Respiratory System Pulmonary Ventilation slide 7 is end of material on Exam 3 Pressure Resistance and Airflow slide 8 is start of material on Exam 4 22 1 Pulmonary Ventilation Breathing pulmonary ventilation repetitive cycle of inspiration and expiration one complete inspiration and expiration is a respiratory cycle The action of respiratory muscles on thoracic cavity causes pressure differences in lungs which fills the lungs with air No muscles are directly attached to lungs The smooth muscles of the lungs do not directly cause airflow The smooth muscle of the bronchioles and bronchii affect the diameter of the airway and the speed of the airflow but do not directly cause airflow Lungs are passive structures like balloons Quiet respiration at rest while forced respiration is deep or rapid breathing for example during exercise 22 2 Respiratory Muscles Quiet Inspiration The respiratory muscles are skeletal muscles of the trunk diaphragm and the intercostal muscles Diaphragm prime mover of pulmonary ventilation The diaphragm accounts for about two thirds of pulmonary airflow relaxation allows diaphragm to bulge upward contraction flattens diaphragm enlarging thoracic cavity Internal and external intercostal muscles aid diaphragm synergists that work with the diaphragm action of expanding or contracting thoracic cavity stiffen the thoracic cage during inspiration prevents it from caving inward contribute to enlargement of thoracic cage during quiet inspiration i e expanding the ribs which adds about 1 3 of the airflow ventilating the lungs 22 3 Respiratory Muscles Quiet Inspiration Scalenes during quiet breathing scalenes holds ribs 1 and 2 stationary while external intercostal muscles pull the other ribs upward This makes it easier for the sternum to swing out as intercostals pull the rest of te ribs up increases both transverse and anterioposterior diameters of the chest 22 4 Quiet Expiration passive energy saving process Because the ribcage and lungs have elastic recoil inwards the bronchial tree tendons of diaphragm and other respiratory muscles and rib attachments to spine and sternum spring back when the muscles relax muscles relax gradually preventing the lungs from recoiling too suddenly 22 5 Respiratory Muscles Forced Breathing Scalenes elevate upper ribs during forced inspiration Rectus abdominus pulls down on sternum and lower ribs during forced expiration Internal intercostals pull other ribs downward during forced expiration These muscle actions reduce thoracic cavity and expel air more rapidly Abdominal and lumbar muscles also contribute to forced expiration greatly increase abdominal pressure pushing viscera up against diaphragm forcing air out quicker 22 6 Respiratory Muscles 22 7 Pressure and Airflow Respiratory airflow is determined by the same principles of flow pressure and resistance as blood flow F P R the flow of a fluid is directly proportional to the pressure difference between two points the flow of a fluid is inversely proportional to the resistance 760 mm Hg at sea level 1 atmosphere atm All pressures in resp system are given in relative pressure which is the difference btw atm pressure and the resp pressure The pressure difference that drives airflow in and out of lungs is difference between intrapulmonary alveolar pressure Palv internal lung pressure and atmospheric barometric pressure Patm This is not the same as the pressure that actually expands the lungs If Palv and Patm are equal then relative Palv is zero If relative Palv is for example 3 mmHg then absolute Palv 9e g 757 is 3 mmHg below Patm e g 760 22 8 Intrapleural Transpulmonary Pressure intrapleural pressure Pip pressure outside the lungs due to the pressure of the intrapleural fluid surrounding the lungs negative Between the two layers parietal and visceral of pleura Pip is a slight At rest at end of unforced expiration natural lung recoil inward is opposed by natural chest wall recoil outward which pulls on the ribs so that the ribs recoil in more slowly than the lungs At rest both forces pulling on the Pip cavity in opposite directions which helps create the negative Pip Two layers of pleurae stick together like wet paper The slight neg pressure in pleural cavity is similar to suction from trying to pull apart two class slides with a drop of water btw transpulmonary pressure Palv Pip pressure across the wall of the lungs which holds lungs open or expands lungs The more negative is Pip the more positive Ptp becomes 22 9 Inspiration At the beginning of inspiration no movement of the thoracic cavity occurs and no diff btw air pressure inside and outside Palv Patm and thus no airflow Boyle s Law at a constant temperature the pressure of a given quantity of gas is inversely proportional to its volume Gas pressure changes when the volume of space changes As the thoracic cavity expands due to inspiratory muscle action it pulls parietal layer of pleura outward while visceral layer is held inward Pip now decreases even more becomes more negative which increases Ptp becomes more positive Ptp now becomes greater than the inward elastic recoil of the 22 10 lungs and Ptp epands lungs Inspiration Charles s Law the volume of a gas is directly proportional to its absolute temperature during inspiration outside air is warmed to 37 C by the time it inflation of lungs aided by warming of inhaled air which expands reaches the alveoli the air As lung vol increases Palv decreases the less than Patm and the air will start to flow into the lungs At the end of inspiration Ttp equeals elastic recoil inward Because Ptp equals elastic revoil inward lungs stop expanding Recoil is now at is greatest because lungs are furthest apart Because lung volume is now greatest lungs want to recoil inward as air rushes in Palv rises again and inspiration stops when the even more now Palv is equal to Patm 22 11 Expiration During quiet breathing expiration achieved by elastic recoil inward of lungs and ribcage Ribs move in more slowly slowing lung recoil inward As respiratory muscles relax elastic recoil makes thoracic cavity smaller which leads to less negative intrapleural pressure decreasing transpulmonary pressure which decreases lung size Ptp becomes less positive and now is smaller than the elastic recoil exerted by the more and more expanded lungs at start of expiration The decreasing lung volume raises Palv above Patm and air flows out of the lungs Air flows out of the lungs until intrapulmonary pressure equals atmospheric