Breathing cycle and regulation

Definitions & Key takeaways

The respiratory cycle is the process of inhaling and exhaling air. The main purpose of the respiratory cycle is to bring fresh oxygen into the body and to expel carbon dioxide. Breathing is made possible thanks to a muscle called the diaphragm. When the diaphragm contracts, it pulls downward, which causes the chest cavity to expand. This expansion creates a vacuum that draws in air from outside the body, this is called inhalation. When the diaphragm relaxes, it pushes upward, which causes the chest cavity to contract and expels air from within the body, this is referred to as exhalation.

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Introduction0:00–0:26

Breathing, also known as ventilation, is how the air moves into and out of the lungs. It consists of repetitive cycles of inspiration, when air flows into the lungs; expiration, when air leaves the lungs; and a brief pause, called the rest period, between these two.
Now, the direction of airflow throughout the breathing cycle depends on the difference between the atmospheric pressure, which is the pressure of the air in the environment, and the alveolar pressure, or the pressure inside the alveoli, which are the tiny sacs of air where gas exchange happens in the lungs.

Pressures in the lungs0:26–1:22

An additional parameter is the intrapleural pressure, also called the intrathoracic pressure, which is the pressure of the fluid inside the pleural cavity that surrounds the lungs.
Intrapleural pressure is usually negative compared to the alveolar or atmospheric pressure, and this is important because the alveolar pressure minus the intrapleural pressure gives the transmural pressure.
As long as the transmural pressure stays positive, the airways remain open throughout all of the phases of the breathing cycle.
Ok, now, normal, quiet breathing involves inspiration and expiration of a tidal volume, or VT for short, of about 500 mL, which includes the volume of air that fills the alveoli plus the volume of air that fills the airways.

Tidal volume1:22–1:59

Now, according to what is known as Boyle’s law, at a constant temperature, pressure and volume are inversely related to each other, so when the alveolar pressure decreases, more air will enter the lungs, increasing the air volumeWith that in mind, let’s establish the starting point for these variables by looking at the lungs during the rest phase of the breathing cycle.

Breathing cycle1:59–3:03

During rest, the diaphragm is at its balanced position. The alveolar pressure equals the atmospheric pressure to a value of zero centimeters H2O, so there is no pressure gradient, and no air is moving into or out of the lungs.
The intrapleural pressure is negative, approximately -5cm H2O because the lungs and the chest wall act as opposing forces, meaning the lungs have a tendency to collapse during rest, while the chest wall has a tendency to expand.
Because alveolar pressure, which is 0 cm H2O, minus the intrapleural pressure, which is -5 cm H2O, equals a transmural pressure of +5 cm H2O, that means that the airways are open during rest.
Now, inspiration, and a new breathing cycle, start when there’s a variation in the arterial pressure of oxygen, or PaO2, which is normally around 100 mmHg; the arterial pressure of carbon dioxide, or PaCO2, normally around 40 mmHg; and the arterial pH, which is normally 7.4.

