CARDIOVASCULAR PHYSIOLOGY LECTURE 4: ARTERIES, ARTERIOLES & THE DISTRIBUTION OF BLOOD FLOW

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  • Dr. Lucinda Krause teaches at the Department of Physiology, Monash University.

CARDIOVASCULAR PHYSIOLOGY LECTURE 4: ARTERIES, ARTERIOLES & THE DISTRIBUTION OF BLOOD FLOW

Learning Outcomes

  • After completing this module and the associated textbook chapter, students should be able to:

    • Explain the functions of arteries as low-resistance conduits and pressure reservoirs for maintaining blood flow during diastole.

    • Explain the pulsatile nature of arterial pressure.

    • Define the terms:

    • Diastolic pressure: The lowest pressure in the arteries during diastole.

    • Systolic pressure: The highest pressure in the arteries during systole.

    • Pulse pressure: The difference between systolic and diastolic pressure.

    • Mean arterial pressure (MAP): The average pressure in a person’s arteries during one cardiac cycle.

    • Explain the theoretical basis for different methods to measure arterial pressure: auscultatory, palpatory, and automatic blood pressure monitoring methods.

    • Describe the role of arterioles in controlling local blood flow.

    • Explain the factors regulating arteriolar diameter.

Reading Texts

  • Vander’s Human Physiology by Eric Widmaier, Hershel Raff & Kevin Strange, 16th Edition: Chapter 12, Sections 12.9-12.10

Structure of Blood Vessels

Overview of Blood Vessel Types

  • Aorta: Main artery, large diameter, and elastic walls to absorb pressure.

  • Arteries: Thick-walled, low resistance, conducting vessels.

  • Arterioles: Major resistance vessels controlling blood distribution.

  • Capillaries: Site of exchange of gases, fluids, nutrients and waste products.

  • Venules: Smaller than veins, low resistance, capacitance vessels.

  • Veins: Larger diameter, high capacitance vessels.

Layers of Blood Vessel Walls

  1. Inner Layer (Tunica Interna):

    • Composed of endothelial cells lining the lumen.

    • Functions: Provides a smooth surface to minimize friction and regulates the diameter of the blood vessel by releasing substances like nitric oxide, prostacyclin, and endothelin-1.

  2. Middle Layer (Tunica Media):

    • Contains circular smooth muscle and elastic tissues.

    • Functions: Can change the blood vessel diameter through contraction and relaxation, allowing for regulation of blood flow and pressure.

  3. Outer Layer (Tunica Externa):

    • Made of connective tissue (collagen).

    • Functions: Protects and reinforces the blood vessel, helping to anchor it to surrounding tissues.

  • Capillary walls consist solely of endothelial cells.

Arteries and Arterial Pressure

Major Roles of Arteries

  • Specialized Functions:

    • Serve as low-resistance pathways for blood from the heart to organs.

    • Act as pressure reservoirs, maintaining blood flow during diastole.

Pulsatile Nature of Arterial Pressure

  • Arterial pressure fluctuates during the cardiac cycle, with peak pressure during systole and lowest pressure during diastole.

Characteristics of Arteries

  • Large diameter (radius), which means low resistance to blood flow.

  • Resistance is inversely proportional to the fourth power of the radius, which implies minimal pressure loss as blood moves through arteries.

  • Example: Similar pressure in the femoral artery compared to the aorta or carotid artery.

Pressure Reservoir Function of Arteries

  • During systole, about 2/3 of blood ejected stretches the large arteries, while 1/3 flows downstream into arterioles and capillaries.

  • This pressure enables continuous blood flow to tissues during diastole, as the arteries recoil and maintain pressure.

Arterial Blood Pressure Metrics

  • Systolic and Diastolic Pressures:

    • Systolic pressure: Peak pressure during systole.

    • Diastolic pressure: Lowest pressure during diastole.

  • Pulse Pressure: Calculated as Systolic Pressure - Diastolic Pressure.

  • Mean Arterial Pressure (MAP): Approximated by the equation:
    MAPext≈DiastolicPressure+rac13PulsePressureMAP ext{ ≈ } Diastolic Pressure + rac{1}{3} Pulse Pressure

  • Normal values: Systolic pressure ~ 120 mmHg; Diastolic pressure ~ 80 mmHg; MAP ~ 93 mmHg.

Importance of Measuring Blood Pressure

  • Arterial blood pressure is a modifiable risk factor for cardiovascular disease (CVD), contributing to significant morbidity and mortality globally.

Methods for Measuring Arterial Blood Pressure

  • Direct Measurement: Invasive method involving insertion of a pressure device.

  • Indirect Measurement: Non-invasive methods using a sphygmomanometer, involving:

    • Auscultation (listening with a stethoscope).

    • Palpation (feeling for a pulse).

    • Automated Devices (sphygmomanometer with an inbuilt pressure sensor).

Auscultation Method for Blood Pressure Measurement

  • Involves detecting blood flow sounds using an inflatable cuff and a stethoscope.

  • Developed by Nikolai Korotkov.

Palpation Method for Measuring Blood Pressure

  • Using a finger pulse transducer to detect the pulse.

Automated Blood Pressure Monitor

  • Utilizes an inflatable cuff with microcomputer-controlled inflation/deflation.

  • Converts pressure waveforms into digital readings using algorithms, clinically validated for accuracy.

Arterioles and Regulation of Blood Flow

Role of Arterioles

  • Arterioles serve as key regulators of blood flow distribution to various organs/tissues based on metabolic demand.

  • They are the primary site of vascular resistance.

  • Adjust their diameter to control blood flow.

Blood Flow Redistribution During Exercise

  • Blood flow increases during strenuous activity, with marked changes in flow to different organs, e.g., muscle, heart, and skin.

    • Example: Skeletal muscle flow during rest ~ 650 mL/min; during exercise ~ 12,500 mL/min, indicating a 3.5-fold increase.

Factors Affecting Blood Flow Dynamics

  • Local Resistance: Determines proportional blood flow to each organ.

  • Arteriolar Diameter Control: Influenced by smooth muscle contraction (vasoconstriction) and relaxation (vasodilation).

  • Intrinsic/Basal Tone: Continuous state of slight contraction allowing for dynamic regulation of blood flow.

Local Control of Arterioles

  • Chemical Signals:

    • Increased blood flow correlates with tissue metabolic activity (active hyperemia), causing dilation due to factors like decreased oxygen and increased carbon dioxide or metabolites.

    • Vasodilators (NO and prostacyclin) promote dilation; vasoconstrictors (endothelin-1) promote constriction.

Autoregulation of Blood Flow

  • Autoregulation allows tissues to maintain stable blood flow despite changes in systemic pressure through myogenic responses to stretch (increased stretch ⇒ contraction; decreased stretch ⇒ dilation).

Extrinsic Control of Arterioles

  • Sympathetic Nervous System:

    • Most arterioles receive sympathetic nerve input controlling vascular smooth muscle via release of noradrenaline, causing constriction.

    • Variations in nerve supply density between vascular beds affect regulation.

Hormonal Influences on Blood Flow Regulation

  • Hormones:

    • Adrenaline acts on different receptors causing varying responses (constriction or dilation) in different tissues.

    • Other hormones such as Angiotensin II and vasopressin promote constriction, while Atrial natriuretic peptide promotes dilation.

Role of Endothelium in Arteriolar Control

  • Endothelial cells are crucial in regulating arteriolar diameter through the release of both vasodilatory and vasoconstrictor substances, triggered by shear stress from blood flow.