The heart, vascular system and respiratory systems serve to create temperature stable environment for the dog's body.
Cardiovascular System--The heart is little more than a high pressure high volume pump that pumps blood to the lungs for a two way exchange of oxygen and carbon dioxide, through the kidneys for selective filtration, through the intestines and liver where nutrients are picked up and transported throughout the body by a system of arteries, veins and capillaries. Additionally, variable flow rates to the skin aid in regulating heat loss and retention. Aging of the cardiovascular system also results in a progressive loss of organ reserve and adaptability.xi Degenerating heart valves heart muscle serve to reduce cardiac output by about 1/3 in the last third of a dog's life. This is further complicated by the muscles of older dogs having less myoglobin. Myoglobin is a form of hemoglobin found in muscle tissue and which gives muscles its red color of muscle and its ability to store oxygen necessary for muscle contraction. With decreased cardiovascular capacity and lower maximum heart rate, many dogs become exercise intolerant.
Cardiac filling and stroke volume are reduced by loss of elasticity of the heart muscle, though with ventricular hypertrophy (enlargement of the ventricular chamber) may partially offset what would otherwise be a greater loss in function. Inelastic heart valves, common in older dogs allow backflow, further reducing performance. Interestingly, arteriosclerosis (arterial plaque) so common in humans, is rare in dogs. Parasitic infection with heartworm (Dirofilaria immitis) is common in dogs and can cause extensive cardiovascular damage, further exasperated by aging.