Respiratory System PDF

Presented by Dr. Karima El-Sayed Ismail Assistant Professor of Medical Physiology FOMSCU 2023-2024  Introduction At rest, a normal human breathes 12-15 breath per minute. About 6-8 L/min, is inspired and expired. This air mixes with...

Presented by Dr. Karima El-Sayed Ismail Assistant Professor of Medical Physiology FOMSCU 2023-2024  Introduction At rest, a normal human breathes 12-15 breath per minute. About 6-8 L/min, is inspired and expired. This air mixes with the gas in the alveoli, and, by simple diffusion, O2 enters the blood in the pulmonary capillaries while CO2 enters the alveoli.  Organization of the Respiratory System The airways are all the tubes through which air flows between the external environment and the alveoli. The Airways Upper Lower Airways Airways Trachea Nose Bronchial Pharynx tree larynx Alveoli Organization of the Respiratory System Organization of the Respiratory System Upper Airways Lower Airways  Respiratory System The respiratory system is formed of 2 ZONES: 1- CONDUCTING ZONE; where no respiration takes place: They are: nose, larynx, pharynx, trachea, bronchi, bronchioles and terminal bronchioles. 2- RESPIRATORY ZONE; actual sites of gas exchange They are: respiratory bronchioles, alveolar ducts and alveoli.  Organization of the Respiratory System 1- The upper respiratory tract: 1- the nose 2- the pharynx: (pharynx is a passage common to both air and food). 3- the larynx: The pharynx branches into two tubes, the esophagus through which food passes to the stomach, and the larynx, which is part of the airways. 2- The lower respiratory tract: 1- The trachea: the larynx opens into a long tube called the trachea. 2-The bronchi: Trachea branches into two bronchi , each one enters a lung. - Within the lungs, there are more than 20 generations of branching bronchi, each resulting in narrower, shorter, and more numerous tubes. - The walls of the trachea and bronchi contain cartilage, which gives them their cylindrical shape and supports them. 3- The bronchioles: the first airway branches that no longer contain cartilage are termed bronchioles. Organization of the Respiratory System Figure showing the branches of the airways.  There are two lungs, the right and left, each divided into several lobes. The lungs consist mainly of tiny air containing sacs called alveoli, which number approximately 300 million in the adult. The total area of the alveolar walls in contact with capillaries in both lungs is about 70 m2. The alveoli are the sites of gas exchange with the blood. The alveolar sacs are clusters of alveoli.  Relation of the Lungs to the Thoracic (Chest) Wall - The lungs, like the heart, are situated in the thorax. -The thorax is a closed compartment that is bounded at the neck by muscles and completely separated from the abdomen by the diaphragm. -The wall of the thorax is formed by the spinal column, the ribs, the sternum, and several groups of muscles that run between the ribs (collectively termed the intercostal muscles). Pleura and pleural fluid: Each lung is surrounded by a completely closed sac, called pleura. The visceral pleura: the pleural surface coating the lung. It is firmly attached to the lung. The parietal pleura: the outer layer of plural sac. It is attached to and lines the interior thoracic wall and diaphragm. Organization of the Respiratory System  The two layers of pleura in each sac are so close to each other, but they are not attached to each other. The visceral and parietal pleura are separated by only a few milliliters of intrapleural fluid. The intrapleural fluid lubricates the pleural surfaces so that they can slide over each other during breathing. The intrapleural pressure (Pip): Changes in the pressure of the intrapleural fluid ➔ cause the lungs and thoracic wall to move in and out together during normal breathing. Definitions: Inspiration (inhalation): is the movement of air from the external environment through the airways into the alveoli during breathing. Expiration (exhalation): is movement of air in the opposite direction; i.e. from alveoli to the external environment. Respiratory cycle: an inspiration and an expiration. **During inspiration: Passage of air through the airways - Air passes through either the nose (the most common site) or mouth ➔ pharynx ➔trachea and through the bronchioles ➔ respiratory bronchioles and alveolar ducts to the alveoli (where gas exchange occurs). Internal Respiration External versus Internal Respiration External Versus Internal respiration External Respiration Internal Respiration  External Respiration The alveolo-capillary membranes: - The alveoli are surrounded by pulmonary capillaries. - In most areas, air and blood are separated only by the alveolar epithelium and the capillary endothelium, so they are about 0.5 um apart. - Gases diffuse from the alveoli to the blood in the pulmonary capillaries or vice versa across the thin alveolocapillary membrane. Regulation of respiration  Respiration is a reflex process. Normally, quiet regular breathing occurs because of two regulatory mechanisms: 1. Nervous mechanism. 