In humans and other mammals, pulmonary ventilation depends on the intercostal muscles (located between the ribs) and the diaphragm.
The air entering the lungs, the inspiration, if the contraction of the muscles of the diaphragm and intercostal muscles. The diaphragm lowers and the ribs rise, which increases the volume of the rib cage, forcing air into the lungs.
The airflow from the lungs, the expiration, occurs by relaxing the muscles of the diaphragm and intercostal muscles. The diaphragm rises and the ribs lower, which decreases the volume of the rib cage, forcing air out of the lungs.
With each breathing movement, a young man inhales and exhales, on average, about half a liter of air; This figure is slightly lower for the average woman.
The maximum volume of air that can be inhaled and exhaled in a forced breath is called vital capacity, around 4 to 5 L, for a young man. The lungs, however, contain more air than their vital capacity, as it is impossible to exhale all the air contained in the alveoli. Even when exhalation is forced to the maximum, about 1.5 L of air remains in the lungs; This is the residual air.
More than 10,000 liters of air in and out of our lungs every 24 hours. During this period the lungs absorb between 450 and 500 L of oxygen gas and expel between 400 and 450 L of carbon dioxide.
What would happen to a person if they tried to hold their breath voluntarily for a while?
Immediately, a command located in the bulb - or oblong medulla (an organ component of our central nervous system) would send the message to the respiratory muscles, causing them to contract. This command center, known as bulbar respiratory center, is highly sensitive to CO increase2 in blood and the decrease in blood pH due to the accumulation of this gas.
Remember that CO2 in aqueous solution forms H2CO3, carbonic acid, which ionizes in H+ and H2CO3-. Increased acidity and CO itself2 in physical solution in plasma stimulate respiratory center neurons.
Consequently, nerve impulses follow the nerve that innervates the diaphragm and intercostal musculature, promoting contraction and involuntary breathing movements.
At first, hyperventilation occurs, ie the rate of breathing movement increases in an attempt to expel excess carbon dioxide. Slowly, however, the situation normalizes and breathing returns to normal levels.