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Histology - Circulatory System

What composes the circulatory system?
The cardiovascular system and the lymphatic vascular system.
Describe three general aspects of arteries.
1. Thick-walled, muscular tubes.
2. Carry high-pressure blood.
3. Branch into smaller arterioles.
Describe four general aspects of veins.
1. Thinner-walled and more fibrous than arteries.
2. Carry low-pressure blood.
3. Capacitance vessels = carry most of blood in body.
4. Branch into venules.
Describe three general aspects of capillaries.
1. Smallest of the blood vessels.
2. Only one cell thick!!!
3. Allow for exchange of materials.
What are the four size classifications of arteries?
1. Large artery - elastic artery/blood conducting artery.
2. Medium artery - muscular artery/distributing artery. (Most common in body).
3. Small artery.
4. Arteriole.
What are the actual sizes of the four classifications of arteries?
1. Large artery = >1cm
2. Med. = 2-10mm
3. Small = 0.1-2mm
4. Arteriole = 10-100um.
What are the four size classifications of veins?
1. Large vein - vena cava, pulmonary vein.
2. Med vein - external jugular vein. (most common in body).
3. Small vein.
4. venule.
What are the actual sizes of the four classifications of veins?
The same as for arteries!!!

1. Large vein = >1cm
2. Medium vein = 1-10mm
3. Small vein = 0.1 - 1mm
4. venule = 10-100um
What are the three layers that compose the vascular walls of the arteries and veins?
1. Tunica intima (inner)
2. Tunica media (middle)
3. Tunica adventitia (outer)
What structural differences exist between veins and arteries?
The vascular walls of veins is thinner and the lumen is wider.
Which vascular layers are prominent in veins and which are prominent in arteries?
In arteries have a prominent tunica media. In veins have a prominent tunica adventitia.
In arteries, in what direction are tunica intima oriented?
Longitudinally. So won't see concentric rings as do in tunica media.
In arteries, what is the tunica intima composed of?
It's just an endothelial layer. A single layer of squamous cells.

Below the the endothelial layer are fibroblasts, collagen, and elastic fibers.

Also have an INTERNAL elastic membrane. This is distinctive of muscular arteries.
In arteries what is the direction of the tunica media?
It is circularly oriented. So in a histo slide will see many rings.
What is the tunica media composed of in arteries?
1. Smooth muscle cells.
2. Elastic fibers.
3. EXTERNAL elastic membrane. This is distinctive of muscular arteries.
In arteries what is the tunica adventitia composed of?
1. Fibroblastic connective tissue.
2. Elastic and collagen fibers.
3. VASA VASORUM - nourishing the blood vessels.
Which layers of the artery undergo degenerative changes in pathology like atherosclerosis or arteriosclerosis?
The tunica intima and the tunica media.

So the more inner layers.
In a histo slide, is the tunica intima prominent?
No, it is a thin layer. Much smaller than the tunica media.
Where do you see fenestrated membranes in arteries?
In the tunica media. Have fenestrated lamellae of elastin. So have layers of elastic fibers, but the fibers are not tightly in association with one another.

This is where you have oxygen exchange for the artery.
When looking at a histo slide what will you see in the tunica media in a large artery?
Clastic laminae and some smooth muscle cell nuclei. The smooth muscles are really hard to see. But you can easily see the sea of elastic fibers.

You see this because large arteries are often called elastic arteries. They have the highest pressure, so need elastic character.
What type of vessels will be in the vasa vasorum?
Venules and arterioles.

So really small.
In a histo slide, what will you see in the tunica media in a medium sized artery?
These arteries are also called muscular arteries. So will have lots of smooth muscle in the tunica media.

Will have internal elastic membrane and external elastic membrane prominent (in the tunica intima and media respectively).

Will also see elastic and collagen fibers in the tunica adventitia.
Where do you find vasa vasorum and nerve bundles? What do they look like on a histo slide?
Find both in the tunica adventitia.

Vasa vasorum looks like a round circle. The nerve bundle looks like an elongated tear drop.
What is the difference between conducting, large, and elastic arteries?
They are all the same.
What do the elastic walls of the arteries help to do?
Reduce changes in blood pressure associated with cardiac cycles. This ensures an uninterrupted flow of blood through the capillaries. This also helps maintain the blood pressure high.

Without elastic walls the pressure would suddenly rise and drop along with the cardiac cycles.
What is atherosclerosis?
Characterized by deposits of yellow (fibrofatty) plaques in the tunica intima of large AND medium arteries.

