Microcirculation

· “the flow of blood throughout the system of smaller vessels of the body, those with a diameter of 100 mm or less”

• The microcirculation is that part of the circulatory system concerned with the exchange of gases, fluids, nutrients and metabolic waste products

• In different tissues, the structure of the microcirculation varies to meet specific functional requirements

Arterial component of microcirculation:

• Blood enters the capillary bed by a small (terminal) arteriole

• Blood from arteriole enters into a series of metarterioles [structure midway between arteriole & capillary] before entering capillaries

• Some arterioles directly enter capillaries

• The arterioles, and precapillary sphincters (muscular sphincters at the arteriolar-capillary junctions) control blood flow within capillary networks

Capillaries

• Exchange occurs mainly within the capillaries

• Average length: 1 mm; average lumen diameter 8-10 mm

• Composed of only of a single layer of endothelial cells

· some of the capillaries are large and course almost directly to to the venule: throughfare channels

· most capillaries ‘true capillaries’ branch mainly from the metarterioles and then finally terminate in a venule

Venous component of the microcirculation

• Blood leaves the capillary bed by a small venule

• The capillary networks drain into a series of vessels of increasing diameter, post-capillary venules, collecting venules and small muscular venules.

*Structure*	*Muscle content*	*Constrictor control*
Arteriole	strong muscular coat	sympathetic ns
Metarteriole	sparse muscular coat	local control
Precapillary sphincter	single spiraling smooth muscle fibre	local control
Venule	sparse muscular coat	sympathetic ns