Cardiovascular System

OBJECTIVES: At the end of this laboratory session, you should be able to:

1. Identify and describe the light and electron microscopic structure of the various types of blood and lymphatic vessels.

2. Describe the structure modifications in the three types of capillaries, and to relate this to differences in permeability.

3. Identify the layers of the atrial and ventricular walls and understand how differences in the thickness of these layers contribute to heart function.

4. Identify and describe the microscopic structure of a heart valve

5. Identify the components of the cardiac skeleton and understand its structural and functional relationship with the musculature of the heart.

SLIDES FOR THIS LABORATORY: 18, 47, 55, 59, 64, 66, 67, 78, 80, 81, and Supplemental Slides 115-121, 128, 129.


Blood vessels are usually composed of three layers: the tunica intima, tunica media, and tunica adventitia. The tunica intima consists of a layer of endothelial cells lining the lumen of the vessel, as well as a subendothelial layer made up of mostly loose connective tissue. Often, the internal elastic lamina separates the tunica intima from the tunica media. The tunica media is composed chiefly of circumferentially arranged smooth muscle cells. Again, the external elastic lamina often separates the tunica media from the tunica adventitia. Finally, the tunica adventitia is primarily composed of loose connective tissue made up of fibroblasts and associated collagen fibers.

  Tunica intima

Tunica media

Tunica adventitia

1. Endothelial cell lining

2. Subendothelial layer

3. Internal elastic lamina


1. Smooth muscle cells, collagen fibers, and ground substance

2. Elastin in the form of fenestrated elastic lamellae (esp elastic arteries)

3. External elastic lamina

1. Mostly collagenous fibers

2. Elastic fibers (not lamellae)

3. Fibroblasts and macrophages

4. Vasa vasorum  


Slide 66 Elastic artery (aorta).

In the aorta, the tunica media constitutes the greater part of the vessel wall. It is made up of multiple elastic lamellae alternating with thin layers of circularly oriented smooth muscle . The boundary between the tunica intima and media is not readily defined, and the internal elastic lamina is merely the innermost of the many elastic lamellae within the wall. The tunica adventitia essentially lacks elastic lamellae, and is mainly loose connective tissue and blood vessels (vasa vasorum).

Slide 67 Elastic artery (carotid) and large vein (jugular).

Contrast the artery with the vein . Observe the intima, media, and adventitia. In the artery, the elastic lamellae are readily observed. The aorta compared to the carotid artery has a thicker intima and more elastic lamellae in the media.

Supplemental Slide 115 Aorta, human 40+ years, Verhoeff Van Gieson stain.

Note that elastic and collagen fibers are easy to visualize in this slide.

Examine these slides to further reinforce your ability to recognize elastic arteries and their features. Note the relationship between age and histology.

Supplemental Slide 116 Aorta, human new born.

Supplemental Slide 117 Aorta, human new born, Verhoeff Van Gieson stain.

Supplemental Slide 118 Aorta, human 7 years.

Supplemental Slide 119 Aorta, human 7 years, Verhoeff Van Gieson stain.

Supplemental Slide 120 Aorta, human 22 years.

Supplemental Slide 121 Aorta, human 22 years, Verhoeff Van Gieson stain.


In muscular arteries, smooth muscle becomes the predominant constituent of the tunica media. Internal and external elastic laminae are prominent. The tunica intima is thinner than the intima of elastic arteries.

Slide 81 Spermatic cord.

This slide has several muscular arteries mixed in with atypical veins of the pampiniform plexus . These veins contain a greater amount of smooth muscle than most veins of this size. Also note the outer longitudinal arrangement of smooth muscle in these veins. Observe the characteristic three layers of the arteries present. Note the large amount of circularly arranged smooth muscle in the media of the muscular arteries.

Slide 47 Submaxillary gland, Verhoeff's hematoxylin; mucicarmine & methyl green.

Find the muscular arteries . Internal and external elastic laminae (or membranes) are stained black and therefore readily observed. Look carefully in the media of the arteries and you should be able to resolve elastic fibers also.


In arterioles, the tunica intima consists of a continuous endothelium and a very thin subendothelial layer. In addition, a thin, fenestrated internal elastic lamina is present in larger arterioles but absent in terminal arterioles. Generally, the tunica media consists of 2 layers of smooth muscle cells. But in the smallest arterioles there is a single layer. The tunica adventitia is a thin sheath of connective tissue, which is not easily defined.

Slide 18 Uterine tube.