2. Chemical mechanism. NERVOUS MECHANISM Nervous mechanism that regulates the respiration includes: Respiratory centers. RESPIRATORY CENTERS Respiratory centers are group of neurons, which control the rate, rhythm and force of respiration. Depending upon the situation in brainstem, the respiratory centers are classified into two groups: A. Medullary centers consisting of: 1-Dorsal respiratory group of neurons 2-Ventral respiratory group of neurons B. Pontine centers: 1-Apneustic center 2-Pneumotaxic center. MEDULLARY CENTERS 1. Dorsal Respiratory Group of Neurons present in the upper part of the medulla oblongata. these neurons are collectively called inspiratory center. Dorsal group of neurons are responsible for basic rhythm of respiration. In the dorsal respiratory group (DRG) of neurons, controls the basic rhythm for breathing by setting the frequency of inspiration. This group of neurons receives sensory input from peripheral chemoreceptors via the glossopharyngeal (cranial nerve [CN] IX) and vagus (CN X) nerves and from mechanoreceptors in the lung via the vagus nerve. The inspiratory center sends its motor output to the diaphragm via the phrenic nerve. 2-Ventral Respiratory Group of Neurons The ventral group neurons were collectively called expiratory center. Normally, ventral group neurons are inactive during quiet breathing and become active during forced breathing. During forced breathing, these neurons stimulate both inspiratory muscles and expiratory muscles. PONTINE CENTERS 1. Apneustic Center Apneustic center increases depth of inspiration by acting directly on dorsal group neurons. 2-Pneumotaxic Center Pneumotaxic center inhibits the apneustic center so that the dorsal group neurons are inhibited. Because of this, inspiration stops and expiration starts. pneumotaxic center influences the switching between inspiration and expiration. CHEMICAL MECHANISM Chemical mechanism of regulation of respiration is operated through the chemoreceptors. Changes in Chemical Constituents of Blood which Stimulate Chemoreceptors: 1. Hypoxia (decreased pO2) 2. Hypercapnea (increased pCO2) 3. Increased hydrogen ion concentration. Types of Chemoreceptors 1. Central chemoreceptors 2. Peripheral chemoreceptors. CENTRAL CHEMORECEPTORS present in the brain. Mechanism of Action Central chemoreceptors are connected with respiratory centers. These chemoreceptors act slowly but effectively. Main stimulant for central chemoreceptors is the increased hydrogen ion concentration. Hydrogen ions stimulate the central chemoreceptors. From chemoreceptors, the excitatory impulses are sent to dorsal respiratory group of neurons, resulting in increased ventilation (increased rate and force of breathing). Because of this, excess carbon dioxide is washed out and respiration is brought back to normal. 1. In the blood, CO2 combines reversibly with H2O to form H+ and HCO3 − by the familiar reactions. Because the blood-brain barrier is relatively impermeable to H+ and HCO3 −, these ions are trapped in the vascular compartment and do not enter the brain. 2. CO2 also is permeable across the brain-CSF barrier and enters the CSF. 3. In the CSF, CO2 is converted to H+ and HCO3 −. Thus increases in arterial PCO2 produce increases in the PCO2 of CSF, which results in an increase in H+ concentration of CSF (decrease in pH). The central chemoreceptors are in close proximity to CSF and detect the decrease in pH. A decrease in pH then signals the inspiratory center to increase the breathing rate (hyperventilation)  In summary, the goal of central chemoreceptors is to keep arterial PCO2 within the normal range, Thus increases in arterial PCO2 produce increases in PCO2 in the brain and the CSF, which decreases the pH of the CSF. A decrease in CSF pH is detected by central chemoreceptors for H+, which instruct the DRG to increase the breathing rate. When the breathing rate increases, more CO2 will be expired and the arterial PCO2 will decrease toward normal PERIPHERAL CHEMORECEPTORS Peripheral chemoreceptors are the chemoreceptors present in carotid and aortic region. Mechanism of Action Hypoxia is the most potent stimulant for peripheral chemoreceptors. In addition to hypoxia, peripheral chemoreceptors are also stimulated by hypercapnea and increased hydrogen ion concentration. However, the sensitivity of peripheral chemoreceptors to hypercapnea and increased hydrogen ion concentration is mild Decreases in arterial PO2. The most important responsibility of the peripheral chemoreceptors is to detect changes in arterial PO2. Surprisingly, however, the peripheral chemoreceptors are relatively insensitive to changes in PO2: They respond dramatically when PO2 decreases to less than 60 mm Hg. Thus if arterial PO2 is between 100 mm Hg and 60 mm Hg, the breathing rate is virtually constant. However, if arterial PO2 is less than 60 mm Hg, the breathing rate increases. In this range of PO2, chemoreceptors are sensitive to O2 and , they respond so rapidly. Increases in arterial PCO2. The peripheral chemoreceptors also detect increases in PCO2, but the effect is less important than their response to decreases in PO2. Detection of changes in PCO2 by the peripheral chemoreceptors also is less important than detection of changes in PCO2 by the central chemoreceptors. Decreases in arterial pH. Decreases in arterial pH cause an increase in ventilation, mediated by peripheral chemoreceptors for H+. Thus in metabolic acidosis, in which there is decreased arterial pH, the peripheral chemoreceptors are stimulated directly to increase the ventilation rate. References: 1-Costanzo LS: Physiology: with Student consult Online Access (Costanzo Physiology) 5th Edition(2013). 2- Sherwood L. Human Physiology: From Cells to Systems (5th ed). Belmont, CA: Wadsworth, 2021. 3- Guyton and Hall Textbook of Medical Physiology (Guyton Physiology) 14th Edition by John E. Hall PhD (Author), Michael E. Hall MD MSc. 4- Ganong's Review of Medical Physiology, Twenty sixth Edition 26th Edition by Kim Barrett, Susan Barman, Jason Yuan, Heddwen Brooks.