1. Identify the 3 layers found in blood vessels.
2. Distinguish between arteries and veins.
3. Identify large, named, arteries and veins.
4. Distinguish between arterioles and venules.
5. Identify lymphatic vessels.
6. Distinguish between a blood capillary and lymphatic capillary.
7. Identify the three layers of the heart. 

slides:

s10 mesentery (orcein)
s16 aorta and mesenteric artery (orcein)
s19 heart (atrium and ventricle)
s25 lymph node
s38 vena cava

s55 duodenum and pancreas
s99 colon
Demonstration slide: Purkinje fibers

Index of images

The cardiovascular system consists of a pump, a distribution system (arteries), exchange system (capillaries), and a return system (veins). There are two of these systems (pumps and plumbing) in the body consisting of the pulmonary and systemic system. The the right atrium and ventricle pump blood into the pulmonary system that supplies blood to lungs for gas exchange while the left atrium and ventricle pump blood into the systemic system that supplies blood to the rest of the body for metabolite exchange. The high-pressure distribution side contains substantial amounts of elastic fibers within the walls to act as an elastic element to minimize pressure changes caused by the beating of the heart. The more proximal to the to the heart on the distribution side (arteries), the more elastic connective tissue present. The layers [coats] of the vessels include the innermost or [tunica] intima, a muscle layer called the [tunica] media that contains smooth muscle, and an outer layer called the [tunica] adventitia that is mostly connective tissue. Distinguishing between arteries and veins is based upon comparison of wall thickness to lumen diameter, the relative thickness of the different layers, and the organization of the outer 2 layers of the vessels. Capillaries are noted by their very thin (1 cell layer) wall. The lymphatic system is another circulatory system of the body that carries lymph (interstitial fluid and lymphocytes). It differs dramatically from the blood vessels in that it is an open system, it does not contain red blood cells, has much thinner walls, is unidirectional, not directly connected to a pump, and typically does not contain a muscular wall.

The first part of the cardiovascular system that we will study is the heart that pressurizes the system. Our primary focus is to define the layers of the heart. Examine s19 (atrium and ventricle, left assumed) and focus your study on the section of the atrium. There are three layers of the heart similar to those found in the vasculature. However, these layers have different names. Study the lumenal side of the atrium. This layer is defined as the endocardium and includes the collagenous connective tissue lying just beneath the endothelial cells (the sub-endothelial and myoelastic layers). The muscular layer is defined as the myocardium and contains the cardiac muscle cells. The outer layer is called the epicardium and comprises the visceral layer of the pericardium.  It consists of a mesothelial lining underlain by connective tissue layers containing blood vessels and nerves. The epicardium of the atrium does not contain as abundant a supply of vessels and nerve as is found in the ventricle. The collagenous connective tissue presents as thicker bundles of collagen fibers in the epicardium than in the endocardium. The section of the ventricle on this slide does not present a good example of the epicardium. The endocardium is present and appears as a thin layer relative to that observed in the atrium. Slide 20 (ventricle) may contain examples of all three layers. Study at low magnification to see if your slide contains the three layers. If present, study the layers carefully. The heart, as well as most arteries and veins, receives its blood supply from the systemic vasculature, not the lumen of the heart. A consequence of this design is that an ischemic (no blood flow) insult to the heart is most damaging or seen first in the subendocardial region of the wall even though it is very near oxygenated and moving blood. Why not design the heart to receive its blood supply directly from the lumen to minimize this problem?

The only part of the heart’s conduction system which is routinely seen in routine heart sections are the Purkinje fibers.  Although visible in H&E sections they are more easily observed in a demonstration slide stained with Weigert’s (Iron) hematoxylin.  While most common in the subendocardial space (low, high), they can also be observed in the myocardium (low, high).  These large cells contain one or two nuclei in a clear central mass.  Myofibrils are sparse and displaced to the periphery. 

We begin studying the vessels by viewing arteries and veins found in the orcein-stained mesentery. Study s10 and you will observe vessels, usually in pairs of an artery and a vein. The arteries are most easily distinguished by their prominent internal elastic membrane. This presents as a dark stained convoluted thin layer (elastin) that lines the lumen of the vessel. On the lumenal side of the internal elastic membrane is the endothelium which may not be apparent. This layer is the [tunica] intima. The next layer of the vessel is the [tunica] media or muscular layer comprised of smooth muscle cells which are oriented around the tube (cells appear in longitudinal section when vessels is cut in cross-section). The outer boundary of the media is marked by another layer of elastin called the external elastic membrane.  The outermost layer is the [tunica] adventitia which, in these sections, is distinguished as having a loose arrangement of elastic connective tissue intermeshed with collagenous connective tissue. [  N.B.  According to Terminologia Histologica it is perfectly acceptable to omit the "tunica' in naming the layers of the vessels; hence intima, media and adventitia.]  Veins are distinguished by their less robust internal elastic membrane and less distinction between the tunica media and adventitia; also, often their lumen is irregularly shaped. When both arteries and veins are present in good cross-section, the artery has a greater wall thickness relative to the lumen diameter. Additionally, arteries have a more prominent internal elastic membrane and more readily defined tunica media or muscle layer as you have just observed. Be able to distinguish arteries from veins even when not presented in good cross-section.

