Pulmonary ventilation
Dr.Suroor Mohamed PhysiologyObjectives: What s mean by Ventilation , Inspiration , Expiration & Boyle s Law? To identify the muscles used during ventilation. Mechanism of ventilation of the lung. Types of respiratory pressures
Ventilation : Movement of air into and out of lungs Air moves from area of higher pressure to area of lower pressure Pressure is inversely related to volume
Mechanical process that moves air in and out of the lungs.● [O2] of air is higher in the lungs than in the blood, O2 diffuses from air to the blood.● C02 moves from the blood to the air by diffusing down its concentration gradient.● Gas exchange occurs entirely by diffusion:◦ Diffusion is rapid because of the large surface area and the small diffusion distance.
Pulmonary ventilation or breathing is the exchange of air between the atmosphere and the lungs. As air moves into and out of the lungs, it travels from regions of high air pressure to regions of low air pressure. Taking in of air is inspiration & giving out of air is expirationNormal respiratory rate (RR) is 12-16/min.↑RR→ tachypnea, e.g., exercise, fever.Arrest of respiration → apnea, e.g, deglutition apnea, sleep apnea etc.Difficulty in breathing → dyspnea, e.g, bronchial asthma.
Lungs normally remain in contact with the chest walls.● Lungs expand and contract along with the thoracic cavity.● Intrapulmonary pressure: ◦ Intra-alveolar pressure (pressure in the alveoli).● Intrapleural pressure: ◦ Pressure in the intrapleural space. ◦ Pressure is negative, due to lack of air in the intrapleural space. Transpulmonary Pressure ● Pressure difference across the wall of the lung. ◦ Keeps the lungs against the chest wall I,e (Intrapulmonary pressure – intrapleural pressure.) Boyle’s Law● Changes in intrapulmonary pressure occur as a result of changes in lung volume. ◦ Pressure of gas is inversely proportional to its volume.● Increase in lung volume decreases intrapulmonary pressure.( Air goes in).● Decrease in lung volume, raises intrapulmonary pressure above atmosphere.(Air goes out).
At the beginning of inspiration, the alveolar pressure is equal to atmospheric pressure. As the thoracic cage volume rises at the beginning of inspiration, it pulls the parietal pleura away from visceral pleura causing a further decrease in the intrapleural pressure , the reduction in the intrapleural pressure in turn pulls the visceral pleura that cover the lung and consequently the alveoli to expand. As the alveoli are expanded, alveolar pressure decreases to less than atmospheric pressure (i.e. becomes negative) The pressure gradient between the atmosphere and the alveoli lead to air flow into lungs, and airflow will continue until the pressure gradient dissipates. Because lung pressures are expressed relative to atmospheric pressure, alveolar pressure is said to be zero (i.e., equal to atmospheric pressure) at the beginning of inspiration (i.e., at the end of expiration).
During expiration, the alveolar pressure becomes larger than atmospheric pressure (i.e., becomes positive) because alveolar gas is compressed by diaphragm, cage, and lung. Thus, alveolar pressure is higher than atmospheric pressure, and the pressure gradient is reversed, and air flows out of the lungs.. In normal quiet breathing, inspiration is an active process and expiration is passive
Pneumothorax is the presence of air in the pleural space, which causes collapse of the lung on that side (atelectasis) away from the chest wall The pleural space is only a potential space because the serous fluid keeps the pleural membranes adhering to one another . the intrapleural pressure is always slightly sub atmospheric pressure. Should air at atmospheric pressure enter the pleural cavity, the suddenly higher pressure outside the lung will contribute to its collapse (the other factor is the normal elasticity of the lungs). When air is introduced into the pleural space, the pleural pressure becomes equal to atmospheric pressure-the chest wall springs outward and the lungs collapse.
Muscles of respiration: Muscles of Inspiration Normal inspiration: Diaphragm External intercostal muscles In forced inspiration (exercise, asthma) accessory muscles of inspiration act which are sternocleidomastoid, scalene
Muscles of Expiration Normal quite expiration No muscles are actively involved (expiration is passive) However, there is some contraction of inspiratory muscles in the early part of expiration. This contraction exerts a braking action on the recoil force and slows expiration. The inspiratory muscles that have contracted, relax. In forced expiration Muscles of the anterior abdominal wall (recti) Internal intercostals
Mechanism of ventilation of lungsContraction of inspiratory muscles ↓ Thoracic cage moves out↓Parietal pleura moves out (adherent to the thoracic cage )↓Draws along with it visceral pleura (adherent to the lungs)↓The lungs expand (Due to dragging force exerted by the visceral pleura and also to elastic property of lung) ↓The inside volume is increased ↓Alveolar pressure ↓ (Boyle’s Law)↓Air from outside is sucked into the lungs (INSPIRATION)
Relaxation of inspiratory muscles ↓ Lungs retract due to elastic recoil↓ Inside volume decrease ↓ Alveolar pressure >atmospheric pressure ↓ Air driven out (expiration)The duration of each normal respiratory cycle is 4-5 seconds
Movements of the thoracic cage:During inspiration, dimensions of the thoracic cage ↑ in three ways:↑ Vertical dimension↑ Anteroposterior dimension↑ Transverse dimensionMechanism of ↑ in vertical dimension Contraction of the diaphragm→ moves downwards like a piston for about 1.5 cm ( normal quiet inspiration), 7cm(forced inspiration)75% of air entry during inspiration is by the activity of diaphragm.Diaphragm is supplied by phrenic n (C3, 4 & 5)In diaphragmatic paralysis, respiration is seriously impaired. Clinically notes :Transection of the spinal cord above C3 is fatal without artificial respiration → paralysis of all respiratory musclesTransaction of the spinal cord below the C5 leaves the nerves that innervate the diaphragm intact and its activity is adequate to maintain life.
