Principles:
Pulmonary Volumes
The air which may be in the lungs at any one time is easily divisible into 4 distinct volumes, three of which can be measured with a spirometer. The distinct air volumes are listed and defined as follows:
1. Tidal Volume (TV) - the quantity of air moved in and out in a single breath.
2. Inspiratory Reserve Volume (IRV) - the extra quantity of air which can be inspired by maximal forced inspiration.
3. Expiratory Reserve Volume (ERV) - the extra quantity of air which can be expired by maximal forced expiration.
4. Residual Volume (RV) - the quantity of air remaining in the lungs after one has forced out all the air he can get out. This air is removable only if the lung is collapsed and thus cannot be easily determined.
It is impossible to measure residual volume with a spirometer or air flow transducer. However, a number of measurements of this volume have been made using special techniques. The residual volume for the average male is about 1200 ml (1000 for the female).
In addition to the above listed volumes there are several lung capacities which are combinations of 2 or more of the volumes. These are:
1. Vital Capacity (VC) - the maximal tidal volume (the amount of air that one can move in and out in a single breath).
2. Total Capacity (TC) - the maximal volume of air that the lungs can hold (at the end of a maximal inspiration). It is determined as the sum of vital capacity and reserve volume TC = VC + RV.
Regulation of Pulmonary Ventilation
All inspired air in the nasal cavity, the pharynx, the larynx,
the trachea, and the bronchi is referred to as "dead space air" since it
does not come into contact with alveolar membranes where gas diffusion
can occur. In addition, any part of the alveolar membrane which because
of disease, etc., does not participate in gas exchange will add to the
effective amount of dead space. Certain standard measurements are
made to determine the efficiency of a person's respiratory system.
Maximum Minute-Volume (MMV is one of these and is a measure of the maximum
volume of air that can be moved in and out of the lungs each minute. Unless
strenuous exercise is undertaken, this maximum effort will require considerable
voluntary effort. By comparing the MMV with the normal amount of air one
breathes per minute at rest, an indication of respiratory reserve is derived.
The quantity of air breathed per minute is known as the Minute-Volume.
The minute-volume, however, is not a clear indication of the actual ventilation
of the lungs, as a certain part of the air taken in and out with each breath
is dead space air. The air actually brought into contact with the alveolar
membranes is known as the Alveolar Ventilatory Air and is equal to the
minute-volume less the dead space air volume multiplied by the breathing
rate. It is alveolar ventilation that is being controlled by the neuronal
centers concerned with respiration.
Changing the amount of dead space (experimentally) will alter the depth and rate at which one must breathe in order to secure the necessary alveolar ventilation. For example, increasing dead space will increase the tidal volume and/or the breathing rate accordingly. This is due to the fact that normal ventilation of the alveoli is required in order to keep the concentrations of oxygen and carbon dioxide constant in the blood stream. Elevated carbon dioxide levels stimulate the respiratory center which in turn increases the mechanical aspects of breathing. The breathing rate will also be altered upward. Similar effects are apparent when one breathes into a closed chamber where no renewal of atmospheric gases is possible. The continual build-up of carbon dioxide and the simultansous reduction in oxygen in the closed chamber in this case are the factors preventing normal alveolar ventilation.
Some indications of the operation of the respiratory center in
controlling alveolar ventilation are brought out in experiments involving
breath holding and hyperventilation. In breath holding, alveolar ventilation
(breathing) is strongly stimulated due indirectly to the accumulation of
CO2 in the blood. In addition, the lowered oxygen concentration in the
blood eventually adds to this stimulus. The respiratory center is especially
sensitive to carbon dioxide concentrations. On the other hand, hyperventilation
causes excessive "blow-off" of carbon dioxide, greatly reducing the internal
stimulus to respiration. In hyperventilation the subject normally experiences
considerable difficulty in maintaining the excessive alveolar ventilation;
as a matter of fact, the desire to breathe at all is greatly diminished
if not absent. The respiratory center is thus responding to the lowered
concentrations of carbon dioxide in the blood and hydrogen ion in the cerebrospinal
fluid.
