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Lung Capacity and Stimulation of Breathing

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The normal volume moved in or out of the lungs during quiet breathing is called tidal volume.
When we are in a relaxed state, only a small amount of air is brought in and out, about 500 mL.
You can increase both the amount you inhale, and the amount you exhale, by breathing deeply. 


Breathing in very deeply is Inspiratory Reserve Volume and can increase lung volume to 3400 mL, which is quite a bit more than the tidal volume of 500 mL. We can also increase expiration by contracting our thoracic and abdominal muscles.
This is called expiratory reserve volume and is about 1200 ml of air. Vital capacity is the total of tidal, inspiratory reserve and expiratory reserve volumes; it is called vital capacity because it is vital for life, and the more air you can move, the better off you are. 



There are a number of illnesses that we will discuss later in the chapter that decrease vital capacity. Vital Capacity can vary a little depending on how much we can increase inspiration by expanding our chest and lungs. Some air that we breathe never even reaches the lungs! Instead it fills our nasal cavities, trachea, bronchi, and bronchioles. These passages aren't used in gas exchange so they are considered to be dead air space. 

To make sure that the inhaled air gets to the lungs, we need to breathe slowly and deeply. Even when we exhale deeply some air is still in the lungs,(about 1200 ml) and is called residual volume. This air isn't useful for gas exchange. There are certain types of diseases of the lung where residual volume builds up because the person cannot fully empty the lungs. This means that the vital capacity is also reduced because their lungs are filled with useless air.

Stimulation of Breathing

There are two pathways of motor neuron stimulation of the respiratory muscles. The first is the control of voluntary breathing by the cerebral cortex. The second is involuntary breathing controlled by the medulla oblongata.

There are chemoreceptors in the aorta, the carotid body of carotid arteries, and in the medulla oblongata of the brainstem that are sensitive to pH. As carbon dioxide levels increase there is a buildup of carbonic acid, which releases hydrogen ions and lowers pH. 

Thus, the chemoreceptors do not respond to changes in oxygen levels (which actually change much more slowly), but to pH, which is dependent upon plasma carbon dioxide levels. In other words, CO2 is the driving force for breathing. The receptors in the aorta and the carotid sinus initiate a reflex that immediately stimulates breathing rate and the receptors in the medulla stimulate a sustained increase in breathing until blood pH returns to normal.

This response can be experienced by running a 100 meter dash. During this exertion (or any other sustained exercise) your muscle cells must metabolize ATP at a much faster rate than usual, and thus will produce much higher quantities of CO2. The blood pH drops as CO2 levels increase, and you will involuntarily increase breathing rate very soon after beginning the sprint. 

You will continue to breathe heavily after the race, thus expelling more carbon dioxide, until pH has returned to normal. Metabolic acidosis therefore is acutely corrected by respiratory compensation (hyperventilation).

Lung Capacity and Stimulation of Breathing Lung Capacity and Stimulation of Breathing Reviewed by gafacom on June 04, 2019 Rating: 5

1 comment:

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