Vascular system

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Vascular System

Lecture Outline

  • Blood Vessels and Arteries

  • Arterioles, Resistance, and Blood Pressure

  • Veins and Capacitance

  • Pulmonary Blood Pressure

Aims I

  • Define vaso and veno constriction.

  • Describe how elastic arteries maintain blood flow during diastole.

  • Define resistance and total peripheral resistance.

  • Describe the role of arterioles in the regulation of total peripheral resistance.

  • Describe how changes in cardiac output and total peripheral resistance alter mean arterial pressure.

  • Describe the actions of sympathetic tone on arterioles.

Aims II

  • Define capacitance.

  • Describe the role of veins as capacitance vessels.

  • Describe the function of the venous valves.

  • Describe how the sympathetic tone alters venous capacitance and venous return.

  • Know typical values for venous pressure and pulmonary arterial pressure.

Outline of the Circulation

Vessel Size and Composition

  • Blood Vessels:

    • Lined with endothelium.

    • Elastic and connective tissue contains the pressure.

    • Smooth muscle contraction changes vessel size.

    • Sympathetic nerves to most blood vessels.

    • Adrenaline/noradrenaline contract blood vessels via α-receptors.

Arterial Elasticity

  • Stores Pressure and Maintains Flow:

    • Thick walls with lots of elastic and connective tissue.

    • Stretch during systole and store pressure (and energy).

    • Maintain arterial pressure and blood flow during diastole.

Resistance to Flow

  • Flow (Q) depends on a pressure difference and the resistance of the vessel.

  • High resistance leads to low flow.

  • Small vessels have more resistance than large vessels.

  • Equation: $Q = (Pa - Pv) / R

Mean Arterial Pressure

  • Equation: MAP = Cardiac Output x Total Peripheral Resistance

  • Factors:

    • Heart rate.

    • Stroke volume.

    • Total peripheral resistance.

Arterioles

  • Characteristics:

    • Thick walls with lots of smooth muscle.

    • Most resistance to blood flow.

    • Control total peripheral resistance and distribution of cardiac output.

    • Active control of diameter through vaso-constriction.

Vaso-constriction

  • Sympathetic Tone:

    • At rest, sympathetic tone increases resistance.

    • Global sympathetic tone increases total peripheral resistance and mean arterial blood pressure.

    • Local sympathetic tone alters resistance and blood flow.

Capacitance (Compliance)

  • Blood vessels are not rigid.

  • Volume of a vessel depends on the pressure in the vessel and the vessel's stiffness.

  • Stiff vessels have low capacitance (e.g., arteries).

  • Veins are not stiff, contain a large volume of blood, and are capacitance vessels.

  • Veno-constriction reduces capacitance, increases venous return, and cardiac output.

Venous Valves

  • Function:

    • Create one-way flow.

    • Prevent blood flow away from the heart.

    • Combine with skeletal muscle to assist pumping of venous blood.

Systemic vs. Pulmonary Pressure

  • Systemic Blood Pressure: 120/80 mmHg.

  • Pulmonary Blood Pressure: 22/8 mmHg.

  • Lower total resistance in the pulmonary circuit.

  • Pa ≈ Q∙R

Summary

  • Arteries are elastic vessels that store energy (pressure) during systole and maintain flow during diastole.

  • Mean arterial pressure is controlled by cardiac output and total peripheral resistance.

  • Arterioles are high resistance vessels that control total peripheral resistance and the distribution of cardiac output.

  • Sympathetic stimulation constricts arterioles, increasing blood pressure by restricting blood flow.

  • Veins are high volume (compliance/capacitance) vessels that can act as a blood store.

  • Sympathetic stimulation constricts veins, reducing the volume of blood they hold, increasing venous return and cardiac output.

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