Vascular System - Arterial System and Venous System

Three Components of CV System

Pump (heart), conduits (blood vessels), and transport medium (blood).

Arteries Function

Transport oxygenated blood away from the heart to the capillaries.

Capillaries Function

Primary site of nutrient, gas, and metabolic product exchange.

Veins Function

Return blood from tissues back to the heart.

Highest Resistance Blood Vessels

Arterioles.

Greatest Pressure Drop Location

Occurs across the arterioles.

Control of Arteriolar Resistance

Regulated by the sympathetic nervous system.

Lowest Blood Velocity Location

Capillaries, due to their large total cross-sectional area.

Blood Velocity Formula

v = Q / A (Velocity = Flow divided by Cross-Sectional Area).

Systolic Pressure (Ps)

Maximum pressure in systemic arteries during a cardiac cycle.

Diastolic Pressure (Pd)

Minimum pressure in systemic arteries during a cardiac cycle.

Determinants of Systolic Pressure

Volume of blood in arterial system near end of systole and arterial compliance.

Determinants of Diastolic Pressure

Volume of blood in arterial system at end of diastole and arterial compliance.

Pulse Pressure (PP) Formula

PP = Ps - Pd (Systolic Pressure minus Diastolic Pressure).

Pulse Pressure Definition

The pressure wave felt as a pulse during cardiac contraction.

Mean Blood Pressure (MBP) Formula (Pressures)

MBP = Pd + (PP / 3).

Mean Blood Pressure (MBP) Formula (Flow/Resistance)

MBP = CO × TPR (Cardiac Output times Total Peripheral Resistance).

Diastolic vs Systolic Duration Ratio

Approximately 2/3 of cardiac cycle is spent in diastole, 1/3 in systole.

Why MBP is Below Midpoint

Heart spends more time in diastole than in systole during each cycle.

Age Effect on Arterial Pressures

Both systolic and diastolic normal values increase with age.

Age Effect on Arterial Compliance

Arterial compliance decreases with age, making arteries stiffer.

Age Effect on Pulse Pressure

Pulse pressure widens/increases with age due to stiffer arterial walls.

Age Effect on Ventricular Afterload

Increases with age as arterial walls become stiffer.

MBP Response to Exercise

Rises because Cardiac Output increases proportionately more than TPR decreases.

Essential Hypertension Mechanism

Elevated MBP driven primarily by increased total peripheral resistance (TPR).

Compliance (C) Definition

Ratio of change in volume to change in pressure (C = ΔV / ΔP).

Stiffness Definition

The reciprocal of compliance (C⁻¹ or ΔP / ΔV).

Pressure Required to Fill Stiff Vessel

Higher pressure is required compared to a compliant vessel.

Venous Compliance Value (Example)

Cv = 300 mL / 5 mmHg = 60 mL/mmHg (high compliance).

Arterial Compliance Value (Example)

Ca = 250 mL / 100 mmHg = 2.5 mL/mmHg (low compliance).

Arteries vs Veins Compliance

Veins have much higher compliance and lower stiffness than arteries.

Primary Pump of CV System

Left ventricle of the heart.

Secondary Pump Action of Aorta

Elastic recoil during diastole drives continuous blood flow to organs.

Role of Aorta Compliance

Buffers stroke volume, preventing excessive surge in systolic pressure.

Arterial Blood Volume Determinants

Inflow rate (Cardiac Output) versus outflow rate into capillaries (TPR).

Effect of Increased TPR on Arterial Volume

Less blood shifts into capillaries, increasing arterial blood volume and pressure.

Systemic Venous Blood Volume Fraction

Approximately two-thirds (2/3) of total blood volume is in systemic veins.

Venous System Function as Reservoir

Acts as an emergency blood reservoir that can be mobilized.

Central Venous Pressure (CVP) Range

Low pressure, normally less than 12 mmHg.

Mean Systemic Arterial Pressure Range

High pressure, approximately 90 mmHg.

Sympathetic Effect on Veins

Causes venoconstriction, reducing compliance and shifting volume into circulation.

Sympathetic Effect on Arteries

Constricts arterioles, increasing Total Peripheral Resistance (TPR).

Venous Return Definition

Volume of blood flowing back into the heart per minute.

Cardiac Output Formula

CO = SV × HR (Stroke Volume times Heart Rate).

Frank-Starling Mechanism in Venous Return

Increased venous return → increased preload → optimal sarcomere overlap → increased SV.

Gravity Effect on Bipedal Circulation

Causes blood pooling in lower limbs due to hydrostatic pressure in compliant veins.

CV Pressure Sources

Heart contraction force plus hydrostatic weight of the blood column.

Effect of Gravitational Pooling on Preload

Decreases blood volume in heart, lowering preload and cardiac output.

