Anatomy of Breathing.pdf

Respiratory System Anatomy of Breathing Welcome to this lecture on the anatomy of breathing.  Respiratory System Anatomy of Breathing Part 1: The Thoracoabdominal Pump The lecture is divided into three parts, and in this first...

Respiratory System Anatomy of Breathing Welcome to this lecture on the anatomy of breathing.  Respiratory System Anatomy of Breathing Part 1: The Thoracoabdominal Pump The lecture is divided into three parts, and in this first part we’ll consider the role of both the chest and the abdomen in breathing.  Learning Outcomes After this lecture you should be able to: ▪ Describe the thoraco-abdominal pump, and give a brief account of the effects of posture on respiratory mechanics ▪ Describe the action of the diaphragm, and contrast this with the part played by the chest wall in respiratory movements ▪ Know the stabilising role of quadratus lumborum for diaphragmatic movements ▪ List the muscles that stabilise the 1st and 2nd ribs. ▪ Give an account of the intercostal muscles and their actions ▪ Discuss the significance of ‘flail-chest’ on respiratory function ▪ List the accessory muscles of respiration These are all of the learning outcomes for this lecture.  Learning Outcomes After this lecture you should be able to: ▪ Describe the thoraco-abdominal pump, and give a brief account of the effects of posture on respiratory mechanics ▪ Describe the action of the diaphragm, and contrast this with the part played by the chest wall in respiratory movements ▪ Know the stabilising role of quadratus lumborum for diaphragmatic movements ▪ List the muscles that stabilise the 1st and 2nd ribs. ▪ Give an account of the intercostal muscles and their actions ▪ Discuss the significance of ‘flail-chest’ on respiratory function ▪ List the accessory muscles of respiration However, the learning outcomes for this part of the lecture are that afterwards you should be able to; Describe the thoraco-abdominal pump, and give a brief account of the effects of posture on respiratory mechanics Describe the action of the diaphragm, and contrast this with the part played by the chest wall in respiratory movements Know the stabilising role of quadratus lumborum for diaphragmatic movements List the muscles that stabilise the 1st and 2nd ribs. The remaining outcomes will be dealt with in subsequent sections. Thoracic and Diaphragmatic Breathing As a recap from the last lecture, I mentioned that breathing can be divided into diaphragmatic breathing which follows movement of the diaphragm, and thoracic breathing which involves movement of the chest wall. I also divided these movements into quiet breathing when the diaphragm moved by only a few centimetres, and forced breathing when the diaphragm moved more, and the ribcage movements were brought into play. Contraction of the diaphragm and elevation of the chest causes an expansion of the thoracic cavity, and hence an expansion of the lungs. Since the thoracic cavity is a closed space, an increase in volume results in a decrease in pressure and since air flows from an area of high pressure to low pressure, air is sucked into the lungs. This is inhalation or inspiration. Similarly, elevation of the diaphragm and depression of the ribcage results in a decrease in volume and a corresponding increase in pressure. Hence the pressure inside the chest is greater than atmospheric pressure and air is forced out of the lungs. This is exhalation or expiration. Thoracic and Diaphragmatic Breathing By this mechanism the lungs inflate and deflate.  The Diaphragm This slide serves as a quick reminder of the attachments of the diaphragm. It attaches to the boundaries of the thoracic outlet. From the frontal view shown here on the right, we can see that the two domes of the diaphragm rise up as high as the 4th rib on the right and 5th intercostal space on the left. During inspiration, the domes descend and in forced breathing the ribs ascend. This presents a conflict, so to prevent the diaphragm ascending with the ribcage, the 12th rib is held down.  Quadratus Lumborum This is achieved by contraction of the quadratus lumborum muscle. This is attached to the iliac crest of the hip bones below and the 12th rib above. It also arises from the transverse processes of the lower lumbar vertebrae and iliolumbar ligament, but the key attachment, that you need to remember, is to the iliac crest. It is innervated by the anterior rami of T12-L3.  Posture and Breathing Normally, in “quiet” (unforced) breathing, the rib cage does not need to move much. The expansion of the thoracic cavity needed to cause inspiration of air is normally achieved through descent of the diaphragm. In the upright position, gravity assists the diaphragm by causing descent of the abdominal organs, notably the liver. Here we can see an exaggerated movement of the liver. Unfortunately, my animation fails to show the accompanying movement of the diaphragm. The abdominal wall undergoes reflex relaxation to assist this process, by allowing the abdominal organs to be displaced. To expire, the opposite occurs – the abdominal wall undergoes reflex contraction to force the abdominal organs and diaphragm upwards. This is known as the thoraco-abdominal pump or abdominal breathing.  