Pulmonary edema, or fluid accumulation in the lung extravascular spaces, results in impaired gas exchange and hypoxemia. When the lung fails to maintain fluid balance, liquid initially accumulates in the interstitium and ultimately invades the alveolar airspaces. The restrictive barrier properties of the pulmonary microvascular endothelium are established by tight junctions and adherens junctions between neighboring endothelial cells. Breakdown of the endothelial barrier leads rapidly to lung edema, the hallmark of acute lung injury and ARDS (acute respiratory distress syndrome). This chapter will focus on the mechanisms that regulate the barrier properties of the lung, highlighting the role of the transcellular and paracellular permeability pathways in edema formation.
term (Pc - P) gives the transmural hydrostatic pressure difference and the product o(nc - p) the effective transmural oncotic pressure drop; the difference between these parenthetical terms defines the "driving pressure" for net fluid filtration or reabsorption through pulmonary microvessel walls. Note that higher (Pc - P) favors fluid filtration (increased Jv), whereas higher o(nc - p) favors fluid reabsorption. Normally, an overall equilibrium is achieved between fluid filtration and fluid reabsorption at proximal and distal segments of capillaries, and little or no net filtration occurs through capillary walls. The lymphatic system in the lung maintains a negative interstitial pressure by continuously withdrawing fluid from interstitium. Edema develops when fluid filtration substantially exceeds its reabsorption and the capacity of the lymphatic system to remove fluid from the pulmonary interstitium.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.