The cardiovascular system is made up of the heart, blood and blood vessels. Though it's not uncommon to consider the vessels merely piping that carries blood to the tissues and returns it to the heart, in fact, the vessels are much more complex. They help maintain physiological parameters and prevent blood from moving in a backward direction.
Function
The major function of blood vessels is to deliver blood to the tissues and return it to the heart. Actually, however, the vessels serve a variety of functions. The arteries, or vessels that lead away from the heart, can expand or contract to help regulate blood pressure, notes Dr. Lauralee Sherwood in her book, "Human Physiology." The capillaries, or smallest vessels, are thin-walled and allow gases to pass through to the tissues.
Features
The three major types of vessels have features that help them function. Arteries are thick-walled, notes Dr. Gary Thibodeau in his book, "Anatomy and Physiology." The muscular walls allow the arteries to contract to increase blood pressure, and remain contracted against the force of blood. Veins have valves that prevent backward flow of blood. The capillaries are so thin as to be made up of only a single layer of cells. These cells don't join completely at the edges, meaning that fluid and white blood cells can enter and exit capillaries.
Considerations
One important consideration with regard to blood vessel function has to do with the perforated nature of the capillaries. Because fluid can exit the vascular system at the tissues, it's important that the fluid reenters the system, or a person would quickly lose blood volume. Proteins called albumins help ensure that fluid gets sucked back into vessels at the end of a capillary bed, notes Dr. Thibodeau. This means that although fluid can exit and bathe the cells with nutrients, overall, blood volume stays constant over time.
Expert Insight
The vessels work in part with help from other body systems. For example, the veins have valves to prevent backward flow of blood, but it's difficult for blood to return to the heart from the lower extremities. As such, notes Dr. Thibodeau, muscular contractions in the legs help move blood upward through the veins, and valves prevent the blood from falling again once it's been moved upward. Regular movement of the legs keeps blood from pooling and assists in vascular function.
Warning
Blood vessels must be open to be operational. High levels of blood cholesterol can clog arteries and harden them, in a disease process called atherosclerosis, warns Dr. Sherwood. This puts the person at risk for heart disease, heart attack, and stroke, because occluded arteries don't deliver blood as well, and hardened arteries can crack and bleed, leading to clot formation. Healthy levels of blood cholesterol help blood vessels continue to function properly.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