In the experiments to follow, you will measure by spirometry
these volumes and determine the capacities. In studying group data
you will note the considerable variation in these volumes from one person
to another.
Procedures:
Instrumentation.
The Biopac Data Acquisition System will used in this exercise.
Follow the instructions in Exercise 8 and 12. As in other Biopac
exercises, the software's dialogue box will "cue" the student as to what
to do.
Students will work in pairs for all measurements. One student
will be the "subject" while the other is the "operator." While the subject
is being prepared for consumption measurements (see below under procedures),
the operator will check the Biopac and make final preparations of the instrument.
(NOTE: During respiration measurements every possible effort should be made to avoid disturbing the subject.)
Exercise 8:
Attach the thermister sensor just below the subjects
nostril and the respiration transducer (a strain gauge) around the chest
of the subject as instructed in the BIOPAC manual. After calibration,
examine normal breathing for 2-3 minutes and examine the effects of hyperventilation
and hypoventilation as instructed by the software. Remove the temperature
transducer and respiration transducer and procede to Exercise 12.
Exercise 12: Pulmonary Volume Measurements.
Attach a filter to the Air flow transducer
and perform the calibration using the "giant syringe" according to instructions.
BE SURE TO HOLD THE SYRINGE AND NOT THE AIR FLOW TRANSDUCER WHILE PUMPING
THE SYRINGE. After the calibration, follow the instructions to produce
a "spirogram". Repeat this operation several times until it is felt
that a
maximum effort has been achieved. The maximum movement of air in a
single breath is a measure of Vital Capacity. There is no need to rush
this forced inhalation and exhalation. From the Spirogram, so produced
by these breathing exercises, one may either read directly or calculate
all the pulmonary volumes and capacities needed.
Determine the following by analyzing the trace:
1. Average, resting Tidal Volume (measure 3 or 4 excursions
and average).
2. Average Breathing Rate per minute at rest (count over several
minutes and determine the average).
3. Expiratory Reserve Volume
4. Vital Capacity (take the largest of all attempts).
Calculate the following from the determinations:
5. Inspiratory Reserve Volume.
6. Minute Volume: The product of tidal volume and breathing rate.
Added Dead Space.
Prepare the BIOPAC again for measurements (Lesson 12). Record the breathing of the subject for a period of 2 to 3 minutes or until a smooth trace is attained. Proceed to insert a piece of additional tubing between the air flow transducer and the mouthpiece. This preparation will force the subject to breathe through an additional amount of dead space and make a 2-3 minute recording of the subject's responses to the added dead space.
After the recording is finished study the subject's breathing pattern, noting how the subject compensated for the added dead space. The method of compensation may change during the 2-3 minutes. Compare this breathing record with that made without the added dead space. With added dead space, was the extra minute volume attained by an increased tidal volume, an increased breathing rate, or a combination of both? Which do you think would be more efficient, increasing breathing rate or increasing tidal volume (when a minute-volume increase is desired)? After hard exercise, which returns to normal first, tidal volume or respiratory rate?
OPTIONAL
Experiments with the Respiratory Center. Breathe as deeply as possible for 2 minutes with mouth open; stress the depth of breathing, allowing the rate of breathing to be about normal. As you breathe deeply you will notice the following: (1) Deep breathing progressively becomes more difficult; much willpower is required to force this action to completion; (2) a feeling of dizziness develops, this is the direct result of lowering of the hydrogen ion concentration of the blood; (3) immediately following the period of forced breathing no desire to breathe will be experienced. This condition is known as apnea and results from depression of the respiratory center.
Repeat the study with depth breathing, except this time breathe into a plastic bag. Notice that there is no difficulty in maintaining the deep breathing; as a matter of fact, apnea does not develop. If this experiment were to be continued for a long time, what would be the eventual results?
Speck's Experiment. This experiment involves a measurement
of breath holding capacity. Measurements are made under different conditions:
(1) after normal expiration; (2) after normal inspiration; (3) after very
deep expiration; (4) after very deep inspiration; and (5) after several
deep inspirations. Record the results in terms of the time the breath
can be held.