Tunica interna

Tunica media

Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system

Controls vasoconstriction / vasodilation of vessels

Tunica externa 

Collagen fibers that protect and reinforce vessels

Larger vessels contain their own personal blood vessels

– distal to elastic arteries; deliver blood to body organs

have more smooth muscle – have smaller lumen

so they are active in vasoconstriction

these are medium-sized ( from diameter of little finger to pencil lead)

you would take pulse pressure on these most often

smallest arteries; lead to capillary beds

control flow into capillary beds via vasodilation and constriction

called resistance vessels

are the smallest blood vessels 

Walls consisting of a thin tunica interna, one cell thick

Allow only a single RBC to pass at a time 

are abundant in the skin and muscles – most parts of the body

Endothelial cells provide an uninterrupted lining

In a small capillary, the endothelial cell may wrap around the lumen as your hand wraps a small glass

Adjacent cells are connected with tight junctions

Allow the passage of fluids through gaps between cells

Have tight junctions completely around the endothelium

Constitute the blood-brain barrier

Very restricted permeability

Found wherever active capillary absorption or filtrate formation occurs 

Ex. hormones from glands easily enter blood

Ex. nutrients easily enter blood

Ex. in kidneys ions exit the blood

Characterized by: 

An endothelium riddled with pores (fenestrations)

Greater permeability than other capillaries

Highly modified, leaky, fenestrated capillaries with large lumens

Found in the liver, bone marrow, lymphoid tissue, spleen

Ex. macrophages reach in and sample blood as it passes slowly through the spleen

Ex. in liver macrophages are part of lining

Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues

A microcirculation of interwoven networks of capillaries, consisting of:

Vascular shunts – metarteriole–thoroughfare channel connecting an arteriole directly with a  postcapillary venule

True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

Precapillary sphincter

Cuff of smooth muscle that surrounds each true capillary

Regulates blood flow into the capillary

smooth muscle keeps changing the blood flow through capillary beds

happens is response to local chemicals

a type of autoregulation

not all of your capillaries are open at one time

about 25% of capillaries are in use at rest

a way to coordinate blood flow

are formed when capillary beds unite

But still allow fluids and WBCs to pass from the bloodstream to tissues (inflammation

This is where the WBCs characteristically stick

They migrate out of the blood into the tissues here

Formed when venules converge

Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks

Large-diameter lumens, which offer little resistance to flow

Valves (resembling semilunar heart valves), which prevent backflow of blood  

Veins have much lower blood pressure and thinner walls than arteries

To return blood to the heart, veins have special adaptations like

Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins 

Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart 

Merging blood vessels, more common in veins than arteries

provide alternate pathways (collateral channels) for blood to reach a given body region

If one branch is blocked, the collateral channel can supply the area with adequate blood supply 

Cooperation of the heart, blood vessels, and kidneys

Supervision of the brain

Pulmonary circulation 

Systemic circulation 

The vascular system has two distinct circulations

veins to capillaries to veins

the usual is arterioles to capillaries to venules and veins and then back to heart

this vein carries most all of your digested foods directly to the liver and in the liver the vein changes into the wide capillaries (sinusoids) so the liver cells can pick through the incoming materials