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Blood Vessel Layers: Tunica Intima, Tunica Media & Tunica Adventitia

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  • 0:06 Blood Vessels
  • 0:50 Layers of Blood Vessel Walls
  • 1:08 Tunica Intima
  • 2:09 Tunica Media
  • 3:58 Tunica Adventitia
  • 4:30 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

In this lesson, you will take a microscopic look at the three layers, or tunics, that make up your blood vessels. Follow along and learn how the different cells within these tunics help the vessels contract and relax as blood moves through your circulatory system.

Blood Vessels

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.

Tunica Intima

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.

The tunica intima is the innermost layer of arteries and veins
Tunica Intima

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.

Tunica Media

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.

Cross section of the tunica media layer
Tunica Media Cross Section

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.

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