Respiratory system

11 Respiratory system




ANATOMY





LUNGS


The lungs (Fig. 11.2) are conical in shape. They conform to the shape of the pleural cavities. Each lung has a blunt apex which reaches above the sternal end of the first rib, a base related to the diaphragm, a convex parietal surface related to the ribs, and a mediastinal surface which is concave and related to the pericardium.



Each lung is subdivided by an oblique fissure into upper and lower lobes, the right lung being further divided by the horizontal fissure to produce a middle lobe. The surface marking of the oblique fissures is best represented by the line of the vertebral border of the scapula with the arm fully elevated. The horizontal fissure of the right lung passes horizontally and medially from the oblique fissure at the level of the fourth costal cartilage. The equivalent of the middle lobe in the left lung is the lingula which lies between the cardiac notch and the oblique fissure.





PLEURA


Each pleural cavity is composed of a thin serous membrane invaginated by the lung. The visceral layer of pleura is intimately related to the surface of the lung and is continuous over the root of the lung with a parietal layer which is applied to the inner aspect of the chest wall, the diaphragm and the mediastinum. The two pleural cavities are totally separate from each other. Below the root of the lung the pleura forms a loose fold known as the pulmonary ligament which allows for distension of the pulmonary veins. The lungs conform to the shape of the pleural cavities but do not occupy the full cavity, as they would not be able to expand as in full inspiration.



Surface anatomy


The cervical pleura extends above the sternal end of the first rib. It follows a curved-line drawn from the sternoclavicular joint to the junction of the inner third and outer two-thirds of the clavicle, the apex of the pleura arising about 2.5 cm above the clavicle. The line of the pleura on each side passes from behind the sternoclavicular joint to meet in the midline at the level of the second costal cartilage. The right pleura then passes vertically down to the 6th costal cartilage before crossing the 8th rib in the midclavicular line, the 10th rib in the midaxillary line, and 12th rib at the lateral border of the erector spinae. On the left side the pleural edge reaches the 4th costal cartilage, where it arches out lateral to the border of the sternum, the pleura being separated from the chest wall by the protrusion of the pericardium. The medial ends of the 4th and 5th intercostal spaces are, therefore, notcovered by pleura. Apart from this, its relationshipsare the same as those on the right side. The pleura actually descends below the 12th rib at its medial extremity.





THORACIC CAGE


The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side, and the sternum and costal cartilages in front.



Ribs


There are 12 pairs of ribs (Fig. 11.4). The first seven pairs are connected anