Physiology II
Respiratory Physiology
Pulmonary Ventilation

Readings required or recommended: Guyton and Hall (9th edition). Chap. 37; Ganong (19th edition), Chap. 34;

Key Words

ventilation:  The inflow and outflow of air btw the atmosphere and the lung alveoli. The process of exchange of air btw the lungs and the ambient air. Pulmonary ventilation refers to the total exchange, whereas alveolar ventilation refers to the effective ventilation of the alveoli, where gas exchange with the blood takes place.

perfusion:  The act of pouring through or over: especially the passage of a fluid through the vessels of a specific organ. Blood flow through the pulmonary vessels where gas exchange occurs.

oxygen consumption:  The amt of oxygen needed to meet the metabolic needs of the tissues.

diffusion:  The movement of O2 and CO2 btw the alveoli and the pulmonary capillaries which is a random molecular motion of molecules intertwining their ways in both direction through the respiratory membrane.

convection:  The removal of heat from the body by a process called convection so that the heated air moves away from the skin, so that new, unheated air is continually brought in contact with the skin.

inspiration:  Contraction of the diaphragm pulls the lover surfaces of the lungs downward.

 expiration:  The diaphragm simply relaxes and the elastic recoil of the lungs, chest wall, and abd structures compress the lungs.

anatomical dead space:  The nose, pharynx, and trachea where no gas exchange occurs.

physiological dead space:  This is the anatomical dead space and any alveoli that do not take part in gas exchange.

work of breathing:  Work of inspiration can be divided into 3 fractions: 1)That required to expand the lungs against the lungs and chest elastic forces, called compliance work or elastic work. 2) that required to overcome the viscosity of the lungs and chest wall structures, called tissue resistance work. And 3) that required to overcome airway resistance during the movement of air into the lungs, called airway resistance work.

partial pressure:  An absolute number of a concentration of a gas in a mixture of gases or a liquid where the partial pressure is indicated in mm of Hg are represents the concentration of that gas and the pressure that gas exerts on the total volume. Each gas exerts its own partial pressure on the whole and the total of all the partial pressures must add up to the total pressure on that volume.

water vapor pressure:  The pressure that the water molecules exert to escape through the surface is called the vapor pressure of the water. At normal body temp this is 47 mm Hg. Once the gas mixture has become fully humidified – or at equilibrium with the surrounding water – the partial pressure of the water vapor in the gas mixture is 47.

minute ventilation:  The amt of air moved in and out of the lungs in a minute.

frequency:  The respiratory rate per minute.

tidal volume:  The volume of air inspired or expired with each normal breath – usually about 500ml.

dead space ventilation:  Alveoli that are ventilated but not perfused.

alveolar ventilation:  The volume of air entering and leaving the alveoli per minute.

pulmonary volumes and capacities:  

hyperventilation:  Abnormally prolonged and deep breathing, usually associated with acute anxiety or emotional tension. A transient respiratory alkalosis commonly results from hyperventilation.

hypoventilation:  Reduction in the amt of air entering the pulmonary alveoli.

alveolar ventilation equation:  Tells you the measure of alveolar ventilation by measuring the CO2 and the end tidal CO2. It also tells you that alveolar ventilation is very important in CO2 elimination. The amt of CO2 would go up if you couldn’t get it out. Anything that impairs alveolar vent will also increase the partial pressure of CO2.

            CO2
A = -------------- x K
            PACO2

Learning Objectives:

describe the general anatomical features of the lung, chest wall, pulmonary circulation, and bronchial circulation:  To move air into or out of the lungs we must create pressure differences between the atmosphere and the alveoli. To move air into the alveoli we must make alveolar pressure less than atmospheric pressure (except during positive pressure ventilation). Alveoli expand passively in response to an increased transmural pressure gradient. As they expand, their elastic recoil increases. Alveolar pressure = intrapleural pressure + alveolar elastic recoil pressure.

describe in detail the structure of the terminal respiratory units, including the anatomical matching of airway and pulmonary arterial branching as well as the venous admixture of pulmonary venous and bronchial venous blood:  The upper sections of the alveoli in the apex of the lung in an upright human are somewhat dilated and have less blood flow than the bases where the alveoli are more compact and can then expand more with air since they aren’t as full as the alveoli at the apex. There is more blood flow in the bases due to the hydrostatic pressure gradient. The alveoli are surrounded by a thin layer of liquid lining them. The alveoli epithelial cells are flat and thin with a tiny area of interstitial space and a basement membrane separating the alveoli epithelium from the vascular endothelium. The gases easily pass through these membranes. There is a great deal of blood flow around each alveoli to maximize ventilation/perfusion ratios. There is some blood flow that comes from the upper apices that have very little blood flow and therefore have poor perfusion. This blood mixes in with the blood that came from areas that were well perfused and the combination of well perfused and poorly perfused blood has a slight overall decrease in the PaO2.

identify the primary and accessory muscles of inspiration and expiration:  

Muscles of Breathing

  1. Inspiration - expansion of thoracic cavity lowers intrathoracic pressure. "Negative pressure." Normally no true intrathoracic space. Only about 7 - 15 ml pleural fluid.
    1. The Diaphragm. The diaphragm is the primary muscle of inspiration. During supine eupneic breathing it is responsible for at least 2/3 of the tidal volume. The muscle fibers of the diaphragm are inserted into the sternum and the lower ribs, and into the vertebral column by the two crura. The other ends of these muscle fibers converge to attach to the fibrous central tendon. During eupnea, contraction of the approximately 250 cm diaphragm causes its dome to descend 1 to 2 cm into the abdominal cavity, with little change in its shape, except that the area of apposition decreases in length. This elongates the thorax and increases its volume. These small downward movements of the diaphragm are possible because the abdominal viscera can push out against the relatively compliant abdominal wall. During a deep inspiration the limit of the compliance of the abdominal wall is reached and the indistensible central tendon becomes fixed against the abdominal contents. After this point contraction of the diaphragm against the fixed central tendon elevates the lower ribs (the "bucket handle motion").
    2. External and Parasternal Intercostal Muscles - contraction pulls ribs up. Increases antero- posterior diameter of the chest. Innervation from T -1 to T-11.
    3. Accessory muscles - not involved in eupnea but may be called into action during exercise, cough, sneeze, chronic obstructive pulmonary diseases, etc. Include sternocleidomastoid and others. Act to raise the upper ribs (" pump handle motion") and the sternum.
  2. Expiration during eupneic breathing is passive. Relaxation of the inspiratory muscles allows the increased alveolar elastic recoil to decrease the volume of the alveoli, increasing alveolar pressure above atmospheric pressure.
  3. The muscles of expiration are involved in active expiration: exercise, speech, cough, sneeze, forced expiration, etc.
    1. Internal intercostals - Perpendicular to external intercostals. Action pulls rib cage down and inward.
    2. Muscles of abdominal wall - raise intra-abdominal pressure. Displace diaphragm upward into thorax. Includes rectus abdominis, internal and external oblique muscles, and transversus abdominis.
    3. During expiration there is a "braking action" of the inspiratory muscles at high lung volumes.
    4. Although all of the respiratory muscles are usually considered to be completely relaxed at the FRC, diaphragmatic tone probably plays an important role.

identify, in words or by drawing a reasonably accurate graph depicting a spirometric tracing, the four pulmonary volumes and four pulmonary capacities, discuss how these are measured or calculated, and state typical values for these volumes and capacities in a healthy human subject:  


Guyton page 483

Ganong page 622

write from memory an equation representing the relationship between minute ventilation, respiratory frequency, and tidal volume:  

E = VT * F
where E = Minute ventilation, VT = tidal volume, and F = frequency.
E can also = (VD x F) + (VA x F)

compare and contrast the effects of changes in respiratory frequency and tidal volume on alveolar ventilation, including a quantification of these effects using typical values for the parameters involved:  When you change frequency or tidal volume, you can increase or decrease alveolar ventilation. If you doubled frequency both VD and VA would then double. If you increased VT then VA would double but the dead space would not so more air would go to the alveoli and thus more ventilation could occur. Thus to increase tidal volume is a more effective way to increase alv. ventilation.

compare and contrast the anatomical and physiological dead spaces and describe how each might be measured:  Anatomical dead space is the physical structures that are not involved in the gas exchange process. These include the nasopharynx, the larynx, the trachea, brochi, and bronchioles to the edge of the alveolus. Physiological dead space is the anatomical plus any alveoli that do not participate in gas exchange for whatever reason. One way to measure this is the nitrogen washout method that would entail breathing in 100% oxygen and then having your expired breath measured. The first air out would be the 100% O2 sitting in the dead space and then as the expiration continued you would begin to pick up nitrogen. The volume of air that was all oxygen would be the amt of dead space. The other method to measure dead space is to measure the amt of CO2 present on expiration. CO2 is essentially not existent in the room air so if you measured inspired air and the amt of CO2 that comes out in expired air, you can get a measurement of dead space. The equation begins like this: CO2 = A * FACO2 since CO2 isn’t the only gas coming out, you have to consider the fraction of [CO2] in the air in the alveoli. In essence you are measuring the end tidal CO2 and the equation becomes the alveolar ventilation equation which tells us the measure of alveolar ventilation by measuring the CO2 and the end tidal CO2. The significance of this equation is that it tells us that alveolar ventilation is very important in CO2 elimination.

               CO2
A =  -----------------   x   K
                PACO2

define the term "alveolar ventilation", discuss its relationship to dead space and minute ventilation, and state its significance in pulmonary gas exchange:  Since alveolar ventilation is the volume of air entering and leaving the alveoli per minute, any changes in this volume will greatly affect the overall rate of gas exchange in the lungs. Anything that impairs the alv vent will increase the partial pressure of CO2. An increased PaCO2 will be a reflection of decreased alv vent. Alv vent and PaCO2 are reciprocally related. If you decreased A and increased PaCO2 and since this gas mixture is in communication with the outside world, and if you added more CO2 the AO2 would have to decrease since as you add more gas the mixture becomes diluted. A hallmark of low alv vent is to have an increased PaCO2 and a decreased PaO2. See two questions above for a relationship btw dead space, min vent and alv vent.

write from memory the alveolar ventilation equation and use it to demonstrate the effects of changes in alveolar ventilation on the alveolar partial pressure of carbon dioxide:  

               CO2
A =  -----------------   x   K
                PACO2

The body’s way of controlling alv ventilation to properly supply the tissues with adequate oxygenation and CO2 removal is to control PaCO2. The changes in partial pressure of this gas will reciprocally effect a change on the partial pressure of oxygen and total alveolar ventilation.

write from memory the simplified form of the alveolar air equation and use it to calculate alveolar PO2 when given values for inspired PO2, the respiratory exchange ratio, and arterial or alveolar PCO2:  

Alveolar air equation: PAO2 = PIO2 –
Where R = the respiratory quotient or the ratio of VCO2 production divided by the O2 consumption.
PIO2 = 150 mm Hg, PACO2 = .40, the resp quotient is .8, and so you can calculate the PAO2 to be 100.

demonstrate an understanding of the effect of hypoventilation and hyperventilation on the arterial partial pressures of oxygen and carbon dioxide:  Hypoventilation is defined as an increase in arterial PCO2. Hyperventilation is defined as a decrease in arterial PCO2. If someone is hypoventilating, then they are not blowing off enough carbon dioxide or breathing in enough oxygen. The two values are usually reciprocal since the partial pressures of each, along with any other gases present in the arterial system must = the total pressure in the system. Hyperventilating can decrease the CO2 and could thus increase the O2.

Last Updated 09/06/01 08:53:26 PM
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