Active lymphatic pumping and sheep lung lymph flow

Computational fluid dynamic modeling of the lymphatic system: a review of existing models and future directions

Historically, research into the lymphatic system has been overlooked due to both a lack of knowledge and limited recognition of its importance. In the last decade however, lymphatic research has gained substantial momentum and has included the development of a variety of computational models to aid understanding of this complex system. This article reviews existing computational fluid dynamic models of the lymphatics covering each structural component including the initial lymphatics, pre-collecting and collecting vessels, and lymph nodes. This is followed by a summary of limitations and gaps in existing computational models and reasons that development in this field has been hindered to date. Over the next decade, efforts to further characterize lymphatic anatomy and physiology are anticipated to provide key data to further inform and validate lymphatic fluid dynamic models. Development of more comprehensive multiscale- and multi-physics computational models has the potential to significantly enhance the understanding of lymphatic function in both health and disease.

Dichotomous effects of in vivo and in vitro ionizing radiation exposure on lymphatic function

On the one hand, lymphatic dysfunction induces interstitial edema and inflammation. On the other hand, the formation of edema and inflammation induce lymphatic dysfunction. However, informed by the earlier reports of undetected apoptosis of irradiated lymphatic endothelial cells (LECs) in vivo, lymphatic vessels are commonly considered inconsequential to ionizing radiation (IR)-induced inflammatory injury to normal tissues. Primarily because of the lack of understanding of the acute effects of IR exposure on lymphatic function, acute edema and inflammation, common sequelae of IR exposure, have been ascribed solely to blood vessel damage. Therefore, in the present study, the lymphatic acute responses to IR exposure were quantified to evaluate the hypothesis that IR exposure impairs lymphatic pumping. Rat mesenteric lymphatic vessels were irradiated in vivo or in vitro, and changes in pumping were quantified in isolated vessels in vitro. Compared with sham-treated vessels, pumping was lowered in lymphatic vessels irradiated in vivo but increased in vessels irradiated in vitro. Furthermore, unlike in blood vessels, the acute effects of IR exposure in lymphatic vessels were not mediated by nitric oxide-dependent pathways in either in vivo or in vitro irradiated vessels. After cyclooxygenase blockade, pumping was partially restored in lymphatic vessels irradiated in vitro but not in vessels irradiated in vivo. Taken together, these findings demonstrated that lymphatic vessels are radiosensitive and LEC apoptosis alone may not account for all the effects of IR exposure on the lymphatic system.NEW & NOTEWORTHY Earlier studies leading to the common belief that lymphatic vessels are radioresistant either did not characterize lymphatic pumping, deemed necessary for the resolution of edema and inflammation, or did it in vivo. By characterizing pumping in vitro, the present study, for the first time, demonstrated that lymphatic pumping was impaired in vessels irradiated in vivo and enhanced in vessels irradiated in vitro. Furthermore, the pathways implicated in ionizing radiation-induced blood vessel damage did not mediate lymphatic responses.

The health effects of short fiber chrysotile and amphibole asbestos

The potential toxic effects of short chrysotile and amphibole asbestos fibers with lengths <5 to ∼10 µm have been debated over the years. This stems from the large database of epidemiology, toxicology, and in-vitro studies, each of which often provides different information in understanding and differentiating the effects of short fibers. The epidemiology studies in which the cancer potency estimates were based upon relatively high exposure concentrations provide a conservative assessment that shorter fibers would have little if any effect, especially under controlled exposure or environmental conditions that may occur today. The QSAR models have shown that fiber aspect ratio and Mg content are excellent predictors of cancer potency and that short fibers/particles of amphibole would have no effect. The studies of motor vehicle mechanics and in particular workers who serviced chrysotile containing brakes with the majority of the fibers being short provides evidence that motor vehicle mechanics, including workers who were engaged in brake repair, are not at an increased risk of mesothelioma. Several inhalation toxicology studies clearly differentiated that short chrysotile and amphibole asbestos fibers did not produce a significant carcinogenic effect in the lung or pleural cavity. Because of dosing and lack of sensitivity to biosolubility, in vitro studies can be difficult to interpret; however, a number have differentiated short chrysotile and amphibole asbestos fibers from long fibers. Integral to understanding the importance of fiber length in determining possible health effects is an understanding of the biological and physiological function of the respiratory system. Short asbestos fibers, like innocuous dust, can be cleared through the tracheobronchial ciliated mucous transport, phagocytized by macrophages and cleared via the bronchial tree, and can also be removed through the lymphatic system. While the first two methods can remove them from the lung, with lymphatic transport through one-way valves, fibers are removed from the active area of the lung where the fiber-related disease has been shown to develop and can accumulate in lymphatic sumps and lymph nodes. While short asbestos fibers are present in most occupational or environmental exposures, the large body of studies strongly supports that they do not contribute to the health effects of asbestos exposure.

