Capillary Fluid Exchange

The partition of fluid between the vascular and interstitial compartments is regulated by forces (hydrostatic and oncotic) operating across the microvascular walls and the surface areas of permeable structures comprising the endothelial barrier to fluid and solute exchange, as well as within the extracellular matrix and lymphatics. In addition to its role in the regulation of vascular volume, transcapillary fluid filtration also allows for continuous turnover of water bathing tissue cells, providing the medium for diffusional flux of oxygen and nutrients required for cellular metabolism and removal of metabolic byproducts. Transendothelial volume flow has also been shown to influence vascular smooth muscle tone in arterioles, hydraulic conductivity in capillaries, and neutrophil transmigration across postcapillary venules, while the flow of this filtrate through the interstitial spaces functions to modify the activities of parenchymal, resident tissue, and metastasizing tumor cells. Likewise, the flow of lymph, which is driven by capillary filtration, is important for the transport of immune and tumor cells, antigen delivery to lymph nodes, and for return of filtered fluid and extravasated proteins to the blood. Given this background, the aims of this treatise are to summarize our current understanding of the factors involved in the regulation of transcapillary fluid movement, how fluid movements across the endothelial barrier and through the interstitium and lymphatic vessels influence cell function and behavior, and the pathophysiology of edema formation. Table of Contents: Fluid Movement Across the Endothelial Barrier / The Interstitium / The Lymphatic Vasculature / Pathophysiology of Edema Formation

Praised for its concise coverage, this highly accessible monograph lays a foundation for understanding the underlying concepts of normal cardiovascular function and offers a welcome alternative to a more mechanistically oriented approach or an encyclopedic physiology text. Clear explanations, ample illustrations and engaging clinical cases and problems provide the perfect guidance for self-directed learning and prepare you to excel in clinical practice.

This presentation describes various aspects of the regulation of tissue oxygenation, including the roles of the circulatory system, respiratory system, and blood, the carrier of oxygen within these components of the cardiorespiratory system. The respiratory system takes oxygen from the atmosphere and transports it by diffusion from the air in the alveoli to the blood flowing through the pulmonary capillaries. The cardiovascular system then moves the oxygenated blood from the heart to the microcirculation of the various organs by convection, where oxygen is released from hemoglobin in the red blood cells and moves to the parenchymal cells of each tissue by diffusion. Oxygen that has diffused into cells is then utilized in the mitochondria to produce adenosine triphosphate (ATP), the energy currency of all cells. The mitochondria are able to produce ATP until the oxygen tension or PO2 on the cell surface falls to a critical level of about 4–5 mm Hg. Thus, in order to meet the energetic needs of cells, it is important to maintain a continuous supply of oxygen to the mitochondria at or above the critical PO2 . In order to accomplish this desired outcome, the cardiorespiratory system, including the blood, must be capable of regulation to ensure survival of all tissues under a wide range of circumstances. The purpose of this presentation is to provide basic information about the operation and regulation of the cardiovascular and respiratory systems, as well as the properties of the blood and parenchymal cells, so that a fundamental understanding of the regulation of tissue oxygenation is achieved.

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The microcirculation is highly responsive to, and a vital participant in, the inflammatory response. All segments of the microvasculature (arterioles, capillaries, and venules) exhibit characteristic phenotypic changes during inflammation that appear to be directed toward enhancing the delivery of inflammatory cells to the injured/infected tissue, isolating the region from healthy tissue and the systemic circulation, and setting the stage for tissue repair and regeneration. The best characterized responses of the microcirculation to inflammation include impaired vasomotor function, reduced capillary perfusion, adhesion of leukocytes and platelets, activation of the coagulation cascade, and enhanced thrombosis, increased vascular permeability, and an increase in the rate of proliferation of blood and lymphatic vessels. A variety of cells that normally circulate in blood (leukocytes, platelets) or reside within the vessel wall (endothelial cells, pericytes) or in the perivascular space (mast cells, macrophages) are activated in response to inflammation. The activation products and chemical mediators released from these cells act through different well-characterized signaling pathways to induce the phenotypic changes in microvessel function that accompany inflammation. Drugs that target a specific microvascular response to inflammation, such as leukocyte-endothelial cell adhesion or angiogenesis, have shown promise in both the preclinical and clinical studies of inflammatory disease. Future research efforts in this area will likely identify new avenues for therapeutic intervention in inflammation. Table of Contents: Introduction / Historical Perspectives / Anatomical Considerations / Impaired Vasomotor Responses / Capillary Perfusion / Angiogenesis / Leukocyte-Endothelial Cell Adhesion / Platelet-Vessel Wall Interactions / Coagulation and Thrombosis / Endothelial Barrier Dysfunction / Epilogue / References

This book is essential reading for medical or veterinary practitioners who need to understand the new fluid physiology and to apply it to the safe care of patients. The Starling principle is one of medicine’s most important concepts and originates from Ernest Starling’s laboratory research 120 years ago. However, inappropriate fluid therapy is now recognised as harming and even killing thousands of patients every year. In 2004, a landmark study was published which confirmed the hypotheses put forward by Sheldon Weinbaum and Charles Michel, among other physiologists, that, in most tissues and in most situations, capillaries filter fluid to the interstitium, but do not reabsorb it. This book draws together for the first time the evolving science of the steady-state Starling principle and the clinical evidence that reveals its applicability to safer patient care. It is a thorough re-appraisal of the basics of fluid therapy. The mantra of colloid boluses for plasma volume resuscitation and colloid-free isotonic salt solution for extracellular fluid volume does not explain observations from blinded clinical trials, and the expectation of benefit for resuscitation with colloids, particularly in respect of oedema, has not materialised. Now that there is consensus that colloid volume therapy should not be used in critically-ill patients, there is a pressing need for a new paradigm for fluid therapy. This book proposes an improved paradigm that takes into consideration the Starling principle, which has been neglected by clinicians and revised by physiologists in recent years. It retires the view of colloids as preferred plasma substitutes, and focuses instead on the central volume of distribution of an infused fluid, its rate of distribution to a peripheral volume, and its rate of excretion. In short, it emphasises volume kinetics.

The vascular endothelium lining the inner surface of blood vessels serves as the first interface for circulating blood components to interact with cells of the vascular wall and surrounding extravascular tissues. In addition to regulating blood delivery and perfusion, a major function of vascular endothelia, especially those in exchange microvessels (capillaries and postcapillary venules), is to provide a semipermeable barrier that controls blood–tissue exchange of fluids, nutrients, and metabolic wastes while preventing pathogens or harmful materials in the circulation from entering into tissues. During host defense against infection or tissue injury, endothelial barrier dysfunction occurs as a consequence as well as cause of inflammatory responses. Plasma leakage disturbs fluid homeostasis and impairs tissue oxygenation, a pathophysiological process contributing to multiple organ dysfunction associated with trauma, infection, metabolic disorder, and other forms of disease. In this book, we provide an updated overview of microvascular endothelial barrier structure and function in health and disease. The discussion is initiated with the basic physiological principles of fluid and solute transport across microvascular endothelium, followed by detailed information on endothelial cell–cell and cell–matrix interactions and the experimental techniques that are employed to measure endothelial permeability. Further discussion focuses on the signaling and molecular mechanisms of endothelial barrier responses to various stimulations or drugs, as well as their relevance to several common clinical conditions. Taken together, this book provides a comprehensive analysis of microvascular endothelial cell and molecular pathophysiology. Such information will assist scientists and clinicians in advanced basic and clinical research for improved health care.