1) Size of Lumen
The smaller the lumen, the greater the resistance. Vasoconstriction reduces the diameter of the lumen and vasodilation increases it. When resistance decreases, blood pressure drops and as arterioles constrict, resistance increases, and blood pressure rises.
2) Blood Viscosity
Viscosity is the “thickness” of blood and depends mostly on the ratio of RBCs to plasma volume and to the concentration of proteins in the plasma. Higher viscosity results in higher resistance. Conditions such as dehydration or polycythemia increase blood pressure. Depletion of plasma proteins or RBCs due to anemia or hemorrhage decreases viscosity and decreases blood pressure.
3) Total Blood Vessel Length
Resistance is directly proportional to the length of the blood vessel. The longer the vessel, the greater the resistance. Often obese horses have higher blood pressure because of the additional blood vessels in their adipose tissue.
Venous Return
This is the volume of blood flowing back to the heart through systemic veins and occurs due to the pressure generated by contractions of the heart’s left ventricle. In horses, the pressure pushing blood up the veins in the lower limbs is barely enough to overcome the gravitation pressure pushing it back down. Besides the heart, there are 2 other mechanisms that pump blood from the limbs back to the heart.
1) Skeletal Muscle Pump
At rest, both the proximal and distal valves in the veins are open and blood flows toward the heart. Contraction of leg muscles pushes blood through the proximal valve while closing the distal valve. As leg muscles relax, the proximal valve closes, and the distal valve opens. When the vein fills with blood from the foot, the proximal valve reopens. It is believed that the frog in the hoof also helps to pump the blood back up the horses’ lower limb due to the lack of muscle below the carpus.
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