The cardiovascular system is the plumbing of the body that distributes food, oxygen, communication molecules (hormones), repair and maintenance molecules (enzymes) and defense cells (first responders and police investigators). It is also responsible for taking out the trash and delivering it to the body’s filters (liver and kidneys) and returning messages from the body’s organs to the brain or other organs. The constantly monitored system maintains the pressure, increasing it when the body needs more materials (exercise, illness), measuring the density (hydration) and the temperature both locally (heat in a limb) or systemically (fever) and is adjusted.
The heart forces blood through the arteries towards the organs and the veins back to the heart. The materials it carries transfer to surrounding tissue in very thin vessels called capillaries. These are a bed of finely meshed vessels in any tissue.
Fortunately for most horses, this system works their whole lives flawlessly, but a problem in one system can lead to trouble for some horses. These include swelling when the blood can’t return properly or death when the blood no longer provides fresh gasses to the individual cells. Causes include blocked vessels (bruising, thrombi), breakage of vessels (trauma, aneurysm), or a pump that doesn’t effectively pump the blood (defect or damage). Bruising with fluid leakage doesn’t look the same in horses as in humans because of the darker skin color of horses, but the swelling from trauma is the same.