Question

The velocity of blood flow is lowest in capillaries because:
A. The capillaries walls are not thin enough to allow oxygen to exchange with the cells.
B. The capillaries are far from the heart and blood flow slows as the distance from the heart increases.
C. The diastolic blood pressure is too low to deliver blood to the capillaries at a high flow rate.
D. The total cross-sectional area of the capillaries is greater than the total cross-sectional area of the arteries or any other part of the circulatory system.

Answer 1

Hint: The transport of nutrients, gases as well as waste materials are carried out through the circulatory system. Blood is the main medium through which the materials are transported. The blood flows through vessels, such as arteries, veins, and capillaries.

Complete answer:
The movement of blood in unit time is called the velocity of blood and the amount of blood moving in time is called the flow of blood. Blood is transported to the tissues through arteries, arterioles, and capillaries.
Capillaries play an important role in the exchange of respiratory gases i.e oxygen and carbon dioxide. They also form a connection between arteries and veins. The movement of blood from the region of the high-pressure area to the low-pressure area in the blood vessels is called blood flow. Blood flows from arteries to capillaries to veins. The velocity of blood flow is inversely proportional to the cross-sectional area of the blood vessel. The total cross-sectional area of capillaries is greater than the total cross-sectional area of the arteries and veins .Hence blood flow is lowest in the capillaries. This slow rate of blood flow helps in the exchange of nutrients and gases.
Considering the other given options:
Even though the capillaries are thin-walled, the gaseous exchange between the cells does not influence the velocity of the flow of blood. Hence it is not the right option.
The location of the capillaries does not influence the velocity. Hence it is not the right option.
The diastolic blood pressure is also not an indicator of the blood flow in capillaries. Hence it is not the right option.

Therefore from the given options, the most appropriate one is option D. That is Velocity of blood flow is lowest in capillaries because the total cross-sectional area of the capillaries is greater than the total cross-sectional area of the arteries or any other part of the circulatory system.

Note:
Capillaries are small blood vessels that connect arteries with veins and blood flow exerts pressure on the walls of these blood vessels. Due to the highest cross-sectional area of the capillaries blood flow is lowest in them. This slow rate gives time for the exchange of water, gases, and nutrients.