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Alveolar dead space is the part of ventilated alveolar gas that does not provide the effective blood–gas exchange represented by a given dead-space estimate. An alveolus can receive gas but have little or no perfusion; it then differs from a conducting airway contributing to anatomical dead space.^[ref] A regional ventilation measurement alone does not establish regional exchange.

A constructed two-region model makes the distinction quantitative. Let tidal volume be 0.50 L and conducting-airway dead space 0.15 L. Of the remaining 0.35 L, suppose 0.28 L participates in the modeled exchange and 0.07 L comes from a ventilated, nonperfused region. Alveolar dead space is then 0.07 L, while total effective respiratory dead space is $0.15+0.07=0.22$ L.

The denominator changes the fraction. The nonexchanging fraction of the modeled alveolar contribution is $0.07/0.35=0.20$, whereas total dead space as a fraction of tidal volume is $0.22/0.50=0.44$. Neither percentage can replace the other without changing what is being compared.

If exchanging gas has carbon dioxide fraction 0.05 and the other two compartments contribute negligible carbon dioxide, mixed expired fraction isThe two-compartment dilution expression then gives $1-F_E/F_A=1-0.028/0.05=0.44$, consistent with the total dead-space fraction. This is a deliberately idealized construction; real regions can have graded ventilation-to-perfusion relationships rather than a binary exchanging/nonexchanging classification.

A whole-lung gas-balance estimate consequently does not identify the anatomical location of every ineffective contribution. The primary technical chapter distinguishes conducting space from gas-exchange-based wasted ventilation and discusses the assumptions behind arterial carbon-dioxide substitution.^[ref] Alveolar dead space makes that compartment distinction explicit.

Expiratory reserve volume, ERV, is the additional gas that can be exhaled after the end of a quiet expiration, down to maximal expiration. It lies between functional residual capacity and residual lung volume:It is distinct from the ordinary tidal volume and does not include the gas left after the maneuver.^[ref]

For stipulated FRC 2.5 L and residual lung volume 1.4 L, ERV is 1.1 L. Beginning instead at the end of a 0.5 L tidal inspiration permits a 1.6 L expiration down to that same residual endpoint. That larger excursion includes tidal volume as well as expiratory reserve volume, so its starting point changes its name and interpretation.

A shift in the ordinary end-expiratory level also changes ERV without necessarily changing residual lung volume. If that lower endpoint stays at 1.4 L while FRC changes from 2.5 to 2.0 L, ERV changes from 1.1 to 0.6 L. This is an original comparison of reference levels, not a claim about a particular disease or posture.

The identity is useful for combining measurements. A measured FRC supplies an absolute gas-volume reference; a linked expiratory maneuver supplies the increment needed to calculate residual lung volume. The technical standard requires consistent linked maneuvers for this calculation.^[ref] An incomplete expiration changes the observed excursion, so a reported ERV should remain attached to its maneuver and endpoints.

Functional residual capacity, FRC, is the volume of gas remaining in the lungs at the end of a passive expiration during quiet breathing. It includes both expiratory reserve volume and residual lung volume:It is an end-expiratory reference level, not the gas expelled in one breath.^[ref]

An original volume ledger uses residual lung volume 1.2 L and expiratory reserve volume 1.0 L, giving FRC 2.2 L. A subsequent tidal volume of 0.5 L raises the represented lung-gas volume to 2.7 L. These stipulated numbers illustrate the relationships; they are not reference limits for an individual.

Helium dilution lung volume measurement supplies an independent gas-volume scale. In a simplified closed, well-mixed system, a known apparatus volume $V_a$ initially contains helium at fraction $c_1$, while the connected lung gas initially contains none. Conservation of the tracer givesFor constructed $V_a=2$ L, $c_1=0.10$ and equilibrated $c_2=0.04$, $V_L=3$ L. This is a model with equal pressure and temperature and no leaks or interfering gas-volume changes. It measures the volume communicating with the tracer; inaccessible gas cannot be recovered by dilution alone. The technical standard distinguishes that limitation from body plethysmography.^[ref]

The phase of connection matters: connecting at a different lung volume changes the volume being estimated. Defining functional residual capacity therefore requires both an anatomical gas-volume estimate and an identified end-expiratory state.

