During exercise, the increase in ventilation which occurs to meet the increasing oxygen demands (called “hyperpnea”) is not fully explained by the control of the peripheral or central chemoreceptors alone. There are non-chemical controls of ventilation that are required to provide input to the respiratory centre to increase ventilation, especially during the initiation of exercise when ventilation needs to increase quickly.
These “non-chemical controls” of ventilation include:
- the motor cortex (cortical control): feed-forward mechanism to increase ventilation at the onset of exercise
- active muscles and joint receptors: active muscles and joints provide feedback to the respiratory centre to increase ventilation (muscle metaboreflex) in order to meet the higher oxygen demands and to remove carbon dioxide
- core body temperature: higher body temperature stimulates increased ventilation
- stretch receptors in the lungs tissue and bronchioles: when these receptors are stretched, they send a signal to the medulla to stop inhalation and start exhalation. This ensures that the lungs will never exceed their maximal physical capacity.
A) At Rest
The normal respiratory cycle of a healthy individual at rest is constant and predictable. The rate and depth of breathing is considered “automatic” with no conscious input required from the individual. This results in a predictable number of breaths per minute with a similar amount of time between breaths.
Minute Ventilation is determined by the following equation:
Where VT is the tidal volume per breath and fB is the frequency of breaths per minute. Generally a healthy individual should have a minute volume of 6 L/min at rest. This number, of course, depends on size, age, and health status on an individual.
Figure 6 illustrates resting tidal volume, divided into the volumes associated with alveolar ventilation (VA), which participates in gas exchange and deadspace ventilation (VD), which does not participate in gas exchange.
Figure 6.Tidal Volumes of a resting healthy individual. The portion of tidal volume depicted in mint green represents the amount of space in the lungs that can actively inhale air and participate in gas exchange. Grey represents physiological dead space, which is the amount of alveolar tissue capable of participating in gas exchange but unable to because of some physical factor (e.g. lack of blood flow to a region of the lung). Purple shows the amount of anatomical deadspace: this is the portion of the airway that conducts air to the alveoli but cannot participate in gas exchange due to its specific anatomy (e.g. trachea, main bronchi). In a healthy individual the physiologic dead space (grey) is minimized and the alveolar airspace (mint) is large compared to the anatomical deadspace (purple).
B) With acute exercise
In any sort of physical exertion, light or strenuous, the body must compensate for the increased oxygen demand. To get more oxygen into the body during exercise, various sensors within the body will tell the central controller in the brain to increase minute ventilation, this means taking more breaths per minute as well as larger volumes of air per breath. Minute ventilation can increase to over 100 L/min with heavy exercise! This concept is illustrated by Figure 7.
C) Change in Lung Volumes with Acute Exercise
As previously described the minute ventilation of an individual will increase with an increased demand for oxygen during exercise, due to an increase in tidal volume and breathing frequency. The increase in tidal volume comes at the expense of certain volumes within the lungs, such as expiratory and inspiratory reserve volumes. These changes are illustrated in Figure 8.
With exercise, there is a need to increase tidal volume to get more air to participate in alveolar ventilation and increase oxygenation of the blood. “Space” is needed in the lungs to accommodate this extra volume of inhaled air. This extra space comes from out of the inspiratory reserve volume (IRV). The IRV acts as a reservoir of extra lung volume that can become inflated with air as we need it, such as when there is a demand for more oxygen inhalation and CO2 exhalation during exercise. Therefore with exercise, there is a decrease in IRV as tidal volume increases and takes over this “space” in the lung. Eventually the IRV can be used up, coinciding with physical size/stretch of the lungs, however this limit is rarely approached by typical, healthy individuals during exercise.
Essentially we also sacrifice some of our expiratory reserve volume (ERV) in order to accommodate more tidal volume. The ERV decreases during exercise, since there is need to expire more air from the lungs that we typically do at rest. This is because we need a larger volume of lung to be involved in gas exchange with new air coming in with each breath. We also need to expire the increasing amount of CO2 (and heat) that is being produced by the working muscles. The decrease in ERV can be seen in Figure 8.
There is a limit however as to how much we can breathe out. There is a small amount of air that needs to remain in the lungs to keep them inflated, called the residual lung volume. Residual volume does not change very much (if any) during exercise even with increasing tidal volume. At rest we usually don’t need to use our expiratory reserve volume. The ERV and the residual volume together make up the functional residual capacity (FRC); that is the amount of air contained in our lungs after a normal exhalation.
(Summary: The process of respiration during exercise can be thought of as a system designed to get oxygen to tissue and expel CO2 from the body as efficiently as possible. There are several mechanisms involved in this control, involving both the respiratory and circulatory systems.)