MEDICAL BIOLOGY, ANATOMY, HUMAN PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

CHAPTER 6. THE RESPIRATORY PROCESS AND ITS PATHOLOGY

RESPIRATORY PHYSIOLOGY

PULMONARY VENTILATION

2. REGULATION OF RESPIRATION

The normal functioning of all physiological systems requires a continuous supply of oxygen delivered via external Respiration. Respiratory arrest for even 3-5 minutes inevitably leads to death. Therefore, The regulatory mechanisms of breathing that evolved over time ensure a high degree of respiratory reliability.

Neurons involved in The regulation of breathing are located at all levels of the Central Nervous system, from the Spinal Cord to the Cerebral Cortex.

Animal experiments utilizing Brain transection at various levels have demonstrated that the vital structures of the respiratory center reside within the Medulla Oblongata. This constitutes the medullary respiratory center; its damage results in immediate respiratory arrest. The respiratory center of the medulla oblongata comprises two main nuclear groups: the dorsal and ventral respiratory groups. The dorsal Nucleus contains inspiratory neurons, the excitation of which triggers inspiration. The ventral nucleus contains both inspiratory and expiratory neurons that initiate expiration. Furthermore, excitation of the neurons in the dorsal nucleus inhibits the expiratory neurons of the ventral group.

The respiratory center operates based on sensory input received from chemo- and mechanoreceptors. Chemoreceptors are located in the carotid sinuses near the bifurcation of the common carotid Arteries and within the aortic arch. These peripheral chemoreceptors respond to changes in Blood gas tensions as well as hydrogen ion concentrations. They are particularly sensitive to Hypoxia—a decrease in blood oxygen tension. These receptors are also stimulated by hypercapnia—an elevation in carbon dioxide tension accompanied by a drop in blood pH. Central chemoreceptors, localized on the ventral surface of the medulla oblongata, are primarily responsive to decreases in the pH of the CEREBROSPINAL FLUID. Impulses originating from both peripheral and central chemoreceptors are relayed to the neurons of the medullary respiratory center, resulting in an increased respiratory rate and depth.

Mechanoreceptors are embedded within the smooth Muscle of the Trachea and Bronchi. These receptors respond to stretching during deep inspiration. Afferent impulses travel via the Vagus nerve to the inspiratory neurons of the respiratory center, inhibiting them and thereby terminating the inspiratory phase. This mechanoreceptor-mediated reflex, known as the Hering-Breuer reflex, prevents overexpansion of the Lungs when lung volume exceeds approximately 1.5 liters. Irritant receptors are located within the epithelial layer of the Airways. They are stimulated by dust particles, mucus, caustic vapors (such as ammonia or tobacco smoke), cold air, and BIOLOGICALLY ACTIVE SUBSTANCES like histamine. Stimulation of upper airway irritant receptors triggers protective reflex responses—such as coughing and sneezing—which involve forced expiration to expel foreign matter. The centers for these Reflexes are situated in the medulla oblongata. Additionally, irritation of these receptors can cause reflex bronchospasm via vagal pathways, a phenomenon observed, for example, when irritant receptors are triggered by histamine during an asthma attack.

The Pons contains the pneumotaxic center, which coordinates the transition between respiratory phases, regulates the duration of inspiration and expiration, and modulates the rhythmic activity of the respiratory center.