Physiological control of respiration – Components, Methods

Respiration is a vital physiological process that encompasses the cyclic movement of air into and out of the lungs. This mechanism is crucial for facilitating the exchange of gases, specifically oxygen and carbon dioxide, between the external environment and the bloodstream. The efficiency of respiration directly influences the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2) within the blood, maintaining homeostasis in the body’s internal environment.

The initiation of spontaneous respiration arises from the rhythmic activity of motor neurons that innervate the respiratory muscles. This rhythmic discharge of nerve impulses, primarily originating from the brainstem, plays a fundamental role in coordinating the contraction and relaxation of the diaphragm and intercostal muscles. During inspiration, these muscles contract, expanding the thoracic cavity and allowing air to flow into the lungs. Conversely, during expiration, the muscles relax, resulting in the expulsion of air from the lungs.

Chemical regulation of respiration is predominantly influenced by changes in arterial levels of PaO2, PaCO2, and hydrogen ion concentration (H+). Chemoreceptors located in the central nervous system and peripheral arteries detect these alterations and signal the respiratory centers in the brain to adjust the rate and depth of breathing accordingly. For instance, an increase in PaCO2 or a decrease in PaO2 triggers an increase in respiratory rate to enhance gas exchange efficiency.

In addition to chemical control mechanisms, various non-chemical influences modulate respiration. These include neural input from higher brain centers, which can alter breathing patterns in response to emotional states, physical activity, or voluntary control. Furthermore, sensory feedback from lung stretch receptors and irritant receptors also plays a significant role in regulating respiratory rhythm, ensuring the system adapts to both internal and external demands.

Key Components of Respiratory Control

The key components of respiratory control are essential for maintaining effective breathing patterns and ensuring the body’s gas exchange needs are met. This system consists of specialized centers and receptors that monitor and respond to various physiological factors, allowing for adjustments in ventilation to maintain homeostasis.

• Respiratory Centers:
  • The respiratory centers in the brain, particularly the inspiratory center located in the medulla, are crucial for regulating breathing rhythm.
  • Inspiratory Center: This center automatically initiates inspiration by sending nerve impulses to the diaphragm and external intercostal muscles, leading to their contraction and the initiation of inhalation.
  • Expiratory Center: This center plays a role in regulating expiration, ensuring smooth transitions between inhalation and exhalation.
• Chemoreceptors:
  • Central Chemoreceptors: Found in the brain, these receptors monitor changes in arterial carbon dioxide (PCO2), pH, and oxygen (PO2) levels. They are particularly responsive to pH changes due to variations in carbon dioxide levels, which is the most critical factor influencing ventilation.
  • Peripheral Chemoreceptors: Located in the carotid and aortic bodies, these receptors also monitor blood gas levels and provide sensory input to the respiratory centers, contributing to the overall regulation of breathing.
• Factors Influencing Breathing:
  • Chemical Factors: An increase in carbon dioxide levels and a decrease in pH trigger the respiratory centers to enhance both the rate and depth of breathing, ensuring adequate gas exchange.
  • Neural Input: Various neural inputs, including signals from the motor cortex and proprioceptors in muscles and joints, influence respiratory patterns, especially during physical activity. This neural feedback helps modulate ventilation according to the body’s needs.
  • Other Stimuli: Factors such as elevated body temperature, circulating hormones (like epinephrine), and metabolic byproducts (such as lactic acid) can also affect respiration, prompting the respiratory centers to adjust breathing accordingly.
• Homeostasis:
  • The primary function of the respiratory system is to maintain homeostasis by regulating ventilation to restore normal levels of PCO2, PO2, and pH in the blood. This regulatory process involves a feedback loop: chemoreceptors detect changes in these parameters, the respiratory centers process the information, and the effectors (respiratory muscles) adjust breathing patterns as needed.

Regulation of respiration

Regulation of respiration is a sophisticated process that ensures the body maintains appropriate gas exchange and responds effectively to metabolic demands. This regulation is achieved through neuronal feedback loops comprising various components that work in concert.

