Systemic vs Pulmonary Circulation - Anatomy and Physiology

SECTION 1

Historical Context & Motivation

For centuries, scholars struggled to explain how blood moves through the body, a question that sits at the very foundation of physiology and medicine. Ancient physicians such as Galen proposed that blood was produced in the liver and consumed by the tissues in a tidal, back-and-forth motion—an elegant idea for its era, but one that left the relationship between the lungs and the heart profoundly misunderstood. The eventual recognition that blood follows two distinct, closed circuits—the pulmonary circulation through the lungs and the systemic circulation through the rest of the body—required breakthroughs spanning more than a millennium, from the medieval Islamic world to the Royal Society of London.

c. 200 CE • Galen's Model of Blood Flow

Galen of Pergamon proposed that venous blood formed in the liver and arterial blood in the heart, with invisible pores in the interventricular septum allowing blood to pass between ventricles. This model, though erroneous, dominated Western and Islamic medicine for over a thousand years.

1242 • Ibn al-Nafis Describes Pulmonary Transit

The Arab physician Ibn al-Nafis correctly argued that blood must travel from the right ventricle to the lungs and then to the left ventricle, explicitly denying the existence of Galen's septal pores. His commentary on the Canon of Avicenna represents the earliest known description of the pulmonary circulation.

1553 • Servetus Rediscovers Pulmonary Flow

Michael Servetus independently described pulmonary transit in his theological text Christianismi Restitutio. Although the work was suppressed—and Servetus executed—his anatomical reasoning closely paralleled that of Ibn al-Nafis.

1628 • Harvey's De Motu Cordis

William Harvey published Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, providing quantitative evidence that blood circulates in a continuous loop driven by the heart. Harvey's calculations of cardiac output proved that the volume of blood ejected per hour far exceeded what the liver could plausibly produce, dismantling Galenic physiology.

1661 • Malpighi Visualizes Capillaries

Using an early microscope, Marcello Malpighi observed capillaries in the lungs of a frog, providing the missing anatomical link between arteries and veins that Harvey had predicted but never seen. This discovery completed the picture of a closed, dual-circuit cardiovascular system.

With Harvey's framework and Malpighi's microscopic confirmation, the fundamental architecture of the cardiovascular system was established: a single pump divided into right and left halves, each serving a distinct vascular circuit. The critical question that this lesson addresses is straightforward yet clinically and physiologically rich—how do the pulmonary and systemic circuits differ in structure, pressure, resistance, and function, and why must they be coupled in series rather than operating independently?

SECTION 2

Core Principles & Definitions

The mammalian cardiovascular system is organized as two circuits arranged in series, meaning that virtually all blood ejected by one ventricle must eventually pass through the other. The right heart pumps blood through the pulmonary circuit for gas exchange, and the left heart propels that freshly oxygenated blood through the systemic circuit to deliver oxygen and nutrients to every tissue. Because the circuits are in series, the cardiac output of the right and left ventricles must be equal over time—a constraint with profound implications for fluid balance and pathophysiology. Understanding the principles that distinguish these two loops requires attention to four foundational ideas: circuit architecture, pressure gradients, resistance profiles, and the functional coupling of the two ventricles.

Dual-Circuit Architecture

The heart functions as two pumps unified in a single organ. The right heart (right atrium and ventricle) supplies the pulmonary circuit, while the left heart (left atrium and ventricle) supplies the systemic circuit. Deoxygenated blood returns to the right atrium via the venae cavae; oxygenated blood returns to the left atrium via the pulmonary veins.

Pressure Differential

The systemic circuit operates at high pressure (mean arterial pressure ≈ 93 mmHg) because it must perfuse a vast capillary network spanning from the brain to the toes. The pulmonary circuit operates at roughly one-sixth of that pressure (mean ≈ 15 mmHg) because its capillary bed is short and compliant, and excessive pressure would cause pulmonary edema.

Resistance Profiles

Systemic vascular resistance (SVR) is high, reflecting the extensive arterial tree and arteriolar tone that regulate regional blood flow. Pulmonary vascular resistance (PVR) is low, with thin-walled, highly distensible vessels. The ratio of SVR to PVR is approximately 6:1 in a healthy adult.

