Lung Fluid Balance During Development

Abstract

After completing this article, readers should be able to: Curious students of mammalian development recognized more than a century ago that the lungs are filled with liquid during fetal life, (1) but the origin of that liquid remained obscure until 1948, when an unexpected discovery by two French scientists dispelled the prevailing view that fetal lung liquid derived from intrauterine inhalation of amniotic fluid. In studies designed to explore the ontogeny of the pituitary-adrenal axis, tracheal ligation of fetal rabbits for 9 days resulted in fluid distention of the lungs. (2) This serendipitous finding, subsequently confirmed and embellished by others, (3)(4)(5) established that the fetal lung itself, rather than the amniotic sac, is the source of the liquid that fills the lung during development. This liquid forms a slowly expanding structural template that prevents collapse and promotes growth of the fetal lung. Subsequent studies, conducted mostly with sheep, showed that liquid in the lumen of the fetal lung forms as a result of chloride secretion in the respiratory epithelium, (6)(7) a process that can be inhibited by diuretics that block Na,K-2Cl cotransport. (8)(9) In vitro experiments using cultured explants of lung tissue and monolayers of epithelial cells harvested from human fetal lung have indicated that cation-dependent chloride transport, driven by epithelial cell Na,K-ATPase, is the mechanism responsible for liquid secretion into the lumen of the mammalian lung during fetal life. (10)(11)(12) Rapid clearance of liquid from potential airspaces during and soon after birth is essential for establishing the timely switch from placental to pulmonary gas exchange. A sudden gush of fluid from the mouth often punctuates a baby’s birth, signaling the start of extrauterine life. This observation helped to promulgate what has come to be known as the “vaginal squeeze,” a notion that was introduced nearly half a century ago to explain how liquid might be expelled from the lungs as air breathing starts. (13) Since then, however, numerous studies have established that the normal transition from liquid to air inflation is considerably more complex than the characteristic oral gush at delivery might suggest. This review considers some of the experimental work that provides the basis for our current understanding of lung liquid dynamics before, during, and after birth, focusing on the various pathways and mechanisms by which this process occurs. For further enlightenment on this important aspect of perinatal adaptation, the reader is referred to a recent comprehensive review. (14)During fetal development, the lung is a secretory organ that displays breathing-like movements without contributing to respiratory gas exchange, which is exclusively a placental function in the mammalian fetus. In utero, the lungs are filled with liquid and receive less than 10% of the combined ventricular output of blood from the heart. (15) In fetal sheep, this modest blood supply is sufficient to deliver to the lung epithelium the substrate needed to make surfactant and secrete up to 0.5 L of liquid into the lung lumen each day during the last third of gestation. (16)(17) Studies performed with cultured explants of human fetal lung indicate that liquid production by the bronchopulmonary epithelium may occur as early as the sixth week of gestation, with resultant expansion of the lung lumen. (11) Several studies have shown that the presence of an appropriate volume of secreted liquid within the fetal respiratory tract is essential for normal lung growth and development before birth. (3)(4)(5) Conditions that interfere with normal production of lung liquid, such as pulmonary artery occlusion, (18) diaphragmatic hernia with displacement of abdominal contents into the chest, (19) and uterine compression of the fetal thorax from chronic leak of amniotic fluid, (20) also inhibit lung growth.Figure 1 is a schematic diagram showing the fluid compartments of the fetal lung. Potential air spaces are filled with liquid that is rich in chloride (Cl) (∼150 mEq/L [150 mmol/L]) and almost free of protein (<0.03 mg/mL). (21) Studies of water and solute exchange across the pulmonary epithelium and endothelium of fetal sheep showed that the lung epithelium has tight intercellular junctions that provide an effective barrier to macromolecules, including albumin, whereas the vascular endothelium has wider openings that allow passage of large plasma proteins, including globulins and fibrinogen. (22)(23)(24) Consequently, liquid in the interstitial space, which is sampled in fetal sheep by collecting lung lymph, has a protein concentration that is about 100 times greater than the protein concentration of liquid contained in the lung lumen. (25) Despite the large transepithelial difference in protein osmotic pressure, which tends to inhibit fluid flow out of the interstitium into potential air spaces, active transport of Cl ions across the fetal lung epithelium generates an electrical potential difference that averages about −5 mV, luminal side negative. (7) The osmotic force created