Pulse attenuation and countercurrent heat exchange in the internal spermatic artery of some Australian marsupials.

Abstract

The unusual complexity of the testicular blood vessels of eutherian mammals has been shown to have haemodynamic and thermoregulatory significance. The pulse wave of pressure passing down the long, coiled internal spermatic artery of the ram (Waites & Moule, 1960), dog (Blombery, Gow & Waites, unpublished observations) and rat (Setchell, 1969) is attenuated to a small oscillation, but mean blood pressure is only marginally reduced. A similar pressure-damping action was described for the carotid rete mirabile of the dolphin (Nagel, Morgane, McFarland & Galliano, 1968). There is also plentiful evidence for countercurrent heat exchange between neighbouring arteries and veins (see Waites, 1969). As the spermatic veins closely surround the internal spermatic artery, Harrison & Weiner (1949) proposed that heat exchange was occurring between the counterflowing blood streams. This was confirmed by measuring the thermal gradients in the testicular circulation of dogs (Dahl & Herrick, 1959) and rams (Waites & Moule, 1961). The internal spermatic artery of most marsupials divides into many parallel branches which intermingle in the spermatic cord with a similar number of parallel veins (Harrison, 1948, 1949, 1951; Barnett & Brazenor, 1958). One might expect that this different way of increasing vascular surface area, by branching rather than elongation, would have similar haemodynamic and thermal consequences, but experiments to examine this have apparently not been performed. Six adult tammar wallabies (Macropus eugenii Desmarest) weighing between 5-4 and 9 kg, and two adult red kangaroos (Megaleia rufa Desmarest), weighing 36 and 42 kg, were anaesthetized with pentobarbitone sodium. Rectal tempera¬ ture, recorded with a thermocouple, was maintained with an electric blanket at the values recorded immediately after anaesthesia. The testes were exposed and copper constantan thermocouples (45 S.W.G. mounted in the tips of 25 S.W.G. hypodermic needles) were immediately inserted into the testis, and into a testicular vein and a testicular artery on the surface of one testis. Two to four arteries run along the epididymal margin and a similar number of veins along the free margin (Setchell, 1969). It was thus possible to insert the needle into one vessel and push it as far as a junction with another so that blood was flowing past the site of measurement. The arterial thermocouple was within