Blood flow through arteriovenous anastomoses and its thermal function in the laying hen.

1. Total (arteriovenous anastomoses + capillary) blood flow (t.b.f.) and capillary blood flow (c.b.f.) distribution were determined by 52 and 15 micron microspheres respectively, in body organs of six unanaesthetized laying hens, exposed to thermoneutral air temperatures. 2. Statistically significant differences between t.b.f. and c.b.f. were only found in the unfeathered metatarsal skin, comb and wattles. The c.b.f. constituted 22, 42 and 45% of t.b.f. respectively, indicating significant arteriovenous anastomoses (a.v.a.) blood flow. Differences between t.b.f. and c.b.f. were small in the two larger ovarian follicles and digestive tract and were negligible in reproductive tract, liver and kidney. 3. Microspheres of different size have been injected into the external ischiadic artery in order to examine the vasomotor responses of leg tissues in twenty-one hens kept in thermoneutral, cold or hot conditions. 4. In cold, thermoneutral and hot conditions 1, 16 and 35% of whole leg blood flow respectively passed through a.v.a. Only c.b.f. was present in bone and muscle in either treatment; cold and heat reduced it in bones and cold increased it in muscles. Arteriovenous anastomoses blood flow appeared mostly in unfeathered skin; it constituted 17, 53 and 83% of t.b.f. to this tissue in cold, thermoneutral and hot conditions, while t.b.f. increased from 1.4 to 14% and 30% of whole leg blood flow respectively. Blood flow to feathered skin decreased during heat and cold.