Tidal Volume and Alveolar Ventilation

To improve our understanding of alveolar ventilation, we need to incorporate some of the concepts previously described in the conducting zone. Since out ultimate goal is to determine the proportion of minute ventilation that actually reaches the respiratory zone, we need to include those factors that explain minute ventilation in terms of volumes and frequency. Remember minute ventilation (V_E = V_T * f). Before we look at V_dotA in terms of V_E, let us first discuss the volume of a single breath and how it relates to V_D and V_T. We can describe V_D as the portion of the V_T that fills the conducting zone and is not available for respiration, and does not reach the alveoli. By using this convoluted description of V_D, we can then describe V_A as the portion of V_T that is not V_D or more simply:

V_A = V_T - V_D

Or in terms of V_T:

V_T = V_A+ V_D

In general, we will assume that an individual's V_D in the conducting zone is approximately 350 ml; therefore, we can assume:

V_A = V_T - 350 ml

Or in terms of V_T:

V_T = V_A + 350 ml

Up to this point we are using the terms V_A and V_D as volumes in a single breath. Now, go to the next page of this lesson where we add frequency of breaths to the formula to determine minute alveolar ventilation (V_dotA) and minute dead space ventilation (V_dotD) as a function of V_E.

Minute Ventilation and Alveolar Ventilation

Now that we know that alveolar volume is a portion of tidal volume, let us determine what portion of minute ventilation actually reaches the respiratory zone and functions as alveolar ventilation. We have already learned that minute ventilation is the product of the tidal volume and frequencies of breaths, and that tidal volume is the sum of the alveolar volume and dead space volume. We can now extrapolate the formula for minute ventilation to include the alveolar volume and dead space volume. The resulting formula that describes minute ventilation in terms of alveolar ventilation and dead space ventilation is:

V_E = (V_A + V_D) * f

Or

V_E = V_dotA + V_dotD

Or in terms of alveolar ventilation:

V_dotA = (V_T – V_D) * f

So, let us look at a couple of examples of how V_dotA changes due to breathing patterns.

Example 1: Assume that you are in a resting state and fully relaxed. From the previous pages we know that your resting V_T is approximately 500 ml, and frequency of breaths is 12 breaths per minute. For this situation, we can calculate your V_E as:

V_E = 0.5 L * 12 = 6 L/min

Now to determine V_dotA we can replace the tidal variable with V_A and V_D so that:

V_dotA = (0.5 L – 0.35 L) * 12 = 1.8 L/min

From this example you can see that although 6 liters of air enter the lungs in a minute, only 1.8 liters of that air (30% of the total V_E) reaches the alveoli.

Example 2: What about exercise and alveolar ventilation? As work increases the energy demand increases also, therefore, alveolar ventilation must increase. Assume that you are jogging around your neighborhood, and your tidal volume is 3 liters of air and frequency of breaths is 25 breaths per minute. Your V_E is:

V_E = 3.0 L * 25 = 75 L/min

And your V_dotA is:

V_dotA = (3.0 L - 0.35 L) * 25 = 66.25 L/min

From this example you can see that 75 liters of air enter the lungs in a minute, and 66.25 liters of air (83% of the total V_E) reaches the alveoli. Notice the dramatic increase in alveolar ventilation. Now go to the next page to discuss the oxygen content of that alveolar air.

Oxygen Availability and V_E

Now that you have learned how to determine V_E and V_dotA, you may also want to determine the proportion of V_E and V_dotA that is O₂. You may be asking yourself why this is important? By determining the proportional amount of O₂ for any given V_E and V_dotA, you will know the amount of O₂ that enters the lungs and alveoli. By knowing the O₂ content that reaches the alveoli, you the amount of O₂ that is available for gas exchange. To determine the amount of O₂ entering the lungs during any given minute use the formula:

LungO₂ = V_E * 0.2093

And

AlveolarO₂ = V_dotA * 0.2093

Where 0.2093 is the proportion of oxygen in the ventilated ambient air. Lets use the examples from the previous page to determine the O₂ availability for V_E and V_dotA for each of the examples.

Example 1: From this example, minute ventilation was 6 L/min and alveolar ventilation was 1.8 L/min. The O₂ availability of minute ventilation is:

LungO₂ = 6 L/min * 0.2093 = 1.25 L/min O₂

And

AlveolarO₂ = 1.8 L/min * 0.2093 = 0.377 L/min O₂ = 377 ml/min O₂

From this example you can see that although 1.25 liters of O₂ enter the lungs in a minute but only 377 ml of O₂ reaches the alveoli and is available for gas exchange.

Example 2: From this example the minute ventilation was 75 L/min and alveolar ventilation was 66.25 L/min. O₂ availability of minute ventilation is:

LungO₂ = 75 L/min * 0.2093 = 15.7 L/min O₂

And

AlveolarO₂ = 66.25 L/min * 0.2093 = 13.8 L/min O₂

From this example you can see that 15.7 liters of O₂ enter the lungs in a minute and 13.8 L of O₂ reaches the alveoli and is available for gas exchange.

Exercise and Ventilation

Now that we have discussed several variables that are encompassed in pulmonary ventilation, let us now look at how these variables react to different types of exercise. For our purposes, we will look at these variables in terms of light exercise (exercise = 50 % of maximal effort), moderate exercise (exercise = 60 % to 75% of maximal effort), and incremental exercise (light exercise to maximal effort). For the purposes of this lesson, we are only interested in the trends of the variables. We will discuss the variables' reaction to exercise more in depth in lesson 3.

So, how do V_T, f, V_E and VdotA react to light exercise? For a graphical example, click here. During light exercise, ventilation within the first 2 minutes, reaches steady state. The control of these variables will be discussed later in Lesson 3. For our purposes at this time, we can describe V_T as reaching steady state at approximately 2 liters within the first two minutes, and remaining stable at 2 liters until exercise is stopped. Frequency in breathing increases to steady state within 4 minutes of light exercise where it will remain stable until the end of the exercise bout. V_E and V_dotA both increase to steady state as a function of V_T and f, so they both remain stable after 4 minutes of exercise.

When looking at a prolonged bout of moderate exercise, the ventilation variables act much in the same way as they do at light exercise, however for a couple of point. During prolonged moderate exercise, remember that the diaphragm and intercostal muscle are contracting during each inspiration. After about 40 minutes, the muscles begin to fatigue and the strength of the contractions is slightly decreased. The fatigue occurring is related to the slight decrease in tidal volume as seen on the graph. To keep V_E and V_dotA stable, the frequency of breathing increases to make up for the decreased tidal volume.

Finally we can look at the ventilation variables as portion of the percentage of work effort. This is accomplished by tracking the variables during incremental exercise from light to maximal effort. The incremental graph represents each of the variables at a given percent effort. During incremental work, you might assume that there would be a linear relation between V_E percent effort. Look back at the two previous graphs. During light exercise V_E reaches steady state. During prolonged submaximal effort, V_E is greater than it was at light exercise, but it also reaches a steady state. During incremental exercise, we see that V_E seems to have two points during the effort where the slope of the line changes, and steady state is never reached. We understand that steady state can't be achieved because the work effort is increasing with time, but what about these changes in the slope of the line that represents V_E. Each breakpoint or increase in magnitude of the slope of V_E represents a ventilatory threshold. The ventilatory threshold is not fully understood, but many describe the first ventilatory threshold as a representation of the onset of blood lactate (OBLA) that occurs at approximately 60% of maximal effort. OBLA represents an increase in blood lactate above the resting value. Since CO₂ buffers the blood lactate response, this seems to be an acceptable theory to explain the change in the slope of V_E. The second ventilatory threshold point occurs at approximately 75% of maximal effort, but its cause is unknown.

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