05 Structure and Function of Blood Vessels

BIOL10811 Body Systems - Lecture 5: Structure and Function of Blood Vessels

Overview of Blood Vessels

  • All tissues in the body have an arterial supply and a venous drainage.

  • Different types of blood vessels serve distinct functions.

Essential Functions of Blood Vessels

  • Blood vessels must exhibit certain key characteristics:

    • Resilience: Ability to withstand pressure changes.

    • Flexibility: Adapt to allow blood flow under varying conditions.

    • Openness: Must always remain patent to facilitate circulation.

Structure of Blood Vessels

  • Layers of Blood Vessels:

    1. Lumen: The inner open space through which blood flows.

    2. Tunica Intima:

      • Composed of simple squamous epithelium.

      • Contains a basal lamina and subendothelial connective tissue.

    3. Tunica Media:

      • Composed of smooth muscle fibers within loose connective tissue.

      • May contain elastic fibers to allow for expansion and recoil.

    4. Tunica Adventitia (Externa):

      • Composed of connective tissue that merges with surrounding tissues.

      • May contain small vessels (vasa vasorum) supplying the larger blood vessel tissues.

Comparison of Arteries and Veins

  • Arteries: gARden hose

    • Carry blood under high pressure.

    • Thick walls; resemble garden hoses.

    • No valves present; retain their shape.

  • Veins: Vire hose

    • Carry blood under lower pressure.

    • Thinner walls; resemble fire hoses.

    • Have valves to prevent backflow.

  • Key differences when comparing arteries and veins:

    • Thicker wall: Arteries.

    • Smaller lumen: Arteries.

    • Shape maintenance: Arteries are more resilient.

    • Valves: Absent in arteries, present in veins.

Types of Arteries

  1. Elastic (Conducting) Arteries:

    • E.g. aorta, brachiocephalic.

    • Diameter can be up to 2.5 cm.

    • Ensure continuous blood flow by withstanding pressure changes during the cardiac cycle.

    • Structural adaptations: Thick tunica media with many elastic fibers.

  2. Muscular (Distributing) Arteries:

    • Most named arteries (e.g. brachial, femoral).

    • Diameter: 0.5 mm – 0.4 cm.

    • Distributes blood to various muscles and organs, capable of vasodilation and vasoconstriction.

    • Structural adaptations: Thick tunica externa, distinct internal and external elastic laminae, abundant smooth muscle cells in tunica media.

  3. Arterioles (Resistance) Vessels:

    • Diameter: ≤ 30 µm.

    • Control blood flow to organs and involved in regulating blood pressure.

    • Structural adaptations: One or two layers of smooth muscle cells in tunica media, poorly defined tunica externa.

Capillaries

  • Connect arterioles and venules; the primary site for gaseous exchange.

  • Their thin walls facilitate diffusion, allowing for two-way exchange of substances.

  • Diameter is approximately 8 µm, found near almost all cells in the body.

Types of Capillaries

  1. Continuous Capillaries:

    • Most common type (found in skeletal and smooth muscle, connective tissue, lungs).

  2. Fenestrated Capillaries:

    • Have pores that allow rapid exchange of water and solutes (found in kidneys, choroid plexus, and endocrine glands).

  3. Sinusoidal Capillaries: SWISS CHEESE

    • Have spaces between endothelial cells with an incomplete basement membrane.

    • Designed for the exchange of large solutes, such as plasma proteins (found in the liver where phagocytic cells can engulf damaged red blood cells).

Capillary Beds

  • Comprised of a network of capillaries; essential for nutrient and waste exchange.

  • Includes:

    • Metarterioles connecting arterioles to capillary beds.

    • Precapillary sphincters guard the entrance to capillaries to regulate flow.

    • Arteriovenous anastomoses that allow direct communication between arterioles and venules.

Venules and Veins

  • Veins:

    • Collect blood from capillary beds and return it to larger veins.

    • Diameter varies but averages around 20 µm.

  • Classification by Size:

    • Small: < 2 mm

    • Medium: 2-9 mm

    • Large: > 9 mm (e.g. superior and inferior vena cavae).

  • Veins function as capacitance vessels, meaning they can hold more blood at a lower pressure, aided by their thin walls and valves.

Regulation of Blood Flow

  • Valves & Musculovenous Pump: Valves in the veins prevent backflow and assist in venous return against gravity.

  • Pressure Changes: Systolic and diastolic pressure measurements are critical in understanding cardiovascular function.

Distribution of Blood Volume

  • Systemic venous system: 30-35%

  • Pulmonary circuit: 65-70%

  • Systematic arterial system: Contains the remaining volume of blood.

Anatomical Terminology

  • Body Positions and Directions: Anatomical position, anterior, posterior, superior, inferior, medial, lateral, proximal, and distal.

  • Anatomical Planes:

    • Coronal/frontal plane: divides the body into anterior and posterior sections.

    • Horizontal/transverse plane: divides body into upper and lower parts.

    • Sagittal plane: divides body into right and left sides.


Veins function as capacitance vessels

Tunica intima contain simple squamous epithelium