by this secretory process drives liquid from the pulmonary microcirculation through the interstitium into potential air spaces.Lung epithelial Cl transport, which in fetal sheep begins as early as mid-gestation, (26) is inhibited by diuretics that block Na,K-2Cl cotransport. (8)(9) This finding supports the concept that the driving force for transepithelial Cl movement in the fetal lung is similar to the mechanism described for Cl transport across other epithelia. Accordingly, Cl enters the epithelial cell across its basal membrane linked to sodium (Na) and to potassium (K) (Fig. 2). Na enters the cell down its electrochemical gradient and subsequently is extruded in exchange for K (3 Na ions exchanged for 2 K ions) by the action of Na,K-ATPase located on the basolateral surface of the cell. This energy-dependent process increases the concentration of Cl within the cell so that it exceeds its electrochemical equilibrium. Cl then passively exits the epithelial cell through anion-selective channels that are located on the apical membrane surface. Na traverses the epithelium via paracellular pathways. Water can flow either between epithelial cells or through water channels, one of which (aquaporin 5) is expressed abundantly in type I lung epithelial cells, (27) which are highly permeable to water and, therefore, well equipped to facilitate liquid removal from the lung lumen. (28)Although the Cl concentration of liquid withdrawn from the lung lumen of fetal sheep is about 50% greater than that of plasma (Table 1), the Na concentration is virtually identical to that of plasma. (6)(21) The concentration of bicarbonate in lung liquid of fetal sheep is less than 3 mEq/L (3 mmol/L), yielding a pH of approximately 6.3. This finding led to the hypothesis that the lung epithelium of fetal sheep may transport bicarbonate actively out of the lung lumen. (29) The demonstration that acetazolamide, a carbonic anhydrase inhibitor, blocks secretion of lung liquid in fetal sheep supports this view. Both physiologic and immunohistochemical studies have shown that H+-ATPases are present on the respiratory epithelium of fetal sheep, where they likely provide an important mechanism for acidification of liquid within the lung lumen during development. In vitro electrophysiologic studies using fetal rat lung epithelial cells provided evidence that exposure to an acid pH might activate Cl channels, thereby contributing to the production of fetal lung liquid. (30) In fetal dogs and monkeys, however, the bicarbonate concentration of lung luminal liquid is not significantly different from that of fetal plasma. (31) Thus, the importance of lung liquid pH and acidification mechanisms during mammalian lung development in utero remains unclear.The concentration of K in fetal lung liquid exceeds that of plasma and increases further at the end of gestation as lung epithelial cells release surfactant into potential air spaces. (17) Studies in fetal goats showed that mechanical stretch of the lung caused a decrease in secretion and sometimes led to absorption of luminal liquid, with associated fluxes of Na and Cl in the direction of the interstitium and K toward the lung lumen. (32) Direct micropuncture measurements of Cl concentration in the alveolar lining liquid of lambs before and after birth showed that the large Cl gradient between lung luminal liquid and plasma decreased rapidly with the onset of air breathing. (33) This change was accompanied by a three-fold increase in the calcium concentration of alveolar liquid. (34) Studies performed with fetal sheep before and during labor showed that the Cl concentration of lung liquid often decreased during labor, while the K concentration increased as liquid was absorbed from the lungs during labor. (35) Increased K flux into the lung lumen might be linked to the increase in lung epithelial cell Na,K-ATPase that occurs in labor, (36) as discussed later.The volume of liquid within the lung lumen of fetal sheep increases from 4 to 6 mL/kg at mid-gestation (26) to more than 20 mL/kg near term. (24)(25) The hourly flow rate of lung liquid increases from approximately 2 mL/kg body weight at mid-gestation (26) to approximately 5 mL/kg body weight at term. (16)(17)(37) Increased production of luminal liquid during development reflects a rapidly expanding pulmonary microvascular and epithelial surface area that occurs with proliferation and growth of lung capillaries and respiratory units. (26)(38)The observation that unilateral pulmonary artery occlusion decreases lung liquid production in fetal sheep by at least 50% (39) shows that the pulmonary circulation, rather than the bronchial circulation, is the major source of fetal lung liquid. Intravenous infusion of isotonic saline at a rate sufficient to increase lung microvascular pressure and lung lymph flow in fetal lambs had no effect on liquid flow across the pulmonary epithelium. (40) Thus, transepithelial Cl secretion appears to be the major driving force responsible for the production of liquid in the fetal lung lumen. In vitro studies of epithelial ion transport across the fetal airways indicated that the epithelium of the