• Neuronal Feedback Loops: The regulation of respiration relies on a feedback mechanism involving three primary components:
  • Control Centre: This consists of respiratory nuclei located in the cerebral cortex, medulla oblongata, and pons, which govern the overall respiratory activity.
  • Sensors: Mechanoreceptors, as well as central and peripheral chemoreceptors, play crucial roles in monitoring the state of the respiratory system and the surrounding environment.
  • Effectors: The muscles of respiration, primarily the diaphragm and intercostal muscles, serve as effectors that carry out the respiratory movements.
• Neural Control of Respiration: The control of respiration is intricately coordinated through neural pathways:
  • The cerebral cortex provides voluntary control over breathing, allowing individuals to consciously influence their respiratory rate and depth.
  • The medulla and pons are responsible for automatic, involuntary breathing, which occurs without conscious effort.
  • Nerve impulses generated by respiratory neurons in these areas regulate the activity of respiratory muscles by activating motor neurons in the cervical and thoracic spinal cord. Therefore, input from both the cerebral cortex and brainstem is essential in adjusting the rate and depth of breathing.
• Sensing Physical Changes: The lungs undergo physical changes during respiration, which are sensed by:
  • Mechanoreceptors: These receptors detect stretch and pressure changes within the lungs, providing feedback on lung volume and elasticity.
  • Chemoreceptors: Peripheral and central chemoreceptors monitor the levels of carbon dioxide (CO2), oxygen (O2), and hydrogen ions (H+) in the blood. Changes in these levels trigger appropriate adjustments in breathing patterns to maintain homeostasis.
• Higher Brain Control: The higher brain centers, particularly the cerebral cortex, exert significant influence over respiration:
  • The primary motor cortex is pivotal for initiating voluntary breathing movements. Signals from this region travel to the spinal cord through corticospinal tracts, eventually reaching the diaphragm and accessory respiratory muscles.
  • The superior portion of the primary motor cortex initiates the voluntary contraction and relaxation of both the internal and external intercostal muscles, while the inferior portion regulates controlled exhalation.
• Pathways for Voluntary Control: The cortex includes specialized pathways that allow for voluntary control of respiration, bypassing the medullary neurons. These pathways connect directly to motor neurons that innervate respiratory muscles. Therefore, voluntary thoughts can effectively override automatic control. However, this control is not absolute; voluntary actions such as breath-holding can only be maintained for a limited duration.
• Breaking Point: The physiological limit for the inhibition of breathing is termed the breaking point. This occurs when stimulation from chemoreceptors due to hypoxemia (low oxygen levels) or hypercapnia (high carbon dioxide levels) surpasses the capacity for voluntary control, prompting involuntary breathing to resume.

Respiratory control centre

The respiratory control center is a crucial neural network responsible for regulating the rhythm and depth of breathing. It is composed of four primary anatomical regions: the dorsal respiratory group (DRG) and the ventral respiratory group (VRG) located in the medulla, and the apneustic center and pneumotaxic center situated in the pons. Together, these regions coordinate the body’s involuntary and voluntary respiratory demands.

