Temperature Regulation

Cooling of the body is facilitated through radiation, convection, evaporation, and conduction. The specifics of heat loss in the horse have not been well documented, but most normal heat loss is through radiation and convection. When the ambient temperature is high or the horse is enclosed in a poorly ventilated space (such as a trailer) that becomes warm, convection and radiation are less effective means of heat loss, and the main route of heat loss becomes evaporation of sweat. Likewise, during exercise, evaporation of sweat from the skin surface and water from the respiratory tract are significant routes of heat loss. When high ambient temperatures or exercise are combined with high humidity, the process of heat transfer is significantly compromised, because evaporation of sweat and respiratory secretions cannot occur. When the environmental temperature rises and the horse needs to lose heat, peripheral vasodilation causes increased skin blood flow so that more heat is transferred to the extremities. The resulting rise in skin temperature leads to increased sweating, which facilitates heat loss by evaporation.

Increased blood flow to the skin of the head, neck, and limbs warms the overlying air, which increases heat loss by convection. Convective heat loss can occur passively because the warmed air overlying the skin rises and is replaced by cooler ambient air. However, convection is most effective when there is air movement that continually replaces warmed air with cooler air (such as during exercise). A similar increase in evaporative heat loss occurs in the respiratory system during exercise. The cooler inhaled air is warmed and humidified in the nasal cavity, trachea, and larger bronchi and is then exhaled; the rate of heat loss depends on the difference between the temperature of the inspired and expired air. As minute ventilation increases during exercise, respiratory heat loss increases.

Blood flow is an efficient means of transferring heat from the body core to the periphery. By altering blood flow to organs, the circulatory system acts as an important thermoregulatory system. Increased blood flow to the skin transfers heat to where it can be dissipated by conduction and convection. Increased blood flow also provides fluid for sweat production and loss of heat by evaporation. Heat exposure in the horse increases cardiac output and increased blood flow to the skin without a reduction in blood flow to other tissues. During periods of increased heat production, heat is also transferred into the content of the hindgut, where it can be stored for dissipation later. Under most conditions, heat loss from the body matches heat production, resulting in a stable core temperature being maintained. Sweating is initiated at certain core temperatures. In the horse, this mechanism is under β-adrenergic sympathetic control, largely through increased plasma catecholamine concentrations.

Evaporative heat loss is the primary mechanism for the horse to dissipate excessive heat during exercise. Horses are capable of performing exercise at metabolic rates twice those of human athletes, but have a surface area-to-mass ratio approximately half that of humans. During maximal exercise, horses may be required to dissipate approximately four times more heat per unit body surface area than humans. During exercise, heat accumulates quickly, and the core body temperature reaches a plateau as heat dissipation mechanisms balance the heat production. The horse has several mechanisms that enable storage of heat during high-intensity short-duration exercise for dissipation later. Longer duration submaximal exercise can only be tolerated if heat loss during the event proceeds adequately.