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Tutorial on Microcirculation Titles and Abstracts
Presentation materials: PPT The function of the circulatory system is to transport materials throughout the body. Blood flows through an extensive branching network of tubes, driven by the pumping action of the heart. Transport of oxygen from the lungs to other parts of the body is a crucial task of the circulatory system. The solubility of oxygen in water is relatively low, but the presence of a high volume fraction (40-45%) of red blood cells in blood greatly increases its oxygen carrying capacity. Hemoglobin molecules within red blood cells take up oxygen in the lungs and release it in the body. A further consequence of the low solubility of oxygen in water is that the distance that oxygen can diffuse into an oxygen-consuming tissue is relatively short, typically of order 20-100 µm. The circulatory system must therefore deliver blood within a short distance of every point in the tissue, and so all oxygen-consuming tissues are supplied with a dense network of very narrow blood vessels, ranging in diameter from a few hundred µm to about 4 µm, known as the microcirculation. Theoretical models have provided important insights into the mechanics of blood flow in the microvascular networks, and relationship between microvascular structure and oxygen tissue oxygenation.
Presentation materials: PPT The circulatory system is a dynamic structure. Blood vessels grow or regress during development and in a variety of normal and disease states, over time scales of hours, days and longer. Under normal conditions, these structural changes ensure that all parts of the tissue are supplied with blood, and that the network structure is well organized and efficient with regard both to the volume of blood needed and the energy required to drive the flow. In the arteries and arterioles, the relationship between blood flow rate and vessel diameter is found experimentally to be approximately cubic on average. This relationship can be predicted based on an optimality principle (Murray's law) in which a linear combination of blood volume and energy dissipation is minimized. The cubic relationship implies fixed fluid shear stress acting on all vessel walls, and this led to the proposal that each vessel continuously adjusts its diameter to maintain a fixed level of shear stress. However, such a mechanism would lead to instability, and would not adequately meet the functional needs of tissues. Theoretical models have been used to investigate how other factors, including tension in vessel walls, metabolic needs and information transfer along vessel walls, also play a role in structural adaptation. The simulation of angiogenesis (new vessel growth) has been addressed using various theoretical approaches, particularly in the contexts of wound healing and tumor growth.
Presentation materials: PPT The circulatory system is capable of rapidly controlling blood flow, on time scales of seconds, minutes and longer, by active contraction and dilation of smooth muscle cells in vessel walls, particularly in the arterioles. This allows localized short-term flow regulation in response to changing conditions and tissue needs. Two major modes of flow regulation are autoregulation, in which flow to a given tissue is held almost constant independent of changes in blood pressure, and metabolic regulation, in which blood flow can be modulated over a wide range in response to changing metabolic demands, and particularly to changes in oxygen consumption. Regulation of blood flow is achieved by a number of mechanisms, in which individual vessel segments respond to mechanical and metabolic stimuli. In the myogenic response, an increase in wall tension causes vascular contraction. In the shear-dependent response, and increase in wall-shear stress causes dilation. Several metabolic stimuli, including levels of oxygen, potassium ions, adenosine triphosphate and nitric oxide, cause alterations in arteriole diameter. With regard to metabolic regulation of blood flow, the sensing of metabolic status must occur at downstream locations (capillaries and venules), after oxygen has been extracted from the blood, but the controllers are the arterioles. Information is transferred upstream along vessel walls by conducted responses, which involve electrical coupling of the cells making up the walls. Theoretical models provide a means to integrate information about these multiple processes and gain a quantitative understanding of blood flow regulation. |
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