BIOL 2102 Lab 8 Notes: The Blood Vessels Part 1

Blood Vessels: Arteries vs. Veins

  • General Structure of Blood Vessels: All blood vessels (arteries and veins) share a common basic structure consisting of three layers, or tunics:

    • Tunica interna (intima): The innermost layer, composed of simple squamous epithelium (endothelium) and a basement membrane. This layer provides a smooth surface for blood flow.
    • Tunica media: The middle layer, primarily made of smooth muscle. This muscle layer is crucial for controlling the Lumen size (the internal space of the vessel) and, consequently, regulating blood flow.
    • Tunica externa (adventitia): The outermost layer, rich in collagenous connective tissue. Its primary function is to provide structural support and protection to the vessel.

  • Arteries:

    • Tunica Media: Characterized by a thicker tunica media compared to veins, and the presence of elastic laminae (internal and external elastic laminae).
    • Pressure Withstanding: Designed to withstand high blood pressure generated by the heart.
    • Pressure Regulation: Help to reduce fluctuations in blood pressure between systole (contraction phase) and diastole (relaxation phase).

  • Veins:

    • Tunica Media: Possess a thinner tunica media relative to arteries.
    • Blood Pressure: Carry blood under much lower blood pressure.
    • Valves: Contain valves, which are folds of the tunica interna, to prevent the backflow of blood, especially against gravity.
    • Appearance: Often appear collapsed or irregular when empty due to their thinner walls and lower internal pressure.
    • Blood Return Mechanism: Rely significantly on the contraction of surrounding skeletal muscles (known as the skeletal muscle pump) to help push blood back towards the heart.

Histological Comparison: Arteries vs. Veins

  • Arteries:

    • Tunica Media: Have a significantly thicker tunica media, with the smooth muscle layer being the most prominent feature.
    • Lumen Shape: Typically exhibit a rounder lumen because their thicker, more rigid walls help them retain their shape even when empty, reflecting higher internal pressure.
    • Lumen Size: Possess a smaller lumen relative to their overall wall thickness.
    • Elastic Laminae: Internal and external elastic laminae are often visible.
    • Tunica Externa: Generally thinner than the tunica media.

  • Veins:

    • Tunica Media: Have a thinner tunica media with less smooth muscle.
    • Lumen Shape: Feature a larger, often irregular or "collapsed" lumen because their thinner walls readily collapse when empty due to lower internal pressure.
    • Wall Thickness: The wall is thinner relative to the overall lumen size.
    • Tunica Externa: The tunica externa is typically thicker than the tunica media, which is the opposite of arteries.
    • Valves: Valves, which are infoldings of the tunica interna, may be visible within the lumen.

Small Branching of Blood Vessels

  • Arterioles: Small arteries that lead into capillaries.
  • Venules: Small veins that collect blood from capillaries.
  • Capillaries: The smallest blood vessels, consisting only of a tunica interna (endothelium) and a basement membrane. They form the network that connects arterioles to venules, facilitating the exchange of gases, nutrients, and waste products between blood and tissues.

Arterial Supply:

Branches Coming Off the Aortic Arch
  • The Aortic Arch gives rise to three major arteries for the upper body:
    • Right Side (First Branch): The Innominate Artery (also known as the Brachiocephalic Trunk). This trunk then divides into the Right Subclavian Artery and the Right Common Carotid Artery.
    • Middle (Second Branch): The Left Common Carotid Artery.
    • Left Side (Third Branch): The Left Subclavian Artery.
The Common Carotid Arteries
  • Function: Supply oxygenated blood to the brain and head.
  • Branching: Each common carotid artery (Left and Right) branches into:
    • Internal Carotid Artery: Supplies blood to the brain.
    • External Carotid Artery: Supplies blood to the external structures of the head and face.
The Subclavian Arteries
  • Origin: The Left Subclavian Artery originates directly from the aortic arch, while the Right Subclavian Artery originates from the brachiocephalic trunk.
  • Course: Extend from their origin to the first rib.
  • Branches:
    • Each subclavian artery branches off to form the Vertebral Arteries (which move through the transverse foramen of the cervical vertebrae into the brain).
    • It also branches into the Axillary Arteries, which supply oxygenated blood to the arm and chest.
      • The Brachial Artery runs down the upper arm and is formed from the axillary artery.
The Vertebral Arteries
  • Origin: Form from the subclavian arteries.
  • Course: Travel upwards through the transverse foramen of the cervical vertebrae.
  • Destination: Enter the brain, contributing to its blood supply.
Circle of Willis
  • Description: An arterial anastomosis (a circulatory circle) at the base of the brain.
  • Primary Supply: It is supplied by both the Internal Carotid Arteries and the Vertebral Arteries.
  • Function: This critical circle ensures a consistent and redundant blood supply to most areas of the brain, providing alternate routes for blood flow if one of the major arteries becomes obstructed.

Venous Drainage:

Superior Vena Cava and its Major Branches
  • The Superior Vena Cava is formed by the union of the right and left Brachiocephalic (Innominate) Veins.
  • Brachiocephalic Veins: Each brachiocephalic vein (Right and Left) is formed by the union of:
    • Subclavian Vein: Drains blood from the upper limb.
    • Internal Jugular Vein: Drains blood from the brain and deep areas of the head and neck.
    • External Jugular Vein: Drains blood from the more superficial regions of the head and neck.
The Vertebral Vein
  • Course: Travels down through the transverse foramen of the cervical vertebrae.
  • Destination: Drains from the brain back into the subclavian vein.
Dural Venous Sinuses
  • Location: Specialized venous channels located between the dura mater layers within the brain.
  • Function: Collect venous blood from the brain.
  • Drainage Pathway: Blood from the dural venous sinuses connects to the Sigmoid Sinus, which then flows back into the Internal Jugular Vein, ultimately returning to the heart.