Breathing in the Supine Position When lying down, the effects of gravity have a negative effect on breathing, since the liver moves upwards towards the chest. In effect, this is a movement downhill. This is due to the presence of a curve in the vertebral column. The liver weighs approximately 1500g, so it is heavily influenced by gravity (excuse the pun). Patients with respiratory stress therefore, prefer to avoid lying down.  Effect of Posture on Ribcage But there again, standing affects the patient’s ability to elevate the ribcage against gravity. This can be lessened by having a straight back, as this maximises the ability to elevate the ribs. A slouched posture will limit the capacity of the ribs to aid breathing. By flexing the vertebral column, the ribs are brought closer together and makes them more oblique. This also reduces the availability for expansion of the abdomen.  Supine Position Similarly when lying flat, the rib cage is in contact with the ground and its movements are impeded. Hence this is also not good for breathing.  Patient Positioning Hence the ideal position is to be propped up at 45 degrees either backwards (the Fowler position) or forwards. The latter is not ideal due to the flexion of the vertebral column, but some patients feel more comfortable in this position. Others prefer to be propped up at 45 degrees on their sides.  Muscles Attached to the Inlet 1 Sternocleidomastoid (the nosey-parker’s muscle) Action 1: Extends head, flexes neck Action 2: Turns head Action 3: Inspiratory Muscle Innervation: Spinal Accessory CN XI There are some important muscles at the thoracic inlet. These serve to hold up the chest wall, acting against the forces of gravity which tend to depress the chest wall. This facilitates the action of the muscles used for breathing, which would otherwise pull the chest wall downwards. In forced breathing, these muscles can even elevate the chest wall, although this action tends only to occur when every last gasp of breath counts, and the subject is struggling for air. When the chest is stable, these muscles can be used to move the head and neck. The first muscle to be considered is the sternocleidomastoid muscle (often shortened to sternomastoid). Although this sounds a difficult name, it simply describes its attachments (sterno – sternum, cleido – clavicle and mastoid – mastoid process of the skull). This is a muscle which can turn the head to the side, or acting with its partner on the opposite side, pull the head forward and tilt it backwards, as in peering over someone’s shoulder. I often like to think of it as the “nosey parker’s muscle”! With the head stable, it elevates the sternum. This muscle is supplied by the spinal accessory nerve (11th cranial nerve).  Muscles Attached to the Inlet 2 Scalene Muscles Action 1: Lateral flexion of neck Action 2: Flexion of Neck Action 3: Inspiratory Muscle Innervation: Cervical Plexus Scalene Muscles The second muscle to be considered is really a group of muscles, although can be considered as a functional whole. These are the scalene muscles (scalenus anterior, medius and posterior). These muscles attach to the cervical vertebrae proximally and 1st (scalenus anterior and medius) and 2nd (scalenus posterior) ribs distally, and hence elevate them when the neck is stable. These muscles are innervated by nerves of the cervical plexus.  Respiratory System Anatomy of Breathing Part 2: The Intercostal Muscles Welcome to the second part of this lecture on the anatomy of breathing. In this section we will cover the details of the intercostal muscles, including their attachments, innervation and function.  Learning Outcomes After this lecture you should be able to: ▪ Describe the thoraco-abdominal pump, and give a brief account of the effects of posture on respiratory mechanics ▪ Describe the action of the diaphragm, and contrast this with the part played by the chest wall in respiratory movements ▪ Know the stabilising role of quadratus lumborum for diaphragmatic movements ▪ List the muscles that stabilise the 1st and 2nd ribs. ▪ Give an account of the intercostal muscles and their actions ▪ Discuss the significance of ‘flail-chest’ on respiratory function ▪ List the accessory muscles of respiration The outcome for this part of the lecture is that afterwards you should be able to; Give an account of the intercostal muscles and their actions The remaining outcomes will be dealt with in the final part next time.  External Intercostal Muscle When the skin and fascia of the chest wall is stripped away, the muscle fibres between the ribs are visible. They are of course covered by an epimysium, but the direction of the muscle fibres of the external layer of muscle is clearly visible. This is the external intercostal muscle and the fibres are orientated forwards and downwards. Normally the direction of the muscle fibres is an indication of muscle function, so what is the function of this layer of muscle?  