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Chapter 20: Cardiovascular System - Blood Vessels

I. Blood Vessels Structure & Function

  • Blood vessels are a closed system that consists of:
    • Arteries
    • Arterioles
    • Capillaries
    • Venules
    • Veins
    • Vasa vasorum

II. Hemodynamics

  • Hemodynamics refers to the study of blood flow and the forces involved in circulation.

III. Circulatory Routes

  • Exploration of different circulatory routes in the vascular system continues in later sections.

Blood Vessel Structure & Function

  • Blood Vessel Structure:

    • Comprised of three layers (tunics):
    1. Tunica interna (intima): Innermost layer, includes endothelial cells.
    2. Tunica media: Middle layer, primarily smooth muscle and elastic fibers, which adjust the diameter of the vessel.
    3. Tunica externa: Outermost layer, composed of connective tissue.

  • Modifications in blood vessel structure account for five types of blood vessels and their structural and functional differences:

    • Includes adaptations in the three tunics and the presence or absence of valves.
Blood Vessel Layers Detailed
  • Tunica intima:
    • Includes connective tissue and the endothelium.
    • Valves: Present in veins to prevent backflow.
  • Tunica media:
    • Thicker in arteries than in veins.
  • Tunica externa:
    • Thicker in veins relative to the other layers.
Types of Blood Vessels
  • Arteries: Carry blood away from the heart.
  • Veins: Carry blood toward the heart.
  • Capillaries: Microscopic vessels where gas and nutrient exchange occurs.

Comparison Between Arteries and Veins (Table 20.1)

  • Characteristic Differences:
    • Lumen Diameter:
    • Arteries: Narrower than veins.
    • Veins: Wider than arteries.
    • General Wall Thickness:
    • Arteries: Thicker walls compared to veins.
    • Veins: Thinner walls than arteries.
    • Cross-Sectional Shape:
    • Arteries: Shape retained even without blood.
    • Veins: Shape tends to flatten without blood present.
    • Tunica Media:
    • Arteries: Thickest tunic with significant elastic and collagen fibers.
    • Valves:
    • Arteries: None.
    • Veins: Present in most veins.
    • Blood Pressure Range:
    • Arteries: Higher than veins (100 mm Hg in larger arteries to 40 mm Hg in smaller arterioles).
    • Veins: Lower (20 mm Hg in venules to 0 mm Hg in inferior vena cava).
    • Blood Flow:
    • Arteries: Transports blood high in O₂.
    • Veins: Transports blood low in O₂.

Arteries & Arterioles

  • General Function:
    • They carry blood away from the heart to tissues.
    • Wall Structure: Elastic walls with smooth muscle in the tunica media.
Types of Arteries

  1. Elastic arteries (Conducting arteries):

    • Large diameter (>10 mm).
    • More elastic fibers and less smooth muscle in the tunica media.
    • Function: Act as pressure reservoirs to withstand high pressures from heart contraction.

  2. Muscular arteries (Distributing arteries):

    • Medium diameter (0.1-10 mm).
    • More smooth muscle and fewer elastic fibers in the tunica media.
    • Function: Distribute blood to various parts of the body and regulate blood flow rates.

Capillaries

  • Structure & Function:
    • Microscopic vessels located near most cells in the body.
    • Allow for close distances necessary for efficient gas exchange.
Types of Capillaries

  1. Continuous Capillaries:

    • Structure: Uninterrupted lining made of endothelial cells.
    • Locations: Most tissues (e.g., muscles, skin).

  2. Fenestrated Capillaries:

    • Structure: Contain fenestrations or pores.
    • Locations: Small intestine (nutrient absorption), certain endocrine glands.

  3. Sinusoids:

    • Structure: Larger fenestrations.
    • Locations: Red bone marrow, liver, spleen, facilitating the passage of larger substances.

Blood Flow Dynamics Through Capillaries

  • Blood flows from arterioles through:
    • Metarterioles
    • Thoroughfare channel
    • Postcapillary venule
    • Capillary bed
    • Precapillary sphincter: Regulates blood flow into capillary beds.
    • Vasomotion: Alteration of blood flow due to constriction and dilation of precapillary sphincters.

Venules & Veins

  • Venules:
    • Drain blood from capillaries into veins.
  • Veins:
    • Thinner layers (tunica interna and tunica media) and thicker tunica externa relative to arteries.
    • Contain valves formed from folds of tunica intima to prevent backflow.