• Medullary Respiratory Centers: The medulla oblongata is integral to generating the respiratory pattern and managing the various demands placed on the respiratory system. It consists of two key groups of neurons:
  • The Dorsal Respiratory Group (DRG) is located adjacent to the nucleus tractus solitarius, near the root of cranial nerve IX. It receives sensory input from peripheral chemoreceptors through the glossopharyngeal and vagus nerves. The DRG primarily governs the timing of the respiratory cycle and sends motor output to the diaphragm via the phrenic nerve. This group of neurons operates during both quiet and forced respiration, activating lower motor neurons that innervate the diaphragm and external intercostal muscles.
  • The Ventral Respiratory Group (VRG) is a complex network of neurons extending from the spinal cord to the pons-medulla junction. The expiratory center within the VRG is primarily responsible for active expiration, innervating lower motor neurons that control accessory respiratory muscles involved in this process. While expiration is typically passive during quiet breathing, the VRG’s inspiratory center is engaged during activities requiring maximal inhalation, such as gasping. A reciprocal inhibition mechanism exists between the inhalation and exhalation neurons; thus, when one set is active, the other is inhibited. The VRG encompasses:
    • Caudal Ventral Respiratory Group: Primarily handles expiratory functions with upper motor neurons projecting to contralateral expiratory muscles.
    • Rostral Ventral Respiratory Group: Involved in airway dilation through control of the larynx, pharynx, and tongue.
    • Pre-Bötzinger Complex: Responsible for central pattern generation in breathing.
    • Bötzinger Complex: Engaged in widespread expiratory functions.
  The cyclic interaction between the DRG and VRG establishes the basic pattern of respiration. During forced breathing, increased activity in the DRG stimulates VRG neurons that activate accessory muscles, facilitating active exhalation following inhalation. Therefore, the rhythm and rate of respiration result from a collaborative effort of both groups rather than a singular control area, allowing for adaptable and varied breathing patterns.
• Pontine Respiratory Group (PRG): The apneustic and pneumotaxic centers, located in the pons, fine-tune the rhythmic discharges from the medullary neurons, influencing the depth and rate of respiration in response to various stimuli.
  • The Pneumotaxic Center, located in the upper pons, works in concert with the DRG to modulate the depth of inspiration. Increased output from this center shortens the duration of inhalation, thereby elevating the respiratory rate. Conversely, reduced output decreases the rate while enhancing the depth of breathing, as the apneustic center becomes more active.
  • The Apneustic Center, found in the lower part of the pons, provides continuous stimulation to the DRG, intensifying inhalation during quiet breathing. This stimulation is typically inhibited after about two seconds by the pneumotaxic center. During forced breathing, the apneustic center receives input from the vagus nerves regarding lung inflation, thus preventing lung overexpansion by adjusting the activity of the DRG.
• Afferent Input to the Respiratory Center: Multiple inputs regulate the respiratory center, with major influences stemming from chemoreceptors—responsible for the chemical control of respiration. Additionally, various non-chemical inputs play a role in modulating respiratory activity, ensuring that the respiratory system adapts effectively to the body’s needs.

Chemical control of respiration

The chemical control of respiration is a vital physiological mechanism that ensures the body maintains homeostasis regarding oxygen (O₂) and carbon dioxide (CO₂) levels, as well as the pH of blood. This regulation is primarily facilitated by chemoreceptors that monitor changes in the chemical composition of arterial blood. When there are significant fluctuations, such as an increase in CO₂ or hydrogen ions (H⁺), or a decrease in oxygen levels (PaO₂), the activity of respiratory neurons is heightened, resulting in adjustments to the respiratory rate.

• Chemoreceptor Control: The central nervous system employs two distinct types of chemoreceptors—peripheral and central—that together function to regulate pH, PaO₂, and PaCO₂ levels in the blood.
  • Peripheral Chemoreceptors:
    • These are often referred to as arterial chemoreceptors and are situated in the carotid bodies (at the bifurcation of the carotid artery) and aortic bodies (on the aortic arch).
    • The carotid bodies send afferent signals to the respiratory center via the glossopharyngeal nerve, while the aortic bodies transmit signals via the vagus nerve.
    • Peripheral chemoreceptors primarily respond to significant changes in blood chemistry, such as a drop in oxygen levels, a decrease in blood pH (indicating an increase in H⁺ concentration), or a rise in PaCO₂.
    • When stimulated by these changes, they signal the respiratory center to enhance the respiratory rate, thus increasing PaO₂, neutralizing blood pH, and facilitating the elimination of excess CO₂.
    • While both the carotid and aortic bodies respond to elevated PaCO₂, peripheral chemoreceptors account for only about 20% of the body’s response to hypercapnia (increased CO₂ levels). It is noteworthy that low blood pH primarily stimulates the carotid bodies.
    • Additionally, low blood pressure can lead to hypoperfusion of the carotid and aortic bodies, resulting in increased neuronal output from these chemoreceptors.
  • Central Chemoreceptors:
    • Located in the medulla oblongata on its ventral surface, central chemoreceptors operate independently from the VRG (ventral respiratory group).
    • These chemoreceptors are sensitive to changes in the pH of cerebrospinal fluid (CSF). When the pH of the CSF declines, central chemoreceptors are activated.
    • Since charged ions, such as H⁺ and bicarbonate (HCO₃⁻), cannot traverse the blood-brain barrier, the detection process involves several intermediary steps. Carbon dioxide diffuses from the blood into the CSF, where it reacts with water under the action of the enzyme carbonic anhydrase (CA). This reaction produces carbonic acid (H₂CO₃), which subsequently dissociates into H⁺ and HCO₃⁻.
    • The increase in H⁺ concentration within the chemoreceptor tissue stimulates the central chemoreceptors, prompting them to activate the respiratory center, thereby enhancing the respiratory rate.

