Scrotal/testicular thermoregulation in the bull

Abstract

Testicular temperature in bulls must be 2 to 6°C below body temperature to produce fertile sperm. Increasing testicular temperature results in defective spermatozoa, with recovery dependent upon the nature and duration of the thermal insult. The testicular vascular cone, consisting of the pampiniform plexus (a complex venous network) surrounding the highly coiled testicular artery, functions as a counter-current heat transfer system (heat is transferred from the artery to the vein). Heat loss from the scrotal surface (especially from the scrotal neck), scrotal sweating, relaxation of scrotal muscles, and complimentary temperature gradients (associated with blood vessels) in the scrotum and testes also reduce testicular temperature. Despite these features, the testes are very susceptible to temperature increases due to endogenous or exogenous factors (e.g. fever, high ambient temperature). The testes usually operate on the brink of hypoxia; increasing testicular temperature increases metabolism and therefore oxygen requirements, but blood flow changes little, and hence the testes become hypoxic. Future studies of scrotal/testicular thermoregulation in bulls should take into account heat loss from the scrotal surface, blood flow and oxygen saturation in testicular blood vessels and intratesticular tissues.