CVPT

Folie 1

• CHAPTER 1 ANATOMICAL AND PHYSIOLOGICAL BACKGROUND OF RESPIRATORY SYSTEM I. Anatomical aspects of respiratory system A- Anatomy of chest wall / thoracic cage The bony wall of the thoracic cage is formed by the sternum anteriorly, thoracic vertebrae posteriorly and 12 pairs of ribs and costal cartilage on either side. It is conical in shape, being narrow above and wide below. Its anteroposterior diameter is less than transverse diameter due to forward projection of the bodies of the vertebrae. In transverse section the thoracic cage is elliptical in shape. Besides its role in respiration, the thoracic cage provides a skeletal framework to the chest and has a supportive and protective function to the thoracic and upper abdominal viscera. 4 Several muscles are attached to or related to the chest wall and are considered muscles of respiration. These include: 1- Muscles for inspiration: • Main (primary): Diaphragm. • Accessory (secondary): External intercostal, scalenus, Sternocledomastoid, trapezius, serratus anterior, pectoralis minor. Diaphragm: • It is a large dome-shaped musculo-tendinus partition which separates the thoracic cavity from the abdominal cavity. • Its upper surface is convex towards the thoracic cavity while its lower surface is concave towards the abdominal cavity. • Its right side bulges higher up than its left side due to the upward bulge of the underlying Rt. Lobe of the liver. • The diaphragm is the main muscle of inspiration its contraction leads to its descent and increase in the vertical diameter of the thoracic cavity. External intercostal muscles: The external intercostals elevate the ribs during inspiration increasing the antero-posterior and transverse diameters. Scalenus muscle: They elevate the first and second ribs in forced inspiration. During quiet inspiration, they fix the 1st rib acting as an anchor point to which is all the ribs are drawn together and raised towards it by the contraction of the intercostal muscles. Sternocleidomastoid muscle: It elevates the sternum and ribs during forced inspiration, thereby increasing the volume of the thoracic cavity. Trapezius, levator scapulae and rhomboid muscles: They fix the scapulae during forced inspiration, which enables the serratus anterior and pectoralis minor to pull up the ribs. Moreover, quadratus lumborum fixes the 12th rib so that the diaphragm can exert a more powerful downward push on the abdominal viscera. 5 2- Muscles for expiration: Quiet expiration is a passive process involving the elastic recoil of the lung and relaxation of the inspiratory muscles. However, deep expiration is an active process requiring the action of: Internal intercostal muscles: They pull the ribs downward and inward to decrease the volume of the thoracic cavity and force air out of the lungs. Abdominal muscles: Their contraction increases the intra-abdominal pressure which pushes the diaphragm upwards forcing air out of the lungs. B- Anatomy of Respiratory Tract ❖ The respiratory tract is divided anatomically into: 1- The Upper Respiratory Tract The upper respiratory tract extends from the nasal and oral orifices to the false vocal cords in the larynx. It consists of: Nose, Nasal cavity, Pharynx, Larynx 6 Structure Function Nose and nasal cavity Warming, humidifying, filtering and cleaning air Pharynx • Lies posterior to the nasal and oral cavities • Directs air from nose /mouth to the larynx and lungs. • Initiates swallowing and directs food to esophagus and prevents it from moving to the airways. • Immune functions as it contains tonsils and adenoids. False larynx • Protects the airways by closing off the larynx during swallowing. • Protects the true vocal cords from trauma. 2- The lower respiratory tract The lower respiratory tract consists of the true vocal cords in the larynx, trachea, bronchial tree (bronchi and bronchioles) and alveoli in the lungs. Structure Function True larynx (voice box) • Phonation (sound production): air passing through the larynx causes vibration in the vocal cords producing sound waves that are modified by the mouth and nose to create speech. • Controls the airflow to the lungs through precise adjustements to its internal diameter Trachea (windpipe) • Extends from the larynx to the primary bronchi. • It is about 10-12 cm long and 2-3 cm in diameter. • It has 16-20 incomplete C- shaped hyaline cartilage rings. • The base of the trachea just above the bifurcation point is the "carina". • It transports, humidifies, warm and cleans air before reaching the lungs. Bronchi • The trachea branches into right and left 7 main (primary) bronchi entering right and left lungs respectively. Similar to trachea, they are line with C- shaped hyaline cartilage and ciliated epithelium with goblet cells. • Primary bronchi divides into 3 right secondary (lobular) bronchi and 2 left secondary bronchi. They are smaller and have patches of cartilage and allow the flow of air to the lung lobes. • Secondary bronchi the branch to tertiary (segmental) bronchi passing through each lung segment. They are the smallest in diameter and have patches of cartilage. Bronchioles • Terminal bronchioles are cartilage free, ciliated branches of tertiary bronchi. They direct air to the respiratory zone. • Respiratory bronchioles are the final smallest branches of the airways lined by fine cilia. They have small alveoli openings along their walls to begin gas