Blood flows more slowly in the arterioles than in the artery that supplies them because the arterioles
a. Have thoroughfare channels to venules that are often closed off, slowing the flow of blood.
b. Have sphincters that restrict flow to capillary beds.
c. Are narrower than the artery.
d. Collectively have a larger cross-sectional area than does the artery.

Verified step by step guidance

1. Understand the relationship between blood flow and cross-sectional area: Blood flow velocity is inversely proportional to the total cross-sectional area of the blood vessels. This means that as the total cross-sectional area increases, the velocity of blood flow decreases.
2. Recall the structure of the circulatory system: Arteries branch into smaller arterioles, which then branch into capillaries. While individual arterioles are narrower than the artery, the total cross-sectional area of all arterioles combined is much larger than that of the artery.
3. Eliminate incorrect options: Option (a) refers to thoroughfare channels, which are not the primary reason for slower blood flow in arterioles. Option (b) mentions sphincters, which regulate blood flow to capillary beds but do not directly explain the slower flow in arterioles. Option (c) states that arterioles are narrower, but this does not account for the collective cross-sectional area.
4. Focus on the correct explanation: The slower blood flow in arterioles is due to their collective larger cross-sectional area compared to the artery. This larger area reduces the velocity of blood flow, allowing for efficient exchange of nutrients and gases in the capillaries.
5. Conclude with the correct choice: The correct answer is (d), as the collective larger cross-sectional area of arterioles compared to the artery is the primary reason for the slower blood flow.

Key Concepts

Blood Vessel Structure

Blood vessels, including arteries and arterioles, have different diameters and structural features. Arteries are typically wider and have thicker walls to withstand high pressure, while arterioles are narrower and have more muscular walls that can constrict or dilate, affecting blood flow and pressure.

Cross-Sectional Area

The total cross-sectional area of blood vessels plays a crucial role in blood flow dynamics. As blood moves from larger arteries to smaller arterioles, the collective cross-sectional area increases, leading to a decrease in blood velocity due to the conservation of mass, which is essential for efficient nutrient and gas exchange in tissues.

Vascular Resistance

Vascular resistance refers to the opposition to blood flow within the circulatory system, primarily influenced by the diameter of blood vessels. Smaller vessels like arterioles create higher resistance compared to larger arteries, resulting in slower blood flow, which is vital for regulating blood pressure and ensuring adequate perfusion of tissues.