Find the multitude of vessels in the connective tissue which invests the oviduct. This slide is especially good for observing small arteries , veins , arterioles , venules , and capillaries . Identify the layers of the vessels present.


Capillaries average from 9 to 12 µ m in diameter, just large enough to permit passage of cellular components of blood. The wall consists of extremely attenuated endothelial cells. In cross section, the lumen of small capillaries may be encircled by a single endothelial cell, while larger capillaries may be made up of portions of 2 or 3 cells. No smooth muscle is present. Recall that at the EM level, 3 types of capillaries can be distinguished: continuous, fenestrated, and discontinuous.

Capillaries and venules are the principal vessels where exchange between the blood vascular space and the interstitium takes place. A surprising amount of exchange of fluid and cells takes place through the wall of the venule. Venules are similar to capillaries in that they have a very thin wall; but unlike capillaries they have diameters that range from approximately 20 - 60 µm.

Slide 55 Appendix.

In the submucosa and adventitia of the appendix, observe venules , capillaries , and arterioles .

Slide 81 Spermatic cord.

Slide 81 is also useful for small vessels .


Veins are best studied along with their corresponding arteries. Veins have the same 3 layers as arteries, but boundaries are indistinct and elastic components are not as well developed in veins. Histologically, veins are usually collapsed because of the thinner, less elastic walls.

Slide 80 Pancreas.

Use this slide to appreciate the contrasting features of a medium sized artery (muscular artery) and vein . The vein is typical of vessels not surrounded and supported by solid tissue. Veins in these locations have a well developed muscular adventitia (muscle cells are oriented longitudinally). This is also the typical structure of large veins such as the vena cava.

Slide 59 Lymph node.

This slide shows a more typical medium sized vein .


Lymphatics are found in all tissues except the CNS, cartilage, bone and bone marrow, thymus, teeth, and placenta. Lymphatics are sometimes difficult to demonstrate satisfactorily in normal tissues because these large, thin-walled vessels frequently collapse to the point of invisibility during tissue processing. Lymphatics are often most easily visualized and studied during pathological processes (e.g., inflammation). Lymphatics start as blind-ended lymphatic capillaries which coalesce to form lymphatic vessels and finally empty into the circulation via the lymphatic ducts (thoracic and right lymphatic).

Slide 81 Spermatic cord.

Find the lymphatic vessels of the spermatic cord. Like the veins (pampiniform plexus) of the spermatic cord, these lymphatic vessels have unusually thick walls. Note the valve leaflets of this vessel.


Slide 64 Heart and AV valve.

This slide is a full thickness section heart wall which includes atrium , ventricle and an AV valve leaflet . Note the difference in thickness of the atrial and ventricular walls (correlates with differences in pressure and work loads). The musculature of both walls is composed of cardiac muscle cells which exhibit the usual characteristics of cardiac muscle, i.e., branching, anastomosing, central nuclei, myofibrils, cross striations, and intercalated disks. The musculature of the atria is not continuous with that of the ventricles. Note that the atrial endocardium is very thick but that the ventricular endocardium is very thin.

Observe the AV valve leaflet and its attachment to the fibrous skeleton of the heart. The AV valve leaflet is formed by a fold or duplication of the endocardium; note especially the dense connective tissue core which forms the central part of the valve leaflet. The upper or atrial surface of the valve is thick and resembles atrial endocardium, and the lower or ventricular surface of the valve is thin and resembles ventricular endocardium.

Examine the fibrous skeleton and observe that it is composed of very dense connective tissue and that histologically, it resembles a tendon (the fibrous skeleton of the heart is actually made up of circular tendons).

Slide 78 Heart, muscular interventricular septum.

In the subendocardial layer , locate the longitudinally sectioned Purkinje fibers . Note that 1) they are connected by intercalated discs , 2) they are larger than the ordinary cardiac myocytes, and 3) they are separated from the myocardium by a connective tissue layer which is actually a sheath in the intact heart.

Supplemental Slide 128 Heart.

Again note the endocardium and myocardium of the atrial wall. This section shows one leaflet of the semilunar valve and the fibrous skeleton of the heart. Also observe the portion of the aorta present in this slide. The membranous and muscular parts of the interventricular septum are also present near the valve.

Supplemental Slide 129 Heart, Verhoeff Van Gieson stain.

This is the same section of heart in Slide 128. In this slide the collagen forming the connective tissue investments (including the membranous septum ) is red and the elastic fibers are stained black. Note the relatively greater amount of elastic fibers in the atrial wall of the heart.