Rib Elevation Well, this schematic may offer some clue. This is a model of the ribcage with the vertebrae to your left and the sternum to your right. Two ribs are shown.  Rib Elevation Animation The elastic band represents the external intercostal muscle. It’s pull lifts the ribs and elevates the sternum, so it is clear that this is a muscle of inspiration.  An Intercostal Space Now, you may recall that there are 3 layers of intercostal muscles. Deep to the external one is an internal and deep to that an innermost intercostal muscle layer. Sandwiched between the internal and innermost intercostal muscles is the neurovascular bundle, tucked away in the subcostal groove. Remember though, that there may be collateral branches sitting in the lower part of the intercostal space. The intercostal nerves innervate these muscles.  Layers of the Chest Wall These nerves also supply sensory nerves to the skin and superficial fascia overlying the space, and sense pain from the parietal pleura.  Intercostal Muscles and Membranes The external intercostal muscle does not have muscle fibres filling the entire intercostal space. There is a gap at the front that contains the surrounding fascial epimysium but no muscle fibres. This is called the external intercostal membrane.  Intercostal Muscles and Membranes The same is true of the internal intercostal muscle layer, except this time the membrane is at the back of the intercostal space.  Innermost Muscles of the Chest Wall The innermost intercostal muscle layer is very similar in its appearance to the internal intercostal muscle. Indeed the two muscles are often fused in places. Unlike the internal intercostal layer though, the muscle fibres often cross more than one intercostal space. The layer is sometimes partly incomplete but has thickenings anteriorly and posteriorly. Posteriorly, these thickenings are known as the subcostal muscles. These are also known as subcostalis or infracostalis. Anteriorly it forms the transversus thoracis muscle which is sometimes known as the triangularis sternae or sternocostalis. This gives the idea that at least some of the fibres of the innermost intercostal layer run transversely. You would think that muscles that have 3 names would be important, but you’d be wrong. They are all weak expiratory muscles. Intrathoracic Pressures and Breathing One important function of the intercostal muscles, is to prevent the intercostal spaces from bellowing inwards and outwards during respiration. As the muscles contract, the spaces become more rigid, and prevent deformation. Any such deformation would impede the necessary change in thoracic volume during breathing. During inspiration, the diaphragm and ribs would move to increase thoracic volume, whilst the intercostal spaces tend to move inward reducing this volume. As a consequence of the above, the intercostal muscles need to contract during both inspiration and expiration.  Intrinsic Muscles of the Chest Wall So if the external intercostal muscles are inspiratory and the innermost expiratory, what of the internal intercostal muscle layer sandwiched between? The fibres of this muscle run backwards and downwards from the rib above to the rib below. One might well predict that they would be expiratory as they are running in the opposite direction to the external intercostal muscles. Well surprisingly, they may in fact be BOTH inspiratory and expiratory at the same time, although it’s complicated. There are conflicting views on this. The experimental evidence is confused by the fact that one can record electrical activity from intercostal muscles during both inspiration and expiration, but it’s difficult to record the layers in isolation from each other. To add to the confusion, we already know that the intercostal muscles contract in both phases of the respiratory cycle in order to stiffen the intercostal spaces to prevent bellowing, but they can only be classed as either inspiratory or expiratory if the move the ribs. There are some who would argue that because the first rib is held static by the scalene muscles, ANY muscular contraction strong enough to move a rib must do so in an upwards direction.  Intrinsic Muscles of the Chest Wall Although the external and internal intercostal muscles are at right angles to each other, they must both lift the rib below towards the rib above. However, it isn’t that simple because the 12th rib is also fixed by the quadratus lumborum muscle. So perhaps one is used to lift the rib and the other to depress the rib. Also, if the internal intercostal muscle was used to elevate the rib, it would have to lengthen to do so. Hence many researchers conclude that the internal intercostal muscles are expiratory. There is one place on the chest wall where one can elicit electrical activities of the internal intercostal muscle layer, and that is where the external intercostal fibres are absent. That is in the anterior part of the ribcage between the costal cartilages. When one records here, it is clear they contract most during INSPIRATION not expiration. Hence, the most recent conclusion from the literature is that both the external intercostal muscle layer and the inter-chondral portion of the internal intercostal muscle layer are inspiratory muscles, but that the intercostal portion of the internal intercostal muscle is expiratory.  