Peripheral Resistance

Peripheral resistance is the opposition to blood flow produced by the blood vessels of the systemic circulation. It is an important determinant of both arterial blood pressure and the distribution of cardiac output among different tissues.

Resistance is generated throughout the vascular system, but the small arteries and arterioles are particularly important because their smooth muscle can actively change vessel diameter. For this reason, arterioles are commonly described as the principal resistance vessels of the circulation.

Peripheral resistance is not constant. It changes continuously in response to autonomic nervous activity, circulating hormones, local metabolites, endothelial signals, tissue metabolic demand, and structural properties of the vascular system.

Total Peripheral Resistance

Total peripheral resistance (TPR), also called systemic vascular resistance (SVR), represents the overall resistance to blood flow through the systemic circulation.

It reflects the combined resistance of the many systemic vascular beds arranged in parallel.

Relationship Between Pressure, Flow and Resistance

Blood flow through a vascular bed depends on the pressure difference across that bed and its resistance.

The basic hemodynamic relationship can be expressed as:

Flow = Pressure Difference ÷ Resistance

Rearranging this relationship:

Resistance = Pressure Difference ÷ Flow

Systemic Circulation

For the systemic circulation, the relevant pressure gradient extends approximately from the arterial circulation to the right atrium.

Systemic vascular resistance can therefore be related to mean arterial pressure, right atrial pressure, and cardiac output.

Systemic Vascular Resistance Equation

A simplified hemodynamic relationship is:

SVR = (Mean Arterial Pressure − Right Atrial Pressure) ÷ Cardiac Output

Because right atrial pressure is normally much lower than mean arterial pressure, it may have a relatively small numerical effect in some simplified physiological discussions, although it should not be assumed to be zero in precise clinical calculations.

Major Determinants of Vascular Resistance

Factor	Effect on Resistance
Vessel radius	Small changes produce very large changes in resistance
Blood viscosity	Greater viscosity increases resistance
Vessel length	Greater length increases resistance

Vessel Radius

Vessel radius is the most powerful rapidly adjustable determinant of vascular resistance.

Contraction or relaxation of vascular smooth muscle changes the internal radius of small arteries and arterioles, producing major changes in resistance and blood flow.

Poiseuille's Relationship

For ideal laminar flow through a cylindrical tube, resistance is strongly related to the radius of the tube.

Poiseuille's relationship can be expressed conceptually as:

Resistance ∝ Viscosity × Length ÷ Radius⁴

This fourth-power relationship means that relatively small changes in vessel radius can produce very large changes in resistance.

Effect of Radius on Resistance

If vessel radius decreases, resistance rises sharply. If radius increases, resistance falls sharply.

This relationship explains why arteriolar vasoconstriction and vasodilation are so effective at controlling tissue blood flow and arterial pressure.

Vasoconstriction

Vasoconstriction is narrowing of a blood vessel caused primarily by contraction of vascular smooth muscle.

In resistance vessels, vasoconstriction decreases vessel radius and increases vascular resistance.

Vasodilation

Vasodilation is widening of a blood vessel associated with relaxation of vascular smooth muscle.

Arteriolar vasodilation increases vessel radius, decreases resistance, and generally increases blood flow when the pressure gradient is maintained.

Arterioles as Resistance Vessels

Arterioles have a relatively small lumen surrounded by smooth muscle that can actively alter vessel diameter.

Their ability to produce substantial changes in resistance makes them central regulators of systemic vascular resistance and regional perfusion.

Pressure Drop Across Resistance Vessels

A substantial pressure decrease occurs as blood passes through small arteries and arterioles.

This pressure drop reflects their relatively high resistance and helps regulate the pressure reaching downstream capillary networks.

Protection of Capillaries

By adjusting resistance upstream from capillaries, arterioles help control capillary hydrostatic pressure.

This contributes to regulation of capillary exchange and protects delicate microvascular networks from excessive pressure.