exvhnge process. Alveoli • The respiratory bronchioles connect to the alveoli through alveolar ducts lined by thin epithelial layer. • Alveolar ducts open into alveolar sacs (clusters of alveoli). • Alveoli are the functional unit of lungs lined by sipmple squamous epithelium and surrounded by blood capillaries. They responsible for O2 and CO2 exchange between air inside it and surrounding blood. Macrophages within the alveoli engulf and destroy pathogens and other foreign debris that may have escaped the cleaning action of the ciliated cells lining the bronchial tree. 8 ❖ According to function respiratory tract is classified into: 1- Conducting zone / Anatomical dead space: It represents all the structures that inspired air have to pass through before reaching the respiratory zone. The terminal bronchioles are the most distal segment of the conducting zone. These structures warm and humidify the inspired air. They also contain mucus secreting cells and cilia to filter and clean the air by trapping the foreign particles and expectorating them. 2- Respiratory zone It is the region of gas exchange between the air and blood. It is formed of the pulmonary acini ( respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli). 9 ❖ Functions of respiratory system: Respiratory functions Non- respiratory functions Pulmonary ventilation Filtiration Diffusion of O2 and CO2 between the alveoli and the blood Reservoir: around 450 ml of blood is stored in pulmonary capillaries Transportation of O2 and CO2 in the blood and body fluids to and from the body's tissue cells Defense mechanism due to the presence of cilia, alveolar macrophages, B & T lymphocytes and mast cells Regulation of ventilation Water balance Vocalization C- Anatomy of the pleura ❖ Definition of the pleura: It is a completely closed sac which is invaginated by the lung from its medial aspect. ❖ Subdivisions of the pleura: 1. Visceral pleura: It is the part investing the lung and is closely adherent to it. 10 2. Parietal pleura: It is the part lining the thoracic cavity and has cervical, costal, mediastinal and diaphragmatic parts. 3. The pleural cavity: It is the narrow potential space between the parietal and visceral layer of pleura. It contains a thin film of serous fluid which prevents the friction between the 2 layers of pleura. The pressure inside the pleural cavity is negative keeping the lungs always inflated. D- Anatomy of the Lungs The lungs are the organs of exchange of gases between blood and atmospheric air. They have a spongy pinkish gray texture. Each lung looks like half a cone which has an apex, a base, three surfaces (costal, mediastinal and diaphragmatic) and three borders (anterior, posterior and inferior). ❖ Characteristics of each lung: Right lung Left lung Larger and wider. Smaller and narrower. Shorter due to more upward bulging of the diaphragm by the liver. Longer. 11 Its anterior border is nearly straight. Its anterior border presents cardiac notch and lingula. It has 2 fissures (oblique and horizontal). It has one fissure (oblique). It has 3 lobes (superior, middle and inferior). It has 2 lobes (superior and inferior). ❖ Surface markings of the lung: (1) The apex of the lung: It lies 3-4 cm above the level of the 1st costal cartilage and extends about 2.5 cm above the medial 1/3 of the clavicle. (2) The anterior border: It extends from the sternoclavicular joint downward and vertically close to the midline to the level of the 4th costal cartilage below which they differ: • Right lung: The anterior border of the lung continues descending vertically down to the 6th costal cartilage. • Left lung: The anterior border of the lung passes laterally for 3.5 cm and inferomedially to the 6th costal cartilage presenting the cardiac notch. (3) The inferior border: It crosses the 6th rib in the midclavicular line, 8th rib in the midaxillary line and the 10th rib to end 2 cm lateral to the level of the 10th thoracic spine (at scapular line). (4) The posterior border: It extends from the medial end of the inferior border (T10 spine) upwards along the vertebral column to the apex of the lung. (5) The costal surface: It is covered by the costal pleura and is along the sternum and ribs. (6) The medial surface: It is divided anteriorly and posteriorly. Anteriorly it is related to the sternum and posteriorly it is related to the vertebra. (7) The diaphragmatic surface (base): It is concave and rests on the dome of the diaphragm, higher on right than left side. 12 ❖ Blood supply of the lung: The lung is supplied by bronchial arteries and drained by bronchial veins. ❖ Nerve supply of the lung: The lungs and the visceral pleura are innervated by the autonomic pulmonary plexuses. The parasympathetic fibers of the plexus are broncho-constrictor and secretory to the mucous glands of the bronchial tree. The sympathetic fibers are bronchodilators and inhibitory to the mucous gland of the bronchial tree. II- Physiological Aspects of Respiratory System A- Breathing / Pulmonary Ventilation ❖ Definition: It is an automatic, rhythmic activity accomplished without conscious effort that involves the flow of air in to (inspiration / inhalation) and out of (expiration / exhalation) the lung through the conducting airways, resulting from differences in pressure between the alveolar air and atmosphere. 