The respiratory pump: How do we breathe?

When you are thinking about it, you can control your breathing on your own. Most of the time you are alive, however, you will have other things to think about, yet your breathing continues.

So how then do we breathe? (For further reading you can click here.)

The reason we continue to breathe whether we are thinking about it or not is because of the respiratory pump. It consists of the following:

• Respiratory Center located in the brain
• Peripheral and Chemo receptors
• Nerves connecting the respiratory center with the respiratory muscles
• Respiratory muscles

The respiratory pump basically works to perform the following functions. These functions are the reasons why we breathe:

1. Homeostasis: Keeping balance inside your body, or maintaining a normal acid base balance (pH or hydrogen ions).
2. Exchanging gas: Breathing in oxygen, and blowing out CO2.

According to Donald F. Egan's "Fundamentals of Respiratory Care", breathing is controlled by the Central Nervous System, and originates "in the brain stem, mainly from neurons located in the Medulla Oblongata (yet also in the pons). This gland controls breathing by messages it receives from Chemo receptors.

There are two sets of chemo receptors:

1. Central: They sit right on the Medulla
2. Peripheral: They are located in the "bifurcations" of both carotid arteries and the arch of the aorta, or somewhere between your shoulders and above your heart.

These chemo receptors send messages to the brain (the Medulla) based on changes in Carbon dioxide (CO2) the partial pressure of oxygen (PO2) in the blood.

Thus, there are two drives to breathe:

1. The hypoxic drive: It accounts for 10-15 percent of your drive to breathe and no longer functions when your PO2 is greater than 170. When your PO2 drops below 70 a message is sent to the respiratory center to speed up breathing. A normal PO2 is about 104. This function is mainly performed by the peripheral chemoreceptors.


2. The CO2 drive: A normal CO2 is 35-45. When CO2 increases a signal is sent to the brain to increase your respiratory rate to blow off CO2 to maintain homeostasis. If your CO2 decreases a message is sent to slow down respirations so CO2 can build up to normal levels. CO2 is the main drive to breathe. This function is performed by the central chemoreceptors.

The peripheral chemoreceptors (hypoxic drive) is less of a drive to breathe because it sends signals to the brain far slower than central chemoreceptors. Thus, changes in CO2 effect your breathing about 90 percent of the time.

Let me confuse you a minute. The real drive of breathing is actually hydrogen ions . As hydrogen ions increase, your breathing speeds up. But, since hydrogen ions are not allowed to cross the blood brain barrier so that the pH of the brain can be different from the pH of the body, it cannot directly be used to stimulate breathing.

Thus, CO2 is used. CO2 is allowed to cross the blood brain barrier. Excess levels of CO2 arrive in the brain and are received by the Central Chemo receptors. Thus, "elevations in CO2... cause rapid diffusion of the gas into the CSF (Cerebral Spinal Fluid), where it dissociates into hydrogen ions and lowers the CSF, thereby stimulating the central chemo receptors. The central chemo receptors, in turn, signal the medulary centers to increase ventilation."

So you can see, CO2 "indirectly" causes changes in respirations.

Once the respiratory center receives a message from either the chemoreceptors, the message is interpreted and a signal is sent through through peripheral nerves. The signal travels along one of three peripheral nerves:

1. Phrenic nerve: Starts at the top of the spinal cord (C3-C5) and insert in the diaphragm on either side of the heart
2. Intercostal nerves: Start in the spinal cord (T1-T12) and each one travels under a rib and supply messages from the brain to the intercostal muscles
3. Abdominal nerves: Start in both the thoracic and lumbar regions of the spinal cord and supply neural messages to the abdominal wall muscles

Before the message gets to the respiratory muscle it must pass through a synapse, or narrow gap between the neuron and the muscle. The synapse is called a neuromuscular nunction.

Neuromuscular junction: When an impulse is sent from the brain, it travels down a neuron to a synapse. The impulse signals the nerve ending to release acetylcholine, which is sensed by receptors on the muscle side of the synapse. This causes the muscle to contract, and a breath to take place.

To prevent the muscle from contracting too long, acetylcholinesterase destroys the acetycholine.

To learn about the muscles of respiration and how they help to create a breath click here.

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