The lymphatics in kidney health and disease

The mammalian vascular system consists of two networks: the blood vascular system and the lymphatic vascular system. Throughout the body, the lymphatic system contributes to homeostatic mechanisms by draining extravasated interstitial fluid and facilitating the trafficking and activation of immune cells. In the kidney, lymphatic vessels exist mainly in the kidney cortex. In the medulla, the ascending vasa recta represent a hybrid lymphatic-like vessel that performs lymphatic-like roles in interstitial fluid reabsorption. Although the lymphatic network is mainly derived from the venous system, evidence supports the existence of lymphatic beds that are of non-venous origin. Following their development and maturation, lymphatic vessel density remains relatively stable; however, these vessels undergo dynamic functional changes to meet tissue demands. Additionally, new lymphatic growth, or lymphangiogenesis, can be induced by pathological conditions such as tissue injury, interstitial fluid overload, hyperglycaemia and inflammation. Lymphangiogenesis is also associated with conditions such as polycystic kidney disease, hypertension, ultrafiltration failure and transplant rejection. Although lymphangiogenesis has protective functions in clearing accumulated fluid and immune cells, the kidney lymphatics may also propagate an inflammatory feedback loop, exacerbating inflammation and fibrosis. Greater understanding of lymphatic biology, including the developmental origin and function of the lymphatics and their response to pathogenic stimuli, may aid the development of new therapeutic agents that target the lymphatic system.

Lymphadenectomy in pulmonary metastasectomy

Lymph node (LN) removal during pulmonary metastasectomy is a prerequisite to achieve complete resection or at least collect prognostic information, but is not yet generally accepted. On average, the rate of unexpected lymph node involvement (LNI) is less than 10% in sarcoma, 20% in colorectal cancer (CRC) and 30% in renal cell carcinoma (RCC) when radical LN dissection is performed. LNI is a negative prognostic factor and presence of preoperative mediastinal disease usually leads to exclusion of the patient from metastasis surgery. Nonetheless, some authors found excellent prognoses even with mediastinal LNI in colorectal and RCC metastases when radical LN dissection was performed (median survival of 37 and 36 months, respectively). Multiple metastases, central location of the lesion followed by anatomical resections are associated with a higher LNI rate. The real prognostic influence of systematic LN dissection remains unclear. Two positive effects were described after radical lymphadenectomy: a trend for improved survival in RCC patients and a reduction of mediastinal recurrences from 23% to 0% in CRC patients. Unfortunately, there is a great number of studies that do not demonstrate any positive effect of lymphadenectomy during pulmonary metastasectomy except a pseudo stage migration effect. Future studies should not only focus on survival, but also on local and LN recurrence.

A Modern View of the Interstitial Space in Health and Disease

Increases in the volume of the interstitial space are readily recognized clinically as interstitial edema formation in the loose connective tissue of skin, mucosa, and lung. However, the contents and the hydrostatic pressure of this interstitial fluid can be very difficult to determine even in experimental settings. These difficulties have long obscured what we are beginning to appreciate is a dynamic milieu that is subject to both intrinsic and extrinsic regulation. This review examines current concepts regarding regulation of interstitial volume, pressure, and flow and utilizes that background to address three major topics of interest that impact IV fluid administration. The first of these started with the discovery that excess dietary salt can be stored non-osmotically in the interstitial space with minimal impact on vascular volume and pressures. This led to the hypothesis that, along with the kidney, the interstitial space plays an active role in the long-term regulation of blood pressure. Second, it now appears that hypovolemic shock leads to systemic inflammatory response syndrome principally through the entry of digestive enzymes into the intestinal interstitial space and the subsequent progression of enzymes and inflammatory agents through the mesenteric lymphatic system to the general circulation. Lastly, current evidence strongly supports the non-intuitive view that the primary factor leading to inflammatory edema formation is a decrease in interstitial hydrostatic pressure that dramatically increases microvascular filtration.