Inspiratory capacity, IC, is the maximum volume that can be inhaled from the end of a quiet expiration to maximal inspiration. Its lower reference is functional residual capacity, and its upper reference is total lung capacity:The last expression combines tidal volume with inspiratory reserve volume.^[ref]

For stipulated total lung capacity 6.0 L and functional residual capacity 2.5 L, IC is 3.5 L. If tidal volume is 0.5 L, the corresponding inspiratory reserve volume is 3.0 L. The two descriptions use the same maximum inspiratory endpoint but different starting points.

The starting point is essential when interpreting a change. In an original comparison, keep total lung capacity at 6.0 L while moving the quiet end-expiratory level from 2.5 to 3.2 L. IC falls from 3.5 to 2.8 L, although the maximum inspiratory volume has not changed. Thus a lower inspiratory capacity need not imply a lower total lung capacity; an independently specified baseline is required to distinguish the possibilities.

Inhalation through a volume-measuring apparatus can determine an inspiratory increment without directly measuring the gas already present in the lungs. A stable starting level and complete endpoint connect that increment to the defined capacity. The technical standard therefore links capacity maneuvers with an identified FRC-level measurement.^[ref] The values in the example illustrate endpoint bookkeeping rather than reference limits for a particular person.

Inspiratory reserve volume, IRV, is the additional gas that can be inhaled after the end of an ordinary tidal inspiration, up to maximal inspiration. It is the portion of inspiratory capacity beyond tidal volume:The starting point is end-inspiration, rather than the quiet end-expiratory level used for inspiratory capacity.^[ref]

For a stipulated total lung capacity of 6.0 L, functional residual capacity 2.5 L and tidal volume 0.5 L, end-tidal inspiration reaches 3.0 L and leaves IRV 3.0 L. The full 3.5 L inspiratory capacity includes both the ordinary 0.5 L breath and this additional excursion.

A deeper ordinary breath changes the subdivision even if the absolute endpoints remain fixed. Holding functional residual capacity at 2.5 L and total lung capacity at 6.0 L while increasing tidal volume to 1.0 L reduces IRV to 2.5 L. Thus a change in inspiratory reserve volume need not imply that maximum inspiratory volume changed.

The name describes an available volume increment, not a separate anatomical compartment sealed off from tidal gas. In the ideal volume ledger, boundaries between subdivisions are reference states on the same gas-volume coordinate. A useful measurement consequently states the ordinary breathing level and the maximum endpoint, rather than interpreting IRV independently of the preceding breath.

Intrinsic positive end-expiratory pressure is the residual alveolar pressure above the externally imposed end-expiratory pressure, associated with incomplete lung emptying. In a communicating passive compartment,The manufacturer's technical account makes this distinction and explains why a fitted whole-breath estimate can differ from the pressure obtained during an end-expiratory hold.^[ref]

An original repeated-breath model isolates the timing mechanism. Let $V_n$ be excess volume above equilibrium with the outlet just before inspiration, and let each inspiration add tidal volume $V_T$. During exhalation, assume a passive linear compartment with time constant $\tau =RC$. After expiratory time $T_E$,The steady excess volume satisfies $V_{\ast }=a(V_{\ast }+V_T)$, henceFor stipulated $V_T=0.40$ L, respiratory-system compliance $C=0.05$ L/cm H${}_2$O, $\tau =1$ s and $T_E=1$ s, $a\simeq 0.368$, $V_{\ast }\simeq 0.233$ L and intrinsic pressure is about 4.66 cm H${}_2$O. Giving this same ideal compartment two seconds to empty yields about 0.063 L and 1.25 cm H${}_2$O. These are consequences of a model, not patient-specific timing instructions.

A nonzero external baseline does not by itself imply incomplete emptying. Conversely, setting an external baseline to zero does not prove that alveolar pressure has returned to zero. Measurements require attention to communication between regions, leakage and active muscle pressure; a single airway value need not reveal every trapped regional pressure. The recurrence explains why resistance, compliance and available time matter together rather than assigning the phenomenon to one pressure setting alone.

Mechanical ventilation assists or replaces the movement of gas into and out of the lungs using a machine. A ventilator can deliver that assistance through a mask or a breathing tube.^[ref] This supports pulmonary ventilation; it does not replace every function of the lungs, circulation or gas exchange.

