What are the three layers of blood vessel walls?

(except capillaries)

Tunica Intimia: Inner most layer

Tunica Media: Middle Layer

Tunica Externa: Outermost layer

• simple squamous epithelium
• continuous with inner lining of the heart
• Vessels larger than 1mm have subendothelial layer (CT)
• has Internal Elastic Lamina between the Intima and Media

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• Outermost layer
• Mostly Collagen fibers to strengthen the blood vessel and help it attach to surrounding tissues

• Elastic arteries
• Muscular arteries
• Arterioles

• The Aorta and it's major branches: found proximal to the heart
• Elastic helps maintain blood pressure by stretching and recoiling with the pressure: acts as a pressure reservoir.

• Have more muscle than the other arteries
• are smaller than elastic arteries
• deliver blood to organs
• Involved with vasoconstriction and vasodilation

• Smallest of the arteries
• Important in vasoconstriction and vasodilation to direct blood flow to where it is needed and to regulate blood pressure.

• Microscopic vessels- big enough for a single RBC to pass through
• Where exchange in the blood takes place.

• Most common type of capillary
• the least permeable type of capillary
• Exchange happens between intercellular clefts between tight junctions
• For the brain, these capillaries have no clefts             -->blood brain barrier

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• Fenestrations are pores in the cells
• Increases permeability
• Found in the kidneys where a lot of fluid needs to leave the capillaries in the formation of urine.

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• Most permeable
• large fenestrations
• large clefts between cells
• Found in spleen: RBC graveyard
• Phagocytes stick their processes through the fenestrations to remove worn out RBCs from circulation

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What are the components of a Capillary bed?

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• Vascular Shunt: A short vessel that directly connect an arteriole and a venule
• metarteriole: Artery side of shunt
• thoroughfare: Vein side of shunt
• True Capillaries: the actual exchange vessels
• Precapillary Sphincters: smooth muscle that surrounds the root of each true capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary

• Formed from capillaries merging together.
• They are porous and permeable especially to WBCs which can leave to patrol the tissues and fight infection.

What are veins?

• Vessels that return blood to the heart
• have 3 layers but the middle layer is thinner than in arteries.
•  veins are floppy and can be somewhat collapsed but they have a large potential lumen.

Veins are blood reservoirs and can contain 65% of the blood.

During exercise more blood is returned to the heart from the veins due to muscle action.

Valves enforce one-way blood flow

varicose veins are veins that have incompetent one-way valves causing blood to pool in that area.

Usually found in the legs

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Blood pressure measured as mm Hg

(milimieters of mercury)