is the fraction of hemoglobin saturated with oxygen, and PaO
>2 is the partial pressure of the oxygen dissolved in plasma and is trivial compare to the amount of oxygen carried by hemoglobin. Examination of this equation reveals that increasing hemoglobin concentration and increasing cardiac output can enhance oxygen delivery. Saturation is normally greater than 92 % and usually is easily maintained through supplemental oxygen and mechanical ventilation. Cardiac output is supported be insuring adequate fluid resuscitation (cardiac preload) and manipulating contractility and after load pharmacologically (usually cat—echolamines).
New words
Equation – уравнение
Delivery – доставка
Cardiac output – сердечный выброс
Fraction – фракция
Contractility – сократимость
33. Surface tension forces
In a liquid, the proximity of adjacent molecules results large, intermolecular, attractive (Van der Waals) forces that serve to stabilize the liquid. The liquid—air surface produces inequality of forces that are strong on the liquid side and weak on the gas side because of the greater distance between molecules in the gas phase. Surface tension causes the surface to maintain as small an area as possible. In alveoli, the result a spherically—curved, liquid lining layer that tends to be pulled inward toward the center of curvature of the alveolus. The spherical surface of the alveolar liquid lining behaves in manner similar to a soap bubble. The inner and outer surface of a bubble exert an inward force that creates a greater pressure inside than outside the bubble. Interconnected alveoli of different sizes could lead to collapse of smaller alveoli (atelectasis) into larger alveoli, because of surface tension, the pressure inside the small alveolus (smaller radius of curvature) is greater than that of the larger alveolus. Without surfactant, gas would therefore move from smaller to larger alveoli, eventually producing or giant alveolus.
Pulmonary surfactant: Pulmonary surfactant is aphospho—lipid (comprised primari ly of dipalmitoyl phosphatidylcholi—ne) synthesized by type II alveolar epithelial cells. Surfactant reduces surface tension, thereby preventing the collapse of small alveoli. Surfactant increases the compliance of the lung and reduces the work of breathing.
Surfactant keeps the alveoli dry because alveolar collapse tends to draw fluid into the alveolar space. Surfactant can be produced in the fetus as early as gestational week 24, but is synthesized most abundantly by the 35 th week of gestation. Neonatal respiratory distress syn drome can occur with premature infants, and results in areas of atelectasis, filling of alveoli with transudate, reduced lung compliance, and V/Q mismatch leading to hypoxia and CO