Pulmonary Circuit

The pulmonary circuit is the portion of the cardiovascular system that carries blood between the heart and lungs. It transports deoxygenated blood from the right ventricle to the lungs, where gas exchange occurs, and returns oxygenated blood to the left atrium.

The major vessels of the pulmonary circuit are the pulmonary trunk, right and left pulmonary arteries, progressively smaller pulmonary arterial branches, pulmonary capillaries surrounding the alveoli, pulmonary venous channels, and the pulmonary veins.

Unlike the systemic circulation, the pulmonary circulation operates at relatively low vascular pressure and resistance. Nevertheless, under steady-state conditions it receives essentially the entire output of the right ventricle and therefore carries approximately the same volume of blood per unit time as the systemic circulation.

Pathway of the Pulmonary Circuit

The basic route of blood through the pulmonary circulation is:

Right ventricle → Pulmonary valve → Pulmonary trunk → Right and left pulmonary arteries → Pulmonary arterial branches → Pulmonary capillaries → Pulmonary venules and veins → Left atrium

Beginning of the Pulmonary Circuit

The pulmonary circuit begins at the right ventricle.

During ventricular systole, the right ventricle ejects blood through the pulmonary valve into the pulmonary trunk.

Right Ventricle

The right ventricle receives systemic venous blood from the right atrium and pumps it into the pulmonary circulation.

Its wall is thinner than that of the left ventricle because the pulmonary vascular bed normally presents much less resistance than the systemic circulation.

Pulmonary Valve

The pulmonary valve is a semilunar valve positioned between the right ventricle and pulmonary trunk.

It opens during ventricular ejection and closes as right ventricular pressure falls below pulmonary arterial pressure, helping prevent retrograde flow into the right ventricle.

Pulmonary Trunk

The pulmonary trunk arises from the right ventricle and courses superiorly and posteriorly within the pericardial cavity.

It carries deoxygenated blood, despite being classified as an artery because arteries are defined by carrying blood away from the heart rather than by oxygen content.

Bifurcation of the Pulmonary Trunk

The pulmonary trunk divides into the right pulmonary artery and left pulmonary artery.

These vessels carry blood to the corresponding lungs.

Right Pulmonary Artery

The right pulmonary artery travels from the pulmonary trunk toward the right lung.

Because the pulmonary trunk originates toward the left side of the anterior heart, the right pulmonary artery has a relatively long transverse course through the superior mediastinum.

Course of the Right Pulmonary Artery

The right pulmonary artery passes posterior to the ascending aorta and superior vena cava as it travels toward the right lung.

At the lung root, it divides into branches supplying the lobes and bronchopulmonary segments.

Left Pulmonary Artery

The left pulmonary artery travels toward the hilum of the left lung.

It is shorter than the right pulmonary artery and divides into branches that follow the bronchial tree into the lung.

Relationship to the Aortic Arch

The left pulmonary artery lies inferior to the aortic arch.

The ligamentum arteriosum, the postnatal remnant of the fetal ductus arteriosus, extends between the region of the left pulmonary artery and the aortic arch.

Pulmonary Arteries at the Lung Roots

The relationships of the pulmonary arteries to the main bronchi differ on the two sides.

A useful anatomical relationship is that the right pulmonary artery generally lies anterior to the right main bronchus, while the left pulmonary artery is positioned superior to the left main bronchus at the hilum.

Branching Within the Lungs

The pulmonary arteries branch alongside the bronchial tree.

Lobar arteries supply pulmonary lobes, while segmental branches supply individual bronchopulmonary segments.

Pulmonary Arterioles

Repeated arterial branching produces progressively smaller vessels and eventually pulmonary arterioles.

These vessels regulate resistance and distribute blood into the extensive pulmonary capillary network.

Pulmonary Capillaries

Pulmonary capillaries form dense networks around the alveoli.

Their close relationship with alveolar air spaces creates the structural basis for pulmonary gas exchange.

Respiratory Membrane

Gas exchange occurs across a very thin interface between alveolar gas and capillary blood.

This interface includes the alveolar epithelial layer, closely associated interstitial and basement membrane components, and the capillary endothelium.

Gas Exchange

As blood flows through pulmonary capillaries, oxygen diffuses from alveolar gas into the blood while carbon dioxide diffuses from blood toward the alveoli.

The direction and magnitude of gas movement depend on partial-pressure gradients and the properties of the alveolar-capillary interface.

Oxygenation of Blood

Blood entering the pulmonary capillary network through pulmonary arteries has a relatively low oxygen content compared with systemic arterial blood.

After gas exchange, blood leaving ventilated alveolar capillaries has gained oxygen and lost carbon dioxide.

Pulmonary Venules

Capillary blood collects into small pulmonary venules.

These vessels merge into progressively larger veins that ultimately form the major pulmonary veins.

Pulmonary Veins

The pulmonary veins return oxygenated blood from the lungs to the left atrium.

They are veins because they carry blood toward the heart, even though their blood is highly oxygenated under normal conditions.

