Regulation of Respiration
Introduction :-
Regulation of Respiration- Adults typically breathe at a pace of 14 to 18 breaths per minute, with a tidal volume of roughly 500 ml. The pulmonary ventilation per minute (PV/min) rate and depth are modified in accordance with the body’s needs. For example, metabolic processes and the need for oxygen by the working muscles both rise during muscular exercise. The muscles simultaneously produce a significant amount of carbon dioxide, which needs to be expelled. As a result, during muscular effort, breathing increases in both depth and rate. Actually, the amount of muscle exertion directly correlates with the rise in minute ventilation. Conversely, when the metabolic rate is low during sleep, breathing becomes slower.
Since the depth and rate of respiration can be precisely regulated to meet the body’s metabolic requirements, an effective system for regulation must exist.
Breathing fluctuates even under normal physiological conditions. For instance, breathing force and rate are increased by emotions and physical exercise. However, internal control mechanisms ensure that altered breathing patterns return to normal within a short period of time. Normally, calm, regular breathing occurs through two regulating mechanisms:
1. Nervous or neural mechanisms ( Regulation of Respiration)
2. Chemical mechanisms ( Regulation of Respiration)
1. Nervous or neural mechanisms :-
Regulation of Respiration- The neural regulation of breathing involves a complex network of neurons in the brain that control breathing rate and depth. This regulation ensures that the body’s oxygen demand is met and that carbon dioxide (CO₂) levels are kept within safe limits. Here we detail the neural mechanisms involved in regulating breathing.
Respiratory Centers in the Brainstem
Regulation of Respiration – The respiratory centers are groups of neurons that control the rate, rhythm, and force of breathing. These centers are located bilaterally in the reticular formation of the brain stem. Depending on their location within the brain stem, the respiratory centers are divided into two groups:
A. Medullary centers,
1. Dorsal respiratory neuron group
2. Ventral respiratory neuron group
B. Pontine center
3. Apneustic center
4. Pneumotaxic center.
A. Medullary centers
1. Dorsal respiratory neuron group
Location – The dorsal respiratory group is diffusely located in the nucleus tractus solitarii in the upper medulla. These neurons are generally referred to as the inspiratory center. All neurons in the dorsal respiratory group are inspiratory neurons, and generate the inspiratory ramp due to their autorhythmic properties.
Function: – Generation of inspiratory rhythm
– Integration of sensory information
– Regulation of ventilatory response
– Coordination with other respiratory groups
2. Ventral respiratory neuron group
Location: The ventral respiratory group is found in the nucleus ambiguus and nucleus posterior ambiguus. These two nuclei are located in the medulla oblongata, anterior and lateral to the nucleus tractus solitarius. The ventral group of neurons was formerly referred to as the expiratory center. Both expiratory and inspiratory neurons are found in the ventral respiratory group. The inspiratory neurons are located in the central region of the group. Expiratory neurons are located in the caudal and rostral regions of the group.
Function: Neurons of the ventral group are normally inactive during quiet breathing and become active during forced breathing. During forced breathing, these neurons stimulate both inspiratory and expiratory muscles, especially under conditions requiring increased respiratory effort B. Physical activity or shortness of breath. Its functions include:
– Control of forced breathing
– Activation of accessory respiratory muscles
– Coordination with other respiratory centers
– Role in reflex breathing
B. Pontine center
3. Apneustic center
Location: The apnea center is located in the lower part of the pons, which is part of the brainstem. The pons sits above the medulla oblongata and is involved in several important functions, including regulating breathing.
Function: The apnea center is responsible for regulating the depth and duration of inspiration, contributing to the overall pattern and rhythm of breathing. Its main functions are:
– Prolonging inhalation
– Regulating respiratory rhythm
– Responding to sensory input
4. Pneumotaxic center
Location – The aerotaxis center is located in the dorsolateral part of the superior pontine reticular formation. Neurons from the subbrachial parabrachial nucleus and the medial parabrachial nucleus combine to make it.
Function – The main function of the aerotaxis center is to control the medullary respiratory center, especially the dorsal group of neurons. It functions through the apnea center. The respiratory center inhibits the apneic center, which in turn inhibits the neurons of the dorsal group. This causes inhalation to stop and exhalation to begin. The pneumomotor center therefore influences the changes between inhalation and exhalation. The pneumomotor center increases the respiratory rate and shortens the duration of inspiration.
2. Chemical mechanisms :-
Regulation of Respiration – Chemical regulation of breathing involves sensing and responding to changes in the concentration of certain gases and pH in the blood and cerebrospinal fluid. This regulation is primarily mediated by chemoreceptors that sense the concentrations of carbon dioxide (CO₂), oxygen (O₂), and hydrogen ions (H⁺), which indicate pH. These chemoreceptors send signals to the respiratory center in the brainstem to adjust breathing rate and depth accordingly.
Chemoreceptors are divided into two groups:
1. Central chemoreceptors
2. Peripheral chemoreceptors.
1. Central chemoreceptors
Location – The central chemoreceptors are located deep in the medulla oblongata, near the dorsal respiratory neuron group. This area is called the chemosensitive area and the neurons are called chemoreceptors. Chemoreceptors are in close contact with the blood and cerebrospinal fluid.
Functions Mechanism :-
Central chemoreceptors are connected via synapses to the respiratory center, especially the dorsal respiratory neuron group. These chemoreceptors act slowly but effectively. Through chemical regulation systems, 70–80% of the increase in ventilation is attributed to central chemoreceptors. The main stimulant for central chemoreceptors is an increase in hydrogen ion concentration. However, even if the hydrogen ion concentration in the blood increases, hydrogen ions from the blood cannot cross the blood-brain barrier or the blood-cerebrospinal fluid barrier and therefore cannot stimulate the central chemoreceptors. On the other hand, increased carbon dioxide in the blood can easily cross the blood-brain barrier and the blood-cerebrospinal fluid barrier and enter the brain interstitial fluid and cerebrospinal fluid. There, water and carbon dioxide react to create carbonic acid, which is unstable and rapidly separates into bicarbonate and hydrogen ions. The hydrogen ions stimulate central chemoreceptors, which send excitatory impulses to the dorsal respiratory neuron group, resulting in increased ventilation (increased respiratory frequency and force). This flushes out excess carbon dioxide and allows breathing to return to normal. Oxygen deprivation generally has no significant effect on central chemoreceptors, other than impairing overall brain function.
2. Peripheral chemoreceptors.
Location: Peripheral chemoreceptors are found in the carotid body (where the common carotid artery branches off) and aortic body (aortic arch). These receptors are responsive to variations in pH, CO2 levels, and blood O2 (hypoxemia).
Functions Mechanism :-
Hypoxia stimulates peripheral chemoreceptors most potently. This is due to the presence of oxygen-sensitive potassium channels in the peripheral chemoreceptor glomus cells. Hypoxia causes the closure of oxygen-sensitive potassium channels, preventing potassium efflux. This causes the glomus cells (receptor potential) to depolarize, generating action potentials at their nerve endings. These impulses pass through the aortic and herring nerves and stimulate the dorsal group of neurons, which then send excitatory impulses to the respiratory muscles, resulting in increased ventilation.
This provides sufficient oxygen and relieves oxygen deficiency.
Hypoxia, hypercapnia, and increased hydrogen ion concentration all cause peripheral chemoreceptors to fire. However, peripheral chemoreceptors are less sensitive to hypercapnia and increased hydrogen ion concentration.