a-short-review-of-histology

Capillaries:

The capillaries are the smallest of the blood vessels in the body
They have only a tunica intima - that is an epithelium resting on a basement membrane
Generally the epithelium is a simple squamous type, as lines the lumina of most other parts of the cardiovascular system
These are the blood vessels for gaseous exchange between the blood and the tissue of the body
The exchange of proteins and other molecules as well as blood cells may also occur across capillary walls

Three types of capillary are found:

This is the most common type
It is found where there is a need to restrict the passage of molecules, other then oxygen and carbon dioxide, across the capillary wall
Cell junctions bind the adjacent epithelial cells
Proteins and other molecules may cross this type of capillary, either through or between the lining epithelial cells
However, as the lining of these capillaries is "continuous" the passage of molecules across the epithelium is controlled by the epithelial cells

This type of capillary has fenestrations (or pores) in the epithelial cells lining the lumen
These fenestrations will allow large molecules, including proteins to cross
This type of capillary is common in endocrine glands

This type of capillary has gaps between the cells which form the capillary wall
The basement membrane is also incomplete
In this type of capillary, unrestricted passage of fluid and proteins may occur across the capillary wall; blood cells may also cross the capillary wall
This type of capillary is found in the bone marrow and spleen

Micrograph of Continuous Capillary:

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