The structure of a capillary wall differs from that of a vein or an artery because
a. it has two tunics instead of three
b. there is less smooth muscle
c. it has a single tunic—only the tunica intima
d. none of these

Verified step by step guidance

1. Step 1: Understand the basic structure of blood vessel walls. Arteries and veins typically have three layers or tunics: the tunica intima (inner layer), tunica media (middle layer with smooth muscle), and tunica externa (outer connective tissue).
2. Step 2: Recall the function and structure of capillaries. Capillaries are the smallest blood vessels where exchange of gases, nutrients, and waste occurs between blood and tissues.
3. Step 3: Recognize that capillaries have a very thin wall to facilitate this exchange. Unlike arteries and veins, capillaries do not have all three tunics.
4. Step 4: Identify that capillary walls consist only of the tunica intima, which is a single layer of endothelial cells and a basement membrane, allowing for efficient diffusion.
5. Step 5: Conclude that the correct distinguishing feature is that capillaries have a single tunic—the tunica intima—making option (c) the accurate description.

Key Concepts

Structure of Blood Vessel Walls

Blood vessels have walls composed of layers called tunics. Arteries and veins typically have three tunics: tunica intima (inner layer), tunica media (middle muscular layer), and tunica externa (outer connective tissue). These layers provide strength, elasticity, and support for blood flow.

Capillary Wall Composition

Capillaries have a unique wall structure consisting of only a single tunic, the tunica intima. This thin layer is made of endothelial cells and a basement membrane, allowing efficient exchange of gases, nutrients, and waste between blood and tissues.

Role of Smooth Muscle in Blood Vessels

Smooth muscle is primarily found in the tunica media of arteries and veins, controlling vessel diameter and blood flow. Capillaries lack smooth muscle, which makes them more permeable but unable to regulate blood flow actively.