Blood Vessels: Structure, Function, and Circulation

Chapter 18: The Blood Vessels

Introduction to Blood Vessels

Blood vessels are essential components of the cardiovascular system, responsible for transporting blood throughout the body. They form a complex network that ensures the delivery of oxygen, nutrients, and the removal of waste products from tissues.

• Blood vessels include arteries, veins, and capillaries.

• They facilitate systemic and pulmonary circulation.

• Proper vessel function is vital for maintaining homeostasis.

Tunics (Layers) of the Blood Vessel Wall

Structure of Vessel Walls

Blood vessel walls are composed of three distinct layers, known as tunics. Each layer has specialized functions and structural components.

• Tunica intima: The innermost layer, consisting of:

  • Endothelium – a thin layer of simple squamous epithelial cells lining the lumen.

  • Basal lamina – supports the endothelium.

  • Subendothelial connective tissue – provides structural support.

  • Internal elastic lamina – a layer of elastic fibers allowing vessel stretch.

• Tunica media: The middle layer, primarily composed of:

  • Smooth muscle cells – regulate vessel diameter and blood pressure.

  • External elastic lamina – provides elasticity and resilience.

• Tunica externa (adventitia): The outermost layer, containing:

  • Vasa vasora – small blood vessels that supply the walls of larger vessels.

  • Vasomotor nerves – control contraction of smooth muscle in the tunica media.

Comparing Arteries and Veins

Structural and Functional Differences

Arteries and veins have similar basic structures but differ in wall thickness, lumen size, and functional roles in circulation.

• Arteries:

  • Thicker tunica media with more smooth muscle and elastic fibers.

  • Narrower lumen compared to veins.

  • Carry blood away from the heart under higher pressure.

• Veins:

  • Thinner tunica media and less elastic tissue.

  • Wider lumen to accommodate larger blood volume.

  • Carry blood toward the heart under lower pressure.

  • Often contain valves to prevent backflow.

• Histological comparison: Microscopic images show arteries with thick walls and round lumens, while veins have thinner walls and irregular, collapsed lumens.

Feature	Artery	Vein
Tunica media thickness	Thick	Thin
Lumen size	Narrow	Wide
Pressure	High	Low
Valves	Absent	Present (in limbs)

Example: The Aorta and the Vena Cava

• The aorta is the largest artery, with a thick wall to withstand high pressure.

• The vena cava is the largest vein, with a wide lumen to accommodate large volumes of blood returning to the heart.

*Additional info: The tunics of blood vessels are critical for their function, with the tunica media playing a key role in regulating blood pressure and flow through vasoconstriction and vasodilation.*