Series Coupling & Equal Output

Because the circuits are connected in series, the stroke volume of the right and left ventricles must match on a beat-to-beat basis, with minor transient differences buffered by pulmonary blood volume. Persistent mismatch leads to congestion—right-sided failure causes systemic venous congestion, left-sided failure causes pulmonary edema.

Gas Exchange vs. Nutrient Delivery

The pulmonary circuit is dedicated to gas exchange: loading O₂ and unloading CO₂ at the alveolar-capillary membrane. The systemic circuit handles the far more diverse task of delivering O₂, glucose, hormones, and immune cells to tissues while removing metabolic waste—requiring complex autoregulatory mechanisms in each organ.

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KEY TAKEAWAY

Think of the cardiovascular system like a figure-eight track with a single car (the blood volume) that must complete both loops in sequence. One loop passes through a gas station (the lungs) where fuel is loaded, and the other loop distributes that fuel to every building in a city (the systemic tissues). The car cannot skip the gas station, and it cannot deliver fuel without passing through the city—this obligate coupling ensures that every drop of blood is oxygenated before it reaches the tissues, and every drop returns deoxygenated blood to the lungs for renewal.

SECTION 3

Visual Explanation — The Dual-Circuit Overview

This diagram illustrates the figure-eight arrangement of the cardiovascular system. Deoxygenated blood (blue arrows) flows from the right heart through pulmonary arteries to the lungs, where gas exchange occurs. Oxygenated blood (pink arrows) returns via pulmonary veins to the left heart, which ejects it through the aorta into the systemic circulation. After delivering oxygen to systemic tissues, deoxygenated blood returns to the right heart via the venae cavae, completing the cycle.

Several features of this diagram deserve emphasis. First, note that the pulmonary arteries carry deoxygenated blood—a point that frequently trips up students accustomed to equating 'artery' with 'oxygenated.' The defining characteristic of an artery is that it carries blood away from the heart, regardless of oxygen content, and the defining characteristic of a vein is that it carries blood toward the heart. Second, observe that the two circuits share the same total blood volume, approximately 5 liters in an average adult, meaning that at any instant roughly 10–12% of blood volume resides in the pulmonary vasculature and the remainder in the systemic vasculature. Third, the pressure annotations highlight the enormous difference in driving pressure: the left ventricle generates a peak systolic pressure of roughly 120 mmHg, whereas the right ventricle generates only about 25 mmHg—reflecting the dramatically lower resistance of the pulmonary vascular bed.

SECTION 4

Hemodynamic Framework — Pressure, Flow, and Resistance

The relationship between blood flow, pressure, and resistance can be formalized using an analogy to Ohm's law in electrical circuits. In cardiovascular physiology, this hydraulic analog provides a quantitative framework for comparing the pulmonary and systemic circuits and for understanding how changes in vascular resistance or cardiac output affect hemodynamics across both loops.

HEMODYNAMIC ANALOG OF OHM'S LAW

Q = ΔP / R

Where Q = blood flow (cardiac output, in L/min), ΔP = pressure gradient (mean arterial pressure − mean venous pressure, in mmHg), and R = vascular resistance (in mmHg·min/L or Wood units). This equation applies independently to each circuit.

SYSTEMIC VASCULAR RESISTANCE (SVR)

SVR = (MAP − RAP) / CO

Where MAP = mean arterial pressure (≈ 93 mmHg), RAP = right atrial pressure (≈ 3 mmHg), and CO = cardiac output (≈ 5 L/min). Normal SVR ≈ 18 mmHg·min/L (or ≈ 1440 dyn·s/cm⁵ when converted to CGS units).

PULMONARY VASCULAR RESISTANCE (PVR)

PVR = (MPAP − PCWP) / CO

Where MPAP = mean pulmonary arterial pressure (≈ 15 mmHg), PCWP = pulmonary capillary wedge pressure (≈ 9 mmHg), and CO = cardiac output (≈ 5 L/min). Normal PVR ≈ 1.2 mmHg·min/L (or ≈ 100 dyn·s/cm⁵).

The crucial quantitative insight emerges when you compare SVR and PVR. Because both circuits carry the same cardiac output (they are in series), and because the pulmonary circuit operates at roughly one-sixth the pressure gradient of the systemic circuit, PVR must be roughly one-sixth of SVR. This low resistance is achieved through the architectural design of pulmonary vessels: they are short, wide, thin-walled, and highly compliant. Pulmonary arterioles lack the thick smooth muscle layers found in systemic arterioles, which is why the lungs are capable of accommodating large increases in cardiac output (as during exercise) with only modest increases in pulmonary arterial pressure—a phenomenon called recruitment and distension.

