Human respiration


Ventilation or breathing is responsible for providing a continual supply of oxygen and the removal of carbon dioxide from the body. From the simple observation of a normal subject it is evident that inspiration and expiration should follow each other rhythmically, without either phase being unduly prolonged. This basic rhythm of ventilation is established within the respiratory centers of the brain. The basic ventilation rate and rhythm are not static, but vary as neuronal and hormonal mechanisms act to alter the frequency and amplitude of ventilation in response to the physiological demands of the organism.

A common misconception is that variation in the O2 levels within the system cause changes in the
ventilation rate. Actually, the O2 concentration, under normal conditions, has little to do with the
determination of respiratory rate. The critical determining factor is the level of CO2 and/or the level of free protons circulating in the blood. For example, an increase in CO2 or H+ levels will induce changes which result in an acceleration of the ventilation rate and volume until these levels return to the normal range. Conversely, conditions associated with alkalosis and lower than normal CO2 levels depress the ventilation rate.

The receptors that are sensitive to changes in the concentrations of CO2 and H+ are located within the arterial system and the medulla of the brain. Excitation of these receptors trigger neural reflexes which alter the respiratory rate and depth. Additionally, other parts of the nervous system influence the basic ventilation pattern established by the respiratory center. For example, the spinal cord has an overall facilitative effect on respiration. If the activity of the ascending sensory pathways within the spinal cord were increased, we would also expect to see an increase in respiration. If the spinal cord were injured or severed there would be a decrease in respiratory activity.

The funny thing about this lab is that it is hard for you to think about how you breathe without you changinging how you breathe. Take a look at the figure hyperlinked here. In and out normally is tidal breathing, volume about half a liter. There is an expiratory reserve, over a liter, and an inspiratory reserve, maybe 3 l. From the top of inspiration to the bottom of expiration is vital capacity. Even after you empty your lungs as much as possible there is a residual volume, over one liter.

We'll be using a simple spirometer, and a more advanced version is a recording spirometer. I'll never forget the demo I had of this in my first physio class. The prof took out the part that removed carbon dioxide and the volunteer got red in the face and kept breathing harder. At Mizzou, a similar effect was achieved by having volunteers breathe in and out of a bag. Carbon dioxide, reflected across the blood brain barrier from the blood stream, increases H+ (lowers pH) of cerebrospinal fluid (CSF). Chemoreceptors in the medulla detect this to stimulate breathing via medullary centers. Remember this. CO2 and CSF acidity are stimuli for breathing, and that is why oxygen given to patients has CO2 in it.

The principle value of these pulmonary measurements is to track changes in the volumes for use as a diagnostic tool. For example, the vital capacity decreases in left heart disease. This is due to the blood congestion in the lung capillaries which in turn leads to pulmonary edema and a decrease in VC. As the person recovers and the heart becomes stronger, the pulmonary congestion and edema decrease followed by an increase in vital capacity. The vital capacity also decreases in paralytic polio due to the partial paralysis of the respiratory muscles.

Use the spirometer

(1) take a clean cardboard dispo mouth piece
(2) test your vital capacity
(3) test your tidal volume (breath normally for a while and then, from the top to the bottom of a normal breath, out through the mouth piece)
(4) test your expiration reserve (breath normally for a while and then, at the bottom of a normal breath, force out the rest through the mouth piece)
(5) calculate inspiratory reserve (you do the arithmetic!)

Write these down for each person

Heymer Test of Respiratory Reserve.

In this test, the subject takes five deep breaths and then holds his/her breath as long as possible after the last inspiration. The duration of breath holding provides an indication of the subject's functional respiratory reserve and efficiency of the respiratory system. Normal values for an adult male is 50-70 seconds and for adult females is 50-60 seconds. This test should be repeated three times and the average value used for comparison. The Heymer test is often a better index of respiratory reserve than is the traditional VC measurement.

I found this in the Mizzou lab manual and looked it up in several books without success, but I found it at the course's respiration site for another bio instructor (Karen E. Petersen) at the University of Washington.

Try this!

A nomogram

I found this nomogram for vital capacity for females in the Mizzou lab manual (and there was another one for males). To use the nomogram, locate the height in inches (or centimeters) and the age in years. Place a straightedge between these two points: the intersects will give the predicted vital capacity (VC). Although this seemed pretty eseteric, it'sd on Dr. Petersen's site.

This page was last updated 3/11/11

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