Blood Vessels: Arteries, Capillaries and Veins

Arteries

carry blood away from the heat to capillaries

Primary function of arteries

Maintain pressure generated by the heart and control blood distribution to tissues. Arterial system called pressure reservoir.

3 artery types

Elastic, Muscular, and Arterioles

Elastic arteries

Largest, closest to the heart, deal with the highest pressure, have large lumens (2.5 cm-1 cm), and elastic fibers in the tunica intima, tunica media, and tunica externa.

Muscular arteries

Distal to elastic arteries, middle sized (1cm-0.3cm), thickest tunica media relative to lumen size of any vessel, Has prominent Internal and external elastic membranes, Not as elastic as elastic arteries, more for maintaining pressure

Arterioles

Distal to muscular arteries, Smallest diameter arteries ( 0.3 mm to 10 μm), Very thin tunica externa, tunica media only 2-3 smooth muscle layers thick, Branch into capillary beds, Use vasoconstriction and vasodilation to regulate blood distribution and blood pressure.

Capillaries

Only vessel that allow exchange of fluid and molecules between blood and interstitial fluid.

Capillary filtration

movement of fluid and molecules out of the capillary from blood plasma into interstitial fluid.

Capillary reabsorption

movement of fluid and molecules into the capillary from interstitial fluid into blood plasma

Capillary beds

They are usually in networks of many interconnected capillaries called capillary beds.
Pre-capillary sphincters are small circular smooth muscles that constrict and dilate to regulate blood flow within a capillary bed

Continuous capillaries

most common, complete endothelium surrounded by complete basal lamina, found in most locations, allows the typical rate of filtration and reabsorption

Fenestrated capillaries

surrounded by a complete basal lamina, found in location where filtration and reabsorption need to be fast, allow faster faster rate of filtration and reabsorption

Blood sinusoids

Endothelium with large gaps between endothelial cells and large fenestra. Incomplete basal lamina.
No more size restriction = entire cells can move in and out of blood. Rare, only in locations where cell need to move into the blood. Allow faster rate of filtration and reabsorption. Also allow entire
cells to move into and out of the bloodstream

Veins

carry blood from capillaries back to the heart

Venous valves

Veins have folds of the tunica intima that create one-way valves in the lumen of the vein.
These one-way valves only allow blood to move towards the heart in the
vein, they prevent the backward flow of blood away from the heart.
Movement of blood in veins requires movement of the body

Skeletal muscle pump

Veins are positioned between skeletal muscles so that when the muscle contracts it squeezes the vein and acts like a pump.

Blood pools in veins

65% of the total blood volume is in the veins. The venous system is called the blood reservoir.

Venules

Smallest vein type
Basic function: Drain capillary beds and then merge to form larger veins.
Complete tunica intima, tunica media and tunica externa are very thin if they are even present

Medium to Large veins

Basic function: Carry blood back to the heart in a low-pressure environment Large lumens
b. Thick tunica externa
c. Etc...