In positive-pressure ventilation, pressure applied at the airway opening helps drive inhalation. The resulting tidal volume depends on both that pressure history and respiratory mechanics. For a passive single-compartment model, a useful equation isHere $V$ is volume above the selected end-expiratory reference, $C$ is respiratory-system compliance, $R$ is airway resistance, and $p_0$ represents the baseline positive end-expiratory pressure. The model neglects active respiratory muscle pressure and changes in these coefficients.

For constructed $C=0.05$ L/cm H${}_2$O, $R=5$ cm H${}_2$O·s/L, $V=0.5$ L, $\dot{V}=0.5$ L/s and $p_0=5$ cm H${}_2$O, the predicted airway pressure is $5+10+2.5=17.5$ cm H${}_2$O. At zero flow with the same volume, the resistive term vanishes and the model gives 15 cm H${}_2$O. Thus an airway-pressure peak contains different information from a no-flow pressure measurement. These numbers illustrate the equation, not patient settings.

The importance of testing ventilation strategies was demonstrated by a 2000 randomized trial in acute respiratory distress syndrome and acute lung injury. It compared lower and traditional tidal volume strategies indexed to predicted body weight, with different plateau-pressure limits. In 861 participants, reported mortality was 31.0% versus 39.8%.^[ref] The experiment compared complete specified protocols in a defined population; it does not establish one universal setting from tidal volume alone. Physical models explain how pressure and flow interact, while controlled clinical evidence tests consequences that those models cannot supply by themselves.

Minute ventilation, ${\dot{V}}_E$, is the total volume of respiratory gas moved per unit time, conventionally expressed in litres per minute. When breath volumes are represented by mean tidal volume ${\overline{V}}_T$ and respiratory rate $f$ over the same interval,It measures gross ventilation, distinct from the portion renewing gas in effectively exchanging alveolar regions.^[ref]

An original record contains three complete expirations of 0.4, 0.6 and 0.5 L over 15 s. Their total is 1.5 L, giving $1.5\times 60/15=6$ L/min. The rate is 12 breaths/min and the mean tidal volume is 0.5 L, producing the same answer. Variable breath depth is therefore compatible with this identity if both summaries use the same record.

With a fixed effective respiratory dead space $V_D$ per breath, the corresponding model for alveolar ventilation isFor the constructed record and $V_D=0.15$ L, the effective volume is $1.5-3(0.15)=1.05$ L per 15 s, or 4.2 L/min. The remaining 1.8 L/min belongs to the modeled dead-space turnover. This subtraction assumes each included breath exceeds the modeled dead-space volume.

A pneumotachograph can supply signed volumetric flow rate for integration. Integrating only the expiratory phase yields expired volume; integrating a full balanced breath with its sign intact yields approximately zero net volume. A ventilation report must consequently identify the phase, interval and gas-volume conditions rather than interpreting every integral as the same quantity. Gross minute ventilation alone does not establish oxygen uptake, carbon-dioxide removal or their distribution among lung regions.

Peak airway pressure, $P_{\mathrm{peak}}$, is the largest measured airway pressure over the specified breath or inspiratory interval. Its meaning includes the measurement location, reference pressure and temporal response. At the airway opening during positive-pressure inflation, the peak can include the pressure needed for flow through resistance as well as the pressure distending the respiratory system.^[ref]

For a passive single-compartment model with constant airway resistance $R$ and respiratory-system compliance $C$,Here $V$ is volume above the end-expiratory state and $Q$ is inspiratory flow. If constant flow persists to the end of inspiration, the largest volume and the resistive pressure occur together, givingWith varying flow, the peak is instead the maximum of the time-dependent sum; separate maxima cannot automatically be added.

An original example takes total end-expiratory pressure 5 cm H${}_2$O, tidal volume 0.50 L, $C=0.05$ L/cm H${}_2$O and $Q=0.50$ L/s. For $R=10$ cm H${}_2$O·s/L, peak pressure is $5+10+5=20$ cm H${}_2$O. Doubling resistance raises the calculated peak to 25 cm H${}_2$O while the equilibrated plateau airway pressure stays 15 cm H${}_2$O. A larger peak can therefore occur without a larger static distending increment.

An end-inspiratory hold removes ongoing airway flow under appropriate conditions and helps separate these contributions. Active respiratory effort, leaks, nonlinear resistance or delayed redistribution complicate that separation. Peak pressure alone neither partitions lung and chest-wall loading nor establishes a unique cause of a pressure change.

