![]() Blood flow can be affected by changes in viscosity thus, too many red blood cells and not enough plasma can cause a substantial change in viscosity, as well as other factors related to resistance to flow. However, while RCV and PV are usually looked at independently, they are interdependent in the optimization of blood flow during exercise. Much focus has been placed on the need to have red blood cells to carry oxygen to the working muscles. Oxygen uptake and delivery is dependent on both optimal volume to insure cardiac filling pressure and an optimal number of red blood cells to carry oxygen. The eloquent work of Persson 15 and many others 2, 16– 18 has shown that there is a strong relationship between red blood cell volume and aerobic performance in the horse. 1, 2, 11 In the average 450 kg horse, total BV would be about 36 L, PV ~20 L and RCV around 16 L. 2, 15 Across breeds, studies have reported total blood volumes ranging from 61 mL/kg in draft horses to 137 mL/kg in racehorses. Blood volume varies from breed to breed, with age, body composition, hydration status, and training status. Thus, the total blood volume (BV) is the combination of the plasma volume (PV) and the red cell volume (RCV) or stated as a formula: 2 BV = PV + RCV. The vascular space or total blood volume is filled with a mixture of fluid and cells, the latter primarily red blood cells, but also including white blood cells and platelets. 2įig 38.1 Estimated body fluids compartments in a 450 kg horse. This last category includes the fluid content of the gastrointestinal tract, which represents a large reservoir of fluid. According to Carlson, 2 the latter is further compartmentalized into fluid contained within the vascular space, the interstitial fluid space, the lymphatics, and transcellular fluids. 1, 2, 8, 11 Approximately two-thirds of the TBW (~200 L) is contained within the cells of the body, leaving one-third of the water in the ECF space (~100 L) ( Fig. The TBW is divided by cell membranes into two primary fluid compartments, the intracellular fluid compartment (ICF) and the extracellular fluid compartment (ECF). 1, 2, 11 Each of these techniques has advantages and disadvantages, with the use of stable isotope infusion being one of the most accurate but technically demanding and bioelectric impedance technology the least reliable. 1, 2, 11 TBW can be measured using various indicator dilution techniques, stable isotope techniques, and bioelectric impedance technologies. The TBW accounts for 50–70% of bodyweight, or 250–350 kg of the bodyweight of a typical 500 kg horse. ![]() This combination of intracellular and extracellular water is referred to as the total body water (TBW). Those solutions are compartmentalized within and outside the cells. Like all animals, the body of the horse is primarily composed of water and electrolytes. The present chapter reviews the current literature on the effects of exercise and training adaptations on fluid balance and renal function in horses. 1– 8, 11– 14 Data demonstrate that optimal fluid and electrolyte balance delays the onset of fatigue. 1– 3, 5– 7 Work focused on preventing thermal injuries in horses has documented findings similar to those published in the human sports medicine literature: namely, that strategies aimed at maintenance of fluid and electrolyte balance prevent dehydration and provide thermoregulatory and cardiovascular stability. These fluid deficits and their effects are even more pronounced during endurance exercise and exercise performed during periods of high environmental temperature and humidity. 1, 5, 7, 8 Sweating can produce tremendous fluid and electrolyte losses that, if uncompensated for, can lead to cardiovascular and thermoregulatory instability. 9, 10 Horses and humans are the only species that cool primarily through the evaporation of sweat. 7, 8 Similarly, horses have a singular anatomical feature involved in brain cooling their internal carotid arteries are in close contact with the air-filled guttural pouches and it is suggested that blood reaching the brain has a lower temperature compared to mixed venous blood given this unique arrangement (see Chapter 41). 1– 6 Other mammalian species, like the rabbit and dog, have an enhanced countercurrent brain blood-flow mechanism that couples with panting to provide for cooling of the brain while permissively allowing heat storage in the rest of the body and a rise in body temperature. To do this a horse must move heat produced in the muscles to the periphery. ![]() 1– 6 Normal cellular function, however, requires active and efficient ways to keep core body temperature within narrow limits. This byproduct of the transduction of potential energy into kinetic energy can raise core body temperature in the horse from 37☌ at rest to temperatures exceeding 42☌ in a matter of minutes. The exercising horse produces a tremendous amount of metabolic heat.
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