What is the main reason blood flow is slowest in the capillaries?
A.
Capillaries have to fight the pull of gravity.
B.
Capillaries have a great cross-sectional area.
C.
Capillary blood has a higher viscosity than venous blood.
D.
Capillaries have a higher osmotic pressure.
The correct answer and Explanation is :
The correct answer is:
B. Capillaries have a great cross-sectional area.
Explanation:
Blood flow is slowest in the capillaries primarily due to their large total cross-sectional area. The circulatory system can be divided into different segments, each with its own characteristics. The flow of blood through these segments is influenced by factors such as vessel diameter, total cross-sectional area, and blood viscosity.
In the circulatory system, blood flow velocity is inversely related to the cross-sectional area of the blood vessels. This means that as the cross-sectional area increases, the velocity of blood flow decreases.
Here’s how it works in detail:
- Cross-Sectional Area: The capillaries, being the smallest blood vessels, have a significantly larger total cross-sectional area compared to arteries or veins. While each individual capillary is quite narrow, there are millions of them in the body, creating an enormous collective cross-sectional area. This vast network of capillaries reduces the velocity of blood flow considerably.
- Blood Flow Velocity: According to the principle of continuity in fluid dynamics, the flow rate (or volume flow) of blood must remain constant throughout the circulatory system. Therefore, when blood enters the capillaries from the larger-diameter arteries, it must slow down to accommodate the greater cross-sectional area of the capillary network.
- Physiological Significance: The slow flow in the capillaries is essential for effective nutrient, gas, and waste exchange between the blood and tissues. This slower velocity allows more time for the diffusion of oxygen, nutrients, and other substances into the tissues and the removal of carbon dioxide and metabolic wastes from the tissues.
In summary, the slowest blood flow in the capillaries is primarily due to their great total cross-sectional area. This ensures that the blood moves slowly enough to allow for the necessary exchange of substances between the blood and tissues.