Blood Vessels & Circulation

Artery and Vein Histology

•          Walls have 3 layers:

•          Tunica intima

•          Tunica media

•          Tunica externa

Tunica Intima

•          Is the innermost layer near the lumen

•          Includes:

•          The endothelial lining

•          Connective tissue layer

•          Internal Elastic Membrane:  In arteries, is a thick layer of elastic fibers in the     outer margin of the tunica intima

Tunica Media

•          Is the middle layer

•          Contains concentric sheets of smooth muscle in loose connective tissue

•          Binds to inner and outer layers

Tunica Externa (aka: Tunica Adventitia)

•          Is outer layer

•          Contains connective tissue sheath

•          Anchors vessel to adjacent tissues

•          In arteries:

•          Contain collagen

•          Elastic fibers

•          In veins:

•          Contain elastic fibers

•          Smooth muscle cells

Arteries Vs. Veins

•          Arteries and veins run side-by-side

•          Arteries have thicker walls and higher blood pressure

•          Collapsed artery has small, round lumen

•          Vein has a large, flat lumen

Arteries

•          Carry blood away from the heart.

•          Pulmonary arteries: The pulmonary trunk and its branches;  leave the right ventricle of the heart and contain deoxygenated blood.

•          Systemic arteries: The aorta and its branches;  leave the left ventricle of the heart and contain oxygenated blood.

Elastic Arteries

•          Also called conducting arteries, these are the largest arteries

•          Tunica media has many elastic fibers and few muscle cells

•          Elasticity evens out pulse force

•          Examples:

•          Pulmonary trunk

•          Aorta

•          Common carotid arteries

•          Subclavian arteries

•          Common iliac arteries 

Muscular Arteries

•          Also called distribution arteries, are medium-sized (most arteries)

•          Tunica media has many muscle cells

•          Examples:

•          External carotid arteries

•          Brachial arteries

•          Femoral arteries

Arterioles

•          The smallest branches of arteries

•          Feed into capillaries

•          Have little or no tunica externa

•          Have thin or incomplete tunica media   

Capillaries

•          The smallest vessels

•          Structure: Simple squamous epithelium tube

•          Lumen side has a thin basal lamina

•          No tunica media, No tunica externa

•          Location of exchange between blood and interstitial fluid.

•          Gasses and chemicals diffuse across their walls

•          Types:

•          Continuous

•          Fenestrated

•          Sinusoids

Continuous Capillaries

•          Have complete endothelial lining (most common type of capillary)

•          Permit diffusion of:

•          Water

•          Small solutes

•          Lipid-soluble materials

•          Prevent diffusion of:

•          Blood cells

•          Plasma proteins

Fenestrated Capillaries

•          Have pores in endothelial lining

•          Permit rapid exchange of water and larger solutes between plasma and interstitial fluid

•          Are found in:

•          Choroid plexus

•          Endocrine organs

•          Kidneys

•          Intestinal tract

Sinusoids

•          Modified, extremely leaky, fenestrated capillaries

•          Found in locales where large stuff needs to exit/enter the bloodstream.

•          Liver

•          Spleen

•          Bone marrow

•          Endocrine organs

Veins

•          Carry blood to the heart

•          Are larger in diameter than arteries

•          Have thinner walls

•          Contain valves

•          Folds of tunica intima that prevent blood from flowing backward

•          Venules: The smallest veins that carry blood away from the capillaries

•          Medium-sized veins:

•          Thin tunica media and few smooth muscle cells

•          Tunica externa with longitudinal bundles of elastic fibers

•          Large veins:

•          Have all 3 tunica layers

•          Thick tunica externa

•          Thin tunica media

•          Example: Inferior and Superior vena cava