Understanding Blood Flow Dynamics and Hemodynamics

Posted on Feb 1, 2025 in Biology

Speed: Distance traveled per unit time (cm/sec).

Flow: Volume per unit time (cm3/sec).

Global Blood Flow

Blood flow in the global movement of an adult at rest is approximately 5000 ml/minute. This is called Cardiac Output.

Determinants of Blood Flow

  1. Pressure difference between the two ends of the vessel: “pressure gradient” – the force that pushes blood through the vessel.
  2. Impediment to blood flow through the vessel, called “vascular resistance.”

Ohm’s Law: Q = P/R

The pressure difference between the two ends of the vessel, and not the absolute pressure in the vessel, determines the flow rate.

Probability of Turbulence

Turbulence relates to vessel diameter and blood viscosity.

  • If Re (Reynolds number) is less than 2000, the flow is usually laminar.
  • If Re is greater than 3000, turbulence often prevails.
  • To push a flow of liquid through the same tube, it takes more pressure when the flow is turbulent than when it is laminar.
  • The flow tends to become turbulent with high velocity, low fluid viscosity, larger tube diameter, and irregular vessel walls or stenosis.

Blood Pressure

Blood pressure is the force exerted by blood against any unit area of the vessel wall.

Variations in Arterial Pressure

Any change in arterial pressure (Pa) is directly proportional to changes in the volume of arterial blood (Va) and inversely proportional to changes in arterial distensibility (Da).

Pulse Pressure: Systolic Pressure – Diastolic Pressure

Mean Arterial Pressure: Average pressure during the whole cardiac cycle.

Mean Arterial Pressure = Diastolic Pressure + 1/3 Pulse Pressure

Flow Resistance

The major determinant of blood flow resistance in any part of the circulatory system is vessel caliber.

Hematocrit

The percentage of blood that corresponds to cells is the hematocrit. Blood viscosity increases as the hematocrit increases.

Laplace’s Law

The smaller the radius of a blood vessel, the lower the wall tension required to balance the distending pressure.

Vascular Distensibility

  • All vessels are compliant.
  • When pressure increases in the arterioles, they dilate, and thus, their resistance decreases.
  • Outcome: Increased blood flow, not only by increasing the pressure but also by the fall in resistance.
  • Veins are on average 8 times more compliant than arteries.
  • Pulmonary arteries have compliance 6 times higher than systemic arteries.

Vascular Adaptability or Capacitance

Total amount of blood that can be stored in a given portion of the circulation for every millimeter of mercury increase in pressure.

Capacitance of systemic veins is approximately 24 times that of its corresponding artery. It is 8 times more compliant and has approximately a volume 3 times greater.

Pressure-Blood Volume Curves

Small changes in arterial blood volume (750 to 500 ml) cause large changes in pressure.

Venous: Enormous volume changes are needed to alter venous pressure by a few mmHg.

Factors that Increase Venous Return

  1. Increased blood volume.
  2. Increased tone of large vessels throughout the body, with a resulting increase in peripheral venous pressure.
  3. Dilation of the arterioles (peripheral resistance decreases and allows the rapid passage of blood from arterioles to veins).