Hypothetical Rigid Vessels and Gravity

If blood vessels were rigid, gravity would have minimal effect on circulation.

Mechanisms Counteracting Gravitational Pooling

Venous valves and the skeletal muscle pump.

Function of Venous Valves

Prevent retrograde blood flow and break up the fluid column height.

Skeletal Muscle Pump Function

Muscle contraction compresses veins, propelling blood toward the heart.

Cause of Varicose Veins

Incompetent venous valves causing blood pooling and vessel distention.

Intrathoracic Pressure During Inspiration

Decreases (becomes more negative).

Right Ventricle Preload During Inspiration

Increases due to lower right atrial pressure promoting venous return.

Right Ventricle Stroke Volume During Inspiration

Increases due to elevated right ventricular preload.

Left Ventricle Preload During Inspiration

Decreases due to pulmonary vascular pooling.

Left Ventricle Stroke Volume During Inspiration

Decreases due to lower left ventricular preload.

Intrathoracic Pressure During Expiration

Increases (becomes less negative).

Right Ventricle Preload During Expiration

Decreases as intrathoracic pressure rises.

Right Ventricle Stroke Volume During Expiration

Decreases.

Left Ventricle Preload During Expiration

Increases as pulmonary blood is pushed into the left heart.

Left Ventricle Stroke Volume During Expiration

Increases.

Acute Regulation of Venous vs Arterial Compliance

Venous compliance is acutely regulated by nerves; arterial compliance is not.

Venoconstriction Effect on CVP

Increases CVP (filling pressure), shifting blood to active circulation.

Right Atrial Pressure During Inspiration

Decreases transiently, boosting venous return to the right heart.

Right Atrial Pressure During Expiration

Increases transiently, reducing venous return to the right heart.

Vascular Bed Order from Heart

Aorta → Large Arteries → Small Arteries → Arterioles → Capillaries → Venules → Veins → Vena Cava.

Ventricular Afterload Definition

Pressure the ventricle must overcome to eject blood into the aorta.

Aortic Pressure Waveform Peak

Represents the Systolic Blood Pressure.

Aortic Pressure Waveform Trough

Represents the Diastolic Blood Pressure.

Preload Definition

End-diastolic stretch/volume of myocardial fibers before contraction.

Capillary Cross-Sectional Area vs Velocity

Highest cross-sectional area produces the lowest blood flow velocity.

Arteriolar Vasodilation Effect on TPR

Decreases Total Peripheral Resistance.

Arteriolar Vasoconstriction Effect on TPR

Increases Total Peripheral Resistance.

Hydrostatic Pressure in Lower Extremity Veins

High when standing due to column weight of blood.

Primary Driver of Active Circulation Shift

Sympathetically induced venoconstriction reducing venous compliance.

Compliance Symbol Formula

C = ΔV / ΔP.

Stiffness Symbol Formula

Stiffness = ΔP / ΔV = C⁻¹.

Pulse Pressure Determinant (Volume)

Stroke Volume ejected into the aorta.

Pulse Pressure Determinant (Vessel)

Compliance of the arterial walls.

Venous Valve Closure Mechanism

Prevents gravity from pulling blood downward during muscle relaxation.

Systemic Circulation Low Pressure Side

Venous system.

Systemic Circulation High Pressure Side

Arterial system.

Capillary Exchange Drivers

Diffusional and hydrostatic pressure gradients across endothelial wall.

Effect of Decreased Compliance on ΔP

Larger ΔP generated for a given ΔV.

Effect of Increased Compliance on ΔP

Smaller ΔP generated for a given ΔV.

Arterial Recoil Function

Maintains continuous forward blood flow during cardiac diastole.

Venous Return and Preload Relationship

Directly proportional: increased venous return increases right ventricular preload.

Effect of Sympathetic Activity on Heart Rate

Increases HR, contributing to higher Cardiac Output.

Effect of Sympathetic Activity on Stroke Volume

Increases contractility and venous return, enhancing Stroke Volume.

Conduit Component of CV System

Blood vessels (arteries, capillaries, veins).

Transport Medium of CV System

Blood.

Arterial System Compliance Characteristic

Low compliance, high stiffness, acts as pressure reservoir.

Venous System Compliance Characteristic

High compliance, low stiffness, acts as volume reservoir.

Impact of Upright Posture on Venous Return

Transiently reduces venous return due to gravity-induced venous pooling.

Inspiration Effect on Right Heart SV

Increases Right Ventricle Stroke Volume.

Inspiration Effect on Left Heart SV

Decreases Left Ventricle Stroke Volume.

Expiration Effect on Right Heart SV

Decreases Right Ventricle Stroke Volume.

Expiration Effect on Left Heart SV

Increases Left Ventricle Stroke Volume.