13 ❖ Physical properties affecting pulmonary ventilation: 1. Respiratory pressure changes According to Boyle's law at a constant temperature, there is an inverse relationship between the volume occupied by a gas and its pressure. The following pressures affect the ventilation: a) Atmospheric pressure (atm): It is the amount of force that is exerted by gases in the air surrounding the body. It is equal to 760 mm Hg, which is the atmospheric pressure at sea level. Typically, for respiration, other pressure values are discussed in relation to atmospheric pressure. Lower pressures are negative, higher pressures are positive and equal pressures are zero. b) Intra-pulmonary / alveolar pressure (Palv): It is the pressure of air inside the alveoli, which changes during the different phases of breathing. Because the alveoli are connected to the atmosphere via the tubing of the airways, the interpulmonary pressure always equalizes with the atmospheric pressure. c) Intra-pleural pressure (Ppl): It is the pressure within the pleural cavity due to the fluid bond between the visceral and parietal pleura. Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing but has always negative value to avoid alveolar collapse. d) Transpulmonary pressure (TPP): It is the pressure distendng the lungs and considered as the net difference between the intra-pleural and intra-alveolar pressures.(Intrapulmonary-intrapleural pressure) ▪ Mechanism of breathing ➢ During Inspiration: - Motor impulses from the medulla stimulate phrenic and inercostal nerves causing contraction of diaphragm and external intercostal muscles respectively. - The diaphragm contracts and becomes flattened (descends about 1cm in quiet inspiration and 10 cm in deep inspiration) which increases the vertical diameter of chest wall. - The external intercostals contract and pull the ribs upward and outward increasing the antero-posterior and lateral diameters. 14 - Expansion of chest wall increases its volume and thereby lowers both intra pleural and intra-pulmonary pressures. This pressure gradient causes rushing of more air into the lungs which continues until intra-pulmonary pressure equals the atmospheric pressure. - During strenous activity more forceful contraction of the muscles occur which creates more pressure difference permitting more air entry in deep inspiration. ➢ During Expiration - The motor impulses from the medulla decrease causing relaxation of the inspiratory muscles.The elastic fibers of the lung recoil. - Chest wall volume decreases and thereby both intra-pleural and intra-pulmonary pressures increase. This pressure gradient causes rushing of more air out of the lungs which continues until intra-pulmonary pressure equals the atmospheric pressure. - Quiet expiration is a passive process, while deep expiration such as during blowing, coughing and sneezing is an active process requiring the action of abdominal and internal intercostal muscles. The abdominal contraction pushes the diaphragm upward causing more rise in intrapulmonary pressure that forces more air out of the lungs. 15 Summary of pressure changes Phase Intra-pleural pressure (mmHg) Intra-pulmonary pressure (mmHg) Transpulmonary pressure (mmHg) Inspiration From -4 to -6 From 0 to -1 +5 Expiration From -6 to -3 From -1 to +1 +4 2. Compliance (Stretchability) Lung compliance refers to how easily the lungs and chest wall can expand during breathing. When compliance is high, the lungs expand with little effort; when it is low, expansion is more difficult. This ability to stretch depends mainly on two factors: the elasticity of lung tissue and the surface tension within the alveoli. Under normal conditions, lungs have high compliance due to elastic fibers that stretch easily and the presence of surfactant, which lowers surface tension. In technical terms, compliance is defined as the change in lung volume divided by the change in transpulmonary pressure (∆V/ ∆P). 3. Elasticity Elasticity refers to the lung's natural ability to return to its original size after being stretched. This property is largely due to the high amount of elastin protein present in the elastic connective tissue in between the alveoli, which plays a major role in exhalation. 16 4. Surface tension Each alveolus is lined with a thin fluid layer that is essential for gas exchange, as gases must dissolve in liquid to move in or out of cells. Surface tension is the force created by this thin fluid layer inside the alveoli that resists their expansion. However, this necessary fluid also creates a challenge; it can cause alveolar walls to stick together due to surface tension, making it harder to inflate the alveoli. 5. Pulmonary surfactant Pulmonary surfactant is a phospholipid produced by alveolar Type II cells. It is found in the fluid lining the alveoli and plays a key role in controlling surface tension in the lungs. It mixes with the alveolar fluid and reduces surface tension, making it easier for the alveoli to expand during breathing and avoiding its collapse by keeping it open after each breath. 