- Capillaries are extremely narrow segments of the vasculature which are specialized for exchange of gases, nutrients, and wastes between the blood and tissues. Because of their specialization for exchange of molecules, capillaries possess only the tunica intima common to all vessels and no Tunica Media or Adventitia (See: Cardiovascular Histology). Depending on their location and function, capillary beds may possess some specialized structural features that enhance exchange of substances between the blood and tissues.
- Tunica Intima:
- Most capillaries possess a continuous endothelium composed of a layer of flattened endothelial cells that is invested by a continuous, thin basement membrane. Cells known as pericytes are scattered throughout the capillary endothelium and lie outside of the endothelial basement membrane, hugging the capillaries.
- Fenestrated Endothelium
- Certain specialized capillaries possess holes or "fenestrae" in their endothelial cells and are thus termed fenestrated endothelium. It is important to note that the endothelial basement membrane is still continuous across the fenestrations and thus continues to provide a barrier to diffusion of cells and plasma proteins. Fenestrations appear to simply enhance the diffusion rate of molecules which are normally permeable in continuous capillaries. The prototypical fenestrated endothelia are the glomerular capillaries.
- Discontinuous Endothelium
- Certain specialized capillaries are discontinuous, possessing large sections where cells and plasma proteins may diffuse into the surrounding tissue unimpeded. The only location where discontinuous endothelia exist are the sinusoidal capillaries of the hepatic parenchyma.