Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.
We previously learned that blood is carried to your cells through transportation tubes called blood vessels, and your heart is the pumping station that propels that blood. Every time your heart beats, blood is pumped into large arteries to start the one-way journey around your body. From the large arteries, blood moves into smaller arteries called arterioles and then into the capillary beds, and it's in these capillary beds where you have the exchange of gases and nutrients. Your blood is then drained out of the capillary beds and ready for its return trip to the heart by first going into small veins called venules and then merging into larger veins, which eventually flow back into the heart.
Layers of Blood Vessel Walls
Like a busy highway, your blood vessels have to be well-constructed to withstand all the pressure that comes with circulating your blood every minute of every day. To help stand up to the high demand, the walls of the blood vessels are constructed of three layers known as tunics.
The tunica intima is the innermost layer of the arteries and veins. You can easily recall this term by remembering that 'intima' and 'innermost' both start with the letters 'in.' The tunica intima is composed of a thin layer of endothelial cells and lines the entire circulatory system, from your heart and the large arteries all the way down to the very tiny capillary beds.
As we previously learned, capillaries are comprised of only one layer of endothelial cells, and therefore, capillaries only have a tunica intima. This ultra-thin design is what allows for nutrients and gases to be exchanged in the capillary beds, and it's a unique feature of capillaries. The walls of arteries and veins have two additional layers, or tunics, forming too big of a barrier for there to be an exchange of gases and nutrients. So, why don't we go ahead and take a look at those two additional layers found within the walls of your arteries and veins.
Working our way from the inside of the vessel out, we see the next layer, called the tunica media. This is the muscular middle layer of the arteries and veins. You can easily recall this by remembering that 'media' and 'middle' and 'muscular' all start with the same letter, which is 'm.' The tunica media of arteries contains more smooth muscle than the tunica media of their counterpart, the veins, and this allows the arteries to constrict and dilate to adjust the volume of blood needed by the tissues that they feed.
When the smooth muscle found within the tunica media is stimulated, it contracts, squeezing the walls of the artery and narrowing the vessel. The term vasoconstriction is used to describe the narrowing of the blood vessel due to contraction of the muscular wall. When an artery constricts, the flow of blood decreases and the pressure within the vessel rises. Vasoconstriction is a mechanism your body uses to regulate important functions in the body, such as body temperature and blood pressure.
Vasodilation is the opposite of vasoconstriction and is the term used to describe the widening of the blood vessel due to relaxation of the muscular wall. When a blood vessel dilates, blood is able to flow through the vessel with less resistance. Therefore, vasodilation has the opposite effect of vasoconstriction on such functions as body temperature and blood pressure.
The tunica media of arterioles contains a large amount of smooth muscle, and therefore, arterioles are key regulators of blood flow between the arteries and the delicate capillaries. This regulation allows for a more efficient exchange of gases and nutrients when blood is within the capillary beds.
The tunica adventitia is the outermost layer of the arteries and veins. The word 'adventitia' in tunica adventitia comes from the Latin 'adventicius,' which means 'coming from outside.' This strong outer layer is composed of connective tissue that allows the blood vessel to withstand forces acting on the vessel wall. It also contains strong collagen fibers that help anchor the blood vessel to surrounding tissues, and this gives the vessel some stability.
Let's review. All arteries and veins contain three layers. The innermost layer is called the tunica intima. The muscular middle layer is called the tunica media, and the outermost layer is called the tunica adventitia.
Because capillaries are only one cell layer thick, they only have a tunica intima. This ultra-thin design allows for the exchange of gases and nutrients through the capillary walls.
The smooth muscle found in the tunica media can be stimulated to contract, and this results in vasoconstriction and the narrowing of the blood vessel. When the stimulation stops, the smooth muscle relaxes, resulting in vasodilation and the widening of the blood vessel. The ability to contract and dilate helps to control the amount of blood flowing through the vessel.
After seeing this video, you should be able to:
- List the three layers that make up blood vessels
- Describe the structures and functions of the tunica intima, tunica media, and tunica adventitia
- Explain the mechanics and functions of vasoconstriction and vasodilation
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