Why is the blood flow in capillaries slower than arterial and venous vessels?

The blood flow in capillaries is slower than arterial and venous vessels due to several factors:

1. Narrower Diameter:

- Capillaries have a much narrower diameter compared to arteries and veins. The average diameter of a capillary is about 5-10 micrometers, whereas arteries and veins can be several millimeters in diameter.

- The narrower diameter of capillaries increases the resistance to blood flow, leading to a slower velocity.

2. Increased Surface Area:

- Capillaries have an extensive network of interconnected vessels, resulting in a larger total surface area for exchange of substances between the blood and the surrounding tissues.

- The increased surface area creates more friction between the blood and the vessel walls, contributing to slower blood flow.

3. Higher Viscosity:

- Blood viscosity, which is the resistance of a fluid to flow, is higher in capillaries compared to larger vessels.

- The presence of red blood cells, plasma proteins, and other components in the blood contributes to increased viscosity, making it more difficult for blood to flow through the narrow capillaries.

4. Fluid Exchange:

- Capillaries are the primary sites for the exchange of fluids, nutrients, oxygen, carbon dioxide, and waste products between the blood and the surrounding tissues.

- As blood passes through the capillaries, fluid and substances move in and out, further slowing down the blood flow.

Therefore, the combination of narrower diameter, increased surface area, higher viscosity, and fluid exchange collectively contribute to the slower blood flow in capillaries compared to arterial and venous vessels.