Lecture 3 Respiratory System PDF

Respiration functional events Dr.Zina H.Mohammed Objectives : you must understand the following Mechanism of ventilation How Gas exchange? What are respiratory membrane? What are Partial pressures? How Oxygen (O2) transport ? Respiration can be divided into four major functional e...

Respiration functional events Dr.Zina H.Mohammed Objectives : you must understand the following Mechanism of ventilation How Gas exchange? What are respiratory membrane? What are Partial pressures? How Oxygen (O2) transport ? Respiration can be divided into four major functional events: 1. Ventilation: Movement of air into and out of lungs. 2. Gas exchange: between air in lungs and blood. 3. Transport: of oxygen and carbon dioxide in the blood. 4. Internal respiration: Gas exchange between the blood and tissues.  First: Ventilation Pulmonary Ventilation (Breathing): This is the physical movement of air into and out of the lungs. It includes both inhalation (inspiration) and exhalation (expiration), which help to bring oxygen into the lungs and remove carbon dioxide from the body. There are 3pressures in the lungs: Intra-pleural pr. : is the pressure in the intra- pleural space that is generated between the lungs and chest wall. Alveolar pr. : is the pressure within the alveoli. Trans-pulmonary pr. : is alveolar pressure minus intra-pleural pressure. Principles of Ventilation (Breathing): At rest with mouth open external pressure (Pb)= internal pressure (Pi)= 0. Inhalation (Inspiration): Purpose: To bring oxygen-rich air into the lungs. Mechanism: During inhalation, the diaphragm contracts and moves downward, while the external intercostal muscles (between the ribs) contract to expand the rib cage. This increases the volume of the thoracic cavity and decreases the pressure within the lungs. As a result, air flows into the lungs from the environment due to the pressure difference (higher pressure outside the body and lower pressure inside the lungs).  Exhalation (Expiration): Purpose: To expel carbon dioxide-rich air from the lungs. Mechanism: Exhalation is often a passive process during normal breathing. When the diaphragm relaxes and moves upward, and the intercostal muscles relax, the rib cage becomes smaller, reducing the volume of the thoracic cavity. This increase in lung pressure causes air to be pushed out of the lungs. During forced exhalation (e.g., during exercise), additional muscles (such as the abdominal muscles) may contract to help expel air more forcefully. Factors Affecting Pulmonary Ventilation Airway resistance: Narrowed or obstructed airways (e.g., due to asthma or bronchitis) can increase the resistance to airflow, making it harder to breathe. Lung compliance: Decreased compliance (as seen in diseases like pulmonary fibrosis) makes breathing more difficult. Chest wall compliance: The ability of the chest wall to expand during inhalation also affects pulmonary ventilation. Conditions like obesity or deformities can restrict expansion.  In addition to mechanical factors, control of ventilation is regulated by the respiratory centers in the brainstem (medulla oblongata and pons) in response to levels of carbon dioxide, oxygen, and pH in the blood. By ensuring efficient ventilation, the body is able to maintain proper gas exchange, supply oxygen to tissues, and remove metabolic waste products like carbon dioxide Active expiration: 1. Using muscles of expiration 2. Occurs during exercise or in obstructive lung diseases. 3. During heavy breathing, the elastic forces are not powerful enough to cause the necessary rapid expiration 4. So that extra force is achieved mainly by contraction of the abdominal muscles, which pushes the abdominal contents upward against the bottom of the diaphragm, thereby compressing the lungs. 5. The muscles that used during forcible expiration are mainly (1) abdominal recti, and (2) internal intercostal muscles.  Cough reflex The cough reflex is a protective mechanism that helps clear the airways of irritants such as mucus, foreign particles, or pathogens. The reflex involves a coordinated series of events that include sensory input, central processing, and motor output.  