The circulatory system is a closed network of transport vessels responsible for delivering nutrients and removing waste products from all body systems. Shirley A. Jones's "Pocket Anatomy and Physiology" provides a pictorial guide illustrating the nature of the closed circulatory system, adding a visual component to the discussion of the major blood vessels of the human body. The major vessels of the circulatory system are divided into two main categories, arteries and veins, which branch into smaller vessels for the purpose of nutrient exchange.
Arteries
Arteries are vessels in which the blood flows away from the heart. Arteries are large, thick-walled vessels capable of carrying great amounts of blood and withstanding large amounts of pressure. The largest of the arteries is the aorta, which originates from the left ventricle of the heart and branches into the major arteries of the arms, legs and brain. However, for nutrient exchange to take place, small vessels are required. Thus, these major arteries begin to branch into smaller arterioles, and eventually, at the location of nutrient exchange, into capillaries. Capillaries are so small that red blood cells are forced to pass through in single-file order. This allows for greater opportunity for the exchange of nutrients through the vessel wall, which is much thinner than that of the large arteries. The capillaries mark the end of the arterial system. As the blood flows out of an organ system the capillaries turn into the smallest of venous vessels, known as venules.
Veins
The venous system always returns blood to the heart. In normal, resting conditions, the venous system contains greater than 60 percent of the body's total blood volume, thus these vessels are known as capacitance vessels. As illustrated by Drs. David Mohrman and Lois Heller in "Cardiovascular Physiology," vessels of the venous system contain one-way valves that aid in the forward progress of blood flow toward the heart. These valves become a crucial component of the circulatory system while standing and during exercise as blood attempts to flow in opposition to the pull of gravity in order to return to the heart.
Venules are small branches of veins that are in direct contact with capillaries, the smallest arteries. From venules, blood enters the larger veins, the largest of which are the inferior and superior vena cava, also known as the great veins. These vessels lead directly to the right atrium of the heart where they complete the closed circulatory loop.
Exceptions
A common misconception is that arteries always carry oxygen-rich blood, termed oxygenated, and veins always carry oxygen-poor, or deoxygenated, blood. While this is true for the majority of vessels, it is not always the case. Because arteries and veins are named based on which direction blood is flowing relative to the heart, the rule of oxygenation does not apply to the pulmonary vessels, those leading to and from the lungs. The pulmonary arteries allow blood to flow away from the heart and toward the lungs. Because this blood has not reached the level of the lungs yet, it is deoxygenated, but because it is leading away from the heart, it travels in an artery. Likewise, blood returning from the lungs to the heart is oxygenated, yet all vessels leading toward the heart must be veins. Thus, the pulmonary arteries and veins provide a distinct exception to the oxygenated/deoxygenated rule.
References
- "Pocket Anatomy and Physiology"; Shirley A. Jones; 2009
- "Cardiovascular Physiology" (5th ed); David E Mohrman, PhD; 2003
