The cardiovascular system pumps blood around the body. During exercise, this allows the body to transport oxygen & energy substrates, such as glucose & fatty acids, to the muscle fibres and remove waste products, like lactate, water, and carbon dioxide. The blood pumped around the body by the cardiovascular system also plays an important role in thermoregulation.
The heart is the pump which circulates blood around the body. It is situated within the thoracic cavity, in front of the horse’s elbow. It weighs approximately 4kg. A larger, more muscular heart is more desirable in a sports horse, as it allows a greater volume of blood to be pumped around the body with each beat. Heart size is one comparison that can be made between horses of similar ages and types.
The heart is divided into left and right, each of which is subdivided into an atrium (atria – plural) and a ventricle, that are connected by the atrioventricular openings. The atria collect blood being returned to the heart by large veins, and the ventricles are where blood collects ready to be sent back around the body. The left ventricle pumps blood around the body, whereas the right ventricle pumps blood around the pulmonary system, so oxygen can diffuse from the air into the blood, and carbon dioxide can diffuse into air to be breathed out. Contraction of the heart is called systole, and relaxation is called diastole. During systole, the atria contracts first, which is followed, almost immediately, by the ventricles.
The venae cavae returns ‘used’ blood to the right atrium, ready to be returned to the pulmonary system and lungs. During diastole the right atrioventricular valve opens, allowing blood to flow into the right ventricle. As systole begins, the contraction of the right atrium forces any remaining blood into the ventricle below, and then the atrioventricular valve closes again, before ventricular contraction starts. Ventricular contraction then pumps the blood through the pulmonary circuit, where oxygen and CO2 diffuse between the blood in the capillaries and the air in the alveoli.
The pulmonary veins return blood from the lungs to the left atrium, when the atrioventricular valve opens, allowing blood to flow down into the left ventricle. Systole then forces the remaining contents of the left atrium into the left ventricle, before the left atrioventricular valve closes again. Then the left ventricle will contract, pumping blood through the aorta into smaller arteries and arterioles. Then the blood is collected by venules and veins to be returned to the right atrium via the vena cava.
The large arteries, like the aorta, have elastic walls that expand during systole to accommodate the extra blood flow, and recoil during diastole. This helps to maintain blood pressure. The expansion and recoil of arteries is actually the pulse. The smaller arteries and arterioles have muscular walls which allow them to regulate blood flow through vasoconstriction and vasodilation. During exercise, arterioles taking blood to the skeletal muscles dilate, allowing for greater blood flow, while the arterioles to the intestinal tract and some other organs constrict.
The capillary bed is a network of tiny blood vessels with permeable walls, which allow gases, nutrients and waste products to diffuse through them. The pressure in the capillaries is quite low, as the surface area is so large. Capillaries are so small that red blood cells have to travel down them in single file, and may even fold up to get through. If the concentration of blood cells, known as packed cell volume, becomes too high it can result in ‘sludging’ in the capillaries. Conditioning can increase the capillarization of the muscles by up to 50%, which is much more efficient for oxygen delivery.
In the legs, the venous blood is flowing against gravity so the larger veins have valves to prevent the blood from flowing back down again. Muscular contractions during movement can help to push the blood back up to the heart, whereas periods of inactivity, e.g. longer periods of stabling, can result in blood pooling in the extremities. This can cause an accumulation of tissue fluid causing edema (filling) of the lower limbs.