POISEUILLE'S LAW (SIMPLIFIED)

R = 8ηL / πr⁴

Where η = blood viscosity, L = vessel length, and r = vessel radius. This law reveals that resistance is inversely proportional to the fourth power of the radius, meaning small changes in arteriolar diameter produce dramatic changes in resistance—the primary mechanism by which the systemic circuit regulates regional blood flow.

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Clinical Connection

The r⁴ relationship explains why atherosclerotic narrowing of coronary arteries is so dangerous: a 50% reduction in vessel radius increases resistance by a factor of 16 (since 1/0.5⁴ = 16), severely limiting blood flow to the myocardium. Similarly, in pulmonary arterial hypertension (PAH), vascular remodeling increases PVR, forcing the right ventricle to generate abnormally high pressures—eventually leading to right heart failure.

SECTION 5

Detailed Comparison — Pulmonary vs. Systemic Circuits

While the two circuits share the same pump and the same blood volume, they differ in nearly every other hemodynamic and structural parameter. The table below provides a comprehensive side-by-side comparison, and the diagram that follows offers a visual representation of the contrasting pressure profiles in each circuit—a perspective that is essential for understanding clinical conditions such as heart failure, pulmonary hypertension, and shock.

Parameter	Pulmonary Circuit	Systemic Circuit
Pump	Right ventricle	Left ventricle
Peak systolic pressure	≈ 25 mmHg	≈ 120 mmHg
Diastolic pressure	≈ 8 mmHg	≈ 80 mmHg
Mean pressure	≈ 15 mmHg	≈ 93 mmHg
Vascular resistance	≈ 1.2 Wood units (low)	≈ 18 Wood units (high)
Vessel wall thickness	Thin, compliant	Thick, muscular (esp. arterioles)
Blood volume contained	≈ 450 mL (≈ 9%)	≈ 4100 mL (≈ 84%)
Primary function	Gas exchange (O₂/CO₂)	Nutrient delivery and waste removal
Response to hypoxia	Vasoconstriction (unique!)	Vasodilation
Ventricular wall thickness	RV free wall ≈ 3–5 mm	LV free wall ≈ 13–15 mm

Key hemodynamic and structural differences between the pulmonary and systemic circuits.

Bar chart comparing systolic, diastolic, and mean pressures in the pulmonary (cyan) and systemic (pink) circuits. The systemic circuit operates at approximately 5–6 times the pressure of the pulmonary circuit, reflecting its much higher vascular resistance. The right ventricle generates peak pressures of only ≈ 25 mmHg, while the left ventricle reaches ≈ 120 mmHg.

One of the most physiologically significant differences highlighted in the table and diagram is the response to local hypoxia. In the systemic circuit, local tissue hypoxia triggers vasodilation—arterioles relax to increase blood flow to oxygen-starved tissue, a homeostatic response that makes intuitive sense. In the pulmonary circuit, however, the response is the opposite: alveolar hypoxia causes hypoxic pulmonary vasoconstriction (HPV), which diverts blood away from poorly ventilated alveoli and toward well-ventilated regions. This mechanism optimizes ventilation-perfusion (V/Q) matching, ensuring that blood preferentially flows past alveoli that are receiving fresh air. HPV is a unique and clinically important feature of the pulmonary circulation that has no parallel in the systemic circuit.

SECTION 6

Worked Example — Calculating Vascular Resistance

A right heart catheterization on a patient yields the following data: cardiac output = 4.8 L/min, mean pulmonary arterial pressure (MPAP) = 28 mmHg, and pulmonary capillary wedge pressure (PCWP) = 10 mmHg. Simultaneously, the patient's mean arterial pressure (MAP) is 88 mmHg and right atrial pressure (RAP) is 5 mmHg. Calculate both the PVR and SVR, determine the SVR-to-PVR ratio, and comment on whether these values fall within normal ranges.

