The effect of heat stress on capillary blood flow (CBF) distribution was examined in laying hens, using 15 micron microspheres, by determining CBF before and after elevating body temperature by 1-2 degrees C. No changes was evident in unfeathered metatarsal skin, although its temperature increased by 7 degrees C. Breast skin CBF change was 3.5 times larger than that of back skin. Comb CBF increase was larger than in wattles. CBF in upper respiratory tract increased proportionally to increment in respiratory frequency. Digestive system CBF was reduced by hyperthermia: the effect was pronounced in its upper organs (46% of normal) and decreased along the tract. CBF increased 4-fold in an expiratory abdominal muscle, a smaller rise occurred in a pectoral muscle and no change in a leg muscle. CBF in the tibia fell to 64% of normal. In the reproductive system, CBF fell to 58% of control level in the uterus, to 70-80% in the larger ovarian follicles and infundibulum with no significant changes in magnum and isthmus. Cerebral CBF increased during hyperthermia. Heat stress significantly reduced CBF to inner body organs, with marked differences between systems as well as within systems. Changes were more pronounced on 2 degrees C hyperthermia than on 1 degree C hyperthermia.

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