Plateau airway pressure, $P_{\mathrm{plat}}$, is the airway pressure assessed during an end-inspiratory interval with no flow and sufficiently stable conditions. An end-inspiratory hold can allow the airway and communicating alveolar gas pressures to equilibrate when the respiratory system is passive and the circuit is not leaking.^[ref][ref]

The distinction from peak airway pressure follows a simple single-compartment model:During the modeled hold, $\dot{V}=0$, leavingHere $C$ is respiratory-system compliance, $R$ is airway resistance, and total positive end-expiratory pressure supplies the starting reference. The measured difference $P_{\mathrm{plat}}-P_{\mathrm{EEP},\mathrm{total}}$ is the corresponding respiratory driving pressure.

In an original example, total PEEP 8 cm H${}_2$O, tidal volume 0.40 L and compliance 0.04 L/cm H${}_2$O give $P_{\mathrm{plat}}=18$ cm H${}_2$O. If flow immediately beforehand is 0.5 L/s and resistance is 12 cm H${}_2$O·s/L, the model's airway pressure during that flow is 24 cm H${}_2$O. The 6 cm H${}_2$O difference is the modeled resistive contribution, not an extra static distending pressure.

A plateau does not directly measure pleural pressure or isolate the lung from the chest wall. For the same stipulated alveolar pressure 18 cm H${}_2$O, pleural pressures of 12 and 2 cm H${}_2$O imply transpulmonary pressures of 6 and 16 cm H${}_2$O. The airway value alone cannot distinguish those states.

Finally, a stable displayed number is not automatically the same operation as an actual hold. The manufacturer distinguishes measured holds, rapid occlusion and calculated monitoring estimates.^[ref] Reporting the method and its conditions connects plateau airway pressure to the mechanical question it can answer.

Positive end-expiratory pressure, PEEP, is airway pressure maintained above atmospheric pressure at the end of exhalation. It specifies an end-expiratory pressure reference, not a tidal volume or a pressure rise measured from the beginning to the end of inspiration. A set external pressure and the total pressure assessed during an appropriate end-expiratory hold need not be identical.^[ref]

One reason is incomplete emptying. In an original passive single-compartment model, let $V$ be excess volume above equilibrium with the prescribed outlet pressure. With airway resistance $R$ and respiratory-system compliance $C$,The remaining elastic pressure above the outlet is $V(t)/C$. Stipulated $R=20$ cm H${}_2$O·s/L and $C=0.05$ L/cm H${}_2$O give a time constant of 1 s. Starting with 0.50 L excess volume, after 0.5 s the model retains about 0.303 L, corresponding to 6.07 cm H${}_2$O above the outlet pressure. These are constructed mechanics, not ventilator settings for a person.

If the prescribed outlet reference is 5 cm H${}_2$O, the modeled retained elastic pressure raises the equilibrated total to about 11.07 cm H${}_2$O. This illustrates intrinsic positive end-expiratory pressure: a pressure contribution that cannot be identified from the external setting alone. The result assumes a communicating passive compartment; active effort, leakage and heterogeneous time constants require further interpretation.

Respiratory driving pressure compares plateau airway pressure with total PEEP, rather than silently subtracting an external setting that may omit retained pressure. The manufacturer's primary measurement account states that distinction and the required passive, leak-free conditions.^[ref] Naming the pressure reference is therefore part of defining the reported mechanical quantity.

Positive-pressure ventilation assists inhalation by raising pressure at the airway opening, driving gas into the lungs when that pressure exceeds alveolar pressure. It differs mechanically from spontaneous expansion of the thorax or negative-pressure ventilation, which lowers pressure around the lungs to produce the relevant distending difference. A ventilator's mask or tube supplies the applied airway connection.^[ref]

The direction of flow still depends on a pressure difference, not on whether either pressure is above atmospheric. For a simple resistive path,with positive $Q$ directed inward and $R$ the airway resistance. In an original example, pressures 10 and 8 cm H${}_2$O and $R=4$ cm H${}_2$O·s/L give inward flow 0.5 L/s. Pressures 8 and 10 cm H${}_2$O give outward flow 0.5 L/s. Both pairs are positive relative to atmosphere.

This explains why positive end-expiratory pressure can coexist with exhalation. The applied pressure can remain above atmospheric while elastic recoil produces a still higher alveolar pressure and drives gas outward. At equal opening and alveolar pressures, the same ideal resistive model gives zero flow.