Hemodynamic Resistance Calculation

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Step 1 — Identify Given Values

CO = 4.8 L/min, MPAP = 28 mmHg, PCWP = 10 mmHg, MAP = 88 mmHg, RAP = 5 mmHg.
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Step 2 — Calculate PVR

Apply the formula PVR = (MPAP − PCWP) / CO. Substituting: PVR = (28 − 10) / 4.8 = 18 / 4.8.

PVR = 3.75 Wood units (mmHg·min/L)
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Step 3 — Calculate SVR

Apply the formula SVR = (MAP − RAP) / CO. Substituting: SVR = (88 − 5) / 4.8 = 83 / 4.8.

SVR = 17.3 Wood units (mmHg·min/L)
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Step 4 — Calculate the SVR-to-PVR Ratio

SVR / PVR = 17.3 / 3.75 ≈ 4.6. In a healthy adult, this ratio is typically around 15:1. Here the ratio is reduced because PVR is elevated.

SVR : PVR ≈ 4.6 : 1
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Step 5 — Clinical Interpretation

Normal PVR is approximately 1–2 Wood units, and values above 3 Wood units are considered elevated. This patient's PVR of 3.75 is consistent with pulmonary hypertension. The SVR of 17.3 is within normal range (15–20 Wood units). The reduced SVR/PVR ratio confirms that the primary hemodynamic abnormality resides in the pulmonary circuit. The right ventricle is being forced to generate higher pressures to maintain cardiac output through the elevated pulmonary resistance, placing it at risk for hypertrophy and eventual failure.

Conclusion: Elevated PVR indicative of pulmonary hypertension; SVR within normal limits.

SECTION 7

Clinical Relevance — When the Circuits Fail

Because the pulmonary and systemic circuits are arranged in series, failure of either ventricle has downstream and upstream consequences that are predictable once you understand the circuit architecture. The clinical manifestations of heart failure can be mapped directly onto the two-circuit model, making this framework indispensable for differential diagnosis and treatment planning.

Feature	Left-Sided Heart Failure	Right-Sided Heart Failure
Ventricle affected	Left ventricle	Right ventricle
Upstream congestion	Pulmonary veins → pulmonary edema	Systemic veins → peripheral edema, hepatomegaly, JVD
Key symptom	Dyspnea (shortness of breath), orthopnea	Peripheral edema, ascites
Downstream effect	Reduced systemic cardiac output → fatigue, hypotension	Reduced pulmonary perfusion → impaired gas exchange
Auscultation finding	Bibasilar crackles (fluid in lungs)	Clear lungs (no pulmonary congestion)
Common cause	Systemic hypertension, aortic stenosis, MI	Pulmonary hypertension, PE, chronic left HF

Contrasting clinical presentations of left- vs. right-sided heart failure, mapped to circuit anatomy.

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KEY TAKEAWAY

Think of the two-circuit system like a two-stage water treatment plant connected by a single pipeline. If the first pump (right heart) fails, water (blood) backs up in the collection reservoir (systemic veins), causing flooding in the surrounding neighborhood (peripheral edema). If the second pump (left heart) fails, water backs up in the intermediate holding tank (pulmonary veins), flooding the treatment facility itself (pulmonary edema). The serial connection means that prolonged failure of one pump eventually overwhelms the other—explaining why isolated left heart failure often progresses to biventricular failure.

SECTION 8

Connection to Advanced Cardiovascular Physiology

The dual-circuit model introduced in this lesson provides the foundation for more advanced topics in cardiovascular physiology, including the study of pressure-volume loops, ventricular interdependence, exercise hemodynamics, and fetal circulation. Each of these areas builds upon the fundamental distinction between the low-pressure pulmonary circuit and the high-pressure systemic circuit, extending the analysis to include dynamic changes in loading conditions, myocardial contractility, and circulatory adaptations across the lifespan.

Foundational Concept (This Lesson)	Advanced Extension
Series arrangement of pulmonary and systemic circuits	Ventricular interdependence via the shared septum and pericardium; Starling mechanism as the equalizer of left and right output
Pressure gradients (MAP, MPAP)	Pressure-volume (PV) loops for each ventricle; end-systolic pressure-volume relationship (ESPVR) and contractility indices
SVR and PVR calculations	Dynamic changes in SVR/PVR during exercise, hemorrhage, and autonomic reflexes (baroreflex, chemoreflexes)
Hypoxic pulmonary vasoconstriction	V/Q matching theory; multiple inert gas elimination technique (MIGET); altitude physiology and chronic mountain sickness
Dual-circuit anatomy in the adult	Fetal circulation with parallel circuits, foramen ovale, ductus arteriosus; transitional circulation at birth; congenital heart defects (ASDs, VSDs)

Mapping foundational concepts to advanced cardiovascular physiology topics.

