Skin microvasculature is organized as two horizontal plexuses, with one located 1 to 1.5 mm below the skin surface and the other at the dermal-subcutaneous junction. The lower plexus is formed by perforating vessels from the underlying muscles and subcutaneous fat. The arterioles and venules in the fat lobules are identical in size and structure to those of the lower horizontal plexus. Descending venules and ascending arterioles connect the two plexuses and are arranged in pairs and supply the hair bulbs and sweat glands. The nutritive capillary loops of the dermal papillae arise from the upper plexus. The arterioles in the papillary dermis are terminal arterioles and function as part of the resistance vessels in the skin. The other vessels of the papillary dermis are capillaries and postcapillary venules. Postcapillary venules present the majority of vessels and are associated with a variety of functions—migration of inflammatory cells from vessels into the tissue and increasing vascular permeability in response to acute inflammation. Postcapillary venules join larger valve-containing vessels that pass from the deep dermis into the superficial layer of the adipose tissue. These valves in collecting veins at dermal-fat interface ensure the forward propulsion of blood . Direct cannula-tion of human finger nailfold capillaries has demonstrated that the blood pressure is pulsatile in both arterioles and venules with systolic pressures fluctuating between 11 and 75 mmHg.
Skin blood flow is regulated by the nervous system and local factors. Several reflex arcs are involved, including central reflexes, short reflexes through the spinal cord, and local reflex arcs within the skin. Sympathetic nervous system tone usually keeps arteriovenous anastomoses in a constricted state. Loss of sympathetic tone results in opening of these shunts and decreases the blood flow through the skin capillaries . Spectral analysis of vasomotor waves measured by laser-Doppler flowmetry in skin demonstrated that the beat-to-beat rate of the cardiac cycle varies around two means: 0.1 and 0.25 Hz. The 0.1-Hz frequency likely represents sympathetic activity, as it becomes accentuated with position change from supine to erect. These postural responses can be reduced significantly by sympathetic blockade of the limb space .
Laser-Doppler flowmetry involves applying a beam of laser light to the skin. Blood cells moving within the area will alter the wavelength of the light (Doppler shift). The scattered light is converted to an electronic signal and analyzed. The greater the number and velocity of moving cells, the higher the Doppler shift. The laser-Doppler technique gives an estimation of skin perfusion, which is expressed as arbitrary units. It does not measure exact skin perfusion in mL/min/100g of tissue. Nevertheless, the laser-Doppler technique is useful in measuring changes in skin blood flow in response to various stimuli and interventions, such as local administration of vasoactive medications by iontophoresis . The laser-Doppler flowmetry technique gathers signals from 1 to 1.5 mm below the epidermis; therefore, it does not measure blood flow to the structures located 3 to 5 mm below the skin surface, such as sweat glands and hair follicles. However, some researchers have used the laser-Doppler flux technique to measure adipose tissue blood flow by introducing the Doppler probe directly into adipose tissue through a needle.
Iontophoresis and microdialysis are techniques allowing drug delivery through the skin and could be used for extraction of substances from the skin and measuring concentrations of these substances. These techniques are described in detail elsewhere . In addition, intradermal electrode techniques can be used to directly measure nitric oxide in skin. Nitric oxide is involved in skin vasodilatation in response to heat and ischemia-reperfusion.
Dynamic capillaroscopy provides the best information about nutritional status of a circumscribed skin area. This technique involves using a microscope, television camera, monitor, and software to analyze the nutritional blood flow. Several observations can be made with this technique, such as number and type of capillaries in the nailfold, intercapil-lary distance, caliber, tortuosity, and capillary blood cell velocity. Transcapillary diffusion of sodium fluorescein through the wall of a single capillary can be measured by this technique to assess capillary permeability.
Skin adipose tissue blood flow can vary greatly if measured in milliliters per unit of tissue. However, blood flow is relatively constant if expressed per adipocyte. Adipose tissue blood flow expressed in milliliters per amount of tissue decreases with increasing adipocyte mass in lipid accumulation .
A useful technique of local catheterization involves insertion of a catheter into a vein draining anterior abdominal wall, as this adipose depot has separate venous outflow from muscle. Adipose tissue blood flow in humans is determined by the clearance of previously injected radiolabeled molecules. The xenon washout technique involves administration of 133Xe into adipose tissue and serial sampling of venous radioactivity for several hours. Adipose tissue blood flow is thought to be equivalent to the washout of xenon into the venous blood.
An additional methodology is a microdialysis technique that allows measurement of blood flow by introducing ethanol into human adipose tissue through a dialysis probe, and then measuring its escape into the dialysate spectrophotometrically.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.