Stimulation of Cough Receptors The cough reflex begins when sensory receptors in the airways detect an irritant. The receptors involved in cough detection are primarily mechanoreceptors and chemoreceptors. They respond to: Physical irritants (dust, smoke, mucus) Chemical irritants (cigarette smoke, strong odors, pollutants) Inflammation (due to infection or injury)  once the cough receptors are stimulated, they send an afferent (sensory) signal through vagal nerve fibers (mainly the afferent vagus nerve, or cranial nerve X) to the cough center in the brainstem. The cough center is located in the medulla oblongata.  Processing in the Cough Center The cough center integrates sensory input from the vagus nerve and coordinates the appropriate motor responses. This center communicates with motor neurons that regulate: Respiratory muscles (diaphragm, intercostal muscles, abdominal muscles) Laryngeal muscles (to initiate a cough sound) The cough center also receives inputs from higher centers in the brain (e.g., cortex, limbic system), which may modulate the response based on context, emotional state, and learned behaviors (e.g., voluntary suppression of cough).  The motor response to the cough signal consists of a sequence of events: Inhalation: The diaphragm and intercostal muscles contract to rapidly inhale a large volume of air, usually to a deep lung volume. Glottic Closure: The vocal cords close, creating a build-up of pressure in the lungs. Expiratory Effort: After the glottis closes, the abdominal and intercostal muscles contract forcefully to increase the intra- abdominal and intrathoracic pressure. Expulsion of Air (Cough): The sudden opening of the vocal cords and a rapid exhalation forcefully expels air from the lungs, helping to dislodge irritants from the airways. The speed of this exhalation can exceed 100 miles per hour in some cases, and it is accompanied by the sound of a "cough.  The sneeze reflex: Receptors in the nasal passageways. The initiating stimulus of the sneeze reflex is irritation in the nasal passageways. The afferent impulses pass in the fifth cranial nerve to the medulla. A series of reactions similar to those for the cough reflex takes place; the uvula is depressed, so that large amounts of air pass rapidly through the nose.  Gas exchange Gas exchange between the lungs and the blood occurs in the alveoli. This process involves the exchange of oxygen (O₂) from the inhaled air into the bloodstream and the removal of carbon dioxide (CO₂), a waste product of metabolism, from the blood into the lungs to be exhaled. 1.Inhalation: When you breathe in, air enters the lungs and travels down to the alveoli. 2.Diffusion of Oxygen: Oxygen in the alveolar air has a higher concentration than in the blood of the capillaries surrounding the alveoli. Because of this concentration gradient, oxygen moves from the alveoli into the capillaries by diffusion. 3.Oxygen Transport: Once in the blood, oxygen binds to hemoglobin molecules in red blood cells, forming oxyhemoglobin. This allows oxygen to be transported throughout the body. 1.Diffusion of Carbon Dioxide: At the same time, carbon dioxide (a waste product from cellular metabolism) is present in higher concentrations in the blood than in the alveolar air. CO₂ diffuses from the blood into the alveoli, where it is expelled from the body when you exhale. 2.Exhalation: The carbon dioxide-rich air is then exhaled out of the lungs.  This gas exchange relies on the principle of diffusion, where gases move from areas of higher concentration to lower concentration until equilibrium is reached. The thin walls of the alveoli and capillaries, combined with the large surface area of the alveoli, facilitate efficient gas exchange. Respiratory membrane (blood-gas barrier) : Is 0.3 micrometer thickness, it has a surface area of 50 to 100 m2 and contain 60- 140ml blood. and composed of: 1) Fluid (surfactant) in alveoli 2) Epithelium of alveoli 3) Epithelial basement membrane 4) Interstitial fluid 5) Capillary basement membrane 6) Endothelial cells of capillary Factors Affecting Diffusion through the Respiratory Membrane 1. Surface Area of the Respiratory Membrane Greater surface area allows for more gas molecules to diffuse at a given time. The alveoli, with their extensive surface area, are designed to maximize diffusion. Any condition that reduces the surface area, such as emphysema or pulmonary fibrosis, can decrease the efficiency of gas exchange.  