 Contraction of muscles in the nares, pharynx and larynx is necessary to prevent collapse of structures into the air passages in the presence of the negative pressures generated in the respiratory tract during inspiration,For example, muscle relaxation secondary to the effects of sedatives/anesthetics or as a consequence of nerve dysfunction can impair airflow.