Gas Exchange

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  • Topic: Respiratory physiology, Oxygen, Carbon dioxide
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  • Published : March 9, 2013
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An Introduction to Gas Exchange
Lecturer: Sally Osborne, Ph.D. Department of Cellular & Physiological Sciences Email: sosborne@interchange.ubc.ca Useful link: www.sallyosborne.com Required Reading: Respiratory Physiology: A Clinical Approach, Shwarrtzstein & Parker, Chapter 5 (pp 95-100; 111112).

Objectives
1. Distinguish between the following terms: minute, alveolar and dead space ventilation; and anatomic, alveolar and physiologic dead space. 2. Specify the partial pressures of CO2 and O2 in the alveoli, mixed venous and arterial blood in normal individuals. 3. Using the alveolar ventilation equation, discuss the factors that determine the partial pressure of carbon dioxide in the alveoli and define the terms hyperventilation and hypoventilation. 4. Be able to calculate the PAO2 using the simplified alveolar-air equation. 5. Using Fick’s law of diffusion, specify the key factors that affect the exchange of oxygen across the alveolar capillary membrane. 1. Ventilation: Minute, Alveolar and Dead Space Ventilation is a general term for the movement of air into and out of the lung. The symbol for ventilation is V. V stands for volume and the dot for “per unit time”. Minute ventilation is a more specific term referring to the total amount of air moved in or out of the lungs per minute. By convention, minute ventilation is typically measured as the quantity of air expired per minute and symbolized VE. It is helpful to remember that VE is the product of the tidal volume expired, written as VT or VE and the respiratory rate, RR. VDS 150ml

VE = VT X RR

For each tidal volume (VT) of 500 ml air inspired, 150 ml stays in the conducting airways. This volume is called the anatomic dead space (VDS) as it does not participate in gas exchange. It merely enters and subsequently leaves the conducting airways. The remaining 350 ml of the tidal breath enters the alveoli (VA) and participates in gas exchange. Note that this 350 ml inspired air mixes with functional residual capacity (FRC), about 2400ml liters of air already in the lungs. Typical values for dead space and alveolar ventilation are derived on the right.

VA 350 ml

VE = VT X RR = 500X12 = 6.0 L/min VA = VA X RR = 350X12 = 4.2 L/min VDS =VDS X RR = 150X12 = 1.8 L/min

FRC = 2400 ml In this figure, the entire conducting zone is represented as a single tube and the respiratory zone as a single reservoir.

Alveolar ventilation (VA) is the volume of air breathed per minute that: 1) reaches the alveoli and 2) participates in gas 1

exchange. Dead space ventilation (VDS) refers to the portion of minute ventilation that does not participate in gas exchange. Dead space ventilation includes ventilation of 1) the anatomic dead space: the portion of each breath that enters the conducting airways and 2) the alveolar dead space: air that reaches the alveoli but does not participate in exchange (e.g. air that reaches an alveolus that is not perfused). In healthy individuals, alveolar dead space is about 25 ml and considered relatively insignificant. The sum of the anatomic and alveolar dead space is referred to as the “physiologic dead space”.

2. The Significance of Partial Pressure values of Alveolar Carbon Dioxide and Oxygen In the lungs, by the time blood reaches the end of a pulmonary capillary, gas exchange is complete and the partial pressure of oxygen and carbon dioxide in the blood is in equilibrium with the partial pressure of these gases in the alveolus. Therefore, it is important to be aware of the factors that determine the partial pressure of oxygen and carbon dioxide in the alveoli, as they in turn, affect the partial pressure of these gases in the arterial blood supplying the tissues.

dry air PO2 (mmHg) PCO2 (mmHg)
160 0

inspired air
150 0

alveolar gas
100 40

mixed venous blood
40 46

arterial blood
95 40

You need to memorize the normal PCO2 and PO2 values under standard barometric pressure conditions listed in the table above.

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