Intercostal Muscle Function Function of an intercostal muscle fibre is dependent upon its position in the intercostal space. Taken from De Troyer A, Kirkwood PA, Wilson TA. Respiratory action of the intercostal muscles. Physiol Rev 85: 717-756, 2005. Backing up this idea is some research that discusses the observation that function of the intercostal muscles may change depending on their distance from the costovertebral joints where the axis of rotation of a rib occurs. The study shown here suggests that the external and internal intercostal muscles have opposing effects that change as the distance from the costovertebral joint increases. This is indicated by the angle theta on the diagram. The graph shows that at the back of the intercostal space, the external intercostal muscles are inspiratory and internal ones expiratory, but that reverses beyond 120 degrees as the muscles approach the sternum. Of course the amount of this change will vary from one intercostal space to another. Now the take home message here is that intercostal muscle action is far from crystal clear, and all that is asked of you is to appreciate that there are no certainties beyond saying the external intercostal muscle is largely inspiratory and the inter-chondral portion of the internal intercostal muscle is also inspiratory. The rest is guesswork!  Respiratory System Anatomy of Breathing Part 3: Accessory Muscles of Respiration Welcome to the last part of this lecture on the anatomy of breathing. In this section we will consider the extrinsic muscles of the chest that aid movement of the ribcage. In the previous section we discussed the intrinsic muscles. An intrinsic muscle of the chest is one that has all of its attachments within the bony anatomy of the chest. An extrinsic muscle of the chest will have at least one of its attachments outside of the ribcage.  Learning Outcomes After this lecture you should be able to: ▪ Describe the thoraco-abdominal pump, and give a brief account of the effects of posture on respiratory mechanics ▪ Describe the action of the diaphragm, and contrast this with the part played by the chest wall in respiratory movements ▪ Know the stabilising role of quadratus lumborum for diaphragmatic movements ▪ List the muscles that stabilise the 1st and 2nd ribs. ▪ Give an account of the intercostal muscles and their actions ▪ Discuss the significance of ‘flail-chest’ on respiratory function ▪ List the accessory muscles of respiration The learning outcomes for this section are that afterwards you should be able to; Discuss the significance of ‘flail-chest’ on respiratory function List the accessory muscles of respiration  Flail Chest When several ribs are broken in two or more places (e.g. following traumatic injury), the intercostal muscles are unable to prevent the sucking-in of the chest wall during inspiration and blowing-out during expiration. This condition is known as a “flail-chest” and is a clinical emergency. I have tried to replicate that here. The movements of the isolated segment of rib-cage are said to be “paradoxical” - that is, in opposite direction to the other ribs. A flail chest is treated by placing a firm pack over the segment, reducing its movement until surgical intervention can be given.  Paradoxical Rib Movements During inspiration the diaphragm is depressed and in forced breathing the chest is lifted upwards and outwards. Both these movements serve to increase the volume of the chest and to reduce the internal pressure. This causes the flail segment to be sucked in.  Paradoxical Rib Movements During expiration the volume of the chest reduces and the internal pressure is increased. This causes the flail segment to be blown out. The movement of the flail segment is opposite to that of the rest of the chest wall, and this is known as paradoxical chest movement.  Patients with Flail Chest Here we can see a couple of patients with flail chest. The one on the right is an extreme case and will need urgent treatment.  The Grasp of Success Have you ever wondered why runners at the end of the race bend over to grab their knees? The answer is that they are using accessory muscles of respiration to maximise breathing.  Accessory Inspiratory Muscles 1 The muscles at the front of the chest are particularly significant here. These are the pectoralis major and minor. The latter is deep to the major and attaches to ribs 3-5 normally with its origin on the coracoid process of the scapula. Hence it can be used to elevate those ribs. The major has a wide attachment proximally with a distal attachment onto the bone of the upper arm, the humerus. Its normal action is to adduct and medially rotate the humerus. The lower fibres of this muscle are attached to the 5th and 6th costal cartilages and costal margin, and hence can also be used to lift the rib cage. This is what our runners are doing. By grabbing hold of their knees, they are stabilising the upper limb. Now by contracting the pectoral muscles, the ribcage is elevated. Of course to get a firm grip on their knees, they need to bend over and this has another advantage. It reduces the effect of gravity tending to pull the chest wall down.  