Which type of blood vessel typically lacks one or more of the three tunics?

• A. Arterioles
• B. Capillaries
• C. Arteries
• D. Veins

Answer: (B)

Verified step by step guidance

1. Understand the structure of blood vessels: Blood vessels are composed of three tunics (layers) - the tunica intima (inner layer), tunica media (middle layer), and tunica externa (outer layer). These layers vary in thickness and composition depending on the type of blood vessel.
2. Review the characteristics of capillaries: Capillaries are the smallest blood vessels and are specialized for exchange of gases, nutrients, and waste between blood and tissues. Unlike other blood vessels, capillaries lack the tunica media and tunica externa, consisting only of the tunica intima, which is a single layer of endothelial cells.
3. Compare capillaries to other blood vessels: Arteries, veins, and arterioles all have the three tunics, although their thickness and composition differ. For example, arteries have a thick tunica media to withstand high pressure, while veins have a thinner tunica media and valves to prevent backflow.
4. Identify the correct answer: Based on the structural differences, capillaries are the type of blood vessel that typically lacks one or more of the three tunics, as they only have the tunica intima.
5. Conclude the reasoning: Capillaries are uniquely adapted for their function in exchange processes, and their lack of tunica media and tunica externa allows for efficient diffusion of substances between blood and tissues.