Respiratory System Lecture Notes PDF

Physical Properties of the Lungs: Dr.Zina H. Mohammed Objectives : we must know what are — 1. Compliance. — 2. Elasticity. — 3. Surface tension. — 4. Surfactant. — 5. Respiratory function test Compliance Lung compliance, or pulmonary compliance, is a measure of the lung's abili...

Physical Properties of the Lungs: Dr.Zina H. Mohammed Objectives : we must know what are — 1. Compliance. — 2. Elasticity. — 3. Surface tension. — 4. Surfactant. — 5. Respiratory function test Compliance Lung compliance, or pulmonary compliance, is a measure of the lung's ability to stretch and expand (distensibility of elastic tissue) A Change in lung volume per change in transpulmonary pressure. DV/DP The lung is 100 x more distensible than a balloon — The characteristics of the compliance are determined by the elastic forces of the lungs. — These forces can be divided into two parts: (1) elastic forces of the lung tissue; and (2) elastic forces caused by surface tension of the fluid that lines the inside walls of the alveoli and other lung air spaces. — The elastic forces of the lung tissue are determined mainly by elastin and collagen fibers interwoven among the lung parenchyma. — The surface tension of fluid acts to decrease lung compliance. — Lung with low compliance requires greater force for breathing while lung with high compliance requires less force.  Factors That Affect Compliance: Several factors affect lung compliance, and they can be broadly classified into two categories: factors related to the lung tissue itself (elastic properties) and factors related to the external forces that act on the lungs (like the chest wall or airways). Key factors include: 1. Connective-tissue structure of lungs: when elastic fibers replace with collagen fibers as in lung fibrosis lead to decrease lung compliance. 2. Level of surfactant production, surfactant decreases the surface tension in alveoli. It is secreted by special surfactant- secreting epithelial cells called type II alveolar epithelial cells, which constitute about 10% of the surface area of the alveoli. These cells are granular, containing lipid inclusions that are secreted in the surfactant into the alveoli. The surface tension of the fluid lining the alveoli is reduced (but not eliminated) by the action of pulmonary surfactant because surfactant interferes with the hydrogen bonding between water molecules. In this way, surfactant increases lung compliance and decreases the work of breathing. 3. Mobility of the thoracic cage any musculoskeletal disease lead to decrease the lung compliance.  Compliance is reduced when (1) Increase pulmonary venous pressure. (2) Alveolar edema due to insufficiency of alveolar inflation. (3) The lung remains unventilated for a while e.g. Atelectasis. (4) Diseases causing fibrosis of the lung e.g. chronic restrictive lung diseases. On the contrary in chronic obstructive pulmonary disease (e.g. emphysema) the alveolar walls progressively degenerate, which increases the compliance.  Elasticity Lung Elasticity refers to the ability of the lungs to return to their original shape and size after being stretched or inflated. This property is a crucial aspect of normal lung function, as it helps the lungs to expand and contract during breathing.Due to: High content of elastin proteins. The elastic forces caused by surface tension are much more complex. Surface tension accounts for about two thirds of the total elastic forces in a normal lungs.  Compliance vs. Elasticity: Compliance refers to the ease with which the lungs can expand when pressure is applied. Elasticity is the ability of the lung to return to its original shape after being stretched. These two properties work together to allow for normal lung function: compliance helps with expansion, while elasticity ensures the lungs can return to their resting state for efficient exhalation. Surface Tension Pulmonary surface tension refers to the force that acts at the surface of the thin layer of fluid lining the alveoli in the lungs. This surface tension is created by the attraction between water molecules at the air-fluid interface. Lungs secrete and absorb fluid, leaving a very thin film of fluid. This film of fluid causes surface tension. H20 molecules at the surface are attracted to other H20 molecules by attractive forces.  The alveoli are lined with a thin layer of fluid that has a natural tendency to pull together, which can lead to the alveoli collapsing due to surface tension. Without any mechanism to counteract this surface tension, the alveoli would be very difficult to inflate and could collapse, especially during exhalation. Surfactant Surfactant is a mixture of lipids and proteins secreted by type II alveolar cells (pneumocytes) in the alveoli. The primary function of surfactant is to reduce the surface tension of the fluid lining the alveoli, which helps prevent them from collapsing. It does this by disrupting the cohesive forces between water molecules in the alveolar fluid, making it easier for the alveoli to expand and remain open during both inhalation and exhalation.  At the end of the expiration, compressed surfactant phospholipid molecules decrease the surface tension to very low, near-zero levels. Without surfactant, the surface tension would be so strong that it would be very difficult for the alveoli to inflate, and they would likely collapse, particularly during exhalation when the lung volumes are lower. This is why premature infants, who may not produce enough surfactant, are at risk for respiratory distress syndrome (RDS)—a condition where the lungs are unable to maintain proper expansion due to inadequate surfactant production.  What are the advantages of having surfactant and the low surface tension? 1. It increases the compliance of the lung 2. It reduces the work of expanding of the lung with each breath 3. It stabilizes the alveoli (thus the smaller alveoli do not collapse at the end-expiration) 4. It keeps the alveoli dry.  