A particularly illuminating advanced topic is fetal circulation, which deviates dramatically from the adult dual-circuit model. In utero, gas exchange occurs at the placenta rather than the lungs, so the pulmonary circuit carries minimal flow. Three shunts—the ductus venosus, foramen ovale, and ductus arteriosus—divert blood away from the non-functional lungs and liver. At birth, the infant's first breath dramatically reduces PVR, increasing pulmonary blood flow and raising left atrial pressure, which functionally closes the foramen ovale. The rising PaO₂ triggers constriction and eventual fibrosis of the ductus arteriosus. The result is a transition from parallel circulation to the adult series configuration—one of the most remarkable physiological adaptations in human biology.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Pulmonary arteries carry deoxygenated blood, while pulmonary veins carry oxygenated blood. Given this, explain why the naming convention 'artery' and 'vein' is based on direction of flow rather than oxygen content. What confusion would arise if we named vessels by their oxygen content?

Show answer

Arteries are defined as vessels that carry blood away from the heart, while veins carry blood toward the heart, regardless of oxygenation status. If vessels were named by oxygen content, the pulmonary arteries would be called 'veins' (since they carry deoxygenated blood), and the pulmonary veins would be called 'arteries' (since they carry oxygenated blood). This would create a contradictory naming scheme where the same vessel might be called an artery by one criterion and a vein by the other. The directional convention avoids this paradox and is also more structurally meaningful, since arteries have thick, elastic walls designed to withstand pulsatile ejection pressure, while veins have thin walls with valves suited for low-pressure return flow—regardless of oxygen content.

Question 2: BASIC CALCULATION

A patient has a mean pulmonary arterial pressure of 20 mmHg, a pulmonary capillary wedge pressure of 8 mmHg, and a cardiac output of 6 L/min. Calculate the pulmonary vascular resistance in Wood units.

Show answer

Using the formula PVR = (MPAP − PCWP) / CO: PVR = (20 − 8) / 6 = 12 / 6 = 2.0 Wood units. Normal PVR ranges from approximately 1 to 2 Wood units, so this value sits at the upper limit of normal. If we wished to convert to CGS units (dyn·s/cm⁵), we would multiply by 80: 2.0 × 80 = 160 dyn·s/cm⁵. This patient's PVR does not meet the hemodynamic threshold for pulmonary hypertension (>3 Wood units or MPAP ≥ 20 mmHg by current guidelines), but the MPAP of 20 mmHg is borderline and warrants monitoring.

Question 3: INTERMEDIATE

During vigorous exercise, cardiac output increases from 5 L/min to 25 L/min in a healthy individual. Explain why mean pulmonary arterial pressure rises only modestly (from ≈ 15 to ≈ 25 mmHg) despite this fivefold increase in flow. What mechanisms prevent dangerous pressure elevation in the pulmonary circuit?

Show answer

According to Q = ΔP / R, a fivefold increase in Q with only a modest increase in ΔP implies that PVR must decrease substantially. The pulmonary vasculature accomplishes this through two primary mechanisms: recruitment and distension. Recruitment refers to the opening of previously unperfused capillaries—at rest, many pulmonary capillaries are collapsed due to low perfusion pressure, and exercise opens them by increasing transmural pressure. Distension refers to the widening of already-perfused vessels, which reduces resistance per Poiseuille's law (R ∝ 1/r⁴). Together, these mechanisms can reduce PVR by 50–60% during maximal exercise. The thin-walled, highly compliant nature of pulmonary vessels makes them especially capable of this adaptation—a feature that systemic arterioles, with their thick muscular walls, do not share to the same degree.

Question 4: APPLIED

A patient with chronic left-sided heart failure presents with dyspnea, bibasilar crackles on auscultation, and elevated pulmonary capillary wedge pressure (PCWP = 22 mmHg). Using the dual-circuit model, trace the pathophysiological chain of events from left ventricular dysfunction to the patient's respiratory symptoms. Additionally, predict whether this patient is at risk for developing right-sided heart failure and explain your reasoning.

