Is this due to increased frictional resistance which decreases the velocity of the blood? You would think using Bernoulli's principle that the velocity of blood in the capillaries would increase due to the decrease in pressure but the inverse is true.
There are two main reasons why the pressure in the capillaries is lower than in the big arteries:
- Arteries are elastic - they increase in diameter with every pulse, so the blood pressure is falling as the blood is moving along.
- The total cross-sectional area of the capillaries is greater than the one of the big arteries.
The pressure of arterial blood is largely dissipated when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7.5 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total cross-sectional area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases.