Cardiovascular System

Wheater's (6th ed.), Ch 8: Circulatory System
Junqueira's (15th ed.), Ch 11: Circulatory System


The goal of this lab is to examine and describe the structural organization of the heart and the major classes of blood vessels.

Below is a schematic drawing showing the key structural features and relative sizes of skeletal, smooth, and cardiac muscle as you would observe them with high power. 


I. Slide Descriptions


Part I: Heart

A. Cardiac Muscle: Webslide 0026_B: Cardiac Muscle, monkey, H&E [DigitalScope]

Cardiac muscle fibers can be seen in both cross and longitudinal sections.  Measure fiber diameters and note blood vessels filled with RBCs between the fibers.  Fine details of the myocardiocytes can be seen, especially myofibrils, cross striations, intercalated disks, and mitochondria next to the nuclei.
Note also the simple squamous epithelium covering both free surfaces of the heart in this section.  How can you tell whether each surface is endocardium or epicardium?  Based on a low-power view, can you tell whether this section was taken from an atrium or a ventricle?



B. Heart wall
Slide 98HE (heart wall, rt atrium & vent, H&E) [DigitalScope]
Slide 98-N (heart wall, rt atrium & vent, trichrome) [DigitalScope]

Slide 98HE is stained with H&E and Slide 98-N is a similar section stained with Aldehyde Fuchsin-Masson. You should look at both stains. Locate the atrioventricular sulcus that contains a branch of the coronary arterial system (a muscular artery that exhibits moderate intimal thickening) embedded in the epicardial fat.

Look at the connective tissue present between the ventricle and atrium. This is part of the cardiac skeleton into which cardiac muscle inserts. A leaflet of an A-V valve takes origin from the cardiac skeleton.

Look at the atrial and ventricular endocardium, consisting of an endothelial lining and the underlying connective tissue (the endothelium is often stripped away during processing, but there are some areas where it has been preserved). With low power, locate the Purkinje fibers present immediately beneath the ventricular endocardium in the H&E and trichrome-stained sections (note the appearance of these fibers in cross and longitudinal orientations). These conducting fibers are larger and paler staining than the cardiac muscle fibers. Note the meshwork arrangement of the cardiac muscle fibers in the myocardium.

These slides also offer excellent views of capillaries within the myocardium.



C. Interventricular septum and valves
Slide 99H&E (heart septum, H&E) [DigitalScope]
Slide 99HM (heart septum, H&Masson) [DigitalScope]
Slide 99M (heart septum, trichrome) [DigitalScope]

The interventricular septal connective tissue is present, and, in most sections, a distinct unit of specialized cardiac muscle, the A-V bundle (of His), traverses the septal connective tissue (a thin group of muscle fibers surrounded by dense c.t.). The A-V bundle is easiest to see in slide 99HM, although you should also be able to recognize it the H&E-stained section as well. In these slides, the bundle fibers are cut in cross section and they are similar in size and staining to that of normal cardiac muscle fibers, although in some of your sections the fibers may more closely resemble Purkinje fibers (which is what they are). On one side of the section, a leaflet of the aortic valve is present. On the other side, portions of an A-V valve are present, as are bits and pieces of collagenous chordae tendinae. In slide 99HE, there is a piece of chorda tendinae actually attached to the valve, whereas in slide 99M, the pieces are unattached and out in the ventricular lumen (the attachment site is out of the plane of section.


Part II: Blood Vessels

The table below summarizes the size ranges, major features, and important roles of the major blood vessel types that you are expected to bw able to identify in tissues and organs.

Type of Vessel

Outer Diameter (Approx.)

