The Seamless Blood Vessel by i-heart-histo
For fans of the Seamless Gut, Seamless Lung and Seamless Ovary.
A generalized blood vessel has a wall that is composed of three tunics:
- an inner tunica intima (endothelium, subendothelium and internal elastic lamina)
- a middle tunica media (smooth muscle and an external elastic lamina)
- an outer tunica adventitia (connective tissue with a vaso vasorum/nervi vasorum)
As you progress from arterial to venous vasculature, each vessel type of decreasing/increasing diameter differs in the organization of these three layers making classification simple. Here is a guide to help you:
1. Heart wall
The largest ‘vessel’. Its tunica intima is known as the endocardium; its tunica media is known as the myocardium and is composed of cardiac muscle; and its tunica adventitia forms an outer pericardium composed of adipose tissue within which is embedded its vaso vasorum (aka the coronary arteries/cardiac veins). The pericardium is also lined with a visceral serous mesothelium that secretes pericardial fluid into the pericardial cavity.
- Key identifying feature: Myocardium composed of cardiac muscle
2. Elastic artery (large)
The aorta is an example of a large/elastic artery. It has a thick tunica media composed of smooth muscle and distinctive elastin fibers that allow the vessel to stretch during ventricular systole and recoil during diastole to assist in propulsion of blood through the vascular system.
- Key identifying feature: Tunica media contains thick wavy elastin fibers
3. Muscular/Distributing artery (medium)
Medium arteries have a very thick tunica media composed of smooth muscle to maintain vascular resistance and blood pressure. The tunica intima has a prominent circumscribing internal elastic lamina and the tunica media has a prominent circumscribing external elastic lamina.
- Key identifying feature: Prominent internal and external elastic laminae
4. Muscular/Distributing artery (small)
As arteries get smaller in diameter, the smooth muscle layer gets thinner and the internal elastic lamina begins to diminished or is absent. As an arbitrary rule of thumb, if you see more than 6 layers of smooth muscle and an internal elastic lamina it is a small artery (that’s the i-heart-histo rule, but some textbooks would set this number higher or lower – matter of preference).
- Key identifying feature: 6+ layers of smooth muscle and a prominent internal elastic lamina
5. Arteriole (large)
Despite the small size of these vessels the tunica media is the most predominant layer. It is less than 6 smooth muscle cells thick. Larger arterioles often have a distinctive internal elastic lamina.
- Key identifying feature: Less than 6 layers of smooth muscle, may be an internal elastic lamina
6. Arteriole (small)
Once again, these have a large tunica media for their very small size, Think around 3 smooth muscle cells thick. Some may have an internal elastic lamina although it is usually absent at this vessel size.
- Key identifying feature: Around three layers of smooth muscle, indistinct/absent internal elastic lamina.
Capillary (not shown)
Venule (not shown)
7. Muscular vein (small)
Small veins have a distinctly thick tunica adventitia when compared to a thin tunica media which is around 3-4 smooth muscle cells thick. Considering the size of the vessel is equivalent to a small artery, it has the tunica media of a small arteriole. Puny! Oh and if you are very lucky you might even see valves!
- Key identifying feature: Thin tunica media with around 3 layers of smooth muscle, but a very thick tunica adventitia.
8. Muscular vein (medium)
Medium veins are the equivalent in size to a medium artery but only have a tunica media thickness of a large arteriole. The tunica adventitia of these vessels is by far the thickest layer. Once again, fingers crossed to see an elusive valve.
- Key identifying feature: Thin tunica media with around 6 layers of smooth muscle, but a very thick tunica adventitia.
9. Vena cava
Equivalent in size to the aorta but with a thin tunica media. The connective tissue tunica adventitia predominates and contains distinctive arrays of longitudinal muscle fibers.
- Key identifying feature: Thin tunica media. Very thick tunica adventitia that contains longitudinally arranged smooth muscle.