2.11 Rate of Blood Flow in Blood Vessels

• Blood Flow:

  • Defined as the quantity of blood that passes through a given point in the circulation over a specific period.
  • For an adult at rest, the overall blood flow is approximately 5000 ml/min.

• Cardiac Output:

  • This term refers to the amount of blood pumped by the heart in a unit period.
  • It is equivalent to the overall blood flow in the circulation (e.g., 5000 ml/min at rest).

Comparison of Blood Flow Rate in Different Vessel Types

• Blood velocity varies significantly across different segments of the circulatory system.
  • In the aorta, blood travels very rapidly, around 30 cm/sec on average.
  • In the capillaries, blood flow is substantially slower, approximately 0.26 cm/sec.

Mechanism Regulating Blood Flow Velocity: Total Cross-Sectional Area

• The primary factor determining the rate of blood flow in different vessels is not the diameter of individual vessels, but the total cross-sectional area of all vessels of that type combined.

  • Although individual capillaries are extremely narrow, their sheer number results in a vastly larger total cross-sectional area than any other part of the circulatory system (e.g., arteries).

• Relationship between Total Cross-Sectional Area and Blood Velocity:

  • There is an inverse relationship: as total cross-sectional area increases, blood velocity decreases.
  • This explains why blood flow slows dramatically in the arterioles and even further in the capillary beds, where the total cross-sectional area is at its maximum.
  • Conversely, as blood exits the capillaries and enters venules and veins, the total cross-sectional area decreases, causing blood velocity to increase again.

• Visual Representation:

• Regulation of Blood Pressure:
  • The carotid sinus and aortic arch baroreceptor reflexes play a crucial role in the moment-to-moment regulation of blood pressure, which indirectly affects blood flow.
  • Blood Pressure→
  • Baroreceptors→