Abstract
Simple SummaryFluid drainage operated by lymphatic vessels is crucial for a proper volume homeostasis of body compartments. This role is particularly relevant for the pleural cavity, where the hydraulic pressure of the pleural liquid is very subatmospheric and fluid filtering from the blood capillaries into the pleural space must be continuously removed to keep the pleural space volume low and to prevent accumulation of liquid causing impairments of the respiratory mechanics. In order to accomplish this task, lymphatic vessels of the pleural side of the diaphragm and those lying on the pleural surface of the chest wall must possess a negative intraluminal pressure which has to vary during the respiratory cycle to follow the similar variations occurring to the pressure of pleural liquid. This review focuses on the in vivo pressure measurements performed in sedated animal models to understand how these lymphatic networks can accomplish this complex but pivotal role.Lymphatic vessels exploit the mechanical stresses of their surroundings together with intrinsic rhythmic contractions to drain lymph from interstitial spaces and serosal cavities to eventually empty into the blood venous stream. This task is more difficult when the liquid to be drained has a very subatmospheric pressure, as it occurs in the pleural cavity. This peculiar space must maintain a very low fluid volume at negative hydraulic pressure in order to guarantee a proper mechanical coupling between the chest wall and lungs. To better understand the potential for liquid drainage, the key parameter to be considered is the difference in hydraulic pressure between the pleural space and the lymphatic lumen. In this review we collected old and new findings from in vivo direct measurements of hydraulic pressures in anaesthetized animals with the aim to better frame the complex physiology of diaphragmatic and intercostal lymphatics which drain liquid from the pleural cavity.
Highlights
Lymphatic vessels exploit the mechanical stresses of their surroundings together with intrinsic rhythmic contractions to drain lymph from interstitial spaces and serosal cavities to eventually empty into the blood venous stream
The mechanical properties of lungs and chest wall are set so that each of the two structures tends to reach its mechanical resting volume. These volumes are located at two very different values, as lungs tend to the very low minimal air volume (~500 mL, below the in situ residual volume corresponding to zero vital capacity) whereas the chest wall tends to a value ~80% of the vital capacity [21]
Lymphatic vessels of the diaphragm remove liquid filtered from blood capillaries into the interstitial space, as occurs in all skeletal muscle tissue, and allow fluid homeostasis of the pleural cavities, due to their subatmospheric intraluminal pressure
Summary
The lymphatic system drains liquid, macromolecules and cells from the surrounding interstitial space and serosal cavities, to guarantee the proper tissue fluid balance [1]. Lymph formation happens first into lymphatic capillaries, named “initial lymphatics”, which are blind-ended vessels originating in peripheral tissue, devoid of lymphatic muscle cells (LMCs). Those vessels are lined by a single layer of overlapping endothelial cells (LECs) forming primary unidirectional valves, allowing lymph entry and preventing its backflow into surrounding tissue [2–5]. Lymph is propelled through larger vessels, named “collecting lymphatics”, endowed with a more structured vessel wall, surrounded by LMCs displaying unique features [9], in which intraluminal competent valves can be identified separating adjacent lymphangions (i.e., the functional pump units of the lymphatic system) and preventing massive lymph backflow [4,8,10]. Lymphatic spontaneous phasic contractions (intrinsic forces) [16] arising in collecting vessels walls combined to extrinsic mechanisms drive lymph propulsion centripetally, critically affecting intraluminal pressure gradients. Lymph is allowed to unidirectionally flow towards the lymph node chain and merge into the thoracic duct and right lymph duct, eventually emptying into the blood circulatory system through the subclavian veins
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