16/12/2020
The Cardiovascular System: Blood Vessels
Blood Vessels
• Blood is carried in a closed system of vessels that begins and ends at the heart
• The three major types of vessels are arteries, capillaries, and veins
• Arteries carry blood away from the heart, veins carry blood toward the heart
• Capillaries contact tissue cells and directly serve cellular needs
Continuous Capillary Structure
Fenestrated Capillary Structure
Discontinuous Sinusoidal Capillary Structure
Generalized Structure of Blood Vessels
• Arteries and veins are composed of three tunics – tunica interna, tunica media, and tunica externa
• Capillaries are composed of endothelium with sparse basal lamina
• Lumen – central blood-containing space surrounded by tunics
Tunics
• Tunica interna (tunica intima)
• Endothelial layer that lines the lumen of all vessels
• In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present
• Tunica media
• Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system
• Controls vasoconstriction/vasodilation of vessels
• Tunica externa (tunica adventitia)
• Collagen fibers that protect and reinforce vessels
• Larger vessels contain vasa vasorum
Elastic (Conducting) Arteries
• Thick-walled arteries near the heart; the aorta and its major branches
• Large lumen allow low-resistance conduction of blood
• Contain elastin in all three tunics
• Withstand and smooth out large blood pressure fluctuations
• Allow blood to flow fairly continuously through the body
Muscular Arteries and Arterioles
• Muscular arteries – distal to elastic arteries; deliver blood to body organs
• Have thick tunica media with more smooth muscle and less elastic tissue
• Active in vasoconstriction
• Arterioles – smallest arteries; lead to capillary beds
• Control flow into capillary beds via vasodilation and constriction
Capillaries
• Capillaries 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
• Pericytes on the outer surface stabilize their walls
• There are three structural types of capillaries: continuous, fenestrated, and sinusoids
Continuous Capillaries
• Continuous capillaries are abundant in the skin and muscles, and have:
• Endothelial cells that provide an uninterrupted lining
• Adjacent cells that are held together with tight junctions
• Intercellular clefts of unjoined membranes that allow the passage of fluids
• Continuous capillaries of the brain:
• Have tight junctions completely around the endothelium
• Constitute the blood-brain barrier
Fenestrated Capillaries
• Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)
• Characterized by:
• An endothelium riddled with pores (fenestrations)
• Greater permeability to solutes and fluids than other capillaries
Sinusoids
• Highly modified, leaky, fenestrated capillaries with large lumens
• Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs
• Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues
• Blood flows sluggishly, allowing for modification in various ways
Capillary Beds
• 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
MD MBBS DNB Mkhadar