Vascular Smooth Muscle

Vascular smooth muscle in arterial and arteriolar walls provides the contractile mechanism responsible for active changes in vessel diameter.

Its tone is influenced by neural, hormonal, endothelial, mechanical, and local metabolic signals.

Vascular Tone

Vascular tone refers to the baseline degree of contraction present in vascular smooth muscle.

Changes above or below this baseline can produce vasoconstriction or vasodilation and thereby modify resistance.

Sympathetic Regulation

The sympathetic nervous system provides important neural control of systemic vascular resistance.

Many systemic arterioles receive sympathetic vasoconstrictor fibers that release norepinephrine.

Alpha-Adrenergic Vasoconstriction

Norepinephrine acting on alpha-adrenergic receptors in vascular smooth muscle commonly produces vasoconstriction.

Widespread increases in sympathetic vasoconstrictor activity can increase total peripheral resistance and support arterial pressure.

Sympathetic Withdrawal

A reduction in sympathetic vasoconstrictor activity allows vascular smooth muscle in many vascular beds to relax.

This can reduce vascular resistance and increase regional blood flow.

Circulating Catecholamines

Epinephrine and norepinephrine released into the circulation can influence vascular tone.

The response varies among vascular beds according to receptor type, receptor density, hormone concentration, and local physiological conditions.

Local Metabolic Regulation

Tissues can regulate their own blood flow by producing local chemical changes that alter arteriolar resistance.

When tissue metabolism increases, accumulation of vasodilator influences can reduce local resistance and increase perfusion.

Metabolic Vasodilator Influences

Local changes associated with increased metabolism can include reduced oxygen availability and increased concentrations of carbon dioxide, hydrogen ions, potassium, adenosine, and other metabolites.

The relative importance of individual factors varies among tissues.

Active Hyperemia

Active hyperemia is an increase in tissue blood flow associated with increased metabolic activity.

Local vasodilation reduces arteriolar resistance and helps match oxygen and nutrient delivery to tissue demand.

Reactive Hyperemia

Reactive hyperemia is a temporary increase in blood flow after a period of reduced or interrupted perfusion.

Accumulated vasodilator influences and altered vascular conditions contribute to the transient reduction in resistance after flow is restored.

Autoregulation

Autoregulation is the ability of a tissue to maintain relatively stable blood flow despite changes in perfusion pressure over a physiological range.

Local changes in arteriolar resistance are central to this process.

Myogenic Mechanism

Vascular smooth muscle can respond directly to changes in stretch.

In many arterioles, increased transmural pressure promotes contraction, while reduced pressure can favor relaxation. This myogenic response contributes to autoregulation.

Endothelial Regulation

The vascular endothelium produces signaling molecules that influence vascular smooth muscle tone.

These substances allow the endothelial layer to participate actively in regulation of resistance and blood flow.

Nitric Oxide

Nitric oxide is an important endothelium-derived vasodilator.

It diffuses into vascular smooth muscle and promotes relaxation, reducing vascular resistance.

Endothelin

Endothelin is a potent vasoconstrictor peptide produced by endothelial cells.

Its physiological and pathological effects depend on receptor distribution and the balance with opposing vasodilator mechanisms.

Shear Stress

Blood flow creates frictional force, or shear stress, along the endothelial surface.

Changes in shear stress can stimulate endothelial signaling, including nitric oxide production, and thereby influence vessel diameter.

Hormonal Regulation

Several circulating hormones influence vascular smooth muscle and peripheral resistance.

Important examples include angiotensin II, vasopressin, catecholamines, and natriuretic signaling systems.

Angiotensin II

Angiotensin II is a potent vasoconstrictor and an important component of the renin-angiotensin-aldosterone system.

Its vascular actions can increase systemic resistance, while its renal and endocrine effects contribute to longer-term regulation of blood volume and arterial pressure.

Vasopressin

Vasopressin, also called antidiuretic hormone, participates in regulation of water balance and can produce vasoconstriction at sufficiently elevated concentrations.

Its overall cardiovascular role depends on physiological context.

