ARTERIOVENOUS ANASTOMOSES

Direct connections between small arteries and veins exist in many parts of the body where they are used to short-circuit the capillary bed (Figure 7-30). One purpose is to shunt blood away from tissues of intermit-

Figure 7-30 A precapillary arteriovenous anastomosis.

Artery; 2, vein; 3, arteriovenous anastomosis; 4, capillary plexus.

tent activity when they are resting; good examples are supplied by the thyroid gland and the gastric mucosa. Arteriovenous anastomoses are also concerned with temperature regulation. To this end, they are plentiful in the exposed appendages of the body: the digits, exter­nal ears, and nose. Paradoxically, they appear to be used in two ways. They open in a cold environment to prevent local overchilling of the appendages; they also open when the animal is overheated, thus promoting heat loss by increasing the throughput of blood close to the body’s surface. A special example of the last use is pro­vided by the panting dog; the circulation of blood through the many arteriovenous anastomoses within the tongue promotes the evaporation of saliva from the surface, which compensates to some degree for the restricted distribution of sweat glands in canine skin.

The use of radioactive-labeled microspheres has made it possible to estimate the amount of blood that can circumvent the capillary bed. In the pig, up to 30% of the total cardiac output sometimes passes through arteriovenous anastomoses.

The structure of these interconnecting channels is not uniform. Some are distinguished by having very muscular walls, others by the muscle cells taking on a peculiar epithelioid character; these epithelioid cells are believed to swell on response to specific chemical stimuli, thereby closing the channel.