Lec # 2.physiology for doctor of physical therapy

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1. Surfactant

2. Surfactant Surfactant: It is a substance secreted in alveolar epithelium . It reduces surface tension and prevents the collapsing tendency produced by surface tension. Surfactant is a surface acting material or agent that is responsible for lowering the surface tension of a fluid. Surfactant that lines the epithelium of the alveoli in lungs is known as pulmonary surfactant and it decreases the surface tension on the alveolar membrane.

3. Surfactant Source of secretion of pulmonary surfactant Pulmonary surfactant is secreted by two types of cells: 1. Type II alveolar epithelial cells in the lungs, which are called surfactant secreting alveolar cells or pneumocytes . Characteristic feature of these cells is the presence of microvilli on their alveolar surface. 2. Clara cells, which are situated in the bronchioles. These cells are also called bronchiolar exocrine cells.

4. Functions of surfactant Surfactant reduces the surface tension in the alveoli of lungs and prevents collapsing tendency of lungs. Surfactant is responsible for stabilization of the alveoli, which is necessary to withstand the collapsing tendency. It plays an important role in the inflation of lungs after birth. In fetus, the secretion of surfactant begins after the 3rd month. Until birth, the lungs are solid and not expanded. Soon after birth, the first breath starts because of the stimulation of respiratory centers by hypoxia and hypercapnea . Although the respiratory movements are attempted by the infant, the lungs tend to collapse repeatedly. And, the presence of surfactant in the alveoli prevents the lungs from collapsing. Another important function of surfactant is its role in defense within the lungs against infection and inflammation. Hydrophilic proteins SP­A and SP­D destroy the bacteria and viruses by means of opsonization ( Opsonization is a process where pathogens or foreign particles are coated with proteins called opsonins , making them more easily recognized and engulfed by phagocytes (immune cells) for destruction ). These two proteins also control the formation of inflammatory mediators.

5. Effect of deficiency of surfactant – respiratory distress syndrome Absence of surfactant in infants, causes collapse of lungs and the condition is called respiratory distress syndrome or hyaline membrane disease. Deficiency of surfactant occurs in adults also and it is called adult respiratory distress syndrome (ARDS). In addition, the deficiency of surfactant increases the susceptibility for bacterial and viral infections.

6. RESPIRATORY PROTECTIVE REFLEXES Respiratory protective reflexes are the reflexes that protect lungs and air passage from foreign particles. Respiratory process is modified by these reflexes in order to eliminate the foreign particles or to prevent the entry of these particles into the respiratory tract. Following are the respiratory protective reflexes.

7. „ COUGH REFLEX Cough is a modified respiratory process characterized by forced expiration. It is a protective reflex and it is caused by irritation of respiratory tract and some other areas such as external auditory canal. Mechanism: Cough begins with deep inspiration followed by forced expiration with closed glottis . This increases the intrapleural pressure above 100 mm Hg. Then, glottis opens suddenly with explosive outflow of air at a high velocity. Velocity of the airflow may reach 960 km/hour. It causes expulsion of irritant substances out of the respiratory tract.

8. Pathway : Receptors that initiate the cough are situated in several locations such as nose, paranasal sinuses, larynx, pharynx, trachea, bronchi, pleura, diaphragm, pericardium, stomach, external auditory canal and tympanic membrane. Afferent nerve fibers pass via vagus , trigeminal, glossopharyngeal and phrenic nerves. The center for cough reflex is in the medulla oblongata. Efferent nerve fibers arising from the medullary center pass through the vagus , phrenic and spinal motor nerves. These nerve fibers activate the primary and accessory respiratory muscles

9. SNEEZING REFLEX Sneezing is also a modified respiratory process characterized by forced expiration. It is a protective reflex caused by irritation of nasal mucous membrane. Causes Irritation of the nasal mucous membrane occurs because of dust particles, debris, mechanical obstruction of the airway and excess fluid accumulation in the nasal passages. Mechanism Sneezing starts with deep inspiration, followed by forceful expiratory effort with opened glottis resulting in expulsion of irritant agents out of respiratory tract.

10. Reflex Pathway Sneezing is initiated by the irritation of nasal mucous membrane, the olfactory receptors and trigeminal nerve endings present in the nasal mucosa. Afferent nerve fibers pass through the trigeminal and olfactory nerves. Sneezing center is in medulla oblongata. It is located diffusely in spinal nucleus of trigeminal nerve, nucleus solitarius and the reticular formation of medulla. Efferent nerve fibers from the medullary center pass via trigeminal, facial, glossopharyngeal, vagus and intercostal nerves. These nerve fibers activate the pharyngeal, tracheal and respiratory muscles.

11. SWALLOWING (DEGLUTITION) REFLEX Swallowing reflex is a respiratory protective reflex that prevents entrance of food particles into the air passage during swallowing. While swallowing of the food, the respiration is arrested for a while. Temporary arrest of respiration is called apnea. Arrest of breathing during swallowing is called swallowing apnea or deglutition apnea. It takes place during pharyngeal stage, i.e. second stage of deglutition and prevents entry of food particles into the respiratory tract. Refer Chapter 43 for details .