upper respiratory tract also secretes Cl, thereby contributing to lung liquid production. (41)(42)(43) However, most of this liquid forms in the distal portions of the fetal lung, where total surface area is many times greater than it is in the conducting airways.Several studies have demonstrated that both the rate of lung liquid production and the volume of liquid within the lumen of the fetal lung normally decrease before birth, most notably during labor. (25)(35)(37)(44)(45) Thus, lung water content is approximately 25% greater after preterm delivery than it is at term, and newborn animals that are delivered by cesarean section without prior labor have considerably more liquid in their lungs than do animals that are delivered either vaginally or operatively after the onset of labor. (46)(47) In studies of fetal sheep, extravascular lung water was 45% less in mature fetuses that were in the midst of labor than in fetuses that did not experience labor, and there was a further 38% decrease in extravascular lung water measured in term lambs that were studied 6 hours after a normal vaginal birth. (25)Morphometric analysis of sections of frozen lung taken from fetal lambs with and without prior labor showed that the decrease in lung water content that occurs before birth is the result of a decrease in the liquid volume of potential air spaces relative to interstitial tissue volume (Fig. 3). These studies showed that reduced secretion, and perhaps absorption, of luminal liquid before birth decreases lung water by about 15 mL/kg body weight, leaving a residual volume of approximately 6 mL/kg, (25) which must be cleared from potential air spaces soon after birth to allow effective pulmonary gas exchange.Hormonal changes that occur in the fetus just before and during labor may play an important role in reducing secretion of lung liquid and promoting its absorption during labor and after birth. A major focus of research in this area has been on catecholamines, in particular the beta-adrenergic effects of epinephrine, in decreasing lung liquid formation in fetal animals. Studies of fetal sheep late in gestation showed that intravenous infusion of epinephrine or isoproterenol, but not norepinephrine, caused reabsorption of liquid from the lung lumen, an effect that beta-adrenergic blockade with propranolol prevented. (48) A subsequent report showed that intraluminal administration of amiloride, an Na-transport inhibitor, blocked the effect of epinephrine on absorption of lung liquid. (49) This finding indicates that beta-adrenergic agonists stimulate Na uptake by the lung epithelium, which drives liquid from the lung lumen into the interstitium, where it can be absorbed into the pulmonary circulation or transported via lung lymphatics to the systemic venous system.Tracheal instillation of dibutyryl cAMP (db-cAMP) also induces lung liquid absorption in fetal sheep late in gestation. (50) The inhibitory effects of both db-cAMP and epinephrine on net production of lung luminal liquid in fetal sheep increase with advancing gestational age, and both responses are attenuated by prior resection of the thyroid gland. (51) Replacement therapy with triiodothyronine after thyroidectomy restored the inhibitory effect of epinephrine on lung liquid production in fetal sheep. (52) Treatment of preterm fetal sheep with the combination of triiodothyronine and hydrocortisone may stimulate early maturation of epinephrine-induced absorption of lung liquid. (53) Another study showed a synergistic effect of terbutaline, a beta-adrenergic agonist, and aminophylline, a phosphodiesterase inhibitor, in switching lung liquid secretion to absorption in fetal lambs. (54) In these studies, the addition of amiloride to the lung liquid prevented absorption. Similarly, in studies conducted with fetal lambs in spontaneous labor, intrapulmonary delivery of amiloride either slowed or reversed lung liquid absorption. (35)(44) These observations are consistent with the view that birth-related events associated with release of cAMP in the lung may stimulate active transport of Na across the epithelium, which causes liquid to be absorbed from the lung lumen into the interstitium.Studies performed on animals during labor, either spontaneous or induced by oxytocin, have demonstrated an association between increased plasma concentrations of epinephrine and reduced production or absorption of lung liquid. (44)(55)(56) The concentration of beta-adrenergic receptors in lung tissue increases late in gestation, (57)(58) which might render the lungs particularly responsive to the effects of epinephrine during labor. At least two reports have indicated, however, that absorption of lung liquid near birth may not depend on epinephrine. One study showed that irreversible blockade of beta-adrenergic receptors in fetal rabbits did not prevent the normal reduction in lung water that occurs during parturition. (59) Another study reported that inhibition of beta-adrenergic activity with propranolol did not prevent lung liquid absorption in fetal lambs late in labor. (35)A number of other hormones have been shown to inhibit net production of fetal lung liquid. Several investigators have indicated that intravenous infusion of arginine vasopressin can reduce liquid formation in the fetal lung and that this effect can be inhibited by the Na-transport blocker amiloride. (60)(61) It is noteworthy, however, that the dose of vasopressin needed to cause lung liquid absorption in these studies yielded plasma concentrations of the hormone that far exceeded those usually detected during labor. (62) Nevertheless, there is evidence that release of both epinephrine and vasopressin at birth may be additive in stimulating absorption of lung liquid. (63) Studies performed with excised lungs of fetal guinea pigs have shown that epinephrine, cAMP, cortisol, and aldosterone each can cause an abrupt decrease in fetal lung liquid formation. (64)(65)(66) Further studies using a similar experimental model provided evidence that the stimulatory effect of epinephrine on lung liquid clearance is linked to increased postnatal pulmonary expression of amiloride-sensitive Na channels, which is mediated, at least in part, by a perinatal increase in plasma cortisol concentrations. (67) Taken together, these findings indicate that the interaction of multiple hormones of adrenal origin plays a key regulatory role in converting the respiratory epithelium from a predominantly Cl-secreting membrane during fetal development to a predominantly Na-absorbing membrane after birth.In vitro electrophysiologic studies of cultured alveolar epithelial cells harvested from fetal and adult rats have demonstrated that the same cells that secrete surfactant into the air spaces also may pump Na in the opposite direction, thus generating the driving force for absorption of liquid from the lung lumen. (68)(69)(70)(71)(72)(73) These studies have shown that monolayers of cultured distal lung epithelial cells (type II cells), when mounted in an Ussing-type chamber, maintain a transepithelial electrical potential difference (luminal side negative) that increases in response to beta-adrenergic stimulation and decreases in response to the Na transport inhibitors amiloride and ouabain. Amiloride blocks Na transport pathways on the luminal surface of the epithelium, and ouabain blocks Na,K-ATPase activity on the basolateral surface of the epithelium (Fig. 4). Although type II lung epithelial cells occupy only a small portion of the surface area of distal air spaces, numerous microvilli on their apical surface greatly increase their absorptive surface area. Morphometric studies also indicate that there are almost three times as many type II cells per unit of tissue mass that line the interior of the newborn lung compared with the adult lung. (74)More than 95% of the surface area of the adult lung is lined by type I alveolar epithelial cells. Recent studies showed that these expansive cells have abundant Na channels and Na,K-ATPase and that they, too, can transport Na by a process that is inhibited by both amiloride and ouabain. (75) Thus, both type I and type II cells are equipped to play key roles in clearing liquid from the lung lumen during and after birth.Na,K-ATPase activity in distal lung epithelial cells increases around the time of birth. (36)(76)(77) Studies performed with freshly isolated distal lung epithelial cells from fetal, newborn, and adult rabbits showed that Na pump turnover number, an index of Na,K-ATPase enzyme activity, increased fourfold during labor, followed by a threefold increase in the number of Na pumps per cell between newborn and adult stages of lung development. (36)(76) Na,K-ATPase activity was not significantly different in newborn and adult lung epithelial cells. Thus, Na pump activity in distal lung epithelium of rabbits increases at birth, and the number of Na pumps per cell increases postnatally. In related studies, Na pump activity was similar in cells harvested from fetal rabbits and from newborn rabbits that had respiratory distress after preterm birth. (36) These findings suggest that the stress of preterm birth and subsequent respiratory distress fail to increase lung epithelial cell Na absorption, an observation that may help to explain the pulmonary edema that is associated with respiratory distress after preterm birth. (78)Other studies have shown that mRNA expression of the alpha1 and beta1 subunits of Na,K-ATPase in fetal rat lungs increases just before birth. (77)(79) These changes are associated with parallel increases in the expression of epithelial Na and water channels in perinatal rat lung. (80)(81) There is now considerable evidence that glucocorticoids may upregulate expression of Na,K-ATPase, Na channels, and aquaporins in the developing rat lung. (82)(83)(84) A number of reports also indicate that the increased oxygen tension that occurs around the time of birth may have an important role in signaling the switch from Cl secretion to Na absorption in the lung epithelium near birth. (85)(86) The observation that early postnatal death from respiratory failure occurs in the absence of functional epithelial Na channels (87) clearly defines the pivotal role of epithelial Na absorption in clearing liquid from potential air spaces and facilitating