Ventilation

• Ventilation is controlled by the respiratory center in the medulla oblongata.
• The respiratory center is divided into two centers: the inspiratory center, which controls inspiration, and the expiratory center, which controls expiration.
• Inspiration involves the respiratory center sending spontaneous rhythmic impulses to the diaphragm and external intercostal muscles, causing them to contract, increasing the volume of the ribcage and reducing the pressure so air enters from the atmosphere into the lungs.
• Expiration involves the entry of air into the lungs stimulating stretch receptors in the walls of Bronchioles.
• Stretch receptors send inhibitory impulses to the inspiratory center causing relaxation of the diaphragm and external intercostal muscles, and excitatory impulses to the expiratory center causing contraction of the internal intercostal muscles.
• Both impulses cause reduction of lung volume and increase its pressure so air is forced out.
• The ventilation process is an involuntary process that continues during sleep.
• A simple feedback system controls the basic rate of breathing.
• The breathing muscles are the muscles between the ribs (intercostal muscles) and the diaphragm.
• The respiratory center receives impulses from chemoreceptors located in the aortic bodies in the aortic arch, carotid bodies in the carotid arteries, and the medulla oblongata itself.
• Chemoreceptors detect increased CO2 levels, increased H+ ion concentration (resulting from increased dissolved CO2) and decreased O2 concentration.
• Increased CO2 level has a greater effect than decreased O2 concentration.
• When the level of CO2 increases, these chemoreceptors respond by sending excitatory impulses to the inspiratory center which sends impulses to the diaphragm and external intercostal muscles, increasing the rate and depth of breathing leading to more removal of CO2 and more intake of O2.
• When the CO2 and O2 levels are restored to normal, the chemoreceptors stop firing, and the normal rate and depth of breathing are regained (negative feedback).
• Stretch receptors are found in the walls of bronchi and bronchioles, and in muscle fibres.
• As the percentage of CO2 in inspired air increases, both the rate and depth of breathing increase.
• The components of lung volume are measured by the Spirometer, a device in which a person breathes in and out in a closed system so air is trapped in a container.
• The movements of the container are recorded in a chart known as a spirogram or spirometer trace.
• Tidal volume is the volume of air that enters and leaves the lung at each resting natural breath (VT).
• Inspiratory reserve volume (IRV) is the volume of air that can be taken in over and above the normal inspired tidal volume when a person breathes as deep as possible.
• Expiratory reserve volume (ERV) is the volume of air that can be expelled over and above the normal expired tidal volume when a person vigorously forces the air out of his lungs.
• Vital capacity (VC) is the volume of air which can be breathed out by the most vigorous force after being breathed in by the deepest inspiration.
• Residual volume (RV) is the volume of air left in the lungs after the strongest possible expiration.
• Total lung capacity (TLC) is the sum of vital capacity and residual volume, about 6 dm3.
• Inspiratory capacity (IC) is the volume that can be inspired after the end of a normal expiration.
• Ventilation rate is the volume of air breathed in a minute (VE).
• The trace from a spirometer can be used to measure tidal volume and breathing rate.
• The spirometer must first be calibrated for volume and time.
• The subject is kept at rest and asked to breathe in and out through the mouthpiece of the spirometer.
• Tidal volume can be calculated from the trace by recording the distance between a peak and trough.
• Breathing rate is the number of peaks per minute, one peak to peak represents one breath.
• Breathing rate can be calculated by counting number of peaks and measuring distance on the trace and convert it to time.
• The experiment for measuring tidal volume and breathing rate should be repeated for the same individual three more times and the mean calculated to improve reliability.