Other examples of muscular arteries and veins can be found in s25 (lymph node) and the mesenteric artery in s16. Note that the vein in s25 is not presented in good cross section while the muscular arteries are presented very well. You can readily identify the tunicae intima, media, and adventitia. Careful observation of the tunica intima shows the endothelial cells and the sub-endothelial connective tissue resting on the internal elastic membrane. The mesenteric artery in s16 presents as a fantastic example of the wall of a muscular artery with its well defined internal elastic membrane of the tunica intima, the tunica media, and tunica adventitia.

The right atrium receives it blood from the vena cava while the left ventricle pumps its blood into the aorta. These are two vessels that can be identified histologically based solely upon their appearance without being in their anatomical context. Study s16 which contains a cross-section of the mesenteric artery and a cross-section of the aorta. The aorta is the larger vessel. Examine this section and note that the middle layer of the vessel wall, the tunica media, contains mostly elastic fibers. The smooth muscle cells present are very difficult to observe. The vena cava is unique in that it contains bundles of smooth muscle cells in the tunica adventitia with the cells aligned with the long axis of the vessel. What is the functional role of having both longitudinal and circular smooth muscle in this vessel? Cooter will contemplate this for a while!! Study s38 and define the layers of the vena cava. As with other veins, the layers are not as readily distinguished as in arteries. The tunica media is rich in collagen and elastic fibers with a lesser investment of smooth muscle. The tunica adventitia, with its smooth muscle bundles and collagen fibers, is the most prominent layer of this vessel.

These primary vessels connect to large arteries and veins, muscular arteries and medium veins (as seen earlier), small arteries and veins, then to arterioles and venules. Distinguishing smaller arteries and veins from arterioles and venules is based primarily upon the thickness of the tunica media. We will define arterioles as possessing 1-5 concentric layers of smooth muscle cells and venules as possessing 0-2 layers of smooth muscle cells in the tunica media.  Arterioles and venules do not have as prominent a layer of elastic connective tissue in the tunica adventitia or media. Good examples of arterioles and venules can be found in the sub-epithelial layer (termed submucosa) of the colon (s99). Look for pairs of small vessels in the sub-epithelial tissue. Note the differences between the vessels of the pair. The vessel with the more defined muscular tunica media is the arteriole while the other vessel is a venules. This arteriole has an obvious tunica intima (endothelial cells), about 3 layers of smooth muscle cells within the tunica media, and a thin tunica adventitia. Venules present as vessels with a wall thickness two or more cell layers thick, an obvious intima, poorly defined muscle layers in the media and poorly defined adventitia. Study numerous pairs of arterioles and venules that are present in this section and in the mesentery (s10). Be able to identify arterioles and venules in longitudinal section.

The next level of the vasculature is the capillaries in which the metabolite exchange occurs. Capillaries are best observed in adipose tissue and we'll use slide 10 (mesentery) for this purpose. These vessels are one cell layer thick, that layer being made of endothelial cells. The capillary diameter is about the same size as a red blood cell. The capillaries, in longitudinal section, will appear as rows of red blood cells. The endothelial cell nuclei is usually visible and protrudes into the lumen of the vessel while the nuclei the appear on the outer aspect of the wall are likely pericytes. Capillaries can also be readily found in the adipose tissue in s55 (duodenum and pancreas). Lymphatic capillaries have a similar appearance to blood capillaries with one major difference: they do not contain red blood cells. So, if you see a presumptive blood capillary that does not have a red blood cell, it could be a blood or lymph capillary. If it has red blood cells within in it, it is a blood capillary. We will learn more about the lymphatic vessels when we learn about the immune system.

The last thing we will study is vasa vasorum. This term is used to describe blood vessels (vasa) within blood vessels (vasorum). These are readily found in the tunica adventitia of large arteries and veins. The vessels are usually small muscular vessels or arterioles and venules. These serve as the blood supply for the vessel wall as the lumen pressure may be too high (arteries) or too low (veins) for proper function (hint for the design question above). Find vasa vasorum in the adventitia of the aorta (s16) and the vena cava (s38).

Lab:  Circulatory System