Blood Vessels: Structure and Function

The Blood Vessels

  • By Ahmed Badr, MBA, MEP, ACSM-CEP

Structure and Function of Blood Vessels

  • Learning Objectives:
    • Describe the three tunics common to most vessels.
    • Explain the distinguishing features of the tunics found in arteries, capillaries, and veins.
    • Distinguish between elastic arteries, muscular arteries, and arterioles.
    • Describe the general anatomic structure and function of capillaries.
    • Compare the anatomic structure, function, and location of continuous capillaries, fenestrated capillaries, and sinusoids.
    • Trace the movement of blood through a capillary bed.
    • Describe the structure and general function of veins.
    • Explain how veins serve as a blood reservoir for the cardiovascular system.
    • Compare and contrast the simple and alternative pathways of blood vessels.

Types of Blood Vessels

  • Three types of blood vessels:
    • Arteries
      • Convey blood from the heart to capillaries.
    • Capillaries
      • Microscopic porous blood vessels.
      • Exchange substances between blood and tissues.
    • Veins
      • Transport blood from capillaries to the heart.

General Structure of Vessels

  • Vessel composition
    • The space inside a vessel is called its lumen.
    • Walls are composed of three layers called tunics: tunica intima, tunica media, tunica externa.
  • Tunics:
    • Tunica intima
      • Innermost layer of the vessel wall.
      • Endothelium of simple squamous epithelium.
      • Sub-endothelial layer of areolar connective tissue.
    • Tunica media
      • Middle layer of the vessel wall.
      • Circularly arranged layers of smooth muscle cells with elastic fibers.
      • Contraction causes vasoconstriction, which narrows the lumen.
      • Relaxation causes vasodilation, which widens the lumen.
    • Tunica externa
      • Outermost layer of the vessel wall.
      • Areolar connective tissue with elastic and collagen fibers.
      • Helps anchor the vessel to other structures.
      • May contain vasa vasorum, which are small arteries required to supply very large vessels.

Comparison of Different Vessel Types

  • Companion vessels lie next to each other, such as arteries and veins serving the same body region.
  • Arteries:
    • Have a thicker tunica media and narrower lumen than veins.
    • Have more elastic and collagen fibers, allowing them to spring back to shape.
    • More resilient and resistant to changes in blood pressure.
  • Veins:
    • Have a thicker tunica externa and larger lumen than arteries.
    • Have less elastic and collagen fibers.
    • The wall collapses if there is no blood in the vessel.
  • Capillaries:
    • Contain only the tunica intima, with no subendothelial layer.
    • Composed of endothelium and basement membrane.
    • The thin wall allows for rapid gas and nutrient exchange.

Arteries

  • Artery branching:
    • Arteries branch into smaller vessels extending from the heart.
    • Decrease in lumen diameter.
    • Decrease in elastic fibers.
    • Increase in the relative amount of smooth muscle.
    • Three basic types of arteries: elastic arteries, muscular arteries, arterioles.
  • Elastic (conducting) arteries:
    • Largest arteries, with diameters from 2.5 to 1 cm.
    • Conduct blood from the heart to muscular arteries.
    • Have a large proportion of elastic fibers, allowing stretch and recoil, which helps propel blood through the arteries during diastole.
    • Examples include the aorta, pulmonary trunk, common carotid, and common iliac arteries.
  • Muscular (distributing) arteries:
    • Medium arteries, with diameters from 1 cm to 3 mm.
    • Distribute blood to specific body regions.
    • Muscle allows vasoconstriction and vasodilation.
    • Elastic tissue is present in two layers:
      • Internal elastic lamina between the tunica intima and tunica media.
      • External elastic lamina between the tunica media and tunica externa.
    • Most named arteries, such as the brachial artery and coronary arteries.
  • Arterioles:
    • Smallest arteries, with diameters of 3 mm to 10 micrometers.
    • Larger arterioles have three tunics.
    • Smaller arterioles have only thin endothelium and a single layer of smooth muscle.
    • Smooth muscle is usually somewhat constricted, called vasomotor tone, and is regulated by the vasomotor center in the brainstem.
    • Regulate systemic blood pressure and blood flow.

Clinical View: Atherosclerosis

  • Progressive disease of elastic and muscular arteries.
  • Presence of atheroma (atheromatous plaque):
    • Thickening of the tunica intima.
    • Narrowing of the arterial lumen.
  • Most likely due to a response to injury to the endothelium caused by infection, trauma, or hypertension, resulting in an inflammation reaction and atheroma formation.
  • With atheroma enlargement, the lumen becomes narrower.
  • Individuals may be unaware of plaques until they restrict blood flow to a region.
  • Increased cholesterol in the blood (hypercholesterolemia) makes one prone to the disease.
  • Males are more affected than females.
  • Smoking and hypertension increase vascular injury and overall risk.
  • Treatment:
    • Angioplasty to expand the narrowed region of the artery.
    • Coronary bypass surgery.