The resulting tidal volume also depends on respiratory-system compliance, airway resistance, duration and any active respiratory muscle effort. The no-flow interval used to interpret plateau airway pressure removes one resistive contribution only under the required measurement conditions.^[ref] Positive pressure is therefore a mode of applying mechanical assistance, not a guarantee of a particular inward flow, volume or gas-exchange result.

A pulmonologist is a physician specializing in diseases of the lungs, conducting airways and related respiratory function. The work connects clinical observations with respiratory mechanics, ventilation and gas exchange. The American Board of Internal Medicine's primary specialty standards include supervision and interpretation of pulmonary-function testing, ventilator-related skills and bronchoscopy.^[ref]

The scientific question determines the measurement. Spirometry records gas moved at the mouth. Lung compliance relates volume to an internal pressure difference. Gas exchange also depends on the exchange surface and blood flow, so an expired-volume trace does not by itself measure every part of respiratory performance. A pulmonologist interprets these complementary observations alongside the clinical context rather than treating all breathing measurements as interchangeable.

An original ventilation example illustrates why this distinction matters. Let tidal volume be $V_T$, respiratory rate be $f$, and non-exchanging conducting-airway volume be $V_D$, representing anatomical dead space. In a simplified model where the remaining inspired gas reaches functioning exchange regions,The first is total ventilation per minute; the second is the model's alveolar ventilation. OpenStax distinguishes conducting-airway dead space from gas available to alveolar exchange.^[ref]

With stipulated $V_D=0.15$ L, twelve breaths per minute at $V_T=0.50$ L give total ventilation 6 L/min and alveolar ventilation 4.2 L/min. Thirty breaths per minute at $V_T=0.20$ L still move 6 L/min in total, but give only 1.5 L/min in this model. Equal total movement can therefore supply quite different amounts of fresh gas to exchange regions. These chosen numbers explain the quantities; they are not a breathing recommendation or a patient assessment.

Measurement quality is part of the specialty's scientific reasoning. The primary spirometry statement separately evaluates instrument response, maneuver acceptability and repeatability.^[ref] A precise measurement of one function can be informative while leaving the cause of an abnormal result unresolved. Combining anatomy, mechanics and exchange measurements makes those remaining questions explicit.

Residual lung volume, RV, is the gas remaining in the lungs after maximal expiration. It is smaller than the quiet end-expiratory functional residual capacity by the expiratory reserve volume:Thus residual gas is not the tidal volume or a direct measurement of gas exchanged in one breath.^[ref]

For stipulated FRC 2.5 L and expiratory reserve volume 1.1 L, RV is 1.4 L. Since spirometry records changes rather than an absolute lung-gas baseline, exhaling into a spirometer cannot by itself identify the gas still inside.

The 1956 experiment by Arthur B. DuBois and colleagues made that hidden volume accessible through body plethysmography. Their apparatus combined an airtight chamber, a mouthpiece shutter, capacitance pressure transducers and an oscillograph. Respiratory efforts against the closed shutter related mouth-pressure changes to changes in chamber volume.^[ref]

The physical idea follows Boyle's law. For a fixed gas amount at approximately fixed temperature, writeTo first order in the small changes,An original ideal calculation uses effective gas pressure 100 kPa, pressure increase 0.2 kPa and volume decrease 4 mL, giving $V\simeq 2000$ mL. This estimates the gas volume at the instant of airway closure. If closure occurs at quiet end-expiration, the result corresponds to an FRC-level measurement; expiratory reserve volume must still be subtracted to obtain residual lung volume. Pressure equilibration, thermal assumptions and correct volume conventions are part of the physical measurement, not consequences of the algebra alone.

Respiratory dead space describes ventilated gas volume that does not contribute to the gas exchange represented by the chosen measurement. Anatomical dead space concerns conducting airways; alveolar dead space concerns ventilation of alveolar regions with ineffective exchange relative to their perfusion. The definition and estimator must be stated before the number is interpreted.^[ref]

A two-compartment carbon dioxide balance illustrates the gas-exchange estimate. Suppose inspired gas contains negligible carbon dioxide, a dead-space fraction contributes none, and the remaining expired volume carries representative alveolar fraction $F_A$. For mixed expired fraction $F_E$,Stipulated $F_A=0.05$ and $F_E=0.035$ give a dead-space fraction 0.30. With tidal volume 0.50 L, the corresponding effective dead-space volume is 0.15 L. This is a dilution-model result, not proof that a conducting tube of exactly that physical volume exists.