Blood Viscosity

Blood viscosity describes the internal resistance of blood to flow.

Greater viscosity increases vascular resistance when other factors remain unchanged.

Hematocrit

Hematocrit is an important determinant of whole-blood viscosity.

A substantial increase in red blood cell concentration can increase viscosity and resistance, while reduced hematocrit decreases viscosity.

Vessel Length

Resistance increases with vessel length when other factors remain constant.

Unlike arteriolar radius, vessel length generally does not undergo rapid moment-to-moment adjustment and is therefore less important for short-term regulation of vascular resistance.

Laminar Flow

Under many normal conditions, blood moves through vessels in approximately laminar flow, with layers of fluid moving parallel to the vessel wall.

Poiseuille-based relationships are most applicable under idealized laminar-flow conditions.

Turbulent Flow

Turbulent flow involves irregular movement and mixing of blood rather than orderly parallel layers.

Turbulence increases energy loss and cannot be described adequately by the simple Poiseuille relationship.

Resistance in Series

When vascular segments are arranged in series, their individual resistances add together.

Blood passes sequentially through arteries, arterioles, capillaries, venules, and veins within a vascular pathway.

Resistance in Parallel

Most major organ circulations are arranged in parallel within the systemic circulation.

This arrangement allows blood flow to individual organs to be regulated relatively independently while each organ receives blood from the systemic arterial circulation.

Total Resistance in Parallel Circuits

Adding an additional parallel pathway decreases the total resistance of the combined system.

Conversely, widespread constriction across multiple systemic vascular beds can substantially increase total peripheral resistance.

Series and Parallel Resistance

Arrangement	Relationship
Series	Individual resistances add directly
Parallel	Total resistance is less than the resistance of individual parallel pathways

Peripheral Resistance and Mean Arterial Pressure

Systemic vascular resistance is an important determinant of mean arterial pressure.

A useful simplified relationship is:

Mean Arterial Pressure − Right Atrial Pressure ≈ Cardiac Output × Systemic Vascular Resistance

Changes in either cardiac output or systemic vascular resistance can therefore alter arterial pressure.

Baroreceptor Reflex

The arterial baroreceptor reflex can rapidly adjust vascular resistance in response to changes in arterial pressure.

Reduced arterial pressure decreases baroreceptor firing and promotes sympathetic vasoconstriction, while increased pressure generally produces the opposite response.

Peripheral Resistance During Exercise

Exercise produces different vascular responses in different tissues.

Metabolic vasodilation markedly reduces resistance in active skeletal muscle, while sympathetic vasoconstriction can limit flow to some less active vascular beds.

Total Peripheral Resistance During Dynamic Exercise

During substantial dynamic exercise, widespread vasodilation in active skeletal muscle commonly reduces total systemic vascular resistance despite increased sympathetic activity.

Cardiac output rises substantially, allowing arterial pressure and tissue perfusion to be maintained.

Skin Circulation

Cutaneous vascular resistance changes according to thermoregulatory requirements.

Increased skin blood flow facilitates heat loss, while cutaneous vasoconstriction helps conserve heat.

Renal Circulation

The kidneys regulate their vascular resistance through intrinsic autoregulatory mechanisms as well as neural and hormonal influences.

Changes in renal arteriolar resistance affect renal blood flow, glomerular filtration dynamics, and long-term regulation of extracellular fluid volume.

Cerebral Circulation

Cerebral vessels possess strong local regulatory mechanisms that help maintain brain blood flow across a range of perfusion pressures.

Carbon dioxide and local metabolic conditions are particularly important influences on cerebral vascular resistance.

Coronary Circulation

Coronary resistance is strongly influenced by myocardial metabolic activity.

As cardiac work and oxygen demand increase, local vasodilation increases coronary blood flow.

Pulmonary Versus Systemic Vascular Resistance

The pulmonary circulation normally operates at much lower pressure and resistance than the systemic circulation.

