These are delicate tubes whose average diameter is about 7 to 9 microns. Capillaries branch extensively without change in caliber and these branches anastomose to form extensive networks whose meshes vary in size and shape in different tissues and organs. The greater the metabolic activity of the tissue, the denser is the capillary network.
The wall of a capillary consists of a single layer of flattened endothelial cells (simple squamous epithelium) separated by narrow intercellular spaces, which are filled with a little intercellular cement. On surface view the cells appear as a delicate mosaic, which can be demonstrated by precipitation of silver in the intercellular clefts. The cell borders are usually serrated or wavy.
Cells are arranged with their long axes parallel with the long axis of the tube. In capillaries of small caliber the cells are very narrow and their ends pointed. In wider capillaries cell are oval with blunt ends. Cytoplasm is clear or finely granular and nucleus is central and elongated. Cells are thicker in the region of the nucleus.
In fixed preparation, there is pronounced bulging of nuclei into the lumen, due to shrinking of cytoplasm. Two endothelial cells and occasionally even one sufficient to form the circumference of a small capillary. For the larger ones, 3 to 5 cells are required.
That part of a capillary, which is nearest to the arteriole, is called an arterial capillary and that part nearest the venule is called a venous capillary but these designations are based on topography and function.
The sinusoids or sinusoidal capillaries of liver and other organs are larger, (25-30 microns). Lining cells have processes, which extend into the lumen of the sinusoids. The connective tissue which usually surrounds capillaries is so greatly reduced in amount in sinusoids that the lining cells of the latter are closely applied to the parenchyma of the organs, being separated form the latter by a fine network or reticular fibres. Lining cells of sinusoids, consists of phagocytic and non-phagocytic cells the former belonging to the reticulo-endothelial system.
The capillary walls furnish the membrane for diffusion, filtration and osmosis of fluids to and from the blood stream. The passage of fluid through the walls of the capillary is partially dependent on the blood pressure within the capillaries and on the colloid osmotic pressure of the blood. Recent play a significant role in increasing capillary permeability.
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