2. Thickness of the Respiratory Membrane Thinner membranes facilitate faster diffusion as the distance that gases must travel is reduced. An increase in membrane thickness, such as in pulmonary edema (fluid accumulation in the lungs) or fibrosis, makes it harder for gases to diffuse, decreasing the efficiency of gas exchange.  3. Partial Pressure Gradient Diffusion occurs from areas of higher partial pressure to areas of lower partial pressure. The larger the difference in partial pressures of gases between the alveoli and the blood, the faster the diffusion. For example, O₂ diffuses from the alveoli (where the partial pressure is higher) into the blood (where the partial pressure is lower), while CO₂ diffuses from the blood (higher partial pressure) into the alveoli (lower partial pressure).. Partial Pressures in Alveoli Henry’s law: Gases diffuse from high pressure to low pressure. Alveolar PO2 depends on: 1. The rate of O2 absorption into the blood. 2. The rate of entry of new O2 during ventilation. Alveolar PCO2 depends on: 1. The rate of CO2 excretion from the blood. 2. The rate of removal of CO2 during ventilation. Diffusion rate depends upon : Pressure differential and Solubility of the gas in the fluid. (O2 and CO2 Exchange by DIFFUSION PO2 in alveoli ~ 103 mmHg, while in pulmonary capillaries ~ 40 mm Hg. Result: **O2 moves into pulmonary capillaries from high PP in alveoli (~103-104 mmHg) to low PP in capillaries (~ 40 mmHg) till reach the equilibrium stat (~ 104 mmHg). **While in tissues O2 move from high partial pressure in capillaries (~ 95 mmHg) to low PP in tissues (~ 20 mmHg) till reach the equilibrium stat (~ 40 mmHg) While PCO2 is 45mmHg in alveolar capillaries and 40mmHg in alveoli. **So moves of CO2 from high PP in capillaries to low PP in alveoli till reach the equilibrium stat (~ 40 mmHg). **While in tissues CO2 move from high partial pressure in tissues (~ 45 mmHg) to low PP in capillaries (~ 40 mmHg) till reach the equilibrium stat (~ 45 mmHg).  4. Solubility of Gases Gases with higher solubility diffuse more easily through the respiratory membrane. CO₂ is more soluble in water than O₂, meaning it diffuses more quickly despite having a smaller partial pressure gradient. The solubility of gases is important for their transfer through the membrane  5. Molecular Size of the Gases Smaller molecules tend to diffuse more easily than larger ones. Oxygen (O₂) is smaller and diffuses faster than carbon dioxide (CO₂), but due to CO₂'s higher solubility, it diffuses efficiently even with a smaller gradient. 6. Ventilation-Perfusion Ratio Ventilation refers to the airflow into the alveoli, while perfusion refers to the blood flow through the pulmonary capillaries. An optimal ventilation-perfusion ratio is essential for efficient gas exchange.  If ventilation is inadequate (e.g., in cases of airway obstruction), the diffusion of gases is impaired. Similarly, if perfusion is reduced (e.g., due to a blockage in blood vessels), the amount of blood available to exchange gases is less, reducing overall diffusion efficiency. 7. Temperature Higher temperatures can increase the rate of diffusion because the molecules move faster at higher temperatures. However, extreme temperature changes (e.g., fever or hypothermia) can affect the efficiency of gas exchange.  8. Diseases and Pathologies Conditions like asthma, chronic obstructive pulmonary disease (COPD), and pulmonary edema can significantly affect the factors mentioned above, such as the surface area of the respiratory membrane, the thickness of the membrane, and the partial pressure gradients. 9. Barometric Pressure Atmospheric pressure also plays a role in the diffusion of gases. At higher altitudes, the partial pressures of gases (like O₂) are lower, which can reduce the rate of diffusion into the bloodstream.