The alveolar quantity in the equation is important. Replacing it with an arterial carbon-dioxide measurement adds physiological assumptions and can reflect more than conducting-airway volume. The technical account distinguishes the alveolar balance from its arterial substitution.^[ref]

An original comparison makes the distinction concrete. Two modeled systems each have anatomical dead space 0.15 L. In one, all remaining inspired gas reaches effectively exchanging regions. In the other, an additional 0.10 L per breath goes to a ventilated region that does not exchange with perfusing blood. Their effective dead-space volumes are 0.15 and 0.25 L despite the same conducting geometry.

The label therefore belongs to a specified physical or physiological model. A measured dead-space fraction, a geometrical airway volume and an estimate of wasted ventilation are related observations, rather than automatically interchangeable measurements.

Respiratory driving pressure, $\Delta P$, is the difference between plateau airway pressure and total end-expiratory pressure when measured under suitable passive conditions:The total baseline includes intrinsic positive end-expiratory pressure when present; substituting only the externally imposed baseline can overestimate this tidal pressure difference.^[ref]

For the linear static model, respiratory-system compliance is $C_{\mathrm{rs}}=V_T/\Delta P$. An original calculation with tidal volume 0.45 L, plateau pressure 23 cm H${}_2$O and total baseline 8 cm H${}_2$O gives $\Delta P=15$ cm H${}_2$O and $C_{\mathrm{rs}}=0.030$ L/cm H${}_2$O. If the external baseline alone were 5 cm H${}_2$O, incorrectly subtracting it would give 18 cm H${}_2$O and a different apparent compliance.

This pressure difference loads both the lung and chest wall. At static endpoints, transpulmonary pressure gives the partitionThus a stipulated airway increment of 12 cm H${}_2$O corresponds to a lung increment of 4 when the pleural increment is 8, but to a lung increment of 10 when the pleural increment is 2. Equal airway driving pressures do not establish equal lung distension without information about the chest-wall contribution.

The measurement method matters as much as the subtraction. A dynamic estimate during flow can include resistance; an end-inspiratory hold and an end-expiratory occlusion address different endpoints. The primary pilot study explicitly compared static and dynamic ways of obtaining whole-respiratory-system compliance and driving pressure.^[ref] These distinctions explain the quantity's mechanical meaning without turning an illustrative number into an individual ventilation recommendation.

Tidal volume, $V_T$, is the volume of gas inhaled or exhaled during one respiratory cycle. It is a per-breath increment, distinct from the gas remaining at functional residual capacity or the maximum total lung capacity.^[ref]

A flow measurement becomes a volume by integration. If $Q(t)$ is positive during inspiration, thenA stipulated triangular inspiratory flow pulse lasting 1 s with peak 1 L/s has area $\frac{1}{2}\times 1\times 1=0.5$ L. Integrating a signed flow over an entire balanced breath instead gives approximately zero, because expiration cancels inspiration; that signed net transfer is not tidal volume.

Multiplying by respiratory rate $f$ gives minute ventilation, ${\dot{V}}_E=f{V}_T$. Two constructed patterns, 0.5 L at 12 breaths/min and 0.25 L at 24 breaths/min, both move 6 L/min. They need not deliver the same gas exchange. In an ideal fixed-respiratory dead space model,where $V_D$ is the per-breath volume not participating in the modeled exchange. For stipulated $V_D=0.15$ L, the two patterns give alveolar ventilation 4.2 and 2.4 L/min. This comparison assumes each breath exceeds the modeled dead-space volume; it is an accounting example, not an individual breathing prescription.

A spirometry or flow trace therefore needs an identified interval and sign convention before its integral is interpreted. The illustration places a quiet tidal volume within a full lung-volume excursion: it shows increments on a constructed absolute scale rather than claiming that ordinary spirometry directly observes the residual offset.

Total lung capacity, TLC, is the volume of gas in the lungs after maximal inspiration. It combines residual lung volume with vital capacity, or equivalently functional residual capacity with inspiratory capacity:The equivalent subdivision is $\mathrm{TLC}=\mathrm{RV}+\mathrm{ERV}+V_T+\mathrm{IRV}$, using expiratory reserve volume, tidal volume and inspiratory reserve volume.^[ref]

A constructed example assigns 1.2 L to residual lung volume, 1.0 L to expiratory reserve volume, 0.5 L to tidal volume and 3.3 L to inspiratory reserve volume. Their sum gives TLC 6.0 L. The vital capacity is 4.8 L, functional residual capacity is 2.2 L, and inspiratory capacity is 3.8 L. Each identity gives the same total because the compartments share consistent endpoints.

