Gas Exchange Flashcards by Camelita Thrift

How is ventilation calculated?

RR x CO

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What is an average/ normal ventilation?

6000 ml/min or 6L/min

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Boyle’s Law

States that when temperature s constant, pressure and volume are inversely related

P1V1=P2V2

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Dalton’s law

The total pressure exerted by a gas mixture is the sum of the pressures exerted by the individual gases that make it that mixture

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Henry’s Law

States that the amount of a gas absorbed by a liquid to which it is not chemically combined is directly proportional to the positive pressure of the gas in the liquid.

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Composition of Alveolar Air- in comparison to atmospheric air

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What is the significance of slow replacement of alveolar air?

Prevents sudden changes in gas composition in the alveolar air and subsequent concentration in blood
Maintains respirtory control mechanisms which are dependent on stability of tissue oxygenation, carbon dioxide concentration and pH when respiration is temporarily interrupted

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What are primary gases found in inspired air?

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Immediately, upon entry to respiratory passage, air is humidified with __________.

Water vapor

NOTE: Total pressure in alveoli cannot exceed barometric pressure; therefore, other gases are diluted

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What are the partial pressure of oxygen and carbon dioxide dependent on?

Rate of absorption of O2 into the blood
Rate of entry of new O2 into the lungs

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What is the normal alveolar PO2?

104 mm Hg

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What’s the normal alveolar ventilation?

4.2 L/min

NOTE: There is a 4-fold increase in VA required during exercise

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The composition of expired air is determined by the ______ and ___________.

Dead space and alveolar volumes

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What does the first portion of expelled air contain?

Dead space air, which is air that has been humidified and does not participate in gas exchange

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What does the second portion of expelled air contain?

A gradual mixing of dead space with alveolar air until all dead space is washed out and only alveolar air is being expelled

NOTE: For pulmonary tests, collect last amount of air for pure sample of alveolar gas

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Total pulmonary ventilation is greater than alveolar ventilation because of __________.

Dead space

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A portion of tidal volume that remains in the upper conducting airways and does not participate in gas exchange.

Dead space volume

NOTE: Anatomical dead space is determined by the size and number of airways the gas occupies. While, physiological dead space usually occurs indiseased lungs.

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What areas are considered to have physiological dead space?

Ventilated alveoli with no perfusion
Over-ventilated areas relative to the amount of perfusion
Sites of no gas exhange

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Fowler’s Method of Dead Space Measurement

Subject takes a single breath of 100% O2 and then exhales into a tube that continuously measures the [N2] in the expired gas.
Anatomic dead space containing 100% O2 and 0% N2 empties first followed by alveolar emptying.
Alveolar emptying results is a rise in [N2] and a decline in [O2] until a plateau is reached (indicates alveolar gas only).
The volume with initially 0% N2 plus 50% of the rising N2 volume is equal to the anatomic dead space.

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Fick’s Law

States that the diffusion of a gas across a sheet of tissue is directly related to the surface area of the tissue, the diffusion constant for that gas, and the partial pressure difference of the gas on each side of the tissue and is inversely proportional to tissue thickness

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Partial pressure of a gas

The partial pressure of a gas is equal to the fraction of the gas in gas mixture times the barometric pressure

O2 = 21% of Barometric pressure
N2= 79% of Barometric pressure

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How can PO2 be changed?

Fraction of inspired oxygen (air mask)
Changes in Barometric pressure
Supplemental O2
Travel to high altitudes (decreases PO2)

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What are the partial pressire in the alveolar air determined by?

Alveolar ventilation
O2 consumption
CO2 production

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Ideal Alveolar Gas Equation

PAO2= The partial pressure of oxygen in the alveolus
PIO2= Inspired partial pressure of oxygen which is equal to the FIO2 times the difference between the barometric pressure and pressure of H2O
PACO2= The alveolar gas carbon dioxide tension
R= Respiratory exchange ratio= 0.8
- Ratio of CO2 excreted to the oxygen taken up by the lungs

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What does a respiratory exhange ratio of 0.8 indicate?

That there is more O2 consumed than CO2 produced **REMEMBER:** The ratio is CO2/O2

Compartmental Values for Respiratory Gases

Alveolar Carbon Dioxide Equation

FACO2 is a function of cellular metabolism

The fraction of CO2 in alveolus is the function CO2 production from _______ and the rate of elimation by the lungs.

Cellular metabolism

Relationship between alveolar ventilation and PACO2

If you double your V_A; halve your PACO2 If you halve your V_A; double your PACO2

Why is CO2 ventilation increased during exercise?

There is an increase in metabolic activity, meaning more CO2 is being produced. To maintain a normal CO2, ventilation must increase

Hypercapnia

Increased arterial PCO2. \*Hypercapnia is 2nd to _hypoventilation_

Ventilation is best in ______ regions when considering body position.

Dependent **NOTE:** Body position changes ventilation such that ventilation per unit volume is best in lower depedent regions of the lung compared with upper regions

Distribution of alveiolar ventilation

What are the changes in ventilation (in a single breath) for reduced compliance and increased airway resistance?

**Normal- 97%** **Reduced compliance- 80%** **Increased resistance - 50%**

How is the ventilation/ perfusion ratio determined for a single alveolus?

Ratio is equal to **alveolar ventilation** divided by the **capillary blood flow**

How is the ventilation/ perfusion ratio determined for an entire lung?

**Total alveolar ventilation** divided by the **cardiac output**

Ventilation- Perfusion mismatching increases with _______ and \_\_\_\_\_\_\_\_\_.

Age and lung disease

In patients with cardiopulmonary disease, misamtcing of ventilation and perfusion, is the most common cause of \_\_\_\_\_\_\_\_\_\_\_\_.

Arterial hypoxemia

Distribution of Ventilation and Perfusion in the upright lung

In cases of severe emphysema, V/Q ratio is ___________ (low/high). In cases of chronic bronchitis, V/Q ratio is (low/high).

* **High V/Q** in emphysema due to low perfusion * Capillaries are ablated, secondary to alveolar wall deteriotation * **Low V/Q** in chronic broncitis due to high perfusion and low ventilation **NOTE:** A normal V/Q does not necessarily indicate a healthy lung because of compensatory mechanisms

High PO2 environment at apex of the lung affects the distribution of \_\_\_\_\_\_\_\_\_\_\_\_\_.

Pulmonary tuberculosis \*Tuberculosis bacillus thrives in O2- rich environments

Overall V/Q in the normal lung is \_\_\_\_\_\_\_\_\_.

0.8

At the apex of the lung, what happens to the ventilation to perfusion ratio?

V/Q will be greater than one due to an increased ventilation relative to little perfusion in pulmonary circulation due to gravity

At the base of the lung, what happens to the ventilation to perfusion ratio?

V/Q wil be less than one due to the perfusion being greater than ventilation

What are the pressure ratios for alveoli, arteries, and veins, in each zone of perfusion?

* **Zone 1** * ****PA \> Pa \> Pv * Blood vessels can become completely **collapsed by alveolar pressure (PA)** and **blood does not flow through these regions.** * **Zone 2** * Pa \> PA \> Pv * Blood flows in pulses. * At first there is no flow because of obstruction at the venous end of the capillary bed. * Pressure from the arterial side builds up until it exceeds alveolar pressure and flow resumes. * **Zone 3** * ****Pa \> Pv \> PA * Blood flow follows a pressure gradient * Results in distension of the blood vessels and **lowering resistance.** ## Footnote **Alveoli =PA** **Arteries =Pa** **Veins =Pv**

Zone 1 of perfusion does not exist normally. When can it be reached?

* When a subject is mechanically ventilated * Physiological condtion that significantly reduce Pa pressure * Hemorrhage * Under conditions of decreased arterial pressure

Which zone of perfusion has blood existing that is most reflective of PaO2?

Zone 2

Which zone of perfusion has the best blood, follows a pressure gradient and has no collapsed capillaries?

Zone 3

Regional differences in ventilation and perfusion in the normal upright lung

The difference between PAO2 and PaO2 is termed \_\_\_\_\_

AaO2 **NOTE:** A=alveolar and a = arterial. In normal subjects there is little to no difference between the two.

Oxygenated and deoxygenated blood mixing causes a reduction in the \_\_\_\_\_\_.

PaO2

An _______ (increase/decrease) in AaO2 is the hallmark of inefficient or abnormal gas exhange.

Increase

Thebesian vessels that supply the ____ drain directly into the arterial circulation, rather than the coronary sinus in the right atrium.

Left ventricle

Low PaO2 is known as \_\_\_\_\_\_\_\_

Arterial hypoxemia **NOTE:** An abnormal PaO2 can occur in the presence or absence of an abnormal AaDO2. However, the relationship between the PaO2 and AaDO2 is useful in determining the cause of the abnormal PaO2 and predicting the response to supplemental oxygen therapy.

What is the normal AaDO2?

Less than 15mm Hg **NOTE:** Value for the AaDO2 increases 3 mmHg per decade of life, making less than 25 mmHg the upper limit of normal

What is the most common cause of hypoxemia?

V/Q mismatch

Stages of impaired gas exhange

Lung mechanisms of arterial hypoxemia

Diffusion deficits Shunt Ventilation/ Perfusion Mismatch Alveolar hypoventilation

What is the V/Q ratio in cases of a physiological shunt?

**V/Q= 0** * Complete block of airway; all ventilation diverted to adjacent alveolus * Perfusion is equally distributed between both units * Results: Mixed venous blood with **no gas exhange**- it remains mixed venous

Disease states associated with hypoxia

* **Congenital heart defects** * **Anatomic shunts** * **Tetralogy of Fallot** * Pulmonary valve stenosis * Ventricular septal defect * Overriding aorta * **Guillain- Barre syndrome** * Diaphragm doesn't generate a great force, results in decreased minute ventilation

What factors determine alveolar O2?

The balance between the rate of O2 uptake and the rate of O2 replenishment by ventilation

AaDO2 results of hypoventilation

Normal because both PAO2 and PaO2 are decreased

Causes of hypoventilation

1. Respiratory center deficit 2. Interference with nerve 3. Dislocation of cervical spin, disruption of nerves to diaphragm 4. Anterior horn cell problem 5. Neuritis 6. Neuromuscular Junction/paralysis or affect of anesthesia 7. Muscle atrophy