Describe the structure of arteries, veins and capillaries, limited to: relative thickness of wall, diameter of the lumen and the presence of valves in veins

The structure of arteries, veins, and capillaries is adapted to their specific functions in the circulatory system. Here’s a detailed comparison focusing on the relative thickness of walls, the diameter of the lumen, and the presence of valves in veins.

Arteries

Structure

• Wall Thickness: Arteries have thick walls composed of three layers:
  • Tunica intima (inner layer)
  • Tunica media (middle layer, thickest, containing smooth muscle and elastic fibers)
  • Tunica externa (outer layer)
• Diameter of Lumen: The lumen of arteries is relatively narrow compared to their wall thickness. This design helps maintain high blood pressure as blood is pumped from the heart.
• Valves: Arteries do not contain valves because blood is under high pressure from the heart’s pumping action, which prevents backflow.

Veins

Structure

• Wall Thickness: Veins have thinner walls than arteries. They also consist of three layers but with a much thinner tunica media and less elastic tissue.
• Diameter of Lumen: The lumen of veins is wider than that of arteries, allowing them to hold a larger volume of blood. This is important for their role in returning blood to the heart at lower pressure.
• Valves: Veins contain one-way valves that prevent backflow of blood as it travels toward the heart. These valves are crucial, especially in the limbs, where blood must be pushed against gravity.

Capillaries

Structure

• Wall Thickness: Capillaries have extremely thin walls, consisting of only a single layer of endothelial cells. This thinness facilitates efficient gas and nutrient exchange between blood and tissues.
• Diameter of Lumen: The lumen of capillaries is very narrow (about 5-10 micrometers), just wide enough for red blood cells to pass through in single file.
• Valves: Capillaries do not have valves as they are involved in direct exchange processes rather than directional flow.

Summary Table