The Cardiovascular System
The function of the cardiovascular system is to transport water, oxygen, carbon dioxide, fuels for energy production, electrolytes, hormones and metabolic products to and from the cells which make up the body. The cardiovascular system comprises a muscular pump - the heart - and a system of blood vessels.
The heart is a highly specialised muscle. Its role is to pump enough blood to maintain blood pressure and the flow of oxygen to the tissues. Mammals have a 'double circulation' since the blood passes through the heart twice on every circuit of the body. The heart has four chambers, with the right and left sides separated by a muscular wall. Blood from one side of the heart cannot mix with blood on the other side.
How the Blood Circulates
The circulation of the blood relies on a sequence of events that occur every time the heart contracts and relaxes. This cycle depends on precise electrical impulses to the heart. Valves located between the chambers of the heart and the major vessels permit blood to flow only in one direction. When the heart contracts, blood is pumped from the left ventricle into the aorta. The left ventricle of the heart is the largest and most muscular chamber, as it has to propel blood right around the body.
From the aorta blood heads out through the arteries in a continuous flow. Once in the arteries the blood is forced out to the tissues by the compression of the elastic arterial walls. Blood continues through the arteries down progressively smaller tubes called arterioles, until it reaches capillary beds throughout the body. Here products can enter and leave the vessels. Oxygen and other nutrients leave the blood, in exchange for water and waste products like carbon dioxide and lactate.
Blood then moves back towards the heart via veins, the diameter of which progressively increases. The largest vein is the vena cava, which delivers blood to the right atrium, from where it is directed into the right ventricle. Blood is then pumped from the right ventricle into the capillary network of the lungs. Here the carbon dioxide is removed and oxygen is picked up. It then proceeds along the pulmonary vein to the left atrium to start the process again.
Responses to Exercise
The resting heart rate of dogs varies considerably, ranging from 60 to 160 beats per minute. Exercise, fever, heat and inflammation will cause the heart rate to increase, reflecting the body's increased demands for nutrients, oxygen and other essential substances. The cardiovascular system is quick to respond to the demands of exercise, increasing the delivery of oxygen to the muscles.
When a dog starts to exercise, the heart rate increases rapidly before reaching a steady state in 2 - 3 minutes. Blood flow is redirected to the working muscles where it is needed to supply oxygen and nutrients and remove waste products. During exercise, blood flow to other essential organs such as the brain, heart, lungs and adrenal glands also increases, while less blood flows to the kidneys, digestive tract and spleen.
Effects of Physical Conditioning
The ability of a dog to continue to perform exercise at a particular level of intensity is limited by the onset of fatigue. Increased cardiovascular fitness enhances a dog's the ability to deliver oxygen to working muscles.
Long-term physical conditioning also increases the number of capillaries found in skeletal muscle and results in better access to oxygen and nutrients and faster removal of waste products such as carbon dioxide and lactate. These responses mean that a fit dog can exercise for longer while using the aerobic pathway for its energy, while the unfit dog switches to anaerobic sources earlier and thus fatigues earlier.