Intima features

Media features

Adventitia features

Roles in Circulatory System

Elastic arteries

> 10 mm

Endothelium; connective tissue with smooth muscle

Many elastic lamellae alternating with smooth muscle

Connective tissue, thinner than media, with vasa vasorum

Conduct blood from heart and with elastic recoil help move blood forward under steady pressure

Muscular arteries

10-1 mm

Endothelium; connective tissue with smooth muscle, internal elastic lamina prominent

Many smooth muscle layers, with much less elastic material

Connective tissue, thinner than media; vasa vasorum maybe present

Distribute blood to all organs and maintain steady blood pressure and flow with vasodilation and constriction

Small arteries

1-0.1 mm

Endothelium; connective tissue less smooth muscle

3-10 layers of smooth muscle

Connective tissue, thinner than media; no vasa vasorum

Distribute blood to arterioles, adjusting flow with vasodilation and constriction


100-10 µm

Endothelium; no connective tissue or smooth muscle

1-3 layers of smooth muscle

Very thin connective tissue layer

Resist and control blood flow to capillaries; major determinant of systemic blood pressure


10-4 µm

Endothelium only

A few pericytes only


Exchange metabolites by diffusion to and from cells


10-100 µm

Endothelium; valves sometimes present

Pericytes and scattered smooth muscle cells


Drain capillary beds; site of leukocyte exit from vasculature

Small veins

0.1-1 mm

Endothelium; valves sometimes present; connective tissue with scattered smooth muscle fibers

Thin, 2-3 loose layers of smooth muscle cells

Connective tissue, thicker than media

Collect blood from venules

Medium veins

1-10 mm

Endothelium; connective tissue, with valves

3-5 more distinct layers of smooth muscle

Thicker than media; longitudinal smooth muscle may be present

Carry blood to larger veins, with no backflow

Large veins

> 10 mm

Endothelium; connective tissue, smooth muscle cells; prominent valves

> 5 layers of smooth muscle, with much collagen

Thickest layer, with bundled longitudinal smooth muscle

Return blood to heart


A. Muscular (medium) artery and veins
Webslide 0174_B:
Muscular Artery & Vein [DigitalScope]

On the right side of WebSlide 174 is a muscular (medium) artery and its companion medium vein. Focusing on the artery first, note the prominent internal and external elastic laminae, which are stained pink in this slide. The tunica media contains several layers of circumferential smooth muscle mixed with collagen. The tunica adventitia which is slightly thinner than the tunica media grades into surrounding fatty connective tissue. The companion vein is just below and the the left of the artery. Note the thickness of the wall compared to the overall diameter of the lumen.



B. Medium and small arteries and veins
UMich Slide 42 (mesentery, H&E) [DigitalScope]
UMich Slide 95M (mesentery, trichrome-elastin) [DigitalScope]

Slides 42 and 95M are mesentery spreads that contain numerous excellent examples medium and small arteries and their companion medium and small veins. Study the arteries first in slide #42 , and/or #95 (trichrome). Note the thin intima, the distinct internal elastic lamina (IEL) and the media composed of circularly oriented smooth muscle cells. The media also contains some elastin and abundant “reticular” collagen, as well as specific proteoglycans. There is sometimes a condensation of elastic fibers in the outer portion of the media that may be an “external elastic lamina”, which varies from artery to artery. The adventitia is the dense, irregular connective tissue surrounding the media that varies in thickness. Remember that in blood vessels the components of the media are arranged circularly, while those of the adventitia are oriented longitudinally. In vessels where the media is too thick to be supported by simple diffusion, vasa vasorum (e.g. shown here in slide 95M) may also be observed in the adventitia.

The structure of the companion veins in slide #42 and/or #95 (trichrome) is less regular and may be difficult to understand at first, but still consists of the same basic layers as arteries with a tunica intima, media, and an adventitia, although the media is usually much less muscular and less organized compared to the companion artery. Conversely, the adventitia is usually thicker in veins and may often have some bundles of longitudinal smooth muscle as well as vasa vasorum. In some sections, you may be able to see a thin internal elastic lamina beneath the venular endothelium (particularly in slide #95M).



C. Arterioles and venules
Webslide 0098_B:  Urinary Bladder, monkey, H&E [DigitalScope]

Several good examples of arterioles and venules can be observed within the dense connective tissue in the upper left region of the slide.   In particular, examine carefully the series of arterioles and venules about 50 to 100 µm in diameter that are located about one-third of the way across the slide from the left edge of the section.   Identify as specifically as possible each of these vessels based on the amount of smooth muscle in relation to the vessel diameter.  Also note the valves in one of the venules.


D. Capillaries
Slide 98HE (heart wall, rt atrium & vent, H&E) [DigitalScope]

The heart is an excellent organ to see examples of capillaries, which are abundant within the myocardium. Typical capillaries are just small enough to allow single red blood cells (~7um in diameter) to pass.



E. Elastic Artery
Aorta: Webslide 0022_B:
Aorta, monkey, H&E [DigitalScope]

  • Tunica intima with endothelial nuclei, longitudinally-oriented subendothelial fibers of reticulin (type III collagen) and longitudinal smooth muscle.
  • Tunica media with multiple wavy light pink elastic lamina (up to 70 such laminae may be counted in a human aorta) and interleaving circular smooth muscle fibers.
  • Tunica adventitia with some loose fatty areolar connective tissue and inner dense irregular collagen containing vasa vasorum.



F. Large Vein
Vena Cava / Large Vein: Webslide 0023A_B: vena cava, monkey, plastic, c.s., H&E [DigitalScope]

Compare this cross section of a vessel with Slide 22 from same animal. The original circumference of 10-15 mm is only partly sampled in your slide.

Note the following features:

  • Variable intima with some longitudinal fibers, including smooth muscle.
  • Skimpy media (1-3 layers of circular smooth muscle.
  • Thick adventitia with plentiful longitudinal smooth muscle bundles like islands in a sea of collagen, with vasa vasorum and surrounding connective tissue --note that the connective tissue of the adventitia here has frayed apart somewhat, so a lot of the empty spaces seen here are due to this artificial separation. There is also extravasated (escaped) blood found within the connective tissue, which is an artifact from sample preparation.


III. Pathology Correlate

Atherosclerosis - [DigitalScope]

These sections came from a 62-year-old patient with a known diagnosis of coronary artery disease (CAD) who presented with a chief complaint of chest pain (angina) upon exertion. A review of the patient's history was significant for long-standing hypertension, hyperlipidemia, and type II diabetes, for which the patient was receiving care but was nevertheless not well-controlled. The patient was evaluated with angiography and subsequently underwent surgery to have coronary arterial bygrass grafts (CABG) for the left anterior descending and left circumflex arteries. The sections shown are of a portion of the left anterior descending artery that was bypassed and shows evidence of significant atherosclerosis.

As you look at the slide, should note that the MOST of the pathological changes are evident within the TUNICA INTIMA:

  • Focus on the section on the right side of the slide [example] since it's easier to make out what's left of the original lumen of the vessel, which is lined by a simple squamous endothelium [example].

  • About 2/3 of the original lumen has been taken up an athersclerotic plaque, within which you can see "foam cells" [example] that have ingested cholesterol.

  • The cholesterol accumulation is so great that it also collects extracellularly as large, elliptical "crystals" (also called cholesterol clefts) [example] --of course, since they are lipid, they are extracted with tissue processing, so they appear as empty spaces.

  • Within the plaque, evidence of neovascularization can be seen [example] due to the significant collagen deopsition and smooth muscle cell proliferation that has become so extensive that the tissue has outstripped what normally could be supplied by simple diffusion from the lumen.

  • Within the plaque, you may also see amorphous, basophilic debris, often associated with cholesterol clefts [example], which are examples of dystrophic calcification.

By comparison you can refer back to healthy coronary artery shown in slide 98HE to see how a typical artery should appear (note the coronary artery in sldie 98HE shows some degree of "intimal thickening" that happens with age).


The following additional Webslides may be used to review this material:


Webslide 0013_B: Femoral vessels & Nerves, Cat, H&E [DigitalScope]

Webslide 0001_B: Mammal artery and vein (and nerve), H&E [DigitalScope]

Webslide 0002_B: Human artery and vein (and lymph node), H&E [DigitalScope]

Webslide 0150_B: Aorta, TB&AF (note: elastin appears clear in this stain) [DigitalScope]

Webslide 0175_B: Abdominal vein, H&E [DigitalScope]

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Updated 10/5/23 - Velkey