Mechanics of Breathing (Pulmonary Ventilation), Lecture notes of Mechanics

Download Mechanics of Breathing (Pulmonary Ventilation) and more Lecture notes Mechanics in PDF only on Docsity! Mechanics of Breathing (Pulmonary Ventilation) • Completely mechanical process • Depends on volume changes in the thoracic cavity • Volume changes lead to pressure changes, which lead to the flow of gases to equalize pressure • Two phases – Inspiration – flow of air into lung – Expiration – air leaving lung Muscles used for ventilation The muscles of inspiration include the diaphragm, external intercostals, sternocleidomastoids, and scalenes. The muscles of expiration include the internal intercostals and the abdominals. Pressure Relationships • Intrapulmonary pressure and intrapleural pressure fluctuate with the phases of breathing • Intrapulmonary pressure always eventually equalizes itself with atmospheric pressure • Intrapleural pressure is always less than intrapulmonary pressure and atmospheric pressure Pressure Relationships • Two forces act to pull the lungs away from the thoracic wall, promoting lung collapse – Elasticity of lungs causes them to assume smallest possible size – Surface tension of alveolar fluid draws alveoli to their smallest possible size • Opposing force – elasticity of the chest wall pulls the thorax outward to enlarge the lungs. • Lymphatic system drains the pleural fluid, generating a negative pressure (- 5cm H2O pressure ) Pressure Relationships Atmospheric pressure Intrapleural pressure 756 mm Hg (-4 mm Hg) Lung Thoracic wall Parietal pleura Collapsing force of lungs Pleural 4mm Hg cavity Visceral pleura Diaphragm intrapulmonary pressure 760 mm Hg (0 mm Hg) Lung Collapse • Caused by equalization of the intrapleural pressure with the intrapulmonary pressure • Transpulmonary pressure keeps the airways open – Transpulmonary pressure – difference between the intrapulmonary and intrapleural pressures (Ppul – Pip) Pulmonary Ventilation • A mechanical process that depends on volume changes in the thoracic cavity • Volume changes lead to pressure changes, which lead to the flow of gases to equalize pressure Boyle’s Law • Boyle’s law – the relationship between the pressure and volume of gases P1V1 = P2V2 – P = pressure of a gas in mm Hg – V = volume of a gas in cubic millimeters – Subscripts 1 and 2 represent the initial and resulting conditions, respectively “Pump handle" motion increases anterior-posterior dimension of rib cage Vertebrae Sternum “Bucket handle" motion increases lateral dimension of rib cage Expiration • Inspiratory muscles relax and the rib cage descends due to gravity • Thoracic cavity volume decreases • Elastic lungs recoil passively and intrapulmonary volume decreases • Intrapulmonary pressure rises above atmospheric pressure (+1 mm Hg) • Gases flow out of the lungs down the pressure gradient until intrapulmonary pressure is 0 Volume of breath fos ® e 5 S 0 4 seconds elapsed Inspiration | Expiration S ; Intrapulmonary a +2 faz pressure EE —o—— hs. : | 2 Bo Trans- 2 2 ; pulmonary 32-4 — ge ZB 6 | \— Intrapleural ® £ E -g | pressure Lung Compliance • The ease with which lungs can be expanded • Specifically, the measure of the change in lung volume that occurs with a given change in transpulmonary pressure • Determined by two main factors – Distensibility of the lung tissue and surrounding thoracic cage – Surface tension of the alveoli Lung Compliance • A. Compliance of Lungs and Thoracic wall:0.13 liters/cm of H2O • B. Compliance of the Lungs only: 0.22 litre/cm H2O Factors That Diminish Lung Compliance • Examples include: – Deformities of thorax – Ossification of the costal cartilage – Paralysis of intercostal muscles Static Compliance Curves -18-16-14-12-10-8-6-4-202 Fibrosis (low compliance) Normal Emphysema (high compliance) VT FRCN VT FRCF VT FRCE Pleural Pressure, Ppl (cm H2O) L u n g V o lu m e Surface Tension. – At every gas-liquid interface surface tension develops. – Surface Tension is a liquid property – LaPlace’s Law: P T r  2 P1 P2 T T r1 r2 T P r 2 P r 2 1 1 2 2     If r r Then, P P1 2 2 1  Result: Small Bubble Collapses Law of LaPlace: P = 2T/r P = pressure T = surface tension t =radius According to the law of LaPlace, if two bubbles have the same surface tension, the small bubble will have higher pressure. (b) Surfactant reduces surface tension (T). Pressure is equalized in the large and small bubbles. MOG eee e e - % oe e e , eee” & e ~e 8. rae r=1 T=2 T=1 P = (2 x 2)/2 P= (2x 1)/1 P=2 P=2 Physiological Importance of Surfactant • Increases lung compliance (less stiff) • Promotes alveolar stability and prevents alveolar collapse • Promotes dry alveoli: – Alveolar collapse tends to “suck” fluid from pulmonary capillaries – Stabilizing alveoli prevents fluid transudation by preventing collapse. Infant Respiratory Disease Syndrome (IRDS) • Surfactant starts late in fetal life – Surfactant: 23 wks 32-36 wks • Infants with immature surfactant (IRDS) – Stiff, fluid-filled lungs – Atelectatic areas (alveolar collapse) • Collapsed alveoli are poorly ventilated • Effective right to left shunt (Admixture)