Patient Care Technician (PCT) AAH Practice Exam

What is the primary characteristic of capillaries?

1. They have thick walls for strength

2. They carry blood under high pressure

3. They allow exchange of substances between blood and tissues

4. They are responsible for pumping blood

The correct answer is: They allow exchange of substances between blood and tissues

The primary characteristic of capillaries is their ability to allow the exchange of substances between blood and tissues. Capillaries are the smallest blood vessels in the circulatory system, featuring walls that are only one cell thick. This thinness facilitates the diffusion of gases (like oxygen and carbon dioxide), nutrients, and waste products between the blood and surrounding tissues. Their design is crucial for maintaining healthy tissue function, as it enables essential exchanges to occur rapidly and effectively. In contrast, other options refer to characteristics that are more descriptive of other types of blood vessels; for instance, arteries have thick walls to withstand high pressure and are responsible for carrying blood from the heart, while veins return blood to the heart and may have valves to assist with flow. Capillaries fill in the functional gap between arteries and veins, ensuring that nutrient and gas exchange can occur efficiently at the cellular level.