Which statement correctly describes a structural feature of capillaries in the human circulatory system?

Introduction / Context:
Capillaries are the smallest blood vessels in the human body and play a central role in the exchange of gases, nutrients, and waste products between blood and tissues. This question checks whether you know the key structural adaptation of capillaries that enables efficient exchange. Understanding this property is fundamental to human physiology and circulatory system topics.

Given Data / Assumptions:

• The question asks for a correct statement about capillaries.

• Several options describe functions more related to airways or larger blood vessels.

• We assume a basic knowledge of the three main vessel types: arteries, veins, and capillaries.

• Capillaries connect arterioles and venules and are the primary exchange vessels.

Concept / Approach:
Capillaries are composed of a very thin wall made up of a single layer of endothelial cells and a delicate basement membrane. This one cell thickness minimizes the diffusion distance for oxygen, carbon dioxide, nutrients, and waste products. Other structures like cilia and mucus movement are features of the respiratory tract, not of capillaries. Larger vessels such as arteries and veins have multiple layers including smooth muscle, but capillaries do not. By recalling their role in exchange, we can quickly identify the key structural feature of capillaries.

Step-by-Step Solution:
Step 1: Recall that capillaries form a network between arterioles and venules in tissues. Step 2: Understand that their main function is to allow exchange of materials between blood and tissue fluid. Step 3: Since diffusion is more effective over short distances, capillaries have walls that are only one cell thick. Step 4: Compare this to arteries, which have thick muscular walls to withstand high pressure, and veins, which have thinner walls but still multiple layers. Step 5: Recognize that cilia and mucus transport are functions of respiratory tract epithelial cells lining air passages. Step 6: Conclude that the statement about single cell thick walls is the only correct description of capillaries.

Verification / Alternative check:
Standard diagrams of the circulatory system show capillaries as fine vessels with a thin wall composed of a single endothelial layer. Histology slides or micrographs confirm that capillaries lack the smooth muscle and connective tissue layers found in arteries and veins. Physiology explanations also emphasize that the thin wall of capillaries is crucial for exchange. This evidence supports the correctness of the statement about one cell thick walls.

Why Other Options Are Wrong:

Their walls are covered with cilia that move fluid: Cilia are found on certain epithelial cells in respiratory passages, not on capillary walls.

They warm and humidify inhaled air in the respiratory tract: This function is carried out by nasal passages and upper airway mucosa, not by capillaries directly.

They actively move mucus through the lungs: Mucus is moved by ciliated epithelial cells in the airway lining, not by capillary walls.

Their walls are made of multiple layers of smooth muscle: This describes arteries and arterioles more accurately, not capillaries.

Common Pitfalls:
One common error is to generalize features of the respiratory system or larger vessels to capillaries. Because blood vessels and air passages are both tubular, students sometimes mix up their features. Another mistake is assuming that all vessels have multiple tissue layers, forgetting that capillaries are specialized for exchange and are therefore much simpler and thinner in structure.

Final Answer:
The correct description is that capillaries have walls that are only one cell thick to allow exchange.