Non-chemical control of respiration

Non-chemical control of respiration involves various physiological mechanisms that regulate breathing based on sensory inputs rather than changes in blood chemistry. This control system allows the body to respond to various stimuli, such as exercise, pain, and environmental factors, ensuring efficient respiratory function.

• Mechanoreceptors:
  • These receptors play a crucial role in regulating breathing through lung stretch receptors and muscle spindles.
  • Lung Stretch Receptors: Located in the bronchial smooth muscle, these receptors are activated when the lungs become overinflated. The neural impulses generated travel via the vagus nerve to the apneustic center, leading to a reduction in the depth of breathing. This reflex is known as the Hering–Breuer inflation reflex, which causes prolonged expiration when the lungs are steadily inflated. Conversely, a marked deflation of the lungs triggers the Hering–Breuer deflation reflex, resulting in a decrease in expiration duration.
• Muscle Spindles:
  • During physical activity, changes in respiration are initiated by muscle spindle activity.
  • Afferent impulses from proprioceptors in muscles, tendons, and joints stimulate inspiratory neurons, which subsequently signal the respiratory center to increase the breathing rate. This increase in respiration helps eliminate carbon dioxide and acids produced during exercise while enhancing oxygen intake.
• Irritant Receptors:
  • Located in the airway epithelium, these receptors serve as a protective mechanism against inhalation of noxious substances.
  • When activated, they induce bronchoconstriction and stimulate ventilation. Chemicals like histamine can also activate these receptors, leading to the stimulation of rapidly adapting receptors in the trachea, resulting in coughing, bronchoconstriction, and mucus secretion.
• Juxtacapillary Receptors (J-Receptors):
  • These receptors are non-myelinated C-fibers found in the alveolar walls and are situated near pulmonary vessels.
  • They are activated by factors such as lung hyperinflation, dyspnea, bradycardia, and hypotension. Administration of capsaicin can also activate J-receptors, resulting in a reflex response known as the pulmonary chemoreflex, characterized by apnoea followed by rapid breathing, bradycardia, and hypotension. Although the exact role of the pulmonary chemoreflex is not entirely understood, it is believed to be significant in pathological states, such as pulmonary congestion or embolism.
• Pain Receptors:
  • Activation of pain receptors leads to increased ventilation. Hyperventilation observed during acute pain may reflect the respiratory component of the fight-or-flight response, preparing the body for potential injury or attack.
• Thalamus:
  • An increase in core body temperature stimulates ventilation. Research conducted in rats indicates that elevated body temperature enhances the mechanical properties of the respiratory system, improving compliance and reducing airway resistance.
• Limbic System:
  • The connection between emotional stimuli and pain suggests that the limbic system and hypothalamus send signals to respiratory neurons in the brainstem. This interaction may lead to hyperventilation in response to emotional or painful experiences.

What are the main chemical factors that influence respiration?

Several key chemical components play significant roles in regulating respiratory activity, ensuring that adequate oxygen is delivered to tissues while carbon dioxide and acidity levels are effectively managed.

