 Hello and welcome to Physiology Open. In this video we will see some fundamentals of mechanics of respiration. The term mechanics of respiration refers to the forces, pressure and volume changes that occur in the thoracic cage and the lungs so that the air can move into the lungs during inspiration and out during expiration. Fundamentally for any liquid or gas to move from one place to another, there should be some pressure difference between the two sides. So during inspiration for the air to move from the atmosphere into the lungs, atmospheric pressure should be higher than the pressure inside the lungs and during expiration it should be opposite. So for bringing the air inside the lungs, either the outside pressure in the atmosphere to which an individual is exposed can be increased. This is known as positive pressure breathing or pressure inside the lungs can be decreased. This is known as negative pressure breathing. Both ways we will achieve a gradient of higher pressure in the atmosphere to lower pressure in the lungs for inspiration. Our pulmonary system physiologically uses negative pressure breathing. With this concept, let's try to understand the mechanics of respiration. The lungs are housed inside the thoracic cage which is formed posteriorly by the spine, anteriorly and laterally by the rib cage with ribs attached to sternum anteriorly and to spine posteriorly. Now both lungs and thoracic cage are elastic structures that is they can expand and recoil back to their respective resting positions. We will understand this using a schematic diagram. So here this diagram is showing two springs which are elastic structures. In this blue spring represents thoracic cage and red one represents lungs. So when seen independently that is we keep lungs and thoracic cage separate the resting position of lungs is collapsed while thoracic cavity is in much more expanded position. And in case any external force tries to change their position they will expand a bit. Or in case of thoracic cage if external force tries to collapse it also it will move inward. But these elastic structures will also tend to return or recoil to their original resting position and will come back to their resting position once the force is removed. Ok that's when they are seen separate from each other. But in an intact human body lungs are housed inside the thoracic cavity separated by pleural membranes. One layer of pleura that is parietal pleura is attached to thoracic cage while the visceral pleura is attached to lungs. Since the natural tendency of thoracic cage is to move outwards and that of lungs is to move inwards these opposite forces act on each other. Where lungs natural tendency to move inwards pulls thoracic cage inwards while thoracic cage natural tendency pulls lungs outwards. This keeps the lungs bit expanded and thoracic cage a bit collapsed from their resting positions respectively. Something like this. So at this point there is balance of forces. However because of these opposing forces layers of pleura also tend to move to opposite sides. Parietal pleura with thoracic cage and visceral pleura with lungs. This creates a negative pressure inside the pleural cavity. At rest that is with forces balanced this pressure is approximately minus 5 cm water. Note here that till now we have not spoken anything about muscles. It is just the natural tendency of these elastic tissues which is at work here. Now since the lungs are kept expanded at rest due to the pull by thoracic cavity. So obviously this expansion will decrease the pressure inside the lungs or alveolar pressure a bit. And some here will move into the lungs from atmosphere till alveolar pressure is equal to the atmospheric pressure. This causes some air to be present inside the lungs when the forces are balanced. The amount of air which is present inside the lungs at this point is functional residual capacity. So at this resting position when muscles are not at work the lungs are still filled with some air known as functional residual capacity just by the virtue of the elastic forces which are acting against each other. So this is equilibrium position. At this state alveolar pressure is equal to atmospheric pressure or we can say zero which is mentioned with respect to the atmospheric air. That is why we call it zero and no more air flows into the lungs at this point. Pleural pressure is however negative approximately minus 5 cm water. Now how will inspiration occur? Remember about the fundamental concept here that to breathe in air pressure inside the lungs should be decreased. Now since lungs are inside the thoracic cage which is a closed cavity any changes which occur in the volume and pressure of the thoracic cage are communicated to the lungs. But how will these changes be brought about from resting position? It needs some external forces. This external force is provided by the contraction of the skeletal muscles which line the thoracic cage. For quiet inspiration these muscles are diaphragm and external intercostal muscles. Diaphragm covers the bottom of the thoracic cage like this and it flattens on contracting and hence increases the vertical diameter of thoracic cage. Now these external intercostal muscles are arranged like this from one rib to the other like your hands in a pocket and are hinged at top. So when they contract they elevate the ribs causing increase in the antroposterior diameter and transverse diameter of the thoracic cage. So basically the thoracic cage expands in all three directions that is antroposteriorly, transversely and vertically. With increase in thoracic cage volume intraplural pressure decreases. See thoracic cage pulls the plural membrane further outwards isn't it? So intraplural pressure decreases that is up to minus eight centimeters water. It is like an external pull force on lung. Hence this causes a pull on the lungs thus increasing the volume of the lungs and hence decreasing the alveolar pressure. During tidal inspiration in healthy people the decrease in alveolar pressure is up to minus one centimeters of water. Due to the pressure difference generated between atmosphere and the alveoli air moves into the alveoli until the alveolar pressure becomes equal to atmospheric pressure. So again it becomes zero with respect to atmospheric pressure. So with a pressure difference of one centimeter of water between atmosphere and the alveoli approximately 500 ml of air moves into the alveoli in healthy people. Now let's see what happens in tidal expiration. For tidal expiration the changes in thoracic cavity and lung volume caused during tidal inspiration are simply reversed. So the diaphragm and external intercostal muscles relax during expiration. All the events reverse that is decrease in thoracic cage volume occurs then increase in plural pressure to the resting position that is minus five centimeters water causing decrease in lung volume. This brings the thoracic cage and lungs to their original equilibrium position. With the increased volume of air inside the lungs from inspiration the pressure that increases to plus one centimeter of water with respect to the atmospheric pressure. This causes the exhalation of same amount of air which entered the lungs during inspiration at the end of which the alveolar and atmospheric pressure again become equal. So basically tidal expiration is a passive process brought about by relaxation of the inspiratory muscles. But what happens during forced inspiration and forced expiration? Till now we have discussed the changes in plural and alveolar pressures which occur during tidal respiration. But these pressures are not sufficient to bring in more air or exhale more air. For inhaling air over and above the tidal volume the volume of air which needs to move inside the lungs is more and hence there should be more pressure difference between the lungs and the atmosphere. For this the diaphragm and external intercostal muscles contract more forcefully. In fact in tidal inspiration diaphragm moves about one to one point five centimeters only. While for maximum inspiration it can move up to seven centimeters. Also accessory muscles of inspiration contract that is sternocleidomastoid, scaleness. With maximum contraction of the muscles the plural pressure may become as negative as minus 30 centimeters water. Causing much more lung expansion and hence movement of much more air into the lungs. Now for exhaling air over and above the tidal volume the thoracic cage volume decreases from resting state. This is brought about by contraction of the internal intercostal muscles and abdominal wall muscles that is external internal obliques, rectus and transversus abdominis muscles. With decrease in thoracic cage volume plural pressure increases and even becomes positive in forced expiration. This decreases the lung volume increases the alveolar pressure and hence air is exhaled. In summary pressure differences are created by changing the dimensions of the thoracic cage causing change in plural pressure. This affects lung volume and which in turn affects alveolar pressure. This generates pressure gradients causing movement of air. Well hopefully this video help you understand the topic well. If you like the video don't forget to subscribe to the channel Physiology Open. Thank you.