The force with which the blood leaves the arteries causes a pressure that is greater in systole movements, when the heart contracts and expels blood, than in diastole, with the heart relaxed and filling with blood. Therefore, when we measure blood pressure we have a maximum and a minimum blood pressure.
When the blood reaches the capillaries, the pressure is low enough for the exchange of substances with the cells to develop correctly, leaving the nutrients and oxygen from the capillaries, and entering the waste substances, including CO2.
The decrease in blood pressure in arterioles and capillaries is due to two reasons:
- Any fluid flowing through a circuit loses pressure as it moves away from the driving organ (the heart).
- The pressure of a fluid is lower the greater the volume of the enclosure through which it circulates. Although the diameter of the capillaries is very small, as they are very numerous, the total volume through which the blood circulates is greater.