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A significant function of the cardiovascular system is to deliver oxygen and nutrients to tissues via arteries and their branches. Subsequently, the venous system receives carbon dioxide and other waste products expelled by the tissues. A vital transition point exists between the arterioles and the venules known as the capillary, which is ultimately where the exchange occurs. In short, capillaries are thin-walled vessels that allow for the transportation of nutrients and metabolites from the vasculature and into the interstitium to be taken up by cells. Capillaries are ubiquitously organized throughout the human body and exert their function in every tissue. This article strives to illustrate the anatomical characteristics of the capillary and its function in meeting the metabolic demands of tissues and demonstrate features that are evident upon histological staining.[1]

The capillary unit possesses a vital role in allowing the exchange of metabolites between the vascular system and tissues. Capillaries have a unique morphology, consisting of only a single layer of endothelium, enabling them to carry out their function in delivering needed nutrients to organs and tissues. Furthermore, despite the thin diameter of capillaries, they are the most numerous of the vessel types and thus possess the highest total surface area enhancing overall exchange.[9] This fact gives an excellent example of the common theme in biology that structure meets function. However, there can also be associated with dysfunction, as capillaries can be involved in many different disease processes. For instance, during septic shock, the immune system responds to bacterial toxins by releasing cytokines and inflammatory mediators such as histamine and nitric oxide. This reaction ultimately leads to vasodilatation and increased capillary permeability, causing leakage of large proteins and fluid into the tissue interstitium. Patients may develop hypotension refractory to fluid resuscitation due to the profound decrease in peripheral vascular resistance and tissue third-spacing caused by the inflammation.[13] All in all, the capillary unit is an imperative structure for maintaining the viability of living tissues. Furthermore, the assistance of histological technologies has aided in a deeper understanding of the anatomy and function of capillaries.