Recording-2025-03-06T02:56:55.803Z

Hemodynamics

• Definition: Study of the forces controlling blood flow ("hemo" = blood, "dynamics" = forces).

• Key Factors Influencing Hemodynamics:

  • Blood Pressure: Higher pressure increases force behind the blood.

  • Blood Flow: How easily blood moves through vessels, influenced by vessel size and resistance.

  • Vascular Resistance: Caused by vessel diameter, blood viscosity, and vessel length.

  • Cardiac Output: The volume of blood the heart pumps per minute, depends on heart rate and stroke volume.

Key Terms

• Vascular Resistance:

  • Influences blood flow depending on vessel diameter (wide vessels offer less resistance; narrow vessels increase resistance).

  • Viscosity: The thickness of the blood; thicker blood leads to higher resistance.

  • Length of Vessels: Longer vessel lengths increase resistance to blood flow.

• Laws of Hemodynamics:

  • Poussin’s Law: Relates vessel diameter, length, and viscosity to resistance.

  • Ohm’s Law in Blood Flow: Flow (Q) is determined by pressure (P) and resistance (R).

    • If pressure increases, flow increases.

    • If resistance increases, flow decreases.

Blood Pressure Measurement

• Systolic Pressure: Maximum pressure when heart contracts; represented by the higher number on a blood pressure reading.

• Diastolic Pressure: Minimum pressure when heart relaxes; represented by the lower number.

• Mean Arterial Pressure: Average pressure in arteries, important for diagnosing conditions.

• Measurement Locations: Typically measured at the arm using a blood pressure cuff.

Cardiac Output (CO)

• Definition: Total blood volume pumped by the heart each minute. Cardiac output is a key determined of overall blood flow and it’s closely regulated to meet the metabolic demands of the body. For example, during exercise, both heart rate and stroke, volume increased due hands, cardiac output.

• Calculation:

  • CO = Heart Rate (HR) x Stroke Volume (SV). CO=HRxSV.

  • Heart Rate: Number of heartbeats per minute.

  • Stroke Volume: Volume of blood ejected by each ventricle during each contraction/pumped with each heartbeat.

Resistance in Blood Flow

• Factors Affecting Resistance:

  • Vessel Diameter: Smaller diameter increases resistance (think of squeezing a hose).

  • Viscosity: Thicker blood increases resistance; thin blood flows easier.

  • Vessel Length: Longer vessels increase resistance to flow.

• Vasoconstriction vs. Vasodilation:

  • Vasoconstriction: Reduces vessel diameter, increases resistance.

  • Vasodilation: Increases vessel diameter, decreases resistance.

Regulation of Blood Flow

• Short-term Regulation: Sympathetic nervous system responds quickly to emergencies by adjusting vessel diameter through vasoconstriction.

• Long-term Regulation: Hormonal control (like RAAS) influences blood volume and pressure, mainly through sodium and water balance.

Hemostasis

• Definition: Process of stopping blood flow, critical when an injury occurs.

• Three Phases:

  1. Vascular Spasm: Immediate constriction of blood vessels to reduce blood flow at the injury site.

  2. Platelet Plug Formation: Platelets adhere to collagen exposed by injury, activated to become sticky, and aggregate to form a plug.

  3. Coagulation: Fibrin threads are formed to stabilize the platelet plug and lead to full clot formation.

Hemostasis Pathways

• Intrinsic Pathway: Activated by injury within the blood vessel; slower response.

• Extrinsic Pathway: Activated by external injury; rapid response.

• Both pathways converge at Factor X, leading to common pathway.

Coagulation Process

• Key Steps:

  • Thrombin converts fibrinogen into fibrin; fibrin forms a mesh that strengthens the platelet plug.

  • Fibrinolysis: Process of breaking down fibrin once the vessel wall is healed, preventing unnecessary clotting.

  • Plasminogen Activation: Plasminogen activates to plasmin to dissolve the fibrin meshwork.

Summary

• Understanding hemodynamics and hemostasis is essential for examining cardiovascular physiology.

• Key concepts include blood flow dynamics, pressure measurement, and physiological responses to maintain blood vessel integrity during injury.

• Be familiar with physiological relationships, terms, and processes to prepare for assessments.