As a horse turns the first barrel, jumps over the first fence, or bursts out of the starting gate, the body changes to meet increased exercise demands. Red blood cells are an important element in intense exercise, and they contribute to the horse’s natural athleticism. Understanding the structure and function of these cells provides insight into nutritional strategies for a healthy horse, beginning at the cellular level.
Red blood cells in horses are a mere 6-7 µm in diameter, yet they play a vital role in the bloodstream. Red blood cells, also called erythrocytes, are disk-shaped cells that are thicker around the outside and thinner toward the middle. The cells house a protein called hemoglobin that binds and carries oxygen. Hemoglobin is made of four heme groups, each of which is bound to a molecule of iron. The iron attaches to oxygen and carries it from the lungs to various tissues for metabolic processes. Erythrocytes do not have nuclei and require little oxygen for their own metabolism.
Red blood cell membranes are comprised of proteins and lipids. A lipid bilayer lies on a network of cytoskeleton proteins that provide the frame for the disk shape. Another set of proteins sit in the lipid bilayer and anchor it to the cytoskeleton. Cell membranes must be strong, yet flexible enough to fit through small capillaries only 2-4 µm in diameter. These capillaries are smaller than the actual red blood cells. This mutable phenomenon becomes more impressive when exercise intensity increases.
During exercise, the body undergoes changes to accommodate increased oxygen demands. Sympathetic nerve input increases heartbeat and stroke volume in an effort to quicken blood flow. Deoxygenated blood travels from tissues in the body toward the heart, where it passes through the pulmonary artery on its way to the lungs. In the lungs, the blood travels through small capillaries, which are arranged as a network around small sacs, called alveoli, which are full of air. Here, the capillary-alveoli network facilitates gas exchange, where oxygen from the lungs diffuses into the blood, and carbon dioxide in the blood diffuses into the lung for exhalation. The oxygenated blood moves back toward the heart, exits through the aorta, and travels to the rest of the body to deliver oxygen.
Horses have the unique ability to naturally “blood dope.” They store up to one-third of their red blood cells in their spleen and release them upon intense exercise. The added red blood cells increase the amount of oxygen in the bloodstream, contributing to the horse’s innate athleticism. The viscosity of the blood increases from the additional red blood cells, and the pressure rises inside the pulmonary capillaries. Therefore, when blood passes through small capillaries, it is crucial the red blood cells remain flexible. As they pass through the capillaries, the cytoskeleton framework and lipid membrane change the shape of the cell. This reduces the amount of friction on the capillary wall. Failure to remain fluid can result in blockages, a buildup of pressure, and bursting capillaries. This becomes important in the lung capillary network, where horses can develop exercise-induced pulmonary hemorrhaging (EIPH), also known as bleeding. EIPH is seen in racing, polo, eventing, and other sports that require strenuous exercise.
Source: Equinews
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