Number of Pulmonary Veins

There are typically four pulmonary veins entering the left atrium:

• Right superior pulmonary vein
• Right inferior pulmonary vein
• Left superior pulmonary vein
• Left inferior pulmonary vein

Anatomical variation in the number and pattern of pulmonary venous connections is common.

Pulmonary Veins at the Hilum

Pulmonary veins generally occupy relatively anterior and inferior positions within the lung roots compared with the major bronchi and pulmonary arteries.

The exact relationships vary according to level and side.

Termination in the Left Atrium

The pulmonary veins open into the posterior aspect of the left atrium.

Their openings normally lack valves at the junction with the atrium.

End of the Pulmonary Circuit

The pulmonary circuit ends when oxygenated pulmonary venous blood enters the left atrium.

Blood then passes through the mitral valve into the left ventricle, beginning the pumping phase that supplies the systemic circulation.

Pulmonary Versus Systemic Circulation

Feature	Pulmonary Circuit	Systemic Circuit
Pumping chamber	Right ventricle	Left ventricle
Receiving chamber	Left atrium	Right atrium
Major artery	Pulmonary trunk and pulmonary arteries	Aorta and systemic arteries
Major function	Pulmonary gas exchange	Perfusion of systemic tissues
Normal vascular resistance	Low	Higher
Normal operating pressure	Low	Higher

Pulmonary Vascular Resistance

Pulmonary vascular resistance (PVR) is the resistance encountered by blood flowing through the pulmonary circulation.

Under normal conditions, PVR is substantially lower than systemic vascular resistance.

Why Pulmonary Resistance Is Low

The pulmonary vascular bed contains an extensive network of relatively compliant vessels arranged to accommodate the entire output of the right ventricle at comparatively low pressures.

Recruitment and distension of pulmonary vessels can further reduce resistance as pulmonary blood flow increases.

Pulmonary Arterial Pressure

Pulmonary arterial pressure is much lower than systemic arterial pressure.

This low-pressure system is appropriate for perfusing the delicate pulmonary capillary network while limiting excessive fluid filtration into lung tissue and alveolar spaces.

Right Ventricular Afterload

Pulmonary vascular resistance is an important component of right ventricular afterload.

An increase in pulmonary vascular resistance makes it more difficult for the right ventricle to eject blood.

Pulmonary Blood Flow

In a stable circulation, pulmonary blood flow is essentially equal to right ventricular cardiac output.

Because the pulmonary and systemic circulations are connected in series, average right and left ventricular outputs must remain approximately equal over time.

Gravity and Pulmonary Blood Flow

Gravity influences the distribution of pulmonary perfusion, particularly in the upright body.

Hydrostatic pressure differences contribute to greater blood flow in dependent portions of the lungs compared with nondependent regions under many normal conditions.

Ventilation-Perfusion Matching

Efficient gas exchange requires appropriate matching between alveolar ventilation and pulmonary perfusion.

Regional differences in either ventilation or blood flow can alter gas exchange efficiency.

Hypoxic Pulmonary Vasoconstriction

Pulmonary vessels respond to low alveolar oxygen differently from many systemic vessels.

Reduced alveolar oxygen can produce local pulmonary vasoconstriction, redirecting blood away from poorly ventilated alveoli toward better ventilated regions.

Functional Importance of Hypoxic Vasoconstriction

Localized hypoxic pulmonary vasoconstriction can improve ventilation-perfusion matching.

However, widespread alveolar hypoxia can cause generalized pulmonary vasoconstriction and increase pulmonary arterial pressure.

Recruitment of Pulmonary Vessels

When pulmonary blood flow increases, previously minimally perfused vessels can begin carrying more blood.

This process, called recruitment, increases the effective cross-sectional area of the pulmonary vascular bed.

Distension of Pulmonary Vessels

Pulmonary vessels can also expand as intravascular pressure increases.

This distension contributes to the ability of the pulmonary circulation to accommodate increased cardiac output without a proportionate increase in pulmonary vascular resistance.

Pulmonary Circulation During Exercise

Cardiac output can rise severalfold during exercise, requiring a corresponding increase in pulmonary blood flow.

Recruitment and distension of pulmonary vessels allow this increased flow to occur with a relatively modest rise in pulmonary arterial pressure in healthy individuals.

Pulmonary Capillary Pressure

Pulmonary capillary hydrostatic pressure influences movement of fluid between the pulmonary microcirculation and interstitial tissues.

Excessive elevation of pulmonary vascular pressure can promote pulmonary interstitial and alveolar fluid accumulation.

Bronchial Versus Pulmonary Circulation

The lungs receive blood from both the pulmonary circulation and the bronchial circulation.

These systems have different primary functions and vascular origins.

Pulmonary and Bronchial Circulations

Feature	Pulmonary Circulation	Bronchial Circulation
Primary function	Gas exchange	Nutritional supply to bronchial and supporting tissues
Origin	Right ventricle via pulmonary trunk	Systemic arterial circulation
Pressure	Low-pressure circulation	Systemic-pressure circulation
Flow volume	Receives essentially entire right ventricular output	Small fraction of systemic cardiac output

Physiological Shunt

A small amount of blood from the bronchial circulation can enter pulmonary venous pathways and mix with oxygenated blood returning to the left heart.

