lecture recording on 26 February 2025 at 12.04.55 PM

Introduction to Hemodynamics

• The study of blood flow and its physical properties, known as hemodynamics, is crucial for understanding how the human body functions.

• Monitoring vital signs, particularly blood flow and pressure, can indicate overall health and predict potential medical issues.

Blood Pressure Basics

• Blood Pressure Measurement:

  • Average blood pressure typically around 83 mmHg is more accurate than the often quoted 100.

  • Blood pressure is measured as a fraction: systolic over diastolic (e.g. 120/80).

  • Blood pressure fluctuates with heartbeats, peaking during contraction (systole) and lowering during relaxation (diastole).

• Pulsatile Nature:

  • In large arteries, blood pressure displays a pulsatile nature, which becomes more stable in microcirculation.

  • Blood flows forward primarily due to a pressure gradient.

Flow Direction and Pressure Gradient

• Anterograde Flow:

  • Refers to normal forward blood flow.

  • Blood flows from areas of high pressure to areas of low pressure.

• Retrograde Flow:

  • Refers to backward flow, which generally should not occur under normal conditions.

  • Elevated venous pressure may lead to aberrant flow back to the heart.

Measuring Blood Pressure

• Millimeters of Mercury (mmHg):

  • This unit is commonly used due to mercury's density, which allows for a practical measurement scale.

  • Alternative fluids can measure pressure, but are less common.

• Health Risks of Mercury:

  • Mercury is toxic and can cause neurotoxic effects, such as in the historical context of hat-making causing 'mad hatter' syndrome.

Ohm's Law in Physiology

• Analogous Concept:

  • Ohm's law applied to blood flow: blood pressure = flow × resistance (TPR).

  • Understanding of fluid dynamics, including calculating resistance, is crucial for understanding blood pressure and flow in medicine.

• Crouse's Law:

  • Represents blood flow based on various physical parameters including blood pressure, radius of the vessels, and resistance.

  • Flow is directly proportional to the fourth power of the radius, meaning small changes in diameter significantly affect flow.

Viscosity and Blood Flow

• Viscosity Explained:

  • Refers to how thick or thin a fluid is; it affects how easily fluids flow.

  • Examples include comparing water to maple syrup, with thicker fluids resisting flow more.

• Changing Conditions:

  • Increased hematocrit (concentration of blood cells) raises viscosity, decreasing flow rate unless other parameters are adjusted (e.g., increased pressure).

Hydrostatic Pressure in Circulation

• Influence of Gravity:

  • Blood pressure is influenced by gravity and body position; hydrostatic pressure increases on standing.

  • Symptoms like lightheadedness can occur due to rapid positional changes affecting blood flow and pressure regulation.

Pulse and Heart Rate Regulation

• Pacemaker Activity:

  • Specialized cells in the heart (SA node, AV node, and Purkinje fibers) control heart rate independently of the nervous system.

  • The heart can maintain a rhythm and rate (around 60-90 beats per minute) due to its autorhythmic capabilities.

Funny Current and Pacemaker Cells

• Mechanism:

  • Pacemaker cells do not have a true resting membrane potential; they continuously drift towards threshold due to ion channel activity (HCN channels).

  • HCN channels allow sodium influx, which initiates action potentials.

  • The upstroke of action potential in pacemaker cells is primarily driven by calcium influx, not sodium as in typical neurons.

Conclusion

• Physiological Importance:

  • Understanding hemodynamics, blood pressure, and heart regulation is crucial for diagnosing and managing cardiovascular health.

  • Recognizing how blood flow can be regulated through various mechanisms is vital in understanding human physiology.