Mechanism of ↑ in anteroposterior and transverse diameterContraction of external intercostal muscles Originate from the lower border of upper rib and inserted into the upper border of lower ribFibers are directed downwards, forwards and mediallyContraction of external intercostal muscles → upper ribs become more horizontal → Sternum is thrust The lower ribs in addition to moving upwards also swing outwards → ↑ transverse dimension upwards and forwards (moves away from the vertebral column) →↑ anteroposterior dimension
Pressure changes during respiratory cycle: Respiratory pressures are: Intrathoracic / intrapleural pressure Intrapulmonary / intra-alveolar pressure Transpulmonary pressure/ transmural pressure Intrapleural Pressure: In normal breathing is always subatmospheric (-ve) Atmospheric pressure which is equal to 760 mmHg is taken as zero atmosphere Causes of negative intrapleural pressure The natural tendency of lungs is always to recoil inwards. The tendency of thoracic cage is to expand or recoil outwards. These two forces are equal in intensity and act in opposite directions against a closed space so that the pressure in the space becomes subatmospheric. Rapid absorption rate of pleural fluid by pulmonary capillaries and also by the lymphatics
Variation in intrapleural pressureDuring different phases of respirationAt the end of expiration the pressure is –2.5 mmHg in normal quiet breathing. During inspiration (–6 mmHg) During expiration → less negativeTowards the end of expiration (–2.5 mm Hg)In forced inspiration(–12 to–18 mm Hg )Forced expiration → positive pressureIn Muller's maneuver, i.e., forced inspiration with closed glottis as if sucking fluid with a straw → (–40 mm Hg)In Valsalva's maneuver, i.e., forced expiration with closed glottis as in straining → (+40 mm Hg)
Regional variation →effect of gravityNear the apex : (–6 mm Hg)In the middle part of the lung (-2.5mmHg)Near the base it is about (–1 mm Hg)Measurement of Intrapleural PressureDirect:By introducing a needle into the pleural space and connecting it to a manometer.Indirect:By introducing a catheter whose tip contains a thin walled balloon into the esophagus and connecting it to manometer.
It increases venous returnMaintain alveolar stabilityKeep the airways openPrevent collapse of lung, i.e., keeps lungs in an expanded position.If the chest wall is opened, the negative intrapleural pressure is lost and the lungs collapseEffects of positive intrapleural pressurePositive intrapleural pressure compresses the great vessels in the thoracic cavity →↓ venous return to heart →↓ COP → cerebral ischemia → loss of consciousnessThis is the reason for the syncopal attacks following continuous severe bouts of cough especially in old people and in cardiac patients. Importance of negative intrapleural pressure
Intrapulmonary pressurePressure developed inside the alveolialveoli are in connection with the atmosphere, at the end of normal expiration the intrapulmonary pressure = atmospheric pressure, i.e., zero. Quite inspiration → negative Midinspiration = –1 mmHg End of inspiration → zero Quite expiration → positive Mid-expiration = +1 mmHg End of expiration → zero In forced inspiration, i.e., in Muller's maneuver → (–80 mmHg)In forced expiration, i.e., Valsalva's maneuver →(+100 mmHg).
Transpulmonary /Transmural PressureThe pressure difference across the lung= Intrapulmonary pressure - Intrapleural pressure↑Transpulmonary pr → greater stretching of lung →↑ lung volume↑During inspiration & ↓ during expirationValues of transpulmonary pr at the end of normal expiration :At the apex of the lung: 0 - (-6) = 6 mmHgIn the middle of the lung: 0- (-2.5) = 2.5 mmHgAt the base of the lung: 0 - (-1) = 1 mmHgSince the transmural pressure is less at the base of the lung: The lung is less expanded at the baseThis pressure further decreases at the end of forced expiration causing the airways to close at the base