2006 Unit reviewed by: David Moscatelli and Victoria Ort

Link to the Hippocrates Module on the CIRCULATORY SYSTEM


The circulatory system is composed of continuous tubes or vessels that are lined by endothelium, a simple squamous epithelium derived embryologically from mesoderm. As the vessels differentiate during fetal and postnatal life, two main principles of organization are maintained.


Classically, three layers or tunicas are described:

Generally (although not embryologically) speaking the three layers of the heart wall (endocardium, myocardium and epicardium) are analogous to the three layers of the blood vessel walls (tunicae intima, media and adventitia).


The functions of the different segments are:

Blood Circulatory System

Lymphatic Circulatory System


tunica intima
tunica media
tunica adventitia
elastic laminae
atrial natriuretic factor
elastic arteries
muscular arteries
internal elastic membrane
continuous capillary
fenestrated capillary
discontinuous capillary
pericytic venule
post capillary venule
muscular venule
lymphatic vessels
lacteals, lymphatic capillaries
vasa vasorum

Be sure to review the CIRCULATORY SYSTEM study unit.


To be able to:

  1. Compare the components of the walls of the different elements of the circulatory system, i.e. arteries, veins, lymphatics and capillaries of different caliber.

  2. Describe the qualitative and quantitative morphological changes that these components undergo as they progress from the heart to the capillaries and back to the heart.

  3. Describe the most important functional aspects of the circulatory system and the way in which the morphological features of the different vessels correlate with these functions.

  4. Discuss the general ultrastructural characteristics of endothelial cells as well as the differences observed in the ultrastructure of both endothelial cells and their basement membrane in the different types of capillaries: continuous, fenestrated and sinusoidal.


A. Muscular (small and medium sized) Arteries and Veins, Capillaries, Lymphatics. Slides 24, 48, 49, 110, 124, and 125 (See illustrations on CS-4, 5 and 6)

In any one of slides 24, 33, 48 or 49 you will be able to look for the different classes of blood vessels. Use CS-4 and CS-5 as a guide to identification. Very often the vessels are found in bundles that include nerve, artery, vein and lymphatic elements, surrounded by a connective tissue sheath. Capillaries are located in close proximity to muscle fibers in the tongue (slides 48, 49 and 110). They are especially clear in slide 110. The red cell (diameter = 6-8) is a good reference for size. Only one red cell fits into the cross-section of one capillary. Blood vessels may be found in longitudinal, transverse or oblique sections, which you can distinguish if you remember that:

The walls of capillaries consist only of endothelium and underlying basal lamina. Usually only one endothelial cell nucleus is observed in a transverse section. On slide 110, the PAS staining of the capillary basal lamina outlines the rich capillary network associated with the skeletal muscle of the tongue. Capillaries and small lymphatics are most easily identifiable in plastic embedded tissues (slides 124, 125). Chart CS-6 gives features of different types of capillaries seen at the EM level.

Pericytic venules are larger than capillaries; 2-4 endothelial nuclei may be observed in their endothelial lining. Pericytes are difficult to identify conclusively at the LM level. However, if you see a muscle-like cell closely associated with the endothelial cells and the profile of the cell's nucleus is the same as the endothelial cell, it is a likely candidate; many pericytes are oriented with their long axis parallel to the flow of blood. Their nuclei may also appear to be more heterochromatic than those of the endothelial cells.

In studying the larger blood vessels, it is practical to first identify the layers which invest the endothelium (tunica intima, tunica media and tunica adventitia) at low power. Since branches of the arterial and venous systems travel together, it is easiest to study them by comparison. Note the thickness of the three tunic layers in relationship to the diameter of the vessel. The arterial vessels usually have a thicker wall in relation to their diameter than their companion branches of the venous system. Identify the cellular components of the layers and assess their relative proportions in each layer.

Arterioles have slightly larger diameter than capillaries and a thicker wall. They usually possess an internal elastic membrane, which is separated from the endothelium by a very thin layer of connective tissue. The internal elastic membrane is usually not fully distended. It appears wavy and homogenous-staining, often giving the lumen a scalloped appearance. This is due to contraction of the smooth muscle in the arteriole wall. Arterioles have one or two layers of smooth muscle cells. The companion venules of arterioles are of larger diameter, have only a few smooth muscle cells in the tunica media and no internal elastic membrane. Because venules lack large amounts of smooth muscle they may often appear collapsed or be filled with pooled blood.