Mesenteric lymphatic vessels adapt to mesenteric venous hypertension by becoming weaker pumps

Lymphangions, the segments of lymphatic vessels between two adjacent lymphatic valves, actively pump lymph. Acute changes in transmural pressure and lymph flow have profound effects on lymphatic pump function in vitro. Chronic changes in pressure and flow in vivo have also been reported to lead to significant changes in lymphangion function. Because changes in pressure and flow are both cause and effect of adaptive processes, characterizing adaptation requires a more fundamental analysis of lymphatic muscle properties. Therefore, the purpose of the present work was to use an intact lymphangion isovolumetric preparation to evaluate changes in mesenteric lymphatic muscle mechanical properties and the intracellular Ca(2+) in response to sustained mesenteric venous hypertension. Bovine mesenteric veins were surgically occluded to create mesenteric venous hypertension. Postnodal mesenteric lymphatic vessels from mesenteric venous hypertension (MVH; n = 6) and sham surgery (Sham; n = 6) animals were isolated and evaluated 3 days after the surgery. Spontaneously contracting MVH vessels generated end-systolic active tension and end-diastolic active tension lower than the Sham vessels. Furthermore, steady-state active tension and intracellular Ca(2+) concentration levels in response to KCl stimulation were also significantly lower in MVH vessels compared with those of the Sham vessels. There was no significant difference in passive tension in lymphatic vessels from the two groups. Taken together, these results suggest that following 3 days of mesenteric venous hypertension, postnodal mesenteric lymphatic vessels adapt to become weaker pumps with decreased cytosolic Ca(2+) concentration.

Anatomy and physiology of the thoracic lymphatic system

The thoracic lymphatic system is one of the most complex and poorly understood systems of the human body, and much is still to be learned, especially in lymphatic physiology. Knowledge of the normal anatomy of this system as well as of its variations is nevertheless important for thoracic surgeons investigating and treating patients with lung or esophageal neoplasms.

Modelling the lymphatic system: challenges and opportunities

The lymphatic system is a vital part of the circulatory and immune systems, and plays an important role in homeostasis by controlling extracellular fluid volume and in combating infection. Nevertheless, there is a notable disparity in terms of research effort expended in relation to the treatment of lymphatic diseases in contrast to the cardiovascular system. While similarities to the cardiovascular system exist, there are considerable differences in their anatomy and physiology. This review outlines some of the challenges and opportunities for those engaged in modelling biological systems. The study of the lymphatic system is still in its infancy, the vast majority of the models presented in the literature to date having been developed since 2003. The number of distinct models and their variants are few in number, and only one effort has been made thus far to study the entire lymphatic network; elements of the lymphatic system such as the nodes, which act as pumps and reservoirs, have not been addressed by mathematical models. Clearly, more work will be necessary in combination with experimental verification in order to progress and update the knowledge on the function of the lymphatic system. As our knowledge and understanding of its function increase, new and more effective treatments of lymphatic diseases are bound to emerge.

Nonlinear lymphangion pressure-volume relationship minimizes edema

Lymphangions, the segments of lymphatic vessel between two valves, contract cyclically and actively pump, analogous to cardiac ventricles. Besides having a discernable systole and diastole, lymphangions have a relatively linear end-systolic pressure-volume relationship (with slope E(max)) and a nonlinear end-diastolic pressure-volume relationship (with slope E(min)). To counter increased microvascular filtration (causing increased lymphatic inlet pressure), lymphangions must respond to modest increases in transmural pressure by increasing pumping. To counter venous hypertension (causing increased lymphatic inlet and outlet pressures), lymphangions must respond to potentially large increases in transmural pressure by maintaining lymph flow. We therefore hypothesized that the nonlinear lymphangion pressure-volume relationship allows transition from a transmural pressure-dependent stroke volume to a transmural pressure-independent stroke volume as transmural pressure increases. To test this hypothesis, we applied a mathematical model based on the time-varying elastance concept typically applied to ventricles (the ratio of pressure to volume cycles periodically from a minimum, E(min), to a maximum, E(max)). This model predicted that lymphangions increase stroke volume and stroke work with transmural pressure if E(min) < E(max) at low transmural pressures, but maintain stroke volume and stroke work if E(min)= E(max) at higher transmural pressures. Furthermore, at higher transmural pressures, stroke work is evenly distributed among a chain of lymphangions. Model predictions were tested by comparison to previously reported data. Model predictions were consistent with reported lymphangion properties and pressure-flow relationships of entire lymphatic systems. The nonlinear lymphangion pressure-volume relationship therefore minimizes edema resulting from both increased microvascular filtration and venous hypertension.

Bronchial arteries and lymphatics of the lung

Bronchial arteries and bronchial lymphatics participate directly in the normal and pathologic conditions of the lungs and are of more than academic interest. Bronchial arteries and bronchial lymphatics usually are discussed separately, but they are linked when considered anatomically or physiologically. This article describes these highly variable structures.