A spirometry trace alone measures changes at the mouth; it does not establish the volume remaining after complete expiration. Adding an arbitrary constant to the entire gas-volume trace leaves every observed inhaled and exhaled increment unchanged. An independent estimate of residual lung volume or functional residual capacity is therefore needed to fix that offset. The measurement standard obtains capacities by combining an FRC measurement with linked volume maneuvers.^[ref]

This distinction also explains why vital capacity and total lung capacity are not interchangeable descriptors. Two hypothetical systems can both have a 4 L vital capacity, while having residual lung volume 1 and 2 L respectively. Their total capacities are then 5 and 6 L. The arithmetic separates the amount that can be exchanged during the maneuver from the amount present at full inspiration.

A ventilator is a machine that assists or replaces pulmonary ventilation by supplying a controlled gas-delivery path to the lungs. The interface can be a mask or a breathing tube; the machine and interface together support mechanical ventilation.^[ref] Gas delivery, oxygen concentration and respiratory mechanics remain distinct parts of that support.

A useful engineering description separates the commanded quantity from the measured response. A machine can regulate an applied pressure or a flow history while monitoring pressure and gas volume. Its displayed or reconstructed quantities depend on sensors, timing and the selected model. The manufacturer's treatment of plateau airway pressure, for example, distinguishes a hold measurement from computed monitoring estimates and states conditions under which they are available.^[ref]

An original circuit example shows why upstream delivered volume need not equal the volume entering the patient. In a simplified volume balance,If a stipulated source supplies 0.50 L, ventilator circuit compliance stores 0.04 L during pressurization, and 0.06 L escapes through a leak, only 0.40 L reaches the modeled patient compartment. This is an illustrative uncompensated circuit, not a description of every ventilator's displayed tidal volume.

The stored term can be approximated as $\Delta V_{\mathrm{circuit}}=C_{\mathrm{circuit}}\Delta p$. A stipulated compliance of 2 mL/cm H${}_2$O with a 20 cm H${}_2$O pressure rise gives the stated 40 mL. Measuring volume at another location, or compensating for known circuit properties, changes what is being reported.

A ventilator consequently supplies an experimentally testable physical system: gas path, actuation, sensors and control logic. A pressure or volume setting alone is not a complete observation of alveolar gas renewal, especially when flow, leakage or circuit storage changes.

Ventilator circuit compliance, $C_{\mathrm{circuit}}$, quantifies gas volume temporarily stored in the breathing circuit as its pressure rises. Storage includes expansion of tubing and compression of gas already in the circuit. With a stated volume-reference convention, a local linear approximation isIt describes the delivery apparatus, rather than the respiratory-system compliance of the patient.

The ideal gas law explains why even rigid tubing can contribute. For a rigid circuit of volume $V_c$ at fixed temperature, $n=P{V}_c/(RT)$. Expressing the extra gas as volume at the initial absolute pressure $P_0$ givesFor an original illustration, let $V_c=2$ L and $P_0=1000$ cm H${}_2$O. The gas-storage contribution is 2 mL/cm H${}_2$O. If tubing expansion adds 0.5 mL/cm H${}_2$O, a 20 cm H${}_2$O rise stores about 50 mL in this linear model. Absolute pressure belongs in the gas-law denominator; the 20 cm H${}_2$O pressure increment is not that denominator.

Under the same gas-reference convention, integrated source flow must supply both patient volume and the change in circuit storage, plus any leaked volume. A remote flow sensor therefore need not measure only tidal volume. A sensor near the airway opening moves the measurement boundary past much of the circuit storage. The manufacturer's technical account explicitly motivates proximal measurement by this distinction.^[ref]

Correcting storage is still a model-based operation. A single compliance coefficient does not identify an unknown leak, changing tubing properties or every dynamic pressure difference. The technical source's reproduced experimental abstracts show why circuit correction did not eliminate every discrepancy between ventilator-side and airway-side measurements. Those observations motivate checking the measurement location and gas conditions rather than equating a displayed source volume with gas entering the lung.