So getting plaque build up.
What are the symptoms of atherosclerosis?
Have narrowing of the arterial lumen which leads to ischemia and blockage.

Can get as a result a stroke, retinal arterial ischemia (eye problem), and coronary heart disease.
On a histo slide what will you see as a result of atherosclerosis?
Thinning of T. media.
T. intima greatly thickened.
-these two are the opposite of the normal state.

Will see the thrombus blocking the lumen.
What is arteriosclerosis?
Characterized by rigidity and fibrosis of the vascular walls.
On a histo slide what you see as a result of arteriosclerosis?
Accumulation of collagen and elastic fibers in the T. intima. Will see fragmented internal elastic membrane.

Loss of smooth muscle and elastic gibers in the T. media and their REPLACEMENT with collagen fibers and mineral salts. So will also see disorganized fragments here.
What diseases is arteriosclerosis associated with?
Hypertension and diabetes mellitus.
What is an aneurysm?
A ballooning out of an artery. This occurs because of a weakness in the arterial wall. Which may result in a rupture.

This weakness can be caused by age-related displacement of elastic fibers with collagen or genetic conntective tissue disorder like Marfan syndrome or Ehlers-Danlos syndrome.
How many layers are the vascular walls of small arteries and arterioles composed of?
Three layers. Just like the larger arteries.
What are the main resistance vessels in the circulatory system?
The arterioles. They can regulate the flow of blood for particular organs and tissues during exercise or stress.

They do this by controlling blood flow to capillary networks by contraction of the smooth muscle cells.
What is unique about the T. intima of small arteries? How about arterioles?
Has smooth muscles and an internal elastic membrane.

The T. intima of arterioles lacks an internal elastic membrane.
What is unique about the T. media of small arteries? How about arterioles?
Small arteries contain 8-10 layers of smooth muscle. The arterioles contain 1-2 layers.

The big difference is that arterioles lack the collagen fibers that small arteries have.

Both LACK external elastic membranes that the bigger arteries have.
What is unique about the T. adventitia of small arteries? How about arterioles?
The arterioles have a thin, ill-defined sheath of connective tissue.

The small arteries have connective tissue and some elastic fibers.
What is different about the tunica intima of veins in comparison to arteries?
Do not have a subendothelial layer and do not have an internal elastic membrane.
What is different about the tunica media of veins in comparison to arteries?
Lack elastic fibers and external elastic membrane.

But veins have collagen fibers in the T. media that arteries do not.
What is the structure of the T. adventitia in veins?
Have smooth cells! (longitudinal bundles in large veins). Do not see this in arteries. This is what makes the t. adventitia so prominent in veins.

Have collagen fiber layer with fibroblast cells.

Like arteries, have a vasa vasorum and nerve fibers.
If looking at a histo slide, how do you quickly differentiate if you are looking at an artery or a vein?
The artery will have well organized cells while the veins will be loosely organized.
In an H&E stain what color is a vein and what color is a nerve bundle?
A vein is pink while a nerve is light white colored with blue nuclei.
What is the exception to having elastic fibers in veins?
Normally veins do NOT have elastic fibers. But the exception is the muscular vein from the leg. It has elastic fibers and smooth muscle which are numerous in the veins of the extremities together in the t. media and adventitia. So the layers are not separated out and the fibers and muscle is not separated out.

Also have an internal elastic membrane. So really this is like an artery. The only was to distinguish is the fact that in the vein you don't have separation of the t. media and adventitia.
If shown a arteriole and venule or small artery and vein in a histo slide, how do you differentiate the two?
The vascular wall of venules and veins is thinner and the lumen is wider than small arteries and arterioles.
What are the three muscle layers of the heart?
1. Endocardium (inner) - an epithelial layers and underlying subendothelial connective tissue.
2. Myocardium - a thick layer of cardiac muscle cells.
3. Epicardium - the outermost layer with connective tissue. Here have blood supply to heart.
How would you distinguish cardiac muscle in a histo slide?
The myocardium looks like meat.

The endocardium is really thin and connected to the meat of the myocardium.

The epicardium have wholes in it for blood vessels. And it will look like connective tissue because that what it is.
What do capillaries consist of cell wise?
They have a single layer of endothelial cells and their basal lamina.

What is unique is that they only have a single layer of cells.
What factors allow capillaries to perform their tasks?
The low rate of blood flow, thin walls and large surface area allow capillaries to exchange gases, metabolizes and waste products between surrounding tissue.