Inspiration3:03–7:48

Changes related to these markers activate a series of receptors, called chemoreceptors, which are specialized sensory cells that convert the concentration of a chemical substance in the blood, such as carbon dioxide or oxygen, to a biological signal for the respiratory center, located in the brainstem.
The respiratory center consists of three major respiratory groups of neurons. The dorsal respiratory group and the ventral respiratory group are found in the medulla oblongata, while the pontine respiratory group is found in the pons and consists of two areas, known as the pneumotaxic center and the apneustic center.Of these, the dorsal respiratory group, or DRG, is the one that initiates respiration, and it also determines the basic rhythm of breathing by adjusting the frequency of inspiration so as to keep PaO2, PaCO2, and the arterial PH in the normal range.
When the DRG receives information regarding the increase of PaCO2, the DRG sends a command through the phrenic nerve to the diaphragm, which contracts to increase the vertical length of the thoracic cavity, and through the intercostal nerves to the external intercostal muscles, which contract and make the ribs move up and out, increasing the lateral size of the thoracic cavity.
Based on Boyle’s law, as lung volume increases, the alveolar pressure decreases, specifically to -1cm H2O. This is lower than the atmospheric pressure, so now there is a pressure gradient, that makes air rush into the lungs like a vacuum.
Eventually, all this air brings the alveolar pressure back up, and once it equals the atmospheric pressure, airflow into the lung stops.
At the same time, as lung volume increases, this compresses the intrapleural cavity, but the intrapleural pressure always remains lower than the alveolar pressure, reaching the value of -8cm H20 at the end of a normal inspiration.
That means that the transmural pressure remains positive at a value of 8cm H2O, and the airways stay open. Thus, the volume present in the lungs at the end of normal inspiration is the sum of the one tidal volume, and the volume of air left in the lungs at the end of the previous expiration - called the functional residual capacity, or FRC.Now, when oxygen requirements are higher, like during exercise, the apneustic center from the pons is activated.
Stimulation of the neurons in the apneustic center excites the dorsal respiratory group in the medulla, prolonging the period of action potentials in the phrenic nerve, and thereby prolonging the contraction of the diaphragm, leading to prolonged inspiration.
Additionally, the external intercostal muscles found between the ribs and accessory muscles, such as the sternocleidomastoid and scalene muscles in the neck, also contract during vigorous inspiration, making the ribs move up and out.
Finally, let’s take this scenario to the extreme: Say you’re exercising really hard, and your apneustic center was so hyperstimulated that it was still trying to make you inhale when your lungs were already fully inflated.
To prevent that, the pneumotaxic center activates, and it limits the burst of action potentials in the phrenic nerve, making the diaphragm contract less.
This stops inspiration, allowing for expiration to happen, so we can keep ventilating the lungs.Speaking of which: Under normal circumstances, once the air movement stops and the inspiration ends, expiration begins.
Normally, expiration is a passive process, meaning that there is no need for any group of neurons to stimulate muscle contraction.
The diaphragm relaxes and the air leaves the lungs due to their elasticity, which allows them to rebound after the inspiration like a filled balloon that lets the air out if you don’t seal it after inflating.

Expiration7:48–9:16

So, essentially, the lungs squeeze back to their initial size, and put pressure on the volume of air inside the alveoli.
This makes the alveolar pressure increase to +1 cm H2O, which is higher than the atmospheric pressure, creating yet another pressure gradient.
But this time, the pressure gradient pushes the air out of the lungs, but about 500 milliliters of air, representing the FRC, remain inside the lungs to prevent the alveoli from shutting closed.
Now, the intrapleural pressure becomes less negative during expiration compared to inspiration, going back to a value of - 5cm H20.
Basically, at the end of this phase, all volumes and pressures return to their values at rest and a new breathing cycle begins.A particular situation is forced expiration, which is when a person forcibly breathes out, like a vocalist is trying to hold a musical note.
And this is when the ventral respiratory group, or VRG, is activated. The VRG sends inhibitory impulses to the apneustic center, which has the role of increasing the duration of inspiration.

Forced expiration9:16–9:54

So, when VRG inhibits the apneustic center, inspiration will be shorter, leaving more time for a longer expiration.Finally, all phases of the breathing cycle can be voluntarily controlled through commands from the cerebral cortex.
For instance, a person can choose to hyperventilate, which means that the breathing frequency and volume will be increased, or hold their breath and, thus, hypoventilate.Okay, quick recap: Breathing involves inspiration, expiration, and a resting period between the two.
Inspiration is the active phase when air enters the body, while expiration is the passive phase when air leaves the lungs.In order for this cycle to happen, there is a respiratory center, which controls breathing.

Cerebral cortex regulation9:54–10:15

This center receives information from a group of receptors, known as chemoreceptors, which detect any changes regarding the pressures of the gases involved in breathing.

Review10:15–10:52

is the active phase. When air enters, the body will expiration is the passive phase when are leaves the lungs in order for this cycle to happen, there is a respiratory Center, which controls breathing this Center receives information, from a group of receptors known as chemoreceptors, which detect any changes regarding the pressures of the gases