Accessory Inspiratory Muscles 2 Serratus Anterior Another muscle of the upper limb is serratus anterior. This attached to the upper 8 ribs laterally and it inserts into the medial border of the scapula posteriorly. This muscle with its serrated edge, can readily be seen in muscular individuals. It is normally used to pull the scapula forwards on the chest wall (e.g. throwing a punch). However, if the scapula is held in a fixed position, the lower part of this muscle can be used to lift the rib cage and hence assist inspiratory movements.  Accessory Expiratory Muscles External Oblique Abdominis Rectus Abdominis Depression or lowering of the ribs causes expiration. Whilst gravity and elastic recoil of the lungs is the principal mechanism of expiration, if expiration needs to be done quickly (e.g. during a cough), then muscular effort is needed. The principal muscles to aid this are the anterior abdominal wall muscles. The two principal ones are rectus abdominis and external oblique abdominis. These are attached to the hip bone below and the rib cage above. Their contraction pulls the ribs downwards. Another abdominal muscle, the transversus abdominis causes compression of the abdominal cavity which assists the elevation of the relaxed diaphragm towards the thoracic cavity. All these factors aid forceful expiration.  Serratus Posterior Muscles Serratus posterior superior Serratus posterior inferior There are two serratus posterior muscles, one superior and one inferior. The superior serratus posterior muscle arises from the spinous processes and supraspinous ligaments of C7-T2 and inserts onto ribs 2-5. The inferior serratus posterior muscle arises from the spinous processes and supraspinous ligaments of T11-L2 and inserts onto ribs 9-12. These are supplied by the anterior rami associated with these respective ribs. However, with all of that said I wouldn’t commit any of that to memory. By looking at their attachments one could easily come to the conclusion that the superior one was an inspiratory muscle and the inferior one an expiratory muscle. Indeed, this is the conclusion reached in many textbooks and websites. However, it simply isn’t true. Electomyographic studies show that these muscles are inactive during breathing, even during forced breathing in patients with obstructive lung disease. Instead it is thought that they are involved in measuring stresses at the top and bottom of the thoracic spine. That is, they have a proprioceptive role for the vertebral column. Hence, we can omit them from our discussions on respiration here.  Levator Costarum Muscles Another muscle with a close relationship to the thoracic part of the vertebral column is the levator costarum. This also has two forms – a long one and a short one, both supplied by dorsal rami of segmental nerves. These muscles arise from the transverse processes of C7 to T11. Levator costarum brevis inserts onto the rib immediately below the vertebra of origin, and levator costarum longus inserts onto the rib two levels below the vertebra of origin. These muscles do exactly what the say they do and that is to elevate the ribs. However, they are small muscles and located close to the joints around which they operate. Hence, they are very weak and don’t contribute any significant action during respiration. Muscles of Inspiration and Expiration So to summarise forced inspiration…. The 1st two ribs are held stable by the action of the scalene group of muscles. The sternocleidomastoid muscle helps stabilize the sternum. The external intercostal and interchondral part of the internal intercostal muscles elevate the ribs, and the diaphragm contracts forcibly. The accessory muscles of inspiration play their part but aren’t shown on this illustration. Only pectoralis minor is mentioned here, but the pectoralis major and serratus anterior have an important role to play. Quiet expiration requires nothing more than relaxation of the inspiratory muscles. The ribcage is lowered, and the diaphragm elevates, and the lungs deflate. There needs to be minimal contraction of the intercostal muscles to prevent the bellowing of the intercostal spaces. In forced breathing, the intercostal part of the internal intercostal muscle possibly depresses the ribs, but the intercostal muscles must at least contract to prevent bellowing. The abdominal muscles contract to depress the ribcage and compress the abdominal cavity. This aids the return of the diaphragm to its resting position. Muscles of Inspiration and Expiration And here is an animation to illustrate this process.  Respiratory System Anatomy of Breathing Well, I always like to finish with a moving tribute! That brings this lecture to close. In the next lecture, we will discuss the Upper Respiratory Tract. I look forward to you joining me there.