Physiologists are studying the mechanisms of pleural fluid formation and absorption in the normal state. They obtain healthy experimental animals whose pleuropulmonary anatomy and physiology closely resembles that of humans. Intravenous radioisotope tracers are injected into these animals and are monitored closely to track fluid movement into and out of the pleural space. Which of the following pathways of pleural fluid turnover is most likely to be observed?

Question

Quizplus · Accepted Answer

11ec160b_3ce2_7c86_9249_fb72b2fb85d7_MD0001_00 Humans normally have approximately 5-10 mL of pleural fluid in the intrapleural space of each hemithorax, with inflow and outflow of fluid taking place at equal rates to maintain homeostasis. Pleural fluid is believed to enter the pleural space via filtration from systemic circulation in both the parietal and visceral pleura, with the majority of fluid filtered from the intercostal microvessels in the parietal pleura and a lesser amount from the bronchial microvessels in the visceral pleura. Most pleural fluid likely exits the pleural space via holes in the parietal pleura known as lymphatic stoma that drain into the parietal pleural lymphatics. Pleural effusion results from an increase in the rate of pleural fluid inflow (eg, increased fluid filtration as occurs with decompensated heart failure, hypoalbuminemia, or an inflammatory increase in pleural vascular permeability) or a decrease in the rate of pleural fluid outflow (eg, malignant obstruction of the parietal pleura lymphatic stoma). (Choices A, B, and D) Neither the pulmonary capillaries nor the pulmonary parenchymal lymphatics significantly contribute to the uptake of pleural fluid from the pleural space in the normal or pathologic state. Fluid from the pulmonary (alveolar) capillaries (ie, the pulmonary circulation) does not significantly contribute to inflow of pleural fluid into the pleural space in the normal state; however, it can become a primary source of fluid inflow in pathologic states (eg, increased pulmonary capillary hydrostatic pressure with decompensated heart failure). Educational objective: Pleural fluid normally enters the pleural space via filtration from the systemic circulation, primarily from the intercostal microvessels of the parietal pleura, and exits the pleural space via stomata in the parietal pleura that drain into the parietal pleura lymphatics. __________ References: Pleural mechanics and fluid exchange. (http://www.ncbi.nlm.nih.gov/pubmed/15044678)

Physiologists Are Studying the Mechanisms of Pleural Fluid Formation and Absorption