Blood carries nutrients and oxygen to the tissues of the body, including the muscles. A complex network of blood vessels, called arteries, takes the blood from the heart to the muscles. Then a different set of blood vessels, called veins, takes deoxygenated blood from the muscles back to the heart and gets rid of waste products that build up in the muscles.
The heart is the pump for the body’s circulatory system or network of blood vessels. The human heart has four chambers, or compartments. The top two chambers are called atria, and the bottom two are called ventricles. The left ventricle produces the pressure to push oxygenated blood that entered the heart from the lungs out to the rest of the body. In order for muscles to get blood, you need a strong, healthy heart that can push the blood into the circulation.
Arteries, Arterioles, and Capillaries
The blood vessels in the body look like tubes. As they get farther from the heart and closer to the muscle tissue, they get smaller and smaller in diameter.
The largest artery of the body is called the aorta. Other large arteries branch from the aorta, and those large arteries split into other arteries, which then branch into smaller vessels called arterioles and finally into very fine vessels called capillaries that are right inside the muscle fibers.
In order for muscles to get blood, you need flexible, open arteries that can allow blood to travel easily. Arteriosclerosis is the medical term for hardening of the arteries, and atherosclerosis is the term for a blockage of the arteries from a buildup of fatty materials on the walls.
Blood Pressure and Blood Flow
Blood pressure drives blood through the circulation. As the blood vessels narrow to become capillaries, the blood pressure drops. This difference in pressure drives blood flow to the tissues.
Blood flow is the amount of blood that passes a particular point in the circulation in a certain amount of time. For example, the blood flow related to the heart is about 5 to 6 liters per minute (L/min); this is also called cardiac output.
Maintaining a normal blood pressure and good blood flow is essential to getting blood to the muscles.
One of the most essential functions of blood is to carry oxygen to the muscle. Most of this oxygen is transported in the blood attached to a molecule called hemoglobin. Hemoglobin is a very special protein with some unique properties. Hemoglobin binds to oxygen in the blood vessels in the lung, but it lets go of the oxygen in the capillaries when the muscle tissues really need it. Then the hemoglobin carries away carbon dioxide waste that the muscle tissue produces during contraction.
Skeletal Muscle Blood Flow
According to Dr. Klabunde, author of the textbook "Cardiovascular Physiology Concepts," skeletal muscles -- or muscles that are under a person’s voluntary control like those in the arms or legs -- use about 20 percent of the heart’s blood flow at rest and can receive up to 80 percent of cardiac output during extreme exercise.
The blood flow through the skeletal muscle can be affected by the interaction of many complex factors. For example, Klabunde states that “coordinated, rhythmical contractions like running” can increase blood flow. Certain chemical factors can change the diameter of the blood vessels, and the balance of certain electrolytes or carbon dioxide in muscle tissue can also change the amount of blood going to the muscle fibers.