Abstract
From a certain level of exercise-intensity onward, hematocrit increases in horses, which brings more oxygen carriers into the bloodstream. Camels, however, when used in competitive racing could be even in need of iron supplementation and blood transfusions due to a severe reduction of their available hematocrit compared to their resting hematocrit. Since the extrinsic and intrinsic mechanical properties of camel erythrocytes (RBC) are so different compared to RBCs of other mammals, the question arises whether this observation might be a response to endurance exercise aiming at keeping the RBC count low. Rheometry indicated dromedary camel blood to behave almost Newtonian, which is unique amongst mammals. Shear thinning did increase with the hematocrit, but remained marginal compared to horses. As a result, camel whole blood viscosity (WBV) exceeded horse WBV at high shear rates, an effect, which was significantly augmented when the packed cell volume (PCV) was increased. Therefore, in camels any infusion of RBCs into the bloodstream can increase the cardiac work and the energy input into the endothelium more effectively, which should generate vascular remodeling in the long term. Yielding, however, was completely absent in camel blood, confirming low cohesion between its components at quasi-static flow. Camel blood remained a viscous liquid without a threshold even at unphysiologically high PCVs. This can help to washout lactate when camels start to dehydrate and might contribute to the sustained working ability of these animals. The subtle pseudoplastic behavior and the high viscosity contrast across the RBC membrane point to weak coupling between blood flow and red cell behavior. We predict that RBCs flow as separate entities and can show various types of motion, which can lead to friction instead of being collectively aligned to the flow direction. In comparison to horses, this behavior will become relevant at higher RBC counts in front of flow obstacles and possibly cause vascular remodeling if the PCV rises during strenuous exercise, a matter that should be avoided.
Highlights
Camels and horses have been used for comparable purposes
In the red blood cell (RBC) concentrates, shear thickening was present in camel blood samples, but not in horse blood samples
Keeping in mind that the blood samples contacted rigid steel surfaces in our standard rheometrical test setup, which modifies the effects of the physiological containment on the circulating blood volume like RBC migration and deformation (Kumar et al, 2014), a species-specific shear thinning was always present in other mammalian blood samples that have been tested so far (Windberger, 2019)
Summary
Camels and horses have been used for comparable purposes. They transport loads, their dairy products and meat are food sources in several countries (Abrhaley and Leta, 2018; Ugwu et al, 2019), and they are used for riding and competitive racing. Dromedary camels bred to gain maximum racing performance receive optimal support by their food and training, and can even perform like horses. When horses exercise, their hematocrit increases, which identifies them as “natural blood dopers” (Boening et al, 2011). Since the hematocrit has a major impact on blood viscosity, blood fluidity changes in horses during a race, but does not in camels. This may be explained by the mechanical RBC properties of horses and camels, which could not be more different
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