Ultracapillaries

Since the time of Harvey it has been accepted that the capillaries are the smallest vessels in the closed vascular system of the mammal, and on this basis we have our current understanding of tissue nutrition, for it is considered that the tissue fluid is filtered from the blood through the wall of the capillary to ‘bathe and feed’ the tissue cells. The latter concept has many difficulties and has long been suspect since it requires for acceptance a rather simpler belief in osmosis and diffusion than can be held at the present time.

Sten Forshufvud (Arkiv for Zoologi. 1952, Bd. 3(7): 9) has employed the injection of laked blood into arteries to demonstrate that there are vessels smaller than capillaries branching out from the capillaries and extending amongst the tissue cells. The haemoglobin of the laked blood enters these small vessels which are termed ultracapillaries. They are less in diameter than the diameter of erythrocytes so that these are retained in the capillary while the plasma moves into the branching ultracapillaries and so comes to an intimate contact with the tissue cells. Forshufvud demonstrates that the reticulin fibres long recognised among the cells in many tissues and well-represented on the wall of blood-vessels and capillaries are actually the ultracapillaries and illustrates these minute vessels in both ordinary microphotographs and in electronmicroscope photos. The latter show these vessels in dentine where they appear as trunks with buds and lesser vessels down to a micron in diameter branching from them.

If this discovery is sustained, as the evidence indicates it shall be, it will have the greatest significance and will remodel our interpretation of the way in which cells receive their nourishment and are maintained in health.