Blood Vessel Tunics

Tunica Externa

This histological slide is stained with something called Masson’s trichrome stain, which is three in one.  With this stain, collagenA structural protein in the dermis that provides strength and elasticity. appears to be a dark teal color and muscle stains a dark red color.  This slide shows an artery. Its round lumenThe inside space of a hollow organ or structure. is on the right. A vein is also shown, with its collapsed lumen on the left.

In arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr, the tunica externaThe outermost layer of a blood vessel, made of connective tissue, providing support and anchoring. contains a lot of collagen fibers. These collagen fibers secure the arteries to the surrounding tissues. In arteries close to the heart, they expand and contract significantly. This movementA fundamental property of life involving motion of the body or its parts. is to handle the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. from the heart’s pumping. Therefore it’s important that these arteries do not move around too much.

The tunica externa of veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge has those same collagen fibers that attach to the surrounding tissues. However, it also contains a lot of elastic fibers and smooth muscle. These elastic fibers allow veins to change the diameter of their lumen and make the vessel larger or smaller.  For example, during exercise, a lot of blood returns to the heart from skeletal muscle. The veins need to be able to accommodate the blood volume. 

Recall that veins return blood to the heart. They are very far away from the pumping action or the high pressures produced by the heart. The smooth muscle in the tunica mediaThe middle layer of a blood vessel, composed of smooth muscle and elastic tissue, responsible for va allows veins to contract and relax. This movement helps to move the blood along and back to the heart. The large vein that leads directly to the heart is called the vena cava. The vena cava is so large. It has its own vessels in the tunica externa that serve the smooth muscle there. It’s almost as if the vena cava is too large to be able to take blood from its lumen. These vessels in the tunica externa of large veins are called the vaso vasorumSmall blood vessels that supply the outer layers of large blood vessels with nutrients and oxygen.. This is a phrase that literally means vessels of the vessels.

Tunica Media

The tunica media in both arteries and veins contains sheets of smooth muscle. There is no tunica media in capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues..  In arteries, the tunica media has a very thick layer. Veins have a thinner layer of tunica media. The arteries closest to the heart, like the aorta, are incapable of contracting their tunica media. In arteries like the aorta, the tunica media is only able to contract enough to maintain the diameter of the lumen. It is incapable of reducing the diameter of the lumen.

In the tunica media, where it meets the tunica externa, is a layer of elastic connective tissue. This layer is called the external elastic laminaA layer of elastic fibers found in muscular arteries, providing flexibility and recoil.. The tunica media in all of your blood vessels is sympathetically innervated. This means that the nerves controlling the tunica media are coordinated in a sympathetic response. They help trigger a fight or flight response.  When someone tries to steal your fanny pack in a dark parking lot you get an adrenaline rush. An adrenaline rush is just the release of epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla. from your adrenal glands. Epinephrine contracts all of your tunica media. It squeezes blood into your skeletal muscle so you can run away from your attacker. 

The tunica media is responsible for contraction and vasoconstrictionThe narrowing of blood vessels due to contraction of smooth muscle, increasing blood pressure and re of all our vessels. It is most important in arteriolesSmall arteries that regulate blood flow into capillaries through vasoconstriction and vasodilation, the blood vessels that feed capillary beds.  Because there are so few arterioles compared to capillaries, arteriole form something of a bottleneck to blood flow.  Because of this, arterioles are the blood vessels most responsible for blood pressure.

Tunica intima

The tunica intimaThe innermost layer of a blood vessel, consisting of endothelium and a thin connective tissue layer. is also called the endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces f. This word, endothelium, indicates a continuous lining of the circulatory system. The endothelium is continuous with the inner lining of the chambers of the heart. The endothelium consists of simple squamous epithelium. These simple squamous cellsThe basic structural and functional units of life. are flattened like eggs and reduce friction as blood flows through the vessels. You want your blood to flow nice and smoothly, you don’t want bumps in the road. More friction will equal a higher pressure even if the vessel does not vasoconstrictive. The tunica intima can contain elastic protein fibers, just like the tunica media does on its superficialNear the surface of the body. face.  These elastic fibers are sandwiched between the tunica intima and the tunica media.  Again, capillaries don’t have these elastic fibers, which means that capillaries are just a layer of simple squamous cells.  It is.  This is because exchange of gases and nutrients happens at capillaries.  You want there to be very little cell volume between the blood in the blood vessel and the surrounding interstitial fluids so that it is easy for the gases and nutrients to diffuse