Vascular System


transport blood throughout the body.

The vascular system comprises the following components:

  • The arterial system, which includes arteries and arterioles that carry blood away from the heart.
  • The capillary system, which comprises complex networks of capillaries termed ‘beds’ that facilitate molecular exchange between the tissues and circulation.
  • The venous system, which includes postcapillary venules, venules and veins that return blood to the heart.
    • These vessels join to form a closed system through which the heart pumps blood.

Capillary Details:

  • Capillary beds merging into precapillary venules; the precapillary venules facilitate exchange like the capillaries do, but also function as a major transit point for white blood cells (a key component of the immune response) as they move into and out of the circulation.
  • Precapillary venules merge into a venule; venules continue to permit molecular change and white blood cell movement.
  • The venules connect to veins; veins are the largest of the vessels returning blood to the heart.

Vessel Cross-Sections & Physiologic Functions

  • Blood vessel walls are generally organized into three layers:
    • Tunica intima, the innermost layer, which contains a layer of epithelium surrounded by connective tissue
    • Tunica media, the middle layer, which contains variable amounts of smooth muscle and elastic connective tissue
    • Tunica adventitia, the outermost layer, which comprises mainly collagen; elastic lamina supports the tunica adventitia, anchoring blood vessels to nearby organs and providing stability.
  • The thickness and relative compositions of these layers vary between vessels.

Three types of arteries:

  • Elastic arteries, which are the largest.
  • Muscular arteries, which branch from elastic arteries.
  • Arterioles, which are the smallest and branch from muscular arteries.

Cross section of an elastic artery:

  • From deep to superficial, draw:
    • A thin tunica intima, a thick tunica media, and a thin tunica adventitia.
    • The tunica media in elastic arteries contains a notably large amount of elastic connective tissue, which enables them to expand and recoil as the heart contracts and ejects blood into circulation.
    • The aorta is an example of an elastic artery.

Cross section of a muscular artery as follows

  • From deep to superficial
    • A thin tunica intima, a thick tunica media and a thin tunica adventitia.
    • Show that the tunica media in muscular arteries contains abundant smooth muscle, which allows them to regulate blood flow by vasoconstriction (a contraction of smooth muscle that narrows the lumen) and vasodilation (a relaxation of smooth muscle that widens the lumen).

Cross section of an arteriole as follows:

  • A thin tunica intima, a thick tunica media, and a thin tunica adventitia.
    • Tunica media includes a varying amount of smooth muscle.
    • Smooth muscle in the tunica media of arterioles also functions in vasoconstriction and vasodilation, allowing arterioles to regulate the flow of blood into capillary beds.

Components of a capillary bed:

  • 4 types of vessels broadly referred to as capillaries:
    • Metarterioles, or precapillaries, which are not true capillaries but pass through capillary beds
    • Continuous capillaries, which are the most abundant
    • Fenestrated capillaries, which are found in the kidneys, small intestine and endocrine glands
    • Sinusoidal capillaries, which are found in the spleen, liver and bone marrow

Features of true capillaries

  • Comprise endothelial cells.
  • Continuous capillary have continuous walls of endothelial cells connected by tight junctions.
  • Lacks smooth muscle, and a basement membrane surrounds it.
  • This thin endothelial wall facilitates molecular exchange between the lumen of the continuous capillary and the surrounding tissue.

Features of fenestrated capillaries

  • Continuous wall of endothelial cells with a number of pores called fenestrations.
  • Continuous basement membrane surrounds the wall of endothelial cells.
  • Fenestrations produce greater permeability than that of continuous capillaries.
  • Fenestrated capillaries can be found in specialized tissues that require more rapid and extensive molecular exchange.

Features of sinusoidal capillaries

  • Large gaps between endothelial cells.
  • Discontinuous basement membrane surrounds these endothelial cells.
  • The large gaps between endothelial cells and incomplete basement membrane permit even greater permeability than that of fenestrated capillaries.
  • Sinusoidal capillaries facilitate the passage of red and white blood cells and are found in specialized organs within which such movement regularly occurs.

Vessels of the venous system

  • Postcapillary venules, which form when capillaries merge
  • Venules, which form when postcapillary venules merge
  • Veins, such as the venae cava, which are the largest of these vessels and form when venules merge
  • The venous system returns blood to the heart.
  • In general, the vessels of the venous system have thinner walls and larger lumens than the arterial system, with less defined elastic and muscular features.

Cross section of a postcapillary venule

  • Walls are porous and that the tunica media is very thin.
    • The porous walls of postcapillary venules facilitate the movement of white blood cells into and out of circulation. – White blood cells function in the immune response, which will be discussed elsewhere.

Cross section of a venule

  • Walls are thin, but thicker than postcapillary venule.
    • Porous walls for passage of white blood cells.

Cross section of a vein

  • Deep to superfiical: thin tunica media and a thick tunica adventitia.
    • Tunica adventitia is the largest layer in veins and contains a small amount of smooth muscle, unlike arterial walls where smooth muscle is limited to the tunica media.
    • Tunica intima and the tunica media are thinner than in the elastic and muscular arterial walls.
    • The venae cava are the largest veins in the body and return blood to the right atrium of the heart.
    • Many veins, particularly in the limbs and extremities, contain valves formed by the tunica intima, which prevent backflow of blood.

Clinical Correlations:

  • Faulty valves cause a back-flow of blood, which manifests as varicose veins and can result in stasis dermatitis.

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