Show answer

Left ventricular dysfunction reduces the LV's ability to eject blood effectively, causing blood to accumulate in the left atrium and, by extension, in the pulmonary veins—a direct consequence of the series arrangement. Elevated pulmonary venous pressure raises capillary hydrostatic pressure (reflected by PCWP of 22 mmHg, well above the normal 6–12 mmHg). When capillary hydrostatic pressure exceeds plasma oncotic pressure (≈ 25 mmHg), fluid transudates into the alveolar interstitium and eventually into the alveoli, producing pulmonary edema—the bibasilar crackles heard on auscultation. Impaired gas exchange at the edematous alveolar-capillary membrane produces the patient's dyspnea. Over time, chronically elevated pulmonary venous pressure causes reactive pulmonary arterial vasoconstriction and vascular remodeling, raising PVR. The right ventricle must then generate higher pressures to maintain cardiac output through the now-resistant pulmonary circuit. Because the RV is thin-walled and poorly adapted to sustained pressure overload, it eventually dilates and fails—producing right-sided heart failure (jugular venous distension, peripheral edema, hepatomegaly). This progression from left-sided to biventricular failure illustrates the inescapable coupling of the two circuits in series.

Question 5: CRITICAL THINKING

In the fetus, the pulmonary and systemic circuits operate largely in parallel rather than in series, with three shunts (ductus venosus, foramen ovale, and ductus arteriosus) bypassing the non-functional lungs. Analyze what would happen if the ductus arteriosus failed to close after birth. Consider the direction of shunting, the hemodynamic consequences for both circuits, and the long-term effects on the heart and lungs.

Show answer

After birth, when PVR drops below SVR, a patent (unclosed) ductus arteriosus creates a left-to-right shunt: oxygenated blood from the high-pressure aorta flows through the ductus into the lower-pressure pulmonary artery. This recirculates blood through the lungs without delivering it to the systemic tissues, causing pulmonary overcirculation and systemic underperfusion. The pulmonary vasculature receives excessive flow, increasing pulmonary venous return to the left atrium and left ventricle, producing volume overload. The left ventricle dilates and eventually hypertrophies to accommodate the increased preload. Meanwhile, the lungs experience increased capillary hydrostatic pressure, predisposing to pulmonary edema. If the shunt is large and persistent, chronic pulmonary overcirculation causes irreversible vascular remodeling (Eisenmenger syndrome), raising PVR to levels exceeding SVR. At this point, shunt direction reverses (right-to-left), sending deoxygenated blood into the systemic circulation and producing cyanosis. This tragic progression—from a correctable left-to-right shunt to an irreversible right-to-left shunt—demonstrates how disruption of the series arrangement fundamentally destabilizes both circuits. Early surgical or catheter-based closure of the PDA prevents this cascade.

SUMMARY

Lesson Summary

Summary

The mammalian cardiovascular system is organized as two circuits connected in series: the pulmonary circuit, driven by the right ventricle, sends deoxygenated blood through the lungs for gas exchange (loading O₂, unloading CO₂), while the systemic circuit, driven by the left ventricle, distributes oxygenated blood to all tissues for nutrient delivery and waste removal. The pulmonary circuit operates at approximately one-sixth the pressure of the systemic circuit (mean ≈ 15 vs. ≈ 93 mmHg), reflecting its dramatically lower vascular resistance (PVR ≈ 1.2 vs. SVR ≈ 18 Wood units). Both resistances are governed by the hemodynamic analog of Ohm's law, Q = ΔP / R, and the profound influence of vessel radius on resistance via Poiseuille's law (R ∝ 1/r⁴).

Because the circuits are in series, the cardiac output of both ventricles must be equal over time, and failure of one ventricle produces predictable upstream congestion—left heart failure causes pulmonary edema, while right heart failure causes systemic venous congestion. Unique features of the pulmonary circuit include hypoxic pulmonary vasoconstriction (diverting blood to well-ventilated alveoli) and the capacity for recruitment and distension during exercise. Understanding these dual circuits provides the essential framework for interpreting hemodynamic data, diagnosing heart failure, and appreciating advanced topics such as fetal circulation and congenital heart defects.