The smallest blood vessels in the human body, whose walls are only one cell thick to allow exchange of materials, are called what?

Introduction / Context:
The circulatory system contains different types of blood vessels that vary in size, structure, and function. Some vessels are large and muscular, while others are extremely small and thin walled. The smallest vessels are specially adapted for exchange of gases, nutrients, and waste products between the blood and body tissues. This question asks you to name these tiny one cell thick vessels correctly.

Given Data / Assumptions:

• The vessels described are the smallest blood vessels in the body.

• Their walls are only one cell thick, allowing easy diffusion of substances.

• The options include venae cavae, arteries, veins, capillaries, and arterioles.

• We assume knowledge of basic blood vessel types and their structural differences.

Concept / Approach:
Capillaries are microscopic blood vessels that form networks between arterioles and venules in tissues. Their walls are composed of a single layer of endothelial cells and a thin basement membrane, making them only one cell thick. This structure minimises diffusion distance, which is essential for efficient exchange of oxygen, carbon dioxide, nutrients, and waste products. In contrast, arteries and veins have thicker, multi layered walls, venae cavae are large veins returning blood to the heart, and arterioles are small arteries that still have thicker walls than capillaries.

Step-by-Step Solution:
Step 1: Recall that capillaries are the smallest diameter blood vessels, often so narrow that red blood cells pass through in single file. Step 2: Understand that capillary walls consist of a single layer of squamous endothelial cells with a basement membrane, making them one cell thick. Step 3: Recognise that this extremely thin barrier allows rapid diffusion of gases and small molecules between blood and tissue fluid. Step 4: Compare this with arteries, which have thick muscular and elastic walls to withstand high pressure from the heart. Step 5: Veins also have multiple layers, although thinner than arteries, and include valves to prevent backflow of blood. Step 6: Venae cavae are the largest veins in the body, not the smallest vessels. Step 7: Arterioles are small branches of arteries, but they still have thicker walls than capillaries because they help control blood flow and pressure. Step 8: Therefore, the one cell thick smallest vessels described in the question are capillaries.

Verification / Alternative check:
Anatomy and physiology diagrams of the circulatory system show blood flowing from arteries to arterioles, then into capillary networks, and then into venules and veins. Labels on histology images identify capillaries by their tiny lumen and single cell thick walls. Text explanations also describe capillaries as exchange vessels, emphasising their thin walls and short diffusion distance. These observations confirm that capillaries are the smallest, one cell thick blood vessels in the body.

Why Other Options Are Wrong:

Venae cavae: These are large veins (superior and inferior vena cava) that return blood to the heart and have thick walls relative to capillaries.

Arteries: Thick walled vessels that carry blood away from the heart under high pressure, not the smallest vessels.

Veins: Larger, thin walled vessels that return blood to the heart but are still much bigger and thicker than capillaries.

Arterioles: Small branches of arteries that regulate blood flow into capillary beds but do not have one cell thick walls.

Common Pitfalls:
Students sometimes confuse arterioles and capillaries because both are small compared to large arteries. Another mistake is assuming that any small vessel is a capillary without considering wall thickness and function. Remember that arterioles regulate flow with muscular walls, whereas capillaries are specifically adapted for exchange and are only one cell thick.

Final Answer:
The smallest blood vessels with one cell thick walls are called Capillaries.