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RESPIRATORY PHYSIOLOGY

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TABLE OF NORMAL VALUES

ITEM NORMAL VALUE COMMENTS

General Stuff

Pleural Pressure:

Beginning of Inspiration

(At Functional Residual Capacity)

-5 cm H2O 5 cm H2O less than atmospheric pressure
Pleural Pressure:

End of Inspiration

-8 cm H2O With normal inspiration, the change in pleural pressure is very small.
Total Alveolar Surface Area for Gas Exchange 75 m2
Normal alveolar diffusion barrier 1 micron

Airway Resistance and Flow

Overall Airway Resistance 0.5 - 1.5 cm H2O / mL / sec
Fraction of Vital Capacity in the First second of a forced maximal exhalation

(FEV1 / FVC)

70% - 80% The FVC test is used to measure airway resistance. Subnormal value indicates COPD.

Gas Exchange

Atmospheric PO2 21% of 760 mm Hg

160 mm Hg

Inspired PO2

PIO2

= (21%) x (747 - 47 mm Hg)

= 147 mm Hg

Air becomes saturated with water which has partial pressure of 47 mm Hg.

Thus we take the PO2 of dry air = (21%)(700)

Inspired PCO2

PICO2

Virtually 0 Atmospheric air contains virtually no CO2
Alveolar PO2

PAO2

100 mm Hg O2 has gone down from 147 mm Hg because some O2 moves to the blood.
Alveolar PCO2

PACO2

40 mm Hg This is the amount of CO2 that enters alveoli from blood in a healthy lung.
Mixed Venous PO2

PVO2

40 mm Hg 40 torr is the standard O2 concentration of mixed venous blood, as determined by how much O2 the tissues need.
Mixed Venous PCO2

PVCO2

46 mm Hg The CO2 gradient for gas transport is only 6 mm Hg -- difference between 46 and 40.
Arterial PO2

PaO2

90 - 95 mm Hg It is slightly less than PAO2 due to contribution of venous admixture.
Arterial PCO2

PaCO2

40 mm Hg It is virtually the same as PACO2 but theoretically higher due to venous admixture.

Dead Space

Tidal Volume 450 - 500 mL Volume of air during normal inspiration
Anatomical Dead Space 150 mL The individual's weight in lbs = anatomical dead space in mL
VD / VE Ratio

Ratio of dead space to expired air

0.2 to 0.3, i.e. 20% - 30% of expired air is dead space.

When VD/VE ------> 0.6, patients are put on a ventilator.

High dead space occurs with pulmonary embolism, and with low VA/Q ratios.
Respiratory Quotient

VCO2 / VO2

0.8

We exchange about 80% as much CO2 as O2 with each inspiration

The reciprocal is 1.2, the fudge-factor for the alveolar ventilation equation.

Gas Transport and Acid-Base Balance

Plasma Solubility Coefficient of O2 (SO2) 0.003 mL O2 / dL plasma, or

0.003 Vol-%

Plasma Solubility Coefficient of CO2 (SCO2) 0.03 mL CO2 / dL plasma, or

0.03 Vol-%

CO2(g) is thus 10x more soluble than O2(g)
O2 Carrying-Capacity of Hemoglobin 1.34 mL O2 / g Hb

One gram of Hb holds 1.34 mL of O2.

Thus:

Maximum HbO2 = (Hematocrit)(1.34)

Hematocrit 15 g Hb / dL blood
HbO2

Normal O2-Carrying Capacity of hemoglobin

20 vol-% (Hematocrit)(1.34) =

(15 g Hb / dL)(1.34) =

20 mL O2 / dL

Arterial Hb-Saturation

Arterial PaO2 (Oxygen partial pressure)

Arterial CaO2 (Oxygen Content)

100% saturation

PaO2 = 100 mm Hg

CaO2 = 20 mL / dL

Arterial:

100% saturation ------>

100 mm Hg ------>

20 mL / dL O2 content

Venous Hb-Saturation

Venous PvO2 (Oxygen partial pressure)

Venous CvO2 (Oxygen content)

75% saturation

PVO2 = 40 mm Hg

CvO2 = 15 mL / dL

Venous:

75% saturation ------>

40 mm Hg ------>

15 mL / dL O2 content

P50 50% saturation

P50 = 26 mm Hg

P50 is the PO2 required to achieve 50% hemoglobin saturation
Normal plasma HCO3- 24 mM
Normal blood pH 7.4 A 0.1 decrease in pH corresponds to a 1 mM increase in HCO3-, in healthy, uncompensated hypoventilation.

Ventilation / Perfusion of the Lung

VA/Q Ratio, Lung Apex (Top) 3.0 Apex of lung gets less ventilation and less perfusion, but perfusion is substantially less than at base.
VA/Q Ratio, Lung Base (Bottom) 0.5 - 0.6 Base of lung gets greater perfusion and ventilation, but perfusion is substantially greater than at apex.
VA/Q Ratio, Overall 1.0 Overall ventilation / overal perfusion 1
Overall Perfusion of Lung 5.0 - 6.0 L / min That is, overall lung perfusion = Cardiac Output
Overall Ventilation of Lung 5.0 - 6.0 L / min This is equal to (respiratory rate) x (tidal volume)

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MECHANICS of RESPIRATION

PNEUMOTHORAX: A knife wound will equilibrate the pressure between the pleura and lung, making the chest expand and the lungs collapse.

NEGATIVE PRESSURE BREATHING: Lung pressures are usually measured in cm H2O because they are relatively low pressures, and cm H2O is a smaller quantity than mm Hg.

PRESSURE AND FLOW:

LUNG VOLUMES:

STATIC PRESSURE-VOLUME CURVE: A pressure-volume curve of the lung, where each point is measured when there is no airflow. This implies that the total pressure at that instant is equal to the transpulmonary pressure since there is no pressure gradient for airflow.

COMPLIANCE: DeltaV / DeltaP, the slope of the Static Pressure Volume Curve. In other words, it is the slope of a P/V curve when there is no airflow and the pressure thus represents transpulmonary pressure.

AIRWAY RESISTANCE:

BRONCHIAL SMOOTH MUSCLE: Autonomic control of airway resistance.


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PHYSIOLOGY of RESPIRATION

CHRONIC OBSTRUCTIVE PULMONARY DISEASE (COPD): Various pathologies will all lead to the common symptoms of COPD, which is caused by chronic increased airway resistance.

DYNAMIC Pressure-Volume Curve:

VENOUS ADMIXTURE: The pulmonary capillary (freshly oxygenated) blood leaving the lungs, combined with venous blood that did not perfuse the lungs.

EXERCISE:

PHYSIOLOGICAL DEAD SPACE = ANATOMICAL DEAD SPACE + ALVEOLAR DEAD SPACE. Dead space is defined as inspired air that is not perfused by blood and thus "waisted" because it does not contribute to gas exchange.

ALVEOLAR VENTILATION:

PULMONARY DIFFUSION:

, and VO2 then becomes

Thus:

Thus we can measure DLO2 in terms of DLCO:

OXYGEN TRANSPORT:

From above two equations:

CARBON DIOXIDE TRANSPORT:

ACID-BASE BALANCE: Blood pH is determined by the HCO3- : CO2 ratio, according to a derivation of the Henderson Hasselbach Equation:

ACID-BASE IMBALANCES:

VENTILATION / PERFUSION BALANCE:

REGULATION OF RESPIRATION:


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Copyright 1999, Scott Goodman, all rights reserved