Systemic vascular resistance therefore should not be confused with pulmonary vascular resistance, which describes opposition to flow through the pulmonary circulation.

Systemic and Pulmonary Resistance

Feature	Systemic Circulation	Pulmonary Circulation
Resistance	Relatively high	Relatively low
Pumping ventricle	Left ventricle	Right ventricle
Major resistance term	SVR or TPR	PVR

Clinical Significance

Hypertension

Increased systemic vascular resistance can contribute to elevated arterial pressure.

Long-term blood pressure regulation is complex and also depends on renal sodium and water handling, cardiac output, neurohormonal systems, vascular structure, and other factors.

Shock

Peripheral resistance changes substantially in different forms of circulatory shock.

Compensatory vasoconstriction may increase resistance in hypovolemic states, while profound vasodilation can reduce systemic vascular resistance in distributive shock.

Septic Shock

Septic shock commonly involves marked abnormalities of vascular tone and distribution of blood flow.

Systemic vascular resistance may be substantially reduced because of widespread vasodilation, particularly during characteristic phases of the syndrome.

Cardiogenic Shock

When cardiac output falls because of severe pump failure, sympathetic activation commonly produces systemic vasoconstriction.

This can support arterial pressure but also increases left ventricular afterload and may further challenge a failing heart.

Heart Failure

In heart failure, neurohormonal activation can increase systemic vascular resistance.

Although vasoconstriction may initially help maintain arterial pressure, increased afterload can make ventricular ejection more difficult.

Peripheral Arterial Disease

Structural narrowing or obstruction of arteries increases resistance to blood flow within affected vascular territories.

This can reduce downstream perfusion, particularly when metabolic demand increases.

Polycythemia

A marked increase in red blood cell concentration can increase blood viscosity.

Higher viscosity increases resistance to flow and alters cardiovascular workload and microcirculatory dynamics.

Anemia

Reduced hematocrit decreases blood viscosity.

Systemic vascular resistance may decrease in significant anemia, while compensatory increases in cardiac output help maintain tissue oxygen delivery.

Vasodilator Therapy

Medications that relax arterial or arteriolar smooth muscle can reduce systemic vascular resistance.

This can lower arterial pressure and reduce the afterload against which the left ventricle ejects.

Vasoconstrictor Therapy

Vasoconstrictor medications can increase vascular tone and systemic vascular resistance.

They may be used in selected forms of severe hypotension or shock when increasing vascular tone is clinically appropriate.

Key Relationships

Change	Effect on Resistance
Vessel radius decreases	Resistance increases markedly
Vessel radius increases	Resistance decreases markedly
Blood viscosity increases	Resistance increases
Vessel length increases	Resistance increases
Arteriolar vasoconstriction	Resistance increases
Arteriolar vasodilation	Resistance decreases

Key Features of Peripheral Resistance

Feature	Key Point
Definition	Opposition to blood flow through the vasculature
Main resistance vessels	Small arteries and arterioles
Most powerful adjustable factor	Vessel radius
Other determinants	Blood viscosity and vessel length
Neural regulator	Sympathetic nervous system
Local regulators	Metabolites, myogenic responses and endothelial signals
Pressure relationship	SVR contributes importantly to mean arterial pressure
Flow relationship	At a given pressure gradient, greater resistance reduces flow

Physiological Importance

Peripheral resistance provides the cardiovascular system with a powerful mechanism for controlling both systemic arterial pressure and regional blood flow. Small arteries and arterioles can rapidly alter their diameter, allowing resistance to change from moment to moment.

Systemic regulation through sympathetic and hormonal mechanisms helps maintain arterial pressure, while local metabolic, myogenic, and endothelial mechanisms allow individual tissues to adjust perfusion according to their requirements.

The strong dependence of resistance on vessel radius explains why relatively small changes in arteriolar diameter can profoundly alter cardiovascular function. Peripheral resistance therefore links vascular anatomy with blood pressure regulation, cardiac afterload, tissue perfusion, microcirculation, exercise physiology, and many cardiovascular disorders.