Muscular arteries and veins are a class of blood vessels of various sizes. Muscular arteries have between 3-40 concentric layers of smooth muscle. The smaller diameter arteries (3-30 layers of smooth muscle) run with companion vessels known as small veins whereas the larger muscular arteries (30-40 layers) run with companion vessels known as medium veins. Note the thicker wall of the artery and the presence of both internal and external elastic membranes. The wall of the vein is much thinner in relationship to the diameter of the vessel and it has no internal elastic lamina or external elastic lamina.

Lymphatic vessels are extremely thin-walled and difficult to visualize either grossly or histologically. The lymphatic vessels start out as blindly ending lymph capillaries, which have thinner wall and are larger in diameter than blood capillaries. Their endothelium allows passage of molecules too large to be transported into blood capillaries. For example, lipids are transported into lacteals, which are lymph capillaries in intestinal villi (see below slides 58 and 60), and proteins are taken up from the connective tissue compartment. These substances, together with extracellular fluid taken up from the connective tissue and cells produced by the lymphatic tissues and organs, comprise the lymph. Lymph is filtered through lymph nodes and ultimately are contributed to the blood circulation, primarily through communications with the large veins at the base of the neck.

Although lymph capillaries are abundant and larger than blood capillaries, their walls are so thin that they easily collapse. It is often impossible to distinguish them from clefts in the tissue section. Larger lymphatics, well endowed with valves, accompany blood vessels, so that typically arteries, veins and lymphatics travel together. However, even larger vessels may be difficult to identify because their walls are thinner and less well organized than those of either veins or arteries. The best way to identify lymphatic vessels is the presence of lymphocytes and precipitated lymph inside the lumen, but absence of RBCs that are found in the accompanying vessels of the blood system.

B. Elastic Arteries. Slides 25 and 32

Large arteries have abundant elastic tissue in their tunica media. Compare the H&E stained section of a human aorta (slide 25) and a section of human pulmonary artery stained for elastin (slide 32). In the H&E preparation the circularly arranged fenestrated elastic membranes of the tunica media appear as wavy bands of yellow or pink refractile material between the layers of smooth muscle cells. In the preparation stained for elastin they are stained brown to black. A few of your boxes may have a newer slide 25 that has been stained to show both elastin (black) and muscle (pink). Keep in mind that the companion vessels to the elastic arteries would be the large veins like the vena cava and brachiocephalic vein. These large veins do possess an internal elastic lamina. Consult your atlas and the chart in this chapter for other characteristics.

C. Heart: Aortic and Atrioventricular Valves. Slides 30*, 31*, 29, and 34

Using either an inverted ocular or by eye, orient yourself on slides 30 and 31 by comparing your slide to the accompanying diagram (CS-8) of these sections. In the wall of the ventricle identify the endocardium (faded, very pale staining), myocardium and epicardium. The latter includes adipose tissue in which you may find branches of the coronary vessels, the coronary nerve plexus and some terminal ganglion cells.

Slide 30 shows the aortic semilunar valve at the junction between the aorta and the left ventricle. The aorta is cut in longitudinal section. Smooth muscle cells, stained bright red, are seen in cross section. Elastic fibers are paler pink to orange while collagen is blue.

Slide 31 shows a section of the left atrium and left ventricle with one cusp of the mitral valve. Look for coronary vessels in epicardium. Many slides lack a section of the coronary vein. The coronary artery has a thick tunica intima containing longitudinally-oriented smooth muscle cells (seen as red dots in cross section). Such intimal thickening is a part of the aging process.

Review slides 29 and 34 for cardiac muscle structure, including Purkinje cells (slide 29)..

D. Miscellaneous aspects of the circulatory system

  1. Whole mount of mesentary. Slide 100

    Remember that there are two types of slides 100, one sained with Giemsa, the other for elastin. In the Giemsa stained slides 100, medium-sized arteries and companion veins are seen in toto. Arteries are dark blue due to the thickness of their wall. The nuclei of smooth muscle cells in the tunica media are perpendicular to the long axis of the vessel, reflecting their circular arrangement. Veins appear red due to the red blood cells in the lumen, seen through their thinner walls. Lymphatics are recognizable by their large diameter and very thin walls, through which valves and lymphocytes may be seen. Lymphatics also have a bulbous appearance where the valves are located. In the slides 100 stained for elastin, the blood vessels cannot be seen very well.