Remember these three characteristics of capillaries.
What is the diameter of a capillary?
Enough only to fit on RBC at a time.
What are arterioles, metarterioles, and capillaries?
Arterioles branch to form metarterioles which branch to form capillaries.
What do metarterioles contain?
They possess individual smooth muscle cells that act as precapillary sphincters.
What is the capillary wall composed of?
A single layer of squamous endothelial cells connected by tight junctions.
What vascular layers do capillaries have?
Only have a tunica intima and basement membrane!!!!

Very unique.
Where in the capillary is the basement membrane located?
On the outside of the endothelial cells. So this NOT the lumen side. The external surface is surrounded by a basal lamina.
What type of cells surround capillaries?
Pericytes. They surround the basal membrane.
What do pericytes do?
Pericytes are contractile and are able to form collagen fibers, endothelial cells, smooth muscle cells, and new pericytes (so are multipotential).

They function in the regulation of capillary sizes.

They also play repair and support roles.
What are the three classifications of capillaries?
1. continuous - no pores in wall. Possess a continuous endothelial linking throughout.
2. fenestrated - pores in wall.
3. sinusoidal - Discontinuous endothelial cells and contain many large fenestrate. Also have enlarged diameter (30-40um).
Where do you find continuous capillaries?
In muscle, skin, nervous and connective tissue.

So would see in the three types of muscle, in the CNS, in lung and in the mesenteries.
What is the intracellular junction?
Where you have tight junctions in endothelial cells.
What is the marginal fold?
Where you have an overlap or extension of cell layer at an intracellular junction.
In a histo slide what do you use to identify a continuous capillary?
The large number of pinocytotic vesicles in the plasmalemma.

This is sometimes called a field of micro vesicles.
Where are fenestrated capillaries found?
Often in the GI tract, kidney and endocrine glands.

So these are places where will do absorption or excretion. So need the windows in the capillaries.
What is the structure of a fenestrated capillary?
Fenestrae (windows) are present and are covered by diaphragms (except for renal glomeruli). So have a basal lamina then a diaphragm above it and endothelial cells above that. The diaphragm allows for specific movement of ions, hormones and nutrients.

Except at the fenestrae, the endothelium has a constant thickness.
Where are microvesicles located with respect to fenestrated capillaries?
In the endothelial cells will see some small microvesicles. These are like the pinocytosis vesicles that see in cont. capillaries.

So microvesicles will be next to the fenestrae in fenestrated capillaries.
What is the size range of fenestrae pores?
How large are the diaphragm pores?
2-3nm thick. This is thinner than the plasma membrane. This allows for the built movement of ions, hormones and nutrients.

So the size of the fenestrae will vary exposing more or less surface area of diaphragm.
What characteristic distinguishes continuous from fenestrated capillaries?
The presence of fenestrae. Also, important to note that both will have microvesicles, but will just have a lot more in continuous capillaries.
What is another term for sinusoidal capillaries?
Discontinuous capillaries.
Where are sinusoidal capillaries located?
In the liver, bone marrow, and lymphoid organs including the spleen.
What is the lumen diameter of sinusoidal capillaries?
30-40um. This is the largest diameter of the three types of capillaries.
What is the defining characteristic of sinusoidal capillaries?
Gaps separate endothelial cells and also have discontinued gaps in the basal lamina.

Really important to note that the basal lamina has gaps! Very unusual.
Why are discontinuous capillaries also called sinusoidal capillaries?
When look at a cross section of discontinuous capillary, will see a discontinuous basal lamina that will lead to sinusoids, or little gaps.

So because of the presence of sinusoids call the capillaries sinusoidal capillaries.
What type of pore system can the endothelial cells of capillaries contain?

Are talking about the transcapillary molecular transport!
Small pore system: 1-10nm in diameter
Large pore system: 50-70nm

The small pores are discontinuous between endothelial junctions.

The large pores are represented by fenestrae and microvesicle transport.
What type of transport is in intracellular junctions?
Small pore transport. This is between endothelial cells.

Water and small solutes simply diffuse through the intracellular junctions.
What type of transport is in microvesicular transport?
Large pore system.

Have microvesicles moving across the plasmalemma of the endothelial cells of the capillary.

So collect things into the microvesicles and move to the other side. Thus why they are called pinocytotic in nature.

Even can have continuous capillaries, can still have transport of larger things.
How large of molecules can fenestrae transport?
Small molecules, less than 11nm, so the small pore system.

But select fenestrae can transport larger molecules (~25nm glycogen) of the large pore system.

This is due to the molecular transport that occurs at the fenestrae that is due to the diaphragm determining what can be transported.