Along with venous drainage directly into left-sided cardiac chambers, this contributes to the normal small physiological venous admixture.

Fetal Pulmonary Circulation

Before birth, the lungs are not used for placental gas exchange and pulmonary vascular resistance is relatively high.

Only a limited proportion of right ventricular output passes through the fetal pulmonary circulation.

Ductus Arteriosus

In fetal life, the ductus arteriosus connects the pulmonary arterial circulation with the aorta.

This allows much of the right ventricular output to bypass the high-resistance fetal lungs.

Changes at Birth

With lung expansion and the onset of ventilation at birth, pulmonary vascular resistance falls markedly.

Pulmonary blood flow increases, supporting gas exchange in the lungs and increasing pulmonary venous return to the left atrium.

Ligamentum Arteriosum

After functional closure of the ductus arteriosus, its remnant becomes the ligamentum arteriosum.

This fibrous structure extends between the pulmonary arterial region and the aortic arch.

Clinical Significance

Pulmonary Hypertension

Pulmonary hypertension refers to abnormally elevated pressure within the pulmonary circulation and can arise from several different disease mechanisms.

Persistent elevation increases the workload placed on the right ventricle.

Right Ventricular Hypertrophy

Chronic elevation of pulmonary vascular resistance can cause the right ventricle to generate higher pressures.

Over time, this pressure load can contribute to right ventricular hypertrophy and eventually right ventricular dysfunction.

Pulmonary Embolism

A pulmonary embolism occurs when material, most commonly thrombus originating in the systemic venous circulation, obstructs a pulmonary arterial vessel.

The physiological consequences depend on the size, number, and location of emboli and the cardiopulmonary condition of the patient.

Massive Pulmonary Embolism

Large or extensive pulmonary arterial obstruction can abruptly increase pulmonary vascular resistance.

This may cause acute right ventricular strain, reduced left ventricular filling, systemic hypotension, and severe circulatory compromise.

Pulmonary Edema

Pulmonary edema is abnormal accumulation of fluid within pulmonary interstitial tissue and potentially the alveoli.

Elevated pulmonary venous and capillary pressures, particularly from left-sided cardiac dysfunction, can contribute to cardiogenic pulmonary edema.

Left-Sided Heart Failure

When left ventricular or left atrial pressure rises, pressure can be transmitted backward into the pulmonary veins and capillaries.

This can produce pulmonary vascular congestion and, when sufficiently severe, pulmonary edema.

Pulmonary Venous Obstruction

Obstruction of pulmonary venous return can increase pressure upstream within the affected lung territory.

The consequences can include pulmonary congestion, edema, and altered pulmonary hemodynamics.

Patent Ductus Arteriosus

A patent ductus arteriosus is persistence after birth of the fetal vascular connection between the pulmonary arterial circulation and aorta.

The direction and magnitude of flow through the connection depend on pressure relationships and the size of the patent channel.

Intracardiac Shunts

Congenital communications between the right and left sides of the heart can alter pulmonary blood flow.

Depending on the defect and pressure relationships, pulmonary flow may be increased, decreased, or associated with abnormal mixing of oxygenated and deoxygenated blood.

Pulmonary Artery Catheterization

A pulmonary artery catheter can be advanced through the right-sided cardiac chambers into the pulmonary arterial circulation.

It can be used in selected clinical settings to obtain hemodynamic measurements related to right-sided pressures, pulmonary arterial pressure, cardiac output, and downstream filling pressures.

Major Structures of the Pulmonary Circuit

Structure	Function
Right ventricle	Pumps blood into pulmonary circulation
Pulmonary trunk	Carries right ventricular output toward lungs
Pulmonary arteries	Carry deoxygenated blood into right and left lungs
Pulmonary capillaries	Site of blood-gas exchange around alveoli
Pulmonary veins	Return oxygenated blood from lungs
Left atrium	Receives pulmonary venous blood

Key Features of the Pulmonary Circuit

Feature	Key Point
Starting chamber	Right ventricle
Ending chamber	Left atrium
Major outflow vessel	Pulmonary trunk
Gas-exchange vessels	Pulmonary capillaries
Venous return	Usually four pulmonary veins
Primary function	Transport blood through lungs for gas exchange
Vascular resistance	Low relative to systemic circulation
Arterial oxygenation	Pulmonary arteries normally carry deoxygenated blood
Venous oxygenation	Pulmonary veins normally carry oxygenated blood

Anatomical Importance

The pulmonary circuit forms the vascular connection between the right and left sides of the heart. Blood ejected by the right ventricle travels through the pulmonary arterial tree to an extensive capillary network surrounding the alveoli, allowing oxygen uptake and carbon dioxide elimination.

Oxygenated blood then returns through the pulmonary veins to the left atrium, from which it enters the systemic circuit. Because the pulmonary and systemic circuits are connected in series, the two circulations must handle essentially equal average blood flow over time.

The pulmonary circulation is distinguished by its low resistance and pressure, extensive capillary surface area, and specialized response to alveolar hypoxia. These characteristics allow the lungs to accommodate the entire cardiac output while maintaining an environment suitable for efficient gas exchange.