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Microvascular Plasticity Jens C.B. Jacobsen, Niels E. Olesen, and Niels-Henrik Holstein-Rathlou

A basic aspect of evolutionary fitness is the ability to adjust to changing requirements. Part of this ability manifests in the processes of tissue remodeling. Endurance training for instance, associated with a sustained high metabolic activity in skeletal muscle tissue may result in increased capillary density.

11.1 Matching Transport Capacity to Tissue Needs Transport by diffusion is an efficient process on very short distances only. Any organism above a certain (small) size therefore requires some kind of circulatory system that can deliver nutrients to the tissues and can clear out waste products. The circulatory system has a diversity of designs in the animal kingdom. In most cases however, an exchange medium, i.e. the blood, is physically circulated and brought into contact with every part of every tissue in the body in order for the basic process of exchange between blood and tissue to proceed efficiently. This exchange process primarily takes place in the smallest vessels of the microcirculation, the capillaries. The density of these vessels in a given tissue reflects its basal metabolic activity such that high densities and, hence, potentially high exchange rates, are found in tissues with high metabolic activity, e.g. the kidneys and the heart. At the other extreme are avascular tissues such as the cartilage of the joints. Such tissues seem to rely on diffusion over long distances and they are characterized by very low metabolic activity. In a given tissue, capillarity may also change with long-term changes in

J.C.B. Jacobsen ()  N.E. Olesen  N.-H. Holstein-Rathlou Panum Institute, University of Copenhagen, Blegdamsvej 3B, DK-2200 Copenhagen, Denmark e-mail: [email protected]; [email protected]; [email protected] E. Mosekilde et al. (eds.), Biosimulation in Biomedical Research, Health Care and Drug Development, DOI 10.1007/978-3-7091-0418-7 11, © Springer-Verlag/Wien 2012

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average metabolic activity as it has been observed in endurance trained skeletal muscles [8]. The arterial high pressure side of the circulation consists of vessels ranging from large, elastic arteries such as the aorta (1.5–2 cm in diameter) and down to small arterioles with diameters of less than 10 m. The main part of the peripheral resistance R is harbored in the small muscular arteries (some 500 m in diameter) and down to the smallest arterioles. Consequently these vessels also play a main role in determining the arterial blood pressure P which, as for an electrical resistor is given by P D RQ, where Q is the flow. The arterial blood pressure per se is interesting from a patho-physiological point of view since its level is well correlated with the risk of developing cardiovascular disease such as stroke, myocardial infarction and gangrene (tissue death due to insufficient blood supply). A central aspect of evolutionary fitness is the ability to survive in a constantly changing environment. At t