• Oxygen (PaO2):
  • A decrease in blood oxygen levels serves as a critical stimulus for respiration. When PaO2 drops, it activates peripheral chemoreceptors located in the carotid and aortic bodies.
  • This activation leads to an increased respiratory rate, facilitating enhanced oxygen intake. Thus, maintaining adequate oxygen levels is vital for cellular metabolism and overall bodily function.
• Carbon Dioxide (PaCO2):
  • An increase in carbon dioxide levels represents one of the most significant stimuli for respiratory regulation. High PaCO2 concentrations are detected by both peripheral and central chemoreceptors.
  • This heightened detection prompts the respiratory centers in the brain to increase the rate and depth of breathing. By expelling excess carbon dioxide, the body effectively prevents respiratory acidosis, ensuring a balanced internal environment.
• Hydrogen Ions (H+):
  • Changes in blood pH, specifically a decrease that indicates increased acidity, also play a crucial role in regulating respiration. Carotid bodies respond to this decrease in pH by stimulating an increase in respiratory activity.
  • This increase helps to regulate the acid-base balance within the body. By enhancing respiration, the body can reduce carbon dioxide levels, which subsequently lowers hydrogen ion concentrations and helps restore normal pH levels.
• Chemoreceptor Function:
  • These chemical changes—variations in oxygen, carbon dioxide, and hydrogen ion levels—are detected by both peripheral and central chemoreceptors.
  • The information gathered by these chemoreceptors is transmitted to the respiratory center in the brain, which adjusts the breathing rate accordingly. This feedback loop is essential for maintaining the delicate balance of gases and pH in the blood, enabling the body to respond dynamically to changes in metabolic activity.

How do mechanoreceptors contribute to the regulation of breathing?

Mechanoreceptors play a crucial role in the regulation of breathing by responding to mechanical changes within the respiratory system. Their contributions ensure that the body can adapt to varying physiological demands, thereby maintaining homeostasis. The primary types of mechanoreceptors involved in this process include lung stretch receptors, muscle spindles, irritant receptors, and juxtacapillary (J) receptors. Each type has a specific function in respiratory regulation.

• Lung Stretch Receptors:
  • Located in the bronchial smooth muscle, these receptors are sensitive to the degree of lung inflation.
  • When the lungs become overinflated, lung stretch receptors are activated.
  • They send neural impulses to the apneustic center through the vagus nerve, leading to a reduction in the depth of breathing. This response is part of the Hering–Breuer inflation reflex, which prevents excessive lung inflation by prolonging the duration of expiration.
  • Therefore, this reflex helps maintain optimal lung volume and protects against potential lung damage from overinflation.
• Muscle Spindles:
  • Muscle spindles, located within muscles, are proprioceptors that respond to changes in muscle length and tension.
  • During physical activity, as muscle spindle activity increases, it stimulates inspiratory neurons in the respiratory center.
  • This stimulation results in an increased respiratory rate, which helps meet higher oxygen demands and facilitates the clearance of carbon dioxide produced during exercise.
  • Consequently, muscle spindles play a vital role in coordinating respiratory activity with physical exertion.
• Irritant Receptors:
  • Found in the airway epithelium, irritant receptors are activated by inhaled noxious gases and particulate matter.
  • Upon activation, these receptors induce bronchoconstriction and stimulate increased ventilation as a protective reflex.
  • This response helps to expel harmful substances from the airways, thus maintaining airway integrity and function.
  • As a result, irritant receptors serve as essential sensors that help safeguard the respiratory system from environmental hazards.
• Juxtacapillary Receptors (J-receptors):
  • Located in close proximity to pulmonary vessels within the alveolar walls, J-receptors are sensitive to various physiological conditions.
  • They are activated by lung hyperinflation, dyspnoea, and changes in blood pressure.
  • Activation of J-receptors can trigger reflexive responses that modify breathing patterns, such as rapid shallow breathing or apnoea, particularly in response to pathological states.
  • This reflex serves as an important mechanism for adapting respiratory function to changing conditions within the pulmonary circulation.