CB [39]NGU Dependence of blood flow on driving pressure and resistance 2024-2025

Blood Flow Dependence on Driving Pressure and Resistance

Prof. Dr. Sandra Younan

Professor of PhysiologyKasr el Aini Faculty of MedicineCairo University2024-2025

Objectives

• Understand blood flow and its intricate relation to pressure and resistance, crucial for physiological homeostasis.

• Identify the pressure difference between arterial and venous systems and how it affects circulation efficiency.

• Comprehend Ohm's and Poiseuille's laws and their varied applications within the cardiovascular system.

• Explore the effects of gravitational pressure on blood vessels and how it influences circulation dynamics.

• Identify complex mechanisms that facilitate venous return against the force of gravity, ensuring effective blood flow even when standing.

Introduction

• Main Function: The heart and circulatory system are vital for maintaining adequate blood flow to body tissues, delivering oxygen and nutrients necessary for cellular functions while removing metabolic waste.

• Blood Flow Definition: Blood flow is the quantity of blood passing a specific point in the circulatory system within a given timeframe, generally expressed in milliliters per minute.

• Relation to Cardiac Output: The rate of blood flow equates to cardiac output, representing the total volume of blood pumped by the heart per minute and is foundational for understanding cardiovascular health.

Relation of Blood Flow to Pressure and Resistance

• Determining Factors: According to Ohm's law, blood flow (F) can be determined by the equation:[ F = \frac{ΔP}{R} ]where ( ΔP ) represents the pressure gradient (effective perfusion pressure), and ( R ) represents the resistance due to friction between the blood and the endothelial lining of blood vessels.

• Implication: In the absence of a pressure difference between the arterial and venous systems, blood flow ceases, underscoring the critical role of pressure gradients in circulation.

Application of Ohm’s Law in the Body

• Cardiac Output Definition: Cardiac output (CO) can further be defined as:[ CO = Heart Rate \times Stroke Volume ]where heart rate is the number of beats per minute and stroke volume is the amount of blood ejected from the heart with each beat.

• Ohm's Application: The framework of F = ΔP / R is essential in cardiovascular physiology, highlighting the relationship between cardiac output, perfusion pressure, and resistance in systemic circulation.

• Expression:[ CO = \frac{ΔP}{TPR} ]where TPR stands for Total Peripheral Resistance, representing the cumulative resistance of all systemic blood vessels.

• Understanding ΔP: Gaining insight into ΔP necessitates a comprehensive exploration of arterial pressure dynamics, significant for interpreting blood flow regulation.

Mean Arterial Blood Pressure (MABP)

• Systolic Pressure: This is the peak arterial pressure during the cardiac cycle, occurring during ventricular contraction.

• Diastolic Pressure: This reflects the minimum arterial pressure preceding the next heartbeat, occurring during ventricular relaxation.

• MABP Calculation: The mean arterial blood pressure (MABP) is calculated as:[ MABP = Diastolic Pressure + \frac{1}{3} \times Pulse Pressure ]where pulse pressure is the difference between systolic and diastolic pressures. Typical MABP values range from 70-110 mmHg, with ~90 mmHg considered optimal for effective perfusion.

Mean Systemic Pressure

• Pressure Observations: Blood pressure is high immediately after it leaves the left ventricle and drops significantly upon returning to the right atrium, highlighting the nuances of blood pressure dynamics in the systemic circulation.

• Systemic Perfusion Pressure (ΔP): This is defined as MABP minus Right Atrial Pressure (RAP), representing the effective pressure driving blood through the systemic circulation. Typical RAP values range from 0-2 mmHg, emphasizing that MABP is decisive in driving systemic blood flow.

Mean Pulmonary Pressure

• Driving Pressure in Pulmonary Circulation: This refers to the difference between mean pulmonary arterial pressure and left atrial pressure, crucial for efficient gas exchange in the lungs.

• Resistance Calculation (R): Systemic Vascular Resistance can be calculated as:[ R = \frac{(MSABP - RAP)}{CO} \times 80 ]For example, if MSABP = 100 mmHg, RAP = 2 mmHg, and CO = 5L/min, then [ R = \frac{(100 - 2)}{5} \times 80 = 1568 ~dynes/sec/cm^5. ]

Poisseuille’s Law

• Flow Characteristics: Blood flow exhibits laminar characteristics, with the maximum velocity occurring at the center of blood vessels, which is essential for efficient nutrient delivery.

