Physics of the Respiratory System (Part 2) PDF - University of Al-Kafeel

University of Al-Kafeel Dr. Rajaa R. Abbas nd College of Medicine 2 Semester/ 2023-2024 Medical Physics ___________________________________________________...

University of Al-Kafeel Dr. Rajaa R. Abbas nd College of Medicine 2 Semester/ 2023-2024 Medical Physics _______________________________________________________________________________________ Physics of the Respiratory System (Part 2) Learning objectives After studying this lecture, student be able to: 1. Define airway resistance and its reasons. 2. Describe the effect of surface tension property on the alveoli work. 3. Explain the concepts of high and low lung compliance with examples. 4. Describe and explain gas exchange process at alveoli. ___________________________________________________________________________ Physical Factors Influencing Efficiency of Air Flow Three physical factors influence the ease of air passage and the amount of energy required for ventilation; airway resistance, alveolar surface tension and lung compliance 1. Airway Resistance Airway resistance is the opposition to flow caused by the forces of friction. It is defined as the ratio of driving pressure to the rate of air flow: Air resistance → ⁄ Resistance to flow in the airways depends on: whether the flow is laminar or turbulent, on the dimensions of the airway, and on the viscosity of the gas.  Type of flow: For turbulent flow, resistance is large (across the nasal cavity till to the 4th division). While for laminar flow, Rg is quite low. (below the 15th division it is considered to be zero).  Dimensions: The air passages provide resistance like; mucous and diameter of the airway, (diameter ↓ = resistance ↑). While a single small airway provides more resistance than a single large one, resistance to air flow depends on the number of parallel pathways present. So, the large airways actually provide greater resistance to flow than do the more numerous small airways. Chronic bronchitis In Chronic bronchitis the inflamed bronchial tubes produce a lot of mucus. This leads to coughing and difficulty breathing. Cigarette smoking is the most common cause. The air way resistance becomes large. 1 University of Al-Kafeel Dr. Rajaa R. Abbas nd College of Medicine 2 Semester/ 2023-2024 Medical Physics _____________________________________________________________________________________ Asthma The basic problem is expiratory difficulty due to increased airway resistance. This resistance is due to swelling and mucus in the smaller airways and due to contraction of the smooth muscle around the large airways. widespread narrowing of airways, sometimes spontaneously. 2. Surface Tension of the Alveoli The alveoli are the tiny air sacs in the lungs where gas exchange happens. And their walls are lined by a thin film of water, which creates a force at their surface called surface tension. Surface tension tends to collapse the pulmonary alveoli when they become smaller during expiration. The pressure inside the alveolus (Palv) with radius (r) and tension of the wall (T) is given by Laplace's law; According to Laplace's Law, and during inspiration the following events will happen: - The radius of the alveoli ↑ → The pressure ↓ → The pressure is greater in the smaller alveoli → The smaller alveoli will collapse. But, luckily, alveolar cells have found a way to counteract surface tension by producing surfactant by cells called T2 cells which prevent this collapse by reducing the surface tension and keeping the alveoli open so that one can breathe properly. Respiratory distress syndrome (RDS) The absence of surfactant in the lungs of some newborn infants, especially premature, is the cause of the respiratory distress syndrome (RDS). This disease kills more babies than other disease. 2 University of Al-Kafeel Dr. Rajaa R. Abbas nd College of Medicine 2 Semester/ 2023-2024 Medical Physics _____________________________________________________________________________________ 3. Lung Compliance Lung compliance is the ability of the lungs to stretch and expand during inspiration. Where the resisting to the stretching is due to elastic and collagen fibers. Compliance is given by the change in lung volume; ∆V, produced by a given change in pressure; ∆P (Transpulmonary pressure) as following: 𝑽 𝑳𝒊𝒕𝒕𝒆𝒓 𝑪𝒐𝒎𝒑𝒍𝒊𝒂𝒏𝒄𝒆 𝑷 𝒄𝒎 𝑯𝟐 𝑶 Lung compliance is given by the pressure - volume curve: - Lot of fibrous tissues → lungs are not able to stretch → compliance decreases → lungs volume will decrease → stiff lung (Pulmonary fibrosis). - Loss of fibrous tissues → lungs have super ability to stretch → compliance increases → lungs volume will increase → Floppy lung (Emphysema). Emphysema The divisions between the alveoli break down, producing larger lung spaces. The lung become more compliant (a small change in pressure produces a larger than normal change in the volume; floppy lung). Pulmonary fibrosis It is a lung disease that occurs when lung tissue becomes damaged, scarred and less compliant. This thickened, stiff tissue makes it more difficult for the lungs to work properly. As pulmonary fibrosis worsens, the breath becomes progressively more short. 3 University of Al-Kafeel Dr. Rajaa R. Abbas nd College of Medicine 2 Semester/ 2023-2024 Medical Physics _____________________________________________________________________________________ Gas Exchange Process The primary purpose of breathing is gas exchange. To understand this processes it is necessary to review the three physical laws that governed it. Diffusions law Dalton's law Henry's law It is the movement of A mixture of non-reacting Quantity of a gas that will molecules of a particular type gases, the total pressure dissolve in solution is of a gas from higher exerted is equal to the sum of proportional to the partial concentration to lower the partial pressures of the pressure and the solubility concentration regions. individual gases. coefficient. During normal breathing the fresh supply of air does not enter the alveoli which are still filled with stale air from previous breaths, but the following processes are take place: 1. According to diffusion law, due to the higher concentration, the fresh O2 rapidly diffuses through the stale air to reach the internal surface of the alveoli. 2. Because the concentration of O2 in the alveolar air > in the blood of pulmonary capillaries. So and according to Daltons law. 3. As a result to the difference in partial pressure and according to Henrys law, O2 will be dissolved in the moist alveoli wall and diffuses through into the capillary blood until the PO2 in the blood is equal to that in the alveoli. This process takes place less than 0.5 sec. Meanwhile the CO2 in the blood diffuses even more rapidly into the alveolar air until the P CO2 in the blood is the same as in the alveolar air. 4