pulmonary gas exchange at birth.There are two components to the process by which luminal liquid drains from the lungs during and after birth: transepithelial flow of liquid into the interstitium, as described previously, followed by flow of liquid into the bloodstream, either directly into the pulmonary circulation or through lymphatics that empty into the systemic venous system. The development of effective respiratory gas exchange and lung volume soon after birth suggests that the shift of liquid from air spaces into the lung interstitium occurs rapidly, after which there is slower uptake of liquid into the lung vasculature or lymphatics. (25)(88) In sheep, liquid absorption from the lung lumen often begins during labor and accelerates immediately after birth. (35)(44) Studies conducted with fetal sheep during and after parturition have demonstrated a decrease in net Cl secretion and a corresponding increase in Na uptake by the respiratory epithelium. (33)(35) The fact that Na transport inhibitors reverse lung liquid absorption in fetal sheep during labor and slow the rate of lung liquid clearance in newborn guinea pigs (89) underscores the importance of this change in epithelial Na uptake in hastening liquid removal from the lung lumen near birth.Studies performed with fetal lambs at the start of breathing showed a transient postnatal increase in hydraulic conductivity and small solute permeability of the lung epithelium, which may contribute to increased bulk flow of liquid from potential air spaces into the interstitium. (90) Lung inflation with air also reduces hydraulic pressure in the pulmonary interstitium, which may help to drain liquid out of the lung lumen into the interstitial space. (91)(92) As plasma protein concentration increases during the few days before birth, (25)(46) the resultant increase in plasma protein osmotic pressure also helps to draw liquid from the lung interstitium into the circulation.Removal of liquid from the lungs continues for several hours after birth. Studies of fetal and newborn rabbits showed that lung blood volume increases with the onset of breathing, but lung water content does not begin to decrease postnatally until 30 to 60 minutes after birth. (93) When breathing starts, air inflation shifts residual liquid from the lung lumen into distensible perivascular spaces around large pulmonary blood vessels and airways (Fig. 5). Accumulation of liquid in these connective tissue spaces, which are distant from sites of respiratory gas exchange, allows time for small blood vessels and lymphatics to remove the displaced liquid with little or no adverse effect on lung function. In rabbits born at term gestation, perivascular cuffs of fluid are of maximal size 30 minutes after birth, at which time they contain up to 75% of the total amount of extravascular water in the lungs. The fluid cuffs normally disappear by about 6 hours after birth.The perinatal pattern of lung liquid clearance is similar in sheep. (25) As net production of lung liquid decreases before birth, the volume of liquid within the lung lumen also decreases, with a corresponding reduction in the caliber of potential air spaces. After breathing begins, residual liquid shifts into the interstitium and collects around large blood vessels and airways. Perivascular fluid cuffs progressively decrease in size as aeration of terminal respiratory units improves postnatally. Clearance of fetal lung liquid in sheep is complete by about 6 hours after normal vaginal delivery. The process is slower in preterm lambs, (78)(94) as it is in preterm rabbits. (46)Potential routes for drainage of lung liquid at birth include lung lymphatics, the pulmonary circulation, the pleural space, the mediastinum, and the upper airway. Studies of chronically catheterized fetal and newborn lambs showed that the postnatal increase in lung lymph flow is modest and transient, accounting for no more than 15% of the amount of residual liquid that drains from the lung postnatally. (25) In these studies, the concentration of protein in lung lymph decreased with the start of ventilation, presumably the result of protein-poor liquid flowing from within the lung lumen into the interstitium. With subsequent uptake of this liquid into the bloodstream, the concentration of protein in lymph returned to its baseline level. These studies showed that lung lymphatics normally drain only a small fraction of liquid in potential air spaces. In preterm lambs that had respiratory distress, the postnatal increase in lung lymph flow lasted for several hours and was accompanied by a substantial increase in protein clearance, indicative of increased lung vascular permeability to protein. (78)Other studies from the same laboratory showed that either elevated left atrial pressure or reduced plasma protein concentration slows the rate of liquid clearance from the lungs of healthy, mature lambs. (95)(96) These findings support the view that the pulmonary circulation absorbs at least some, and perhaps most, of the residual liquid that drains from the lungs after birth. It is also possible that some liquid enters the bloodstream through the mediastinum and pleural cavity, although other studies indicate that very little luminal liquid drains via the pleural space in normal lambs.For many years it was believed that much of the liquid contained in the lungs at birth was extruded via the upper airway as a result of the “vaginal squeeze.” This concept derived from measurements of intrathoracic pressure taken during delivery of normal term infants, which led to the inference that chest compression associated with vaginal delivery drives liquid from the lungs into the oropharynx. (13) Other studies, however, indicated that increased thoracic pressure during spontaneous birth may have little effect on clearance of fetal lung liquid. Animals that are delivered by cesarean section during labor after tracheal ligation have no more water in their lungs than do animals that are born vaginally. (25)(47) Moreover, studies of lung fluid dynamics in near-term fetal sheep showed that late in labor, as luminal liquid is absorbed across the epithelium, little or no liquid could be withdrawn from the trachea. (35)(44) Thus, although the conducting airways may serve as an escape route for lung liquid during delivery without prior labor, they probably have no more than a minor role in clearing liquid from the lungs during the normal birth process.Clearance of liquid from potential air spaces into the bloodstream occurs quickly, usually within a few hours in most newborns. Sometimes the process is delayed, however, producing the clinical and radiographic features of a condition that has been called transient tachypnea of the newborn or the syndrome of retained fetal lung liquid. Because tachypnea is not a consistent finding in this condition, notably in situations associated with respiratory depression, and because some of the liquid may enter the lungs postnatally from the pulmonary circulation, a more appropriate term for this mild form of neonatal respiratory dysfunction is persistent postnatal pulmonary edema. Although this disorder initially was described in infants who were born at term, (97) only one of whom was delivered by cesarean section, subsequent reports have noted an association with operative delivery and with preterm birth, especially in the absence of prior labor. (98)(99)Preterm birth is associated with several conditions that may contribute to delayed removal of fetal lung liquid, including impaired Na-pump activity in lung epithelial cells, (36) high filtration pressure in the pulmonary circulation (78) (often with persistent patency of the ductus arteriosus) (100), reduced microvascular surface area for fluid absorption, (101) and a low plasma protein osmotic pressure. (102)Unless the lungs are immature, with resultant atelectasis and respiratory failure, absorption of fetal lung liquid usually is complete within 24 hours of birth, and respiratory symptoms disappear accordingly. An increased concentration of inspired oxygen sometimes is needed to maintain a normal partial pressure of oxygen in arterial blood. Usually no other treatment is necessary. Because the condition may be aggravated by exogenous fluid overload, fluid and salt intake of infants who have persistent pulmonary edema should not exceed their insensible losses. Diuretics offer little or no benefit and may produce complicating abnormalities of serum electrolytes.Figure 6 is a schematic drawing of the liquid compartments in the fetal lung and the forces that contribute to liquid clearance. Liquid within the lung lumen contains less than 0.3 mg/mL of protein; pulmonary interstitial liquid has a protein concentration of approximately 30 mg/mL. This transepithelial difference in protein concentration generates an osmotic pressure difference of more than 10 cm H2O, which draws liquid from the lung lumen into the interstitium as Cl secretion decreases. Increased activity of lung epithelial Na,K-ATPase during labor provides the main driving force for liquid absorption into the lung interstitium. Transpulmonary pressure associated with lung inflation also contributes to bulk flow of liquid from the lung lumen into the interstitium. Together, these forces increase the protein osmotic pressure difference between plasma and interstitial fluid. Air entry into the lungs not only displaces liquid, but also decreases hydraulic pressure in the pulmonary circulation and increases pulmonary blood flow, which, in turn, increases lung blood volume and effective vascular surface area for fluid uptake. These circulatory changes facilitate absorption of liquid into the lung vascular bed. About 10% to 15% of the luminal liquid drains from the lungs via lymphatics into the systemic venous system. With spontaneous breathing, the postnatal reduction of intrathoracic pressure decreases systemic venous pressure, which may augment lymphatic drainage, but most of the displaced luminal liquid enters the pulmonary microcirculation or seeps into the mediastinum, with subsequent drainage into the systemic circulation.Thanks to the National Heart, Lung and Blood Institute of the National Institutes of Health for its longstanding generous support of much of the research that is described in this review.