• Rate Determinants: Blood flow rate can be grouped into two equations:[ F = \frac{ΔP}{R} ]and[ R = \frac{8ηL}{πr^4} ]where η is the viscosity of blood, L is the length of the vessel, and r is the radius of the vessel.

• Flow Rate (Q): The flow rate can be expressed as:[ Q = \frac{ΔP × πr^4}{8ηL} ]illustrating how a minor change in vessel radius can drastically influence flow rate and resistance.

Application of Poisseuille’s Law in the Body

• Flow Relation: Blood flow (Q) is directly proportional to the fourth power of the radius (r^4), while resistance (R) is inversely proportional to the fourth power of the radius. Thus, even slight vasodilation can cause significant increases in blood flow.

• Regulation of Cardiac Output: The body can adjust vascular resistance through various mechanisms (e.g., vasoconstriction, vasodilation) to manage blood flow distribution effectively in different physiological states such as exercise or rest.

Resistance Offered by Different Vessels

• Comparison of Vessel Type: Resistance varies by vessel type, with elastic vessels (arteries) exhibiting different flow characteristics compared to rigid vessels (arterioles and capillaries).

• Veins: Capable of storing a substantial volume of blood, veins have thinner walls than arteries, contributing to their role as capacitance vessels, which can adapt to changes in blood volume.

• Distensibility: Pulmonary arteries show greater distensibility compared to systemic arteries, which is crucial for accommodating the variable blood flow during respiration without significant pressure changes.

Resistance to Blood Flow in Series and Parallel Circuits

• Series Arrangement: In a series circuit, blood flow through each vessel is consistent, and the total resistance is the sum of individual resistances, critical for understanding organ perfusion.

• Parallel Arrangement: This arrangement allows individual tissues to regulate their blood supply independently, enhancing overall flow efficiency and adaptability.

• Formula: For parallel circuits:[ \frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} ]demonstrating how resistance decreases when connecting vessels in parallel.

Distribution of Blood Volume at Rest

• Blood Volume Distribution:

  • Veins: 64%

  • Lungs: 9%

  • Capillaries: 5%

  • Arterioles: 7%

  • Large Arteries: 8%

  • Heart: 7%Understanding how blood volume is distributed at rest informs clinical practices and insights into cardiovascular health.

Blood Velocity in Relation to Cross-Sectional Area

• Velocity Dynamics: Blood velocity is inversely related to the total cross-sectional area of blood vessels. The highest cross-sectional area is found in capillaries, resulting in the slowest blood velocity, essential for nutrient and gas exchange.

• Flow Behavior:

  • Aorta: 40 cm/sec

  • Arteries: 15 cm/sec

  • Capillaries: 0.1 cm/sec

  • Veins: Velocity increases as blood returns towards the right atrium, ensuring efficient drainage back to the heart.

Effect of Gravitational Pressure on Veins & Arteries

• Gravitational Impact: The weight of blood exerts pressure on blood vessels, especially in an upright position, complicating venous return and overall circulation.

• Pressure Comparison:

  • Right atrium: 0 mmHg

  • Arterial pressure ~100 mmHg

  • Leg vein pressure can reach +90 mmHg.

  • Neck veins may collapse due to surrounding atmospheric pressure, indicating the variability of pressure effects based on body posture.

Mechanisms Helping Venous Return During Standing

• Sympathetic Stimulation: Activation of the sympathetic nervous system promotes venous constriction, enhancing blood return to the heart.

• Muscle Pump: Skeletal muscle contractions, particularly during movement, compress veins and assist in propelling blood toward the heart.

• Venous Valves: Prevent backflow of blood, ensuring unidirectional flow towards the heart especially during movements and against gravity.

• Thoracic Pump: Changes in intrathoracic pressure during breathing facilitate the return of blood to the heart.

• Suction Action of the Right Atrium: The negative pressure generated during the right atrial diastole aids in drawing venous blood back into the heart, especially when body posture changes.

References

Guyton & Hall: Textbook of Medical Physiology, chapter 14, p. 161-170 and p. 177-178 (11th edition).

Acknowledgments

Thank you for your attention!