ABSTRACT
The microvascular bed includes the terminal arterioles, the precapillary sphincters, the
capillaries and the small venules. The microvascular bed also includes the initial lymphatics,
the lymphatic capillaries and artero-venous anastomoses (AVA). The wall of the arterioles
contains smooth muscle cells and these are the major site of systemic vascular resistance.
The rhythmical contractile activity of the precapillary sphincter causes intermittent flow in
the capillaries (vasomotion) and allows a large number of capillaries to remain open or
closed for long periods. The walls of the capillaries are made up of flat endothelial cells lying
on a basal lamina (BL) and oriented with the vessel’s long axis. In some organs, capillaries
have pericapillary cells (pericytes) imbedded within the BL, which play a contractile role.
Endothelial cells secrete vasoconstrictor, vasodilator, mitotic agents, and their own BL.
