This study investigated mechanisms used by horses and steers to increase O2 uptake and delivery (V̇O2) from resting to maximal rates and identified the mechanisms that enable horses to achieve higher maximal rates of O2 consumption (V̇O(2max)) than steers. V̇O2 and circulatory variables were measured while Standardbred trotting horses and steers (450-kg body mass) stood quietly and ran on a treadmill at speeds up to those eliciting V̇O(2max). As V̇O2 increased in both species, heart rate and circulating hemoglobin (Hb) concentration increased, thereby increasing O2 delivery by the circulation, while cardiac stroke volume remained unchanged. At V̇O(2max) arterial PCO2 increased from its resting value in horses but was unchanged in steers, and arterial PO2 decreased in both species. Although the horses hypoventilated and were hypoxemic at V̇O(2max), no significant decrease in arterial Hb saturation occurred. V̇O(2max) of the horses was 2.6 times higher than that of the steers and was associated with a 100% larger cardiac output, 100% larger stroke volume, and 40% higher Hb concentration, whereas heart rates at V̇O(2max) were identical in the two species. The higher cardiac output of the horses at V̇O(2max) resulted from a 1.2-fold higher mean arterial pressure and 1.6-fold lower peripheral tissue resistance (associated with a larger skeletal muscle capillary bed). Both the magnitude of the difference in V̇O(2max) between horses and steers and the mechanisms used to achieve it are the same as observed in smaller pairs of mammalian species with large variation in aerobic capacity.
|Original language||English (US)|
|Number of pages||9|
|Journal||Journal of Applied Physiology|
|State||Published - 1989|
ASJC Scopus subject areas
- Orthopedics and Sports Medicine
- Physical Therapy, Sports Therapy and Rehabilitation