AAMC FL Practice Exam

Why does blood flow slower in capillaries than in arteries?

1. Capillaries are narrower than arteries.

2. Total cross-sectional area of capillaries is less than that of arteries.

3. Total cross-sectional area of capillaries is greater than that of arteries.

4. Capillaries are longer than arteries.

The correct answer is: Total cross-sectional area of capillaries is greater than that of arteries.

The reason blood flows slower in capillaries compared to arteries is primarily due to the greater total cross-sectional area of capillaries. When blood travels from the relatively narrow diameter of arteries into the vast network of capillaries, the total cross-sectional area increases significantly. This increase in cross-sectional area causes the velocity of blood to decrease as it moves through these smaller vessels. As blood enters the capillary beds, it is distributed among thousands of capillaries, leading to a much larger area for the blood to flow through collectively, rather than flowing through a single conduit as it does in arteries. This slower flow rate within the capillaries is essential for effective exchange of nutrients, gases, and waste products between blood and surrounding tissues. The slower movement allows more time for these processes to occur, which is vital for maintaining cellular health and homeostasis. Other options do not correctly account for the relationship between flow velocity and cross-sectional area in the circulatory system. The statement regarding capillaries being narrower than arteries is true, but it does not fully explain the blood flow dynamics. Similarly, while capillaries are longer and narrower, it is the cumulative cross-sectional area that primarily governs the velocity of blood flow in different types of vessels.