arteries and capillaries

III. Arterial System

a. Elastic Arteries (Conducting Arteries)

  • Characteristics:

    • Large, closest to the heart (e.g., aorta, pulmonary trunk).

    • Contain lots of elastic fibers → stretch during systole, recoil during diastole.

    • Function: Smooth out blood pressure fluctuations, maintain continuous flow.

    • Conducting Arteries: low resistance pathway from heart to medium sized arteries

    • Pressure Reservoirs: expand and recoil function provides continuous flow of blood 

b. Muscular Arteries (Distributing Arteries)

  • Characteristics:

    • Medium-sized; deliver blood to specific organs.

    • More smooth muscle, less elastic tissue.

    • Actively constrict/dilate to control blood flow to tissues.

    • Active in vasoconstriction 

    • delivering blood to specific regions

c. Arterioles (Resistance Vessels)

  • Characteristics:

    • Smallest arteries; control blood flow into capillary beds.

    • Smallest are single layer of smooth muscle around endothelial lining 

    • Resistance Vessels: respond to neural, hormonal, and chemical influence by changing diameter

    • Tunica media with few muscle cells.

    • Vasoconstriction → reduces blood flow, Vasodilation → increases flow.

    • Major contributor to peripheral resistance and blood pressure regulation.

IV. Capillaries

a. Continuous Capillaries

  • Characteristics: abdunent

    • Most common; found in skin, muscles, brain.

    • Endothelial cells tightly joined; small intercellular clefts.

    • Allow passage of water and small solutes only.

    • In brain → forms blood-brain barrier.

    • Major function is exchange of materials between blood and interstitial fluid (exchange vessels) 

b. Fenestrated Capillaries

  • Characteristics:

    • Have pores (fenestrations) in endothelial cells.

    • More permeable for fluid and small molecules.

    • Found in kidneys, small intestine, endocrine glands.

c. Sinusoid Capillaries

  • Characteristics:

    • Leaky with large gaps and irregular shape.

    • Allow passage of large molecules and cells.

    • Found in liver, spleen, bone marrow.

d. Capillary Beds

  • Network of capillaries between an arteriole and venule.

Microcirculation Includes:

  1. True Capillaries: Where exchange occurs.

  2. Vascular Shunt (Metarteriole + Thoroughfare Channel):

    • Directly connects arteriole → venule.

Precapillary sphincters (smooth muscle) regulate blood flow into true capillaries.