The Vascular System

Overview – The Vascular System

The vascular system comprises arteries, capillaries, and veins that work together to circulate blood throughout the body. Each component of the system has a distinct structure and function, from the high-pressure conduits of elastic arteries to the exchange-focused capillaries and the capacitance-based return pathway of veins. Understanding vascular anatomy and physiology is critical for diagnosing and managing cardiovascular conditions in clinical practice.

The Arterial System

Elastic (Conducting) Arteries

  • Includes the aorta and its major branches
  • Thick-walled with large lumens → low resistance to flow
  • High elastin content allows:
    • Dampening of pressure fluctuations
    • Energy storage during systole → blood propulsion during diastole

Muscular (Distributing) Arteries

  • Distal to elastic arteries; supply specific organs
  • Diameter: 0.3 mm to 1 cm
  • Thickest tunica media with high smooth muscle content
  • Highly active in vasoconstriction → less elastic
  • Key regulators of regional blood distribution

Arterioles

  • Smallest arteries
  • Larger arterioles have all 3 tunics (intima, media, externa)
  • Smaller arterioles = 1–2 layers of smooth muscle surrounding endothelium
  • Critical for local blood flow control
    • Respond to:
    • Vasoconstriction → bypasses capillary beds
    • Vasodilation → allows tissue perfusion
  • Major determinant of systemic blood pressure
The 3 types of arteries in the arterial system
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Arterial System Anatomy

Arterial System Anatomy
LadyofHats, Public domain, via Wikimedia Commons; https://commons.wikimedia.org/wiki/File:Arterial_System_en.svg

The Capillary System

Structure and Function

  • Microscopic vessels with thin walls (only tunica intima)
  • Length ≈ 1 mm; diameter just enough to allow RBCs in single file
  • Site of exchange between blood and interstitial fluid
  • Penetrate most tissues except cartilage, tendons, ligaments, epithelia

Capillary Beds

  • Network of capillaries = site of microcirculation
  • Blood flow from arteriole → venule
  • Components:
    • Vascular Shunt:
      • Metarteriole to thoroughfare channel
      • Direct arteriole-to-venule connection
    • True Capillaries:
      • Branch from metarteriole
      • Perform exchange functions
  • Precapillary Sphincters:
    • Smooth muscle cuffs at capillary roots
    • Regulate entry of blood into true capillaries
    • Allow blood to bypass or enter capillary beds based on tissue demand
Capillary bed structure
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Types of Capillaries

  • Continuous Capillaries:
    • Uninterrupted endothelium with tight junctions
    • Intercellular clefts allow limited solute passage
    • Form blood-brain barrier in CNS
  • Fenestrated Capillaries:
    • Contain fenestrations (pores) → ↑ permeability
    • Found in kidneys, intestines, endocrine organs
  • Sinusoids (Sinusoidal Capillaries):
    • Large, irregular lumens
    • Incomplete basement membranes
    • Found in liver, bone marrow, lymphoid tissue
    • Allow passage of large molecules and WBCs
    • Lined with phagocytic Kupffer cells
Capillary Types - Continuous, Fenestrated, Sinusoid
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

The Venous System

Venules

  • Formed from post-capillary capillary union
  • Composed of endothelium (high permeability)
  • Permit WBC migration into inflamed tissues
  • Larger venules:
    • Contain smooth muscle (tunica media)
    • Thin tunica externa

Veins

  • Formed from the convergence of venules
  • Thinner walls and larger lumens than arteries
  • All 3 tunics present, but tunica media poorly developed
  • Tunica externa is the thickest layer → collagen & elastic fibers
  • Serve as capacitance vessels → hold ~65% of total blood volume
  • Low-pressure system requiring adaptations:
    • Valves (from tunica intima):
      • Prevent backflow
      • Function similar to semilunar valves
      • Most important in limbs (against gravity)
      • Failure can lead to varicosities and thrombosis
Vein Walls & Valves
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Venous System Anatomy

Anatomy of the Venous System
LadyofHats, Public domain, via Wikimedia Commons; https://commons.wikimedia.org/wiki/File:Venous_system_en.svg

Foetal Circulation

Shunts Bypassing Certain Organs

  • Ductus Venosus:
    • Bypasses liver sinusoids
    • Directs blood from placenta → inferior vena cava
  • Foramen Ovale:
    • Interatrial septal opening
    • Allows blood to bypass non-functional fetal lungs
  • Ductus Arteriosus:
    • Connects pulmonary trunk to aorta
    • Also bypasses lungs

All fetal shunts normally close at birth due to circulatory pressure changes

  • Foramen ovale may take up to 6 months to fully seal
Foetal Circulation & Shunts
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Summary – The Vascular System

The vascular system comprises a complex network of arteries, capillaries, and veins, each uniquely structured to fulfill transport and exchange roles. Understanding these vascular segments, from high-pressure elastic arteries to low-pressure veins, is critical for mastering cardiovascular physiology. For a broader context, see our Cardiovascular Overview page.

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