Lung Volumes and Capacities  Lung volumes and capacities refer to the various measurements of air in the lungs during different phases of the breathing cycle. These measurements are important for assessing lung function and identifying respiratory disorders. Lung Volumes Lung volumes represent the amount of air in the lungs at different stages of the respiratory cycle. Tidal Volume (TV): The amount of air inhaled or exhaled with each breath during normal, relaxed breathing. Typical value: 500 mL for an average adult.  Expiratory Reserve Volume (ERV): The amount of air that can be exhaled after a normal tidal exhalation. Typical value: 1,200 mL. Residual Volume (RV): The amount of air remaining in the lungs after a maximum exhalation. This prevents the lungs from collapsing. Typical value: 1,200 mL.  Lung Capacities Lung capacities are combinations of two or more lung volumes. These give a broader picture of lung function. Total Lung Capacity (TLC): The total amount of air the lungs can hold. It is the sum of all lung volumes. Formula: TLC = TV + IRV + ERV + RV Typical value: 6,000 mL.  Vital Capacity (VC): The maximum amount of air that can be exhaled after a maximum inhalation. It represents the total amount of usable air in the lungs. Formula: VC = IRV + TV + ERV Typical value: 4,800 mL. Inspiratory Capacity (IC): The maximum amount of air that can be inhaled after a normal tidal exhalation. Formula: IC = TV + IRV Typical value: 3,600 mL. Functional Residual Capacity (FRC): The amount of air remaining in the lungs after a normal tidal exhalation. Formula: FRC = ERV + RV Typical value: 2,400 mL  Pulmonary function tests: is a group of tests that measure how well the lungs work. These tests assess lung volume, capacity, flow, and the efficiency with which oxygen is exchanged between the lungs and the blood. PFTs are often used to diagnose and monitor conditions like asthma, chronic obstructive pulmonary disease (COPD), and other respiratory disorders. What is Spirometry? Spirometry This is the most common pulmonary function test. It measures how much air you can breathe in and out, and how quickly you can exhale. GOALS of spirometer  To predict the presence of pulmonary dysfunction  To differentiate between obstructive and restrictive lung diseases.  To assess the severity of disease  To assess the progression of disease  To assess the response to treatment  To assess lung impairment as a result of occupational hazard. Measurements Obtained from the spirometer — Forced Expiratory Volume in first second (FEV1) is the volume of air that can be forcibly expired in the first second following a deep breath. — It is usually > 70% of the FVC (FEV1/FVC > 70%). — In obstructive lung disease (e.g., asthma and COPD), FEV1is reduced proportionally more than FVC; therefore, FEV1 /FVC < 70%. — In restrictive lung disease (e.g., fibrosis), both FEV1 and FVC are reduced. This means that FEV1 /FVC is normal or increased. FACTORS INFLUENCING VC PHYSIOLOGICAL :  physical dimensions- directly proportional to ht.  SEX – more in males : large chest size, more muscle power, more SURFACE AREA.  AGE – decreases with increasing age  STRENGTH OF RESPIRATORY MUSCLES  POSTURE – decreases in supine position  PREGNANCY- unchanged or increases by 10% ( increase in AP diameter In pregnancy)  PATHOLOGICAL:  DISEASE OF RESPIRATORY MUSCLES  ABDOMINAL CONDITION : pain, dis. and splinting  Dead space: refers to the volume of air that does not participate in gas exchange (the exchange of oxygen and carbon dioxide) between the lungs and the bloodstream. This can occur in various parts of the airways and lungs where gas exchange does not take place, despite the air being ventilated into these areas.  Types of Dead Space: 1. Anatomic Dead Space: 1. This refers to the volume of air in the conducting airways (nose, mouth, trachea, bronchi, etc.) that does not reach the alveoli, where gas exchange occurs. These airways are simply conducting air but do not exchange gases. 2. The typical volume of anatomic dead space in a healthy adult is approximately 150 mL. For example, if tidal volume (TV), the amount of air moved in and out of the lungs with each breath, is 500 mL, then approximately 150 mL of this is in the anatomic dead space, and the remaining 350 mL reaches the alveoli for gas exchange.  Alveolar Dead Space: This is the volume of air that reaches the alveoli but does not participate in gas exchange. This can occur due to poor perfusion (blood flow) to certain alveoli, meaning there is ventilation without corresponding blood flow to facilitate the exchange of gases. This type of dead space can be caused by conditions such as pulmonary embolism, where blood flow to parts of the lung is blocked, or certain lung diseases. The volume of alveolar dead space can vary and may increase in pathological conditions.  Total Dead Space( PHYSIOLOGICAL DEAD SPACE): The total dead space is the sum of the anatomic dead space and the alveolar dead space. The presence of increased dead space (either anatomically or alveolarly) can be a sign of inefficiency in the respiratory system, as it reduces the overall volume of air that participates in gas exchange.  In normal person, physiologic dead space is nearly equal to the anatomic dead space where alveolar ventilation and blood flow are well matched. If physiologic dead space is greater, there is imbalance of ventilation and perfusion.  Total ventilation: The total volume of the gas leaving the lung per unit time. If TV is 500 ml and there are approximately 15 breaths/min the total volume of the gas leaving the lung, total ventilation will be 500 x 15 = 7500 ml/min. Alveolar ventilation: The volume of the gas reaching the respiratory zone of the airways  not all of the total ventilation volume reaches the alveoli. 150 ml of the TV (500 ml) is left behind in the airways, which does not contain alveoli, therefore does not contribute the diffusion (Anatomic dead space). Thus, the volume of gas entering the respiratory zone, alveolar ventilation, is (500-150) x 15 = 5250 ml/min