    Examine the periphery of the whole mount for capillary loops. Size is the best criterion, approximately the diameter of one red blood cell. Larger diameter, simple endothelial-lined tubes are probably venules. Arterioles have one or two layers of smooth muscle cells.

  2. Lacteals. Slides 58 and 60

    Slide 58 is from the small intestine of a monkey fed a fat-rich meal. Fats are absorbed by lacteals and in this slide the distended lacteals or lymphatic capillaries appear as empty spaces, lined by endothelium, in the very cellular loose connective tissue underlying the intestinal epithelium. Their empty appearance is due to the extraction of the fats during tissue preparation. Compare the appearance of the lacteals in slide 58 with those in slide 60 where lacteals are collapsed.

  3. Microvasculature. Slides 113, 125, 65, and 117 (See diagrams on CS-6)

    In slide 113 abundant small blood vessels (capillaries, arterioles and venules) are clearly seen in the connective tissue surrounding the paraffin pellet.

    Slide 125, a plastic section of the trachea or a large bronchus, has very clear arterioles and venules in the connective tissue beneath the lining epithelium.

    Sinusoids can be seen in slide 65 of liver and slide 117 of spleen. They are usually of larger diameter than capillaries and are lined by an endothelium that may have phagocytic cells lying just outside of it. In the spleen, slide 117, the sinusoids are filled with red blood cells, making them easy to locate under low power. Here the endothelial cells are spindle-shaped and oriented longitudinally.

    In liver, the vascular spaces between the plates of hepatocytes can also be identified by the red blood cells within them. These sinusoids are lined in part by the phagocytic Kupffer cells, which can be demonstrated by their ability to phagocytize vital stains such as trypan blue (slide 65) or particulate matters, such as Ti02 (slide 111).

  4. Variability of blood vessels. Slide 27

    Blood vessels vary in structure depending upon their location and functional requirements. For example, in the lung (slide 27) the pulmonary arteries accompanying bronchi have thinner walls than might be expected for arteries of their diameter in the systemic circulatory system. The blood pressure in the pulmonary arteries is lower than that in their systemic counterparts of the same diameter. Pulmonary veins collect oxygenated blood from alveoli and from the pleura. Pulmonary veins run alone in the septa of the lung and have very thin-walls for their diameter. Bronchial arteries accompany pulmonary arteries but do not extend beyond the respiratory bronchioles. Bronchial veins are present only in the hilus of the lobes. Lymphatics accompany the larger airways and, with careful examination, can be found in their walls.

EXERCISE: Blood vessels may be found on slides of all organs; try the spermatic cord, slide 76.


  1. For each of the vessels in the list, match its characteristics from the panel beneath.

    Sinusoids in the liver
    Capillary in the back of the hand
    Arteriole in the tongue
    Capillary in the mucosa of the ileum
    Venule in a skeletal muscle
    The Endothelium is...
    Pericytes wrap the vessel at its narrowest.
    Smooth muscle layer is well developed.
    Smooth muscle layer is tenuous.
    Endothelial cells make tight junctions.
    The basement membrane is...
    Synthesized only by the endothelium.


  2. How is the dural sinus constructed? How does it compare with venous vessels of approximately equal size in the leg?


  3. What are the main structural and functional differences between lymph vessels and vessels of the arterial-venous system?

  4. What regions of which organs in the body lack lymphatics?

  5. Describe the main morphological changes in the aorta and large arteries during aging.

  6. Correlate the classically described layers present in major arteries and veins with their equivalents in the heart. Point out the morphological differences between the layers and correlate them with their function.

  7. In which of the small vessels does most fluid leakage and diapedesis of cells into surrounding tissue occur, normally and during shock?

  8. Why are veins and lymphatics more likely than arteries to be involved in metastatic spread of tumors?

  9. Is the circulatory system a "closed system". Give examples, if any, of where it is "open".

  10. Compare the pulmonary vessels with ones of similar diameter in the systemic circulation.

The following is a POP-QUIZ on the Circulatory System with questions reminiscent in style and difficulty to those to be asked in the exam. Don't waste the quiz by taking it before you are ready.

POP-Quiz on the Circulatory System

Link to the Hippocrates QUIZ on the   CIRCULATORY SYSTEM
Copyright 2006 New York University