Pericyte surrounding a continuous (somatic) capillary

1. Pericyte: large, prominent nucleus
2. Continuous capillary: note the pinocytotic vesicles within the cytoplasm

A: Pericyte    B: RBC    C: Endothelial Cell

1. Pericyte functions
1. has contractile capability to regulate blood flow within an individual capillary
2. Can differentiate into smooth m. or endothelial cell after injury

Arrowheads point to somatic (continuous) capillary

• capillaries are thin-walled to allow diffusion
• continuous capillaries have no breaks in the cell membrane
• utilize pino-/transcytosis
• occluding junctions often seen in somatic capillaries of nervous tissue; they help form blood-brain barrier

Fenestrated capillary

• "windowed" to increase diffusion rate and to allow larger molecules to diffuse
• "windows" may have diaphragm or not
• Found in intestines & kidneys

A: arteriole    B: Pericyte   C: smooth m.

Arterioles:

• only contain 1-3 smooth muscle layers in media
• direct blood flow to capillaries via dilation / constriction

A: arteriole   V: venule

• Note the difference in shape & the presence of RBC's in the venule
• Remember: venules run with arterioles

Both veins & arteries contain:

• endothelium
• basal lamina
• loose CT
• smooth m
• fibroblasts
• elastic/ reticular / collagen fibers
• ground substance

A: Lymph vessel    B: Arterioles   C: venules

Note: Relative size of lumen to vessel wall & contents of each vessel (lymph does NOT have RBC's; veins & arteries do)

A: Vein    B: muscular artery   C: tunica media  

D: tunica adventitia   Arrow: Tunica intima

1. Tunica intima:
1. Composed of endothelium + subendothelium + CT + internal elastic membrane
2. contains von willebrand factor necessary for clotting
2. Tunica media:
1. composed of smooth m. & elastic lamella
2. Thicker in arteries than veins
3. Tunica adventitia
1. composed of collagen, elastic fibers, fibroblasts, & macrophages
2. contains vasa vasorum & vasa nervosum
3. Thicker in veins than arteries

• 3 layers of the artery
• Note: corrugated endothelium due to presence of elastic fibers in the internal elastic membrane (characteristic of muscular arteries)
• Examples: brachial, renal, ulnar, femoral

Arrows: Endothelial layer

• Veins have tunica layers, but are less defined
• Note: tunica adventitia is the largest layer in veins
• veins have high capacitance due to their inability to distend b/c of longitudinally arranged smooth m.
• May contain up to 70% of circulating blood

• Note: although muscular artery is collapsed, it still maintains a regular shape; additional tip: corrugated endothelium w/ elastic staining black
• N = nerves

Left: Muscular artery    Right: Medium vein

• Note: Difference in TM / TA layers
• TM in artery is thicker & more muscular
• TA in vein is thicker

Arrowheads: Endothelium   Arrows: Tunica intima

• This is a muscular artery due to presence of internal elastic membrane
• NOT a large artery b/c the TM is not too thick

Note presence of RBC's in veins

Arrows: Endothelium

Note: large adventitia & lack of elastic fibers

Vein 

Arrows: endothelium

Note: This is NOT a lymph vessel b/c of presence of RBC's in lumen

Wall of large artery (aorta)

Arrows: Elastic fibers

Note:

• LARGE width of TM and presence of elastic fibers within the TM (allows them to withstand large pressures from the heart)
• LACK of corrugated endothelium / internal elastic membrane
• Examples: aorta, subclavian & common carotid

Arrowheads: vasa vasorum

• Are small blood vessels that supply the walls of large arteries
• Found in the tunica adventitia

Wall of large vein (vena cava)

• Thick tunica adventitia containing longitudinal bundles of smooth m. (appear in X section)

A: medium vein   B: Muscular artery  

Arrows: internal elastic lamina 

Green: tunica adventitia  Yellow: Tunica media

Black arrow: arteriole

Blue arrow: venule

Note: venule has mulitiple nuclei, therfore it can't be a capillary

Sinusoid

• function like capillaries in liver, bone marrow, spleen, and endocrine organs
• They have discontinuous, fenestrated endothelium for passage of larger molecules secreted by these organs
• They ARE blood vessels; distinguish from lymph or vein by presence of RBC's / fenestrations, respectively

Atherosclerotic Plaque

• accumulates in tunica intima and protrudes into lumen, blocking blood flow
• Common sites: aorta, coronary / cerebral a.a.
• Can cause heart disease / MI, stroke and abdominal aneurysms
• Mechanism:
• Chronic hyperlipidemia causes LDL to migrate into intima where it becomes oxidized
• Oxidized LDL forms foam cells by either being taken up by macrophages or smooth m. cells
• Accumulation of foam cells = plaque

Arrow: Pericyte

Arrowheads: continuous capillaries

Vein valve

• Is extension of tunica intima
• FYI: blood is drawn in direction of blood flow from veins so as to not puncture the valves

A: Subendocardium  B: Cardiac M. 

C: Purkinje Fibers  D: Endocardium

• Purkinje fibers are modified that conduct electrical impulses for contraction cardiac myocytes
• They are located in the subendocardium
• They appear large & pale due to high glycogen content
• They are NOT contractile

A: subendothelium  B: Purkinje fibers

C: myocardium