Capillary, cut open, with empty lumen. The capillaries are the finest ramifications of arteries and veins, so they connect arterial and venous vascular system. Their diameter is ca. 6 µm, thus they're a little smaller than the erythrocytes, which have to deform during their passage. Their are two forms of capillaries: Continual capillaries (muscular type): Their endothelial lining is unintermitted and they are fully surrounded by a basal membrane. Fenestrated capillaries: They have thin pores in their endothelial cells, where the vascular lumen is only separated from the interstitium by the thin basal membrane. They are mainly found in the gastrointestinal tract and endocrine glands. The capillaries are where the exchange of oxygen, nutrients, and metabolic products between tissues and the blood circulation takes place.