What is Respiration?

o Respiration is exchange of gases (oxygen and carbon dioxide) and process to reach the oxygen to all parts of body and eliminate carbon dioxide as byproduct from the body.

o It is required for the metabolic processes of the body and maintain the homeostasis of the body.

o The complete gas exchange is controlled the system of respiratory control – control centers, effector organs, sensors.

Regulation of Respiration

o Control center is located at the medulla oblongata including many other brain stems. Medulla oblongata is an important location as respiratory rhythm originates. These centers are in charge of the automatic breathing control. Neurons from the cerebral cortex help in control system in instances like breath hold and intentional hyperventilation.

o Chemo receptors and sensory receptors are responsible for the ventilation. Chemo receptor – includes central and peripheral receptors. Central receptors lie in the central nervous system and the peripheral are located at carotid artery bifurcation and aortic arch. Sensory receptor – are sensitive to the local environment but is predominantly mechanosensitive found in the lower and upper airways and in the intercoastal muscles, accessory muscle of the chest walls and the diaphragm.

o Effector organs (respiratory muscles) that is responsible for the air and out of the lungs helping in the gas exchange. Diaphragm, intercoastal muscles are the important respiratory muscle whereas the scalene muscles also act as the necessary muscle with the abdominal muscles. The other accessory muscles are the sternomastoid trapezius muscles.

Respiration Control Centers

I. Medulla

Known to the primary respiratory control center and functions the muscles to control the respiration for breathing to occur. Two parts of medulla that control respiration are – ventral and dorsal respiratory group. The ventral respiratory group is responsible for the expiration and dorsal respiratory group help in inspiration. The non-respiratory movements like cough and sneeze are also controlled by the medulla along with other reflexes like vomiting and swallowing.

I. Pons

o Located under the medulla and function to control the rate of involuntary respiration. The apneustic center signals for the long deep breaths during inspiration, which is it controls the intensity of the breathing and stretch receptors of the pulmonary muscles that inhibit in the depth inspiration.

o Pneumotaxic center controls the respiratory rate by inhibiting the inspiration. This is done by the limiting the phrenic nerve and signaling the apneustic center and decreases the tidal volume. The apneustic and pnuemotaxic center control the respiratory rate by working against each other.

o Nerves that are used for the respiration are as follows that are used in the muscular functions – phrenic nerves, vagus nerve and posterior thoracic nerve.

i. Phrenic nerve – helps in the stimulation of diaphragm activity. It consists the left and right phrenic nerve passing to the left and right side of heart respectively and is a autonomic nerve.

ii. Vagus nerve – it also innervates to the diaphragm but with the movements of pharynx and larynx. It also helps in the parasympathetic stimulations of digestive system and heart.

iii. Posterior thoracic nerves – stimulates the intercoastal muscles found in the pleural region. They are somatic nerves and are the largest nerve groups that help stimulating the thorax and abdomen.

Respiration Control Receptors

Chemoreceptors

1. Chemoreceptors – sensory receptor that signals to the action potential. This action potential is sent to the brain and feedback is provided which are taken into action by the centers. These work by the sensing of pH of their environment by the H⁺ ions. The carbon dioxide levels in the blood stream are also way to measure the pH as carbon dioxide is converted to the carbon acid and bicarbonate.

o Central chemoreceptors – helps in the changes in pH of spinal fluid and also can be anesthetized from chronic hypoxia to increase carbon dioxide.

o Peripheral chemoreceptors – detects the oxygen and carbon dioxide in the blood and cannot be desensitized and do not have much impact to the respiratory rate as central chemoreceptors.

Chemoreceptor Negative Feedback

Three components involved are – sensor (chemoreceptors for blood pH), integrating sensor (medulla and pons) and effector (respiratory muscles).

Case: hyperventilation from an anxiety attack

i. Increased ventilation rate results in loss of carbon dioxide from body. Reduction of carbon dioxide in the body, pH is increased due to the increase of H+ ions causing alkalosis.

ii. Chemoreceptors detect the change in the process and signals the medulla that signals the respiratory muscles to reduce the ventilation rate for the carbon dioxide and pH levels to restore.

Case: diarrhea

Severe diarrhea causes the loss of bicarbonate in the intestinal tract and loss of bicarbonate observed in the plasma. Hydrogen ions remain same but the bicarbonate ions decrease leads to the acidity of the pH

Chemoreceptor’s feedback helps in the helps with the oxygen level to prevent hypoxia by the help of peripheral chemoreceptors. This feedback increases the ventilation to increase oxygen intake.

Proprio-Receptor Regulation of Breathing

1. Heiring – breuer reflex – known as the inflation reflex – helps to prevent the over inflation of the lungs. Presence of stretch receptors in the lungs are the mechanoreceptors acts as sensory receptors, detecting mechanical pressure with the stretch and distortion. It is widely found in the lungs, skin and stomach. When the lungs are at the maximum stretch during the inflation, the stretch receptors send action potential to the center through vagus nerve.

Pneumotaxic nerve acts to the signal that inhibit the apneustic nerve to function. Then the signals are sent to the diaphragm and accessory muscles to stop the inspiration. Then this is followed by the exhalation and these proprio-receptors helps in the exhalation and when to stop the process and begin with inhalation.

2. Sinus Arrhythmia – during the Heiring – breuer reflex the vagus nerve help in the signal to the neural pathway, there is also a signal to cardiovascular pathway present by the vagus nerve as it also innervates to the heart.

During stretch, the signal is also sent to the sinus atrial node of the heart via vagus nerve. This results in the short-term increase of the resting heart rate known as tachycardia. And it normalizes when the exhalation takes place. Sinus arrythmia is said when this process is cyclic and is considered as normal physiological phenomenon, when it occurs short-term tachycardia during inspiration.

You may like to read;

Human Respiratory System: Mechanism, Diagram, and Function

Respiratory Disorders: Definition, Types, and Treatment

Human Respiration: Transport of Gases, Mechanism, and Examples

Exchange of Gases in Respiration: Definition and Mechanism

Krebs Cycle: Definition, Diagram, Steps, and Mechanism

Regulation of Respiration Citations

• Control of the pulmonary circulation in the fetus and during the transitional period to air breathing. Eur J Obstet Gynecol Reprod Biol . 1999 Jun;84(2):127-32.
• Building and Regenerating the Lung Cell by Cell. Physiol Rev . 2019 Jan 1;99(1):513-554.