A precapillary or postcapillary communication between the neighboring vessels is called anastomosis. Circulation of blood through the anastomosis is called collateral circulation.
Types of anastomoses:
Anastomoses are of three types:
Arterial anastomosis is the communication between the arteries, or branches of arteries. It may be actual or potential.
- Actual Arterial Anastomosis: In actual arterial anastomosis the arteries meet end to end. For example, palmar arches, plantar arch, circle of Willis, intestinal arcades, labial branches of facial arteries.
- Potential Arterial Anastomosis: In potential arterial anastomosis the communication takes place between the terminal arterioles. Such communications can dilate only gradually for collateral circulation. Therefore on sudden occlusion of a main artery, the anastomosis may fail to compensate the loss. The examples are seen in the coronary arteries and the cortical branches of cerebral arteries.
Venous anastomosis is the communication between the veins or tributaries of veins. For example, the dorsal venous arches of the hand and foot.
Arteriovenous Anastomosis (Shunt):
Arteriovenous anastomosis (shunt) is the communication between an artery and a vein. It serves the function of phasic activity of the organ. When the organ is active these shunts are closed and the blood circulates through the capillaries. However, when the organ is at rest, the blood bypasses the capillary bed and is shunted back through the arteriovenous anastomosis. The shunt vessel may be straight or coiled, possesses a thick muscular coat, and is under the influence of sympathetic nervous system.
Shunts of simple structure are found in the skin of nose, lips and external ear. They are also common in the mucous membrane of nose and alimentary canal and the erectile tissue of sexual organs.
Preferential ‘thoroughfare channels’ are also a kind of shunts. They course through the capillary network. Many true capillaries arise as their side branches. One thoroughfare channel with its associated capillaries forms a microcirculatory unit. The size of the unit is variable from 1-2 to 20-30 true capillaries. The number of active units varies from time to time.