6. Airway resistance Airway resistance is the opposition to the airflow through the respiratory tract during breathing, caused by frictional forces of the air against the walls of the trachea, bronchi, and bronchioles and by airway dimensions. It is primarily influenced by airway diameters; a smaller diameter leads to higher resistance. It is also affected by the airway length; longer airways have more resistance. The airway diameter control influenced by the autonomic nervous system will alter the resistance; bronchoconstriction increases resistance, while bronchodilation decreases it. B- Respiration: It is the process of exchanging O2 & CO2 between organism and its environment. It takes place in 2 stages: • External respiration (gas exchange / diffusion) • Internal respiration (tissue / cellular respiration) 17 1. External respiration External respiration is the process of bringing air from the external enviroment into the lungs and the subsequent diffusion of oxygen into the blood and carbon dioxide out of the blood, occurring in the alveolar-capillary interface. This process requires adequate ventilation and gas exchange / diffusion. ❖ Diffusion Diffusion involves the movement of a substance in a solution (liquid or air) from higher concentration areas to lower concentration areas. This process is rapid at the alveolar-capillary membrane due to the large surface area of the alveoli and the very thin barrier between air and blood—just a single layer of squamous cells in both the alveolar and capillary walls. Oxygen in the blood capillaries having a low oxygen partial pressure (PO₂ \~40 mmHg), while oxygen in the alveoli has higher partial pressure (PO₂ is \~100 mmHg). Therefore, oxygen diffuses down the concentration gradient from the alveoli into the blood, which then returns to the heart through the pulmonary veins to be pumped into systemic circulation. In healthy lungs, the oxygen level in the pulmonary veins should match that of the alveoli. However, conditions like ventilation-perfusion mismatch, shunts, or slow diffusion can lower the oxygen levels in the pulmonary veins compared to systemic arterial levels. The efficiency of gas diffusion depends on several factors such as the properties of the gas, blood flow rate (perfusion) and the health and structure of the alveolar-capillary membrane. 2. Internal respiration Internal respiration is the exchange of gases at the capillary-tissue interface, where oxygen from the blood moves into the body's tissues for cellular metabolism, and carbon dioxide moves from the tissues into the blood to be transported away. 18 C- Perfusion Lung perfusion refers to the volume and distribution of blood flowing through the pulmonary capillaries that surround the alveoli. This blood flow is critical for supplying the alveoli with blood for effective gas exchange. ❖ Lung Ventilation / Perfusion ratio (V/Q ratio) In respiratory physiology the V/Q ratio is a ratio used to assess the efficiency and adequacy of the matching of two variables: ventilation (V); the air reaching the alveoli and perfusion (Q); the blood reaching the alveoli via the capillaries. Efficient perfusion, working in harmony with airflow (ventilation), is necessary for healthy oxygen levels in the body. ➢ Normal V/Q ratio The ideal V/Q ratio should be one or slightly greater, meaning, there is one to one matching of ventilation to blood flow, which results in optimum gas exchange. Unfortunately the V/Q ratio is generally not equal to one but varies depending on the body's position and also on the position within the lung. The principal factor involved in the genesis of V/Q dishomogenity is the gravity. Normally: • In supine lying position, blood flow becomes uniform within posterior aspect of the lungs. • In prone / lateral positions both ventilation and perfusion increase dorsally. 19 • In upright position, both blood flow and alveolar ventilation is less in the upper part of the lung than in the lower part • Alveoli at the apex are under perfused but over ventilated. • Alveoli at the base are under ventilated but over perfused. If taken as a whole the typical V/Q ratio is approximately 0.8:1 ➢ V/Q Mismatch: It is a condition in which there is a poor balance in the V/Q ratio such that one or more areas of the lung receive oxygen but poor blood flow (high V/Q), or they receive blood flow but poor oxygen (low V/Q) due to some diseases and disorders. This imbalance impairs the lungs' ability to efficiently deliver oxygen to the blood, leading to hypoxemia. D- Dead Space Dead space refers to the portion of inhaled air that does not participate in gas exchange, either because they stays in the conducting airways, or they reach alveoli that are not perfused. In healthy adults, dead space is approximately 150 mL, which is roughly one-third of a resting tidal volume (typically 500 mL). ❖ Types of Dead Space: 1. Anatomical Dead Space It is the volume of air remaining in the conducting airways. Normally, no gas exchange occurs here because these areas do not contain alveoli. Usually equal in mL to a person’s body weight in pounds (e.g., 150 mL in a 150 lb/68 kg adult). 20 2. Alveolar Dead Space It is the amount of air that reaches alveoli but does not exchange gases due to lack of blood flow in nearby capillaries (perfusion). It is negligible in healthy individuals, however, it can increase in chronic lung diseases. 3. Physiological (Total) Dead Space It is the sum of anatomical and alveolar dead space. In healthy people, it is roughly the same as anatomical dead space. If it increases significantly it may signal respiratory problem. NB: Slow, deep breaths are more efficient than rapid, shallow breaths, even if the total air moved per minute is the same. This is because shallow breathing wastes more air in dead space and results in less effective gas exchange.