Cardiovascular System

Learning Objectives:

o   Describe the histological features of the three layers of the heart, and correlate those features with the functions.

o   Correlate the heart conduction system with its histology, location, and function

o   Correlate the histology of the cardiac valves with their anatomical morphology, locations, and functions.

o   Describe the structural composition of the three histological layers (tunics) of a typical blood vessel and correlate it with the vessel’s location and function

o   Compare the histology, location, and functions of different categories of arteries and veins

o   Explain the structures, locations, ad functions of the three types of capillaries

No Slide:

What are the three layers, or “tunics,” of the arterial wall? As you move from arteries to veins, what change would you expect to see in the relative sizes of each of the tunics? Which of the layers of a larger artery is composed of simple squamous epithelium? Which layer is composed of smooth muscle?

Tunica intima, media and adventitia. Arteries have thicker layers of smooth muscle, while veins have thicker adventitia layers. Simple squamous epithelium is tin the tunica intima, while smooth muscle is in the tunica media. 

Compare and contrast the structure of conducting, distributing, and small arteries and arterioles.

See lecture notes for full comparison.

SLU Slide 47: Aorta

Iowa Virtual Slidebox: Aorta (295)

Draw a cross section of the aorta, paying attention to the relative thickness of each layer.

What type of connective tissues exist in the aorta?

The aorta has a thick tunica media and a thick tunica intima. The internal elastic lamina is not prominent, but blends in with the elastic lamina of the tunica media, which has abundant elastic fibers. 

Compare and contrast the cross section of an artery and vein in terms of size of the lumen, thickness of the wall, and any other features you might notice. List three ways that you can differentiate a vein from an artery in a histological section:

Arteries have a smaller lumen and thicker walls (particularly tunica media). Veins will sometimes have valves.

Why do arteries and veins differ in the structure of their walls?

Arteries are under higher pressure. They also have more smooth muscle which enables them to vasodilate or vasoconstrict in response to autonomics.

Slide 66: Vena Cava

Iowa Virtual Slidebox: Vena cava (296)

Compare this section of the vena cava to that of the aorta. What similarities exist, and what differences?

Both have elastic fibers, though the fibers in the vena cava are more sparse and not arranged in concentric lamina. This makes the IEL of the vena cava appear more prominent, though it’s the same size and thickness in both.

The Adventitia is much thicker in the vena cava, while the media is thinner in the aorta. Both have a similarly sized intima.

Variety of tissues: Look for capillaries Try kidney, skin, and small intestine sections.

Capillaries may be difficult for you to find. How could you identify a capillary? Which aspect of capillary structure is directly related to its function?

Capillaries comprise a single endothelial cell. This allows for the passage of oxygen and other particles easily. 

Electron Micrograph: Capillaries

Examine several electron micrographs of capillaries from the EM atlas or another source. What distinguishes them from one another? How can you correlate these images to the ones that you can see in LM?

What specific structure allows us to differentiate skeletal and cardiac muscle tissues? Remind yourself of characteristics of the other types of muscle as well.

Cardiac muscle has Intercalated discs, branching cardiomyocytes, centrally located nucleus in axial section, etc. 

EM: Intercalated Disc

Examine this section to note the cell-cell junctions present at the Intercalated disc. Also note other features of the cytoplasm amd myofibrils.

Iowa Virtual Slidebox: Heart (109)

Explore the myocardium in this slide. Note the numerous capillaries within the connective tissue between cardiomyocytes.

 

Compare the two “surfaces” of this section. The endocardium is a single layer of endothelial cells supported by a layer of small, thin collagen fibers comprising the subendothelial layer. There is an additional subendocardial layer, but it is difficult to distinguish in this section. The opposite surface is the epicardium (in gross anatomy, we call this the visceral layer of the pericardium). The surface is lined by a flat mesothelium, and deep to this iis a layer of connective tissue rich in adipocytes. If you explore this layer you will also find blood vessels and nerves.

 

Which layer is which? Draw or screenshot the two and label the features that distinguish them.

SLU Slide 68: Semilunar valve

Iowa Virtual Slidebox: Semilunar Valve (108)

While the atrioventricular valves have different gross anatomy from the semilunar valves, their microscopic anatomy is similar. What layer of the heart do the semilunar valves extend from? What types of tissue comprise these structures? Examine an anatomical illustration of a semilunar valve and try to put this histological section into context.

The semilunar valves extend from the myocardium, with a core of pale-staining dense connective tissue. The valves are lined on both sides by endocardium, which may be thicker on the ventricular side. You may also see some smooth muscle cells.

SLU Slide 67: Heart with Purkinje fibers (Note that large, blue oval structures are parasites and that this is not human tissue.)

Iowa Virtual Slidebox: Heart – endocardium and purkinje fibers (110)

What are Purkinje fibers? What role do they play in the cardiac conduction system?

Purkinje fibers are modified cardiomyocytes that conduct electrical impulses throughout the myocardium via gap junctions. 

With your microscope, travel along the endocardium of this section. How can you distinguish Purkinje fibers from the surrounding connective tissue and myocardium? (Discuss cytoplasm color, size, nuclei, etc.)  Make a sketch or screenshot comparing the size and morphology of Purkinje fibers with those of regular cardiac muscle cells found in the myocardium.

Purkinje fibers much larger are lighter in color than normal cardiomyocytes. They will also typically have two centrally-placed, large nuclei. The pale staining is due to abundant glycogen in the sarcoplasm. Under high magnification, you may be able to make out some peripherally-located myofibrils, but they are much less abundant in purkinje fibers than in regular cadiomyocytes. Under high magnification you can sometimes make out mitochondria.

Metacognitive tip: Make a table (or tables) comparing the tunica intima, tunica media, and tunica adventita in different classes of arteries, capillaries, and veins.