Clinical View: Aneurysm

  • Part of the arterial wall thins and balloons out.
  • The wall becomes more prone to rupture, which can cause massive bleeding and death.
  • Elastic and muscular arteries, with age, are less able to withstand forces from pulsating blood.
  • Risk increases with age.
  • Most common in the aorta or arteries at the base of the brain.

Capillaries

  • Capillary characteristics
    • Small vessels connecting arterioles to venules.
    • Average length = 11 mm; diameter = 88 to 1010 micrometers.
      • Erythrocytes travel single file (rouleau).
    • The wall consists of an endothelial layer on a basement membrane.
    • The thin wall and small diameter are optimal for exchange between blood and tissue fluid.
    • Three types: continuous, fenestrated, and sinusoid.
  • Continuous capillaries
    • Endothelial cells form a continuous lining.
    • Tight junctions connect cells but do not form a complete seal.
      • Intercellular clefts are gaps between endothelial cells of the capillary wall.
      • Large particles (e.g., cells, proteins) cannot pass, but smaller molecules (e.g., glucose) can pass through the wall.
    • Common: found in muscle, skin, lungs, and the central nervous system.
  • Fenestrated capillaries
    • Endothelial cells form a continuous lining, but the cells have fenestrations (pores).
      • Fenestrations allow movement of smaller plasma proteins.
    • Found in areas where much fluid transport happens, such as in the intestine capillaries absorbing nutrients and kidney capillaries filtering blood to form urine.
  • Sinusoids (discontinuous capillaries)
    • Endothelial cells form an incomplete lining with large gaps.
    • The basement membrane is incomplete or absent.
    • Openings allow transport of large substances (formed elements, large proteins).
    • Found in bone marrow, spleen, and some endocrine glands.
  • Capillary beds
    • Groups of capillaries functioning together.
    • Fed by a metarteriole, a vessel branch of an arteriole.
      • The proximal part is encircled by scattered smooth muscle cells.
      • The distal part, the thoroughfare channel, has no smooth muscle cells and connects to a postcapillary venule, draining the bed.
    • True capillaries
      • Vessels branching from the metarteriole, making up the bulk of the capillary bed.
    • Precapillary sphincter
      • Smooth muscle ring at the true capillary origin.
      • Sphincter relaxation permits blood to flow into the true capillaries.
      • Sphincter contraction causes blood to bypass the capillary bed.
      • Vasomotion: the cycle of contracting and relaxing of precapillary sphincters.
      • At any time, only one-quarter of the body’s capillary beds are open.
    • Perfusion
      • The amount of blood entering capillaries per unit time per gram of tissue.
      • Units are mL/min/g.

Veins

  • Venule
    • The smallest vein, with diameters of 8 to 100 micrometers.
    • Companion vessels with arterioles.
    • The smallest venules are postcapillary venules.
    • Largest venules having all three tunics
    • Merge to form veins.
  • Small, medium-sized, and large veins
    • Small and medium-sized veins are companion vessels with muscular arteries.
    • The largest veins travel with elastic arteries.
    • Most veins of these sizes have numerous valves.
      • Prevent blood from pooling in the limbs and ensure flow toward the heart.
      • Valves are made of tunica intima and elastic and collagen fibers.
      • Similar structure to the heart’s semilunar valves.
  • Systemic veins as blood reservoirs
    • At rest: 70% of blood in systemic circulation
      • Systemic veins: 55%
      • Systemic arteries: 10%
      • Systemic capillaries: 5%
    • The pulmonary circulation has 18% of blood.
    • The heart has 12% of blood.
    • Blood can be moved from veins into circulation via vasoconstriction of veins, e.g., when more blood is needed during exertion.
    • Blood can be shifted back into reservoirs via vasodilation, e.g., when less blood is needed during rest.

Pathways of Blood Vessels

  • Simple pathway
    • One major artery delivers blood to an organ or region.
      • An end artery is one that provides only one path for blood to reach an organ or region.
      • It branches into smaller arteries that become arterioles.
    • Each arteriole feeds into a capillary bed.
    • The capillary bed is drained by a venule.
    • Venules merge to one major vein.
    • E.g., the splenic artery delivers blood to the spleen; the splenic vein drains the organ.
  • Alternative pathways are configured differently
    • Arterial anastomosis (arterial joining)
      • Two or more arteries converge to supply the same region.
      • E.g., superior and inferior epigastric arteries supplying the abdominal wall.
      • If the junction is small, the arteries might be functional end arteries.
    • Venous anastomosis are more common
      • Two or more veins drain the same body region.
      • E.g., basilic, brachial, and cephalic veins draining the upper limb.
    • Arteriovenous anastomosis (shunt)
      • Transports blood from an artery directly to a vein.
      • E.g., in fingers, toes, palms, and ears.
      • Allows areas to be bypassed if the body is hypothermic.
    • Portal system (two capillary beds in sequence)
      • Path: artery → capillary bed → portal vein → capillary bed → vein
      • E.g., hypothalamo-hypophyseal portal system.