Physiology of Respiratory System

Resting Condition of the Lungs

The resting condition of the lungs means when the breath is held at the end of normal expiration. When the breath is held at the end of normal expiration, the residual volume (RV) is present inside the lung along with the expiratory reserve volume (ERV). This happens because expiration has happened normally. A combination of both of these volumes is known as functional residual capacity (FRC). The functional reserve capacity is known as the resting lung volume. In this resting condition, the respiratory muscles remain relaxed, and this volume is hence also known as the relaxation lung volume.

 Thus, the resting or relaxation lung volume is known as functional residual capacity. In case of a forceful expiration, the expiratory reserve volume (ERV) will go outside of the lung. At the level of the lung, all the alveoli are surrounded by elastic tissues. These elastic tissues have an inherent tendency to recoil in the inward direction and try to cause a collapse of the alveoli. The lungs are surrounded by visceral pleura. The elastic tissues are connected to the visceral pleura through the elastic septum. If the alveoli are getting collapsed, the visceral pleura will follow in the same direction. 

Read this blog further to get a quick overview of this important topic for physiology and ace your NEET PG exam preparation.

The parietal pleura is attached to the chest wall and the muscles.

All of these structures present in the chest are elastic in nature which means when they are stretched, they have a tendency to recoil back. The elastic recoiling of the chest wall is in the outward direction. The elastic tissue of the lungs has an inherent tendency to recoil inwards. In between the visceral and parietal pleura, the intrapleural space, it contains little amount of water with a volume of 10 to 20 ml. 

The parietal plural is trying to expand in the outward direction because of the elastic recoiling of the chest wall. The visceral pleura can be seen to recoil in the outward direction due to the elastic tissue of the lung. In between both a little amount of water can be seen. These two pleurae are very thin like a cellophane membrane and the water or the fluid in between acts like glue, making these attach to one another; the pressure in such a case is zero. Now when both of these are being recoiled in opposite directions, there is a creation of space and volume between the two pleura. According to Boyle's law whenever the volume expands, the pressure decreases (PV = Constant). When the two pleurae are pulled in different directions there will be a creation of pressure inside the space that is less than zero which is negative pressure. This is why it is said that intrapleural pressure is lower than 0 mm Hg. The intrapleural pressure or the intrathoracic pressure is negative pressure.

Intrapleural Pressure/Intrathoracic Pressure

The intrapleural pressure is usually - 2.5 mm Hg or - 5 cm H2O. The advantage of this pressure being negative will now be discussed. The alveoli have an inherent tendency to collapse. In order to keep the alveoli open some pressure has to be given from the inside which will try to expand the alveoli. Some pressure has to be given from the outside which will stretch the alveoli open. In order to prevent the alveoli from collapsing the stretching force has to either be from the inside or negative pressure has to be given from the outside. If the alveoli are to be kept open a negative pressure zone is to be maintained around the lung, which is done by the intrapleural pressure. Thus, the negative pleural pressure keeps the alveoli in an open condition. If this pressure becomes 0 or negative the alveoli will collapse Negative pressure is generated by elastic recoiling. At the level of the parietal pleura, capillaries are present from which the intrapleural fluid is generated into the intrapleural space. This fluid is secreted from the capillaries at the level of the parietal pleura. Absorption of the fluid is done by the lymphatics present at the level of the visceral pleura. The net force which is pushing the fluid out of the capillary, is the net pressure, 9 cm H2O. The net pressure for absorption at the level of lymphatic systems is 10 cm H2O. At the intrapleural space, secretion is occurring with a lesser amount of pressure, and absorption of fluid is occurring with more pressure, making the space remain in a negative zone. Hence, the maintenance of the negative pressure in the intrapleural zone is done by lymphatic drainage.

The pressure at the alveoli in a resting condition is in continuity with the atmospheric pressure. The atmospheric pressure (ATM) is 760 mm Hg. The atmosphere and alveoli are connected with one another and thus there will be an equilibrium of pressure making the pressure inside the alveoli, intra-alveolar pressure/ intrapulmonary pressure, 760 mm Hg. Intra-alveolar pressure/ intrapulmonary pressure is normally equal to 0 mm Hg. The zero here is not an absolute zero but is a relative zero, which means that in comparison with the atmospheric pressure if the pressure inside is the same it is written as 0 mm Hg. All of the pressures being discussed here are the relative pressures, which are with respect to atmospheric pressure. The intrapleural pressure is -2.5 mm Hg, which is also a relative pressure. The actual pressure would be 760 - 2.5 mm Hg. This is the actual pressure at the level of intrapleural space. Minus means that the pressure is below atmospheric pressure.  Some books might write + 2.5 mm Hg Sub-ATM. This also means the pressure is below the atmospheric pressure.

Simple Pneumothorax

In case of an injury to the chest wall causing an opening on the chest wall. The opening causes the air to enter into the pleural space because the intrapleural pressure is lower. Air in this space is called Pneumothorax.  Air will continue to enter the intrapleural space unless equilibrium is reached, which is 760 mm Hg, the same as the atmospheric pressure. The entry of air will then stop. Injury to the chest wall causes Pneumothorax, which will create intrapleural pressure that is equal to the atmospheric pressure, which is 0 mm Hg. In the case of simple pneumothorax: Intrapleural pressure will be 0 mm Hg. No one is there to hold the alveoli open since the pressure has become neutral, causing the alveoli to collapse or the lung to collapse. In case there is a full collapse of the lung, the minimum volume of air which will still remain inside the collapsed lung is known as the minimal lung volume. The minimal lung volume is equal to 500 ml.  This happens because the bronchioles collapse earlier than the full collapse of the alveoli. The minimal lung volume is lesser than the residual volume.

Tension Pneumothorax

Tension Pneumothorax develops after a laceration type of lung injury. There is a creation of a valve at the level of a damaged visceral pleura in an injured lung. The visceral pleura will be in continuity with the alveoli because there will be some surface alveoli. On inspiration, the air will also enter the intrapleural space. But on expiration the valve collapses, leading to entrapment of extra air in the pleural space. This cycle of entrapment of air continues on every inspiration. Entrapment of air occurring in the intrapleural space is called Tension pneumothorax, where the Intrapleural pressure is positive. This will cause more and more air to accumulate in the intrapleural space. This will cause the pressure to be more than 0, that is positive pressure or more than 760 mm Hg. Suppose when the intra-alveolar pressure is 760 mm Hg, which is normal, and the intrapleural pressure is positive, which is more than 760 mm Hg, the positive pressure will push the alveoli. The alveoli has a tendency to get collapsed and on top of that positive pressure is being given. This will lead to a severe collapse of the lung along with a mediastinum shift. 

This condition is thus known as tension pneumothorax with mediastinal shift.If this condition is not treated immediately the whole lung may get collapsed, puncture to the intrapleural space is the immediate treatment modality for this condition. The puncture will cause the air that is trapped inside the intrapleural space to get out because the pressure inside is more than the pressure outside (ATM). When the air is getting out the severe collapse will not happen, and it will be converted into a simple pneumothorax. A puncture is made at the second intercostal space to release the air that is trapped. 

When the pressure difference is measured across a wall or a structure this kind of pressure is known as trans-pressure or trans-neural pressure. If the pressure difference is measured between intra-alveolar space (IAP) and intrapleural space (IPP), the lung interstitial tissues are present in between these two regions. When the pressure difference is measured between the lung interstitial tissues, it is known as the trans pressure or the transpulmonary pressure. 

(IAP-IPP) = Trans-Pulmonary Pressure

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[0 – (–5cm H₂)] = +5 cm H₂O

The normal value of transpulmonary pressure in resting condition. Intra-alveolar pressure in resting condition is zero. Intra pleural pressure in resting condition is -5 cm Hg. The difference is + 5 cm H₂O which is the normal transpulmonary pressure in the resting condition. Now the pressure b, the pressure difference between the intrapleural space and outside the chest wall, which has the thoracic wall in between, also known as the trans thoracic pressure. The outside pressure that is zero mm Hg is subtracted from intrapleural pressure is -5 cm H2O, to get the trans thoracic pressure which is the same, that is -5 cm H2O. Pressure C, If the pressure difference from the intra-alveolar region to the outside chest wall, the pulmonary tissue, the intrapleural space, and the thoracic cavity are also present. This means that the pressure difference across all the components of the respiratory system is being measured and is called trans respiratory pressure. The trans respiratory pressure will be zero because the intra-alveolar pressure is zero and the chest wall pressure is also zero, making the difference between these zero. The resting condition or the FRC can be seen. If it is expanded more after this condition, the lung in such a case will be filled with air, known as total lung capacity (TLC). If more expiration takes place, the lung is compressed, and the minimum volume or the residual volume (RV) is reached. It is not possible to go beyond these conditions in a normal state. When there is a full collapse of the lungs, that is a condition of the leftmost balloon here. It can be seen that a certain amount of air remains in the alveoli; this is the minimal volume which is 500 ml. The resting condition of the lung is drawn where the alveoli can be seen along with the visceral pleura surrounding it, and the parietal pleura surrounding it. The volume present inside the lung is the FRC. Suppose the chest wall is removed, and the attached parietal pleura is also removed. Just the lung and outside the lung the atmospheric pressure remains, which is zero pressure, and this will cause the lung to get fully collapsed. It is the negative intrapleural pressure that is keeping the alveoli open. On the opening of the chest wall suddenly the air will enter causing a collapse of the lung. The minimal volume will be remaining in the lungs in such a case which is the minimal volume which is 500 ml. At this point when the lungs are fully collapsed the transpulmonary pressure is 0 mm H2O because the pressure inside the alveoli is zero and outside the alveoli is also zero.

When going in the opposite direction, if the lung is removed from the system. In such a case only the chest wall is remaining. The chest wall has a tendency to expand in the outward direction. The elastic tissue of the lung was pulling it in the inward direction. Since the lung is removed the chest wall will now expand in the outer direction. 

However, this expansion will not be unlimited and it will stop at a certain point. At that point when the expansion is stopped, the total volume inside the thoracic cavity is 70% of the total lung capacity. This is the neutral condition of the chest wall. The lung has an inherent tendency to get collapsed, but even after the collapse minimal volume will be present inside the lung. When the chest wall is fully expanded and the pressure difference is measured across the thoracic cavity, it is the trans thoracic pressure. The transthoracic pressure in this condition will be 0 mm of Hg.

In the resting condition, if the trans respiratory pressure is measured, which is the difference of pressure between the intra-alveolar and outside the thoracic wall, it will also be zero. When the lungs have fully collapsed and the transpulmonary pressure is measured, it will also be zero. When the chest wall is fully expanded, the transthoracic pressure is also zero. The X-axis represents the trans pressure and Y- the axis represents the volume. The maximum volume that is possible, which is the total lung capacity is marked on the Y-axis. The FRC volume is also marked along with the residual volume and minimal volume.  In the first condition when the chest wall is removed and only the lung is there, there will be a collapse of the lung, the transpulmonary pressure will be zero, and the volume will be minimal. If the pressure is increased inside the alveoli, making the transpulmonary pressure positive, there will be alveolar expansion. This is done in positive pressure ventilation. If more and more negative transpulmonary pressure is given, the volume of the lung cannot go beyond the minimal volume. In such a case there will be a plateau in the minimal volume. This is the only lung curve.

The second condition here is the resting condition. 

When the trans pressure is zero, both the lung and chest wall are in normal condition, and the volume present is the FRC. From the FRC if one goes on positive trans-respiratory pressure, the lung and chest wall will both expand, which can be seen from the curve reaching up to the level of TLC. When negative pressure is given, it will go down and reach the level of residual volume. The maximum volume to which the lung and chest wall can be expanded is the TLC and the minimum volume that it can reach is the residual volume. Looking at our respiratory system, it can be seen that the maximum volume that can be reached is TLC, and the minimum volume that can be reached is the residual volume. The neutral condition or wear the trans respiratory pressure is zero is the FRC.

The third condition deals with only the chest wall. This means that when the transthoracic pressure is zero in the neutral condition, the volume is 70% of TLC. When only the chest wall is present the maximum point that can be reached is a little beyond the total limit, but the minimum volume that can be reached is at the level of the residual volume. This is known as the only chest wall curve.

To identify the curve the trans pressure has to be looked at which is zero. In the blue-coloured curve, it is seen that when the trans pressure is zero, the curve is reaching at a minimal point which is below the residual volume, which is the minimal volume. This is the only lung curve. For the green-coloured curve, when the trans pressure is zero, it is cutting at the FRC. It is the lung and chest wall curve. The brown color curve is cutting at approximately 70% of the TLC. This is the only chest wall curve. The only chest wall curve and lung and chest wall curve are reaching a minimal volume, which is the residual volume. The only lung curve is reaching a minimal volume. 

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Mechanism of Normal Inspiration

The main muscle for inspiration is the diaphragm, which contracts during inspiration and is responsible for 75% of the inspiratory effort. External intercostal muscles are also present, which constitute 25% of the inspiration effort. During normal inspiration and the contraction of the diaphragm, it moves downwards. The diaphragm is attached to the parietal pleura. Whenever inspiration takes place, along with the movement of the diaphragm the parietal pleura also moves downward. Expansion of intrapleural volume takes place on inspiration. The intrapleural pressure will decrease from -2.5 to -6 mm Hg.

Negative intrapleural pressure is the expanding force of the alveoli. Increased negative intrapleural pressure leads to stretching of the alveoli. Since the alveolar volume is increasing, the intra-alveolar pressure will decrease from 0 which is normal to -1 mm Hg. This is less than atmospheric pressure, and this will cause the atmospheric air to enter inside, until equilibrium or 0 mm Hg pressure is achieved. This will lead to the end of the entry of air and that is the end of inspiration. At the end of inspiration, the alveoli or expanded pressure inside is 0 mm Hg. 

Expiration

During expiration, which is a passive process when the muscles that have been contracted during inspiration will relax. The diaphragm will relax and will move upward again causing compression of the pleural space. On compression of alveoli, there will be a reduction of alveolar volume. The intra-alveolar pressure will increase to +1 mm Hg. This pressure means that it is more than atmospheric pressure that is zero, and will push the air out of the lung, causing expiration. At the end of expiration, there will be an equilibrium pressure, where the intra-alveolar pressure will be 0.

Intra-alveolar Pressure Graph

The X-axis represents the time and the Y-axis represents the pressure. TThe time duration between the first two curves is the inspiratory phase of respiration, and the duration between the second and the third line is the expiratory duration. Before the start of the inspiration, in the resting condition, intra-alveolar pressure is 0 mmHg. During inspiration the intra-alveolar pressure will be negative. At the end of inspiration, the intra-alveolar pressure will again become 0. Maximum negative intra-alveolar pressure, during normal breathing, is seen during the middle part of the inspiration. During expiration, the alveolar pressure will be increased due to the alveoli being compressed. At the end of expiration, the alveolar pressure will be zero. The maximum positive pressure, in this case, is +1 mm Hg and the minimum pressure reached is -1 mm Hg. This is typical of intra-alveolar pressure.

In resting condition, intrapleural pressure/ intrathoracic pressure is -2.5 mm Hg. IPP drops even more during inspiration, up to -6 mm Hg, which is during normal inspiration. At the end of the inspiration, it reaches its lowest point. During expiration the IPP is returning to the resting condition, thereby becoming less negative intrapleural pressure. But the intrapleural pressure never becomes positive.

Valsalva’s IPP + 100 mmHg 

Muller’s IPP  – 80 mmHg

Forceful inspiration or expiration, for example, during the Valsalva maneuver, which is forceful expiration against a closed glottis, intrapleural pressure will become more and more positive, and can rise up to +100 mm Hg. 

This is severe positive pressure in the intrapleural space. Because this is done against the closed glottis, the alveoli will not collapse. 

In the opposite case, which is Muller's maneuver, forced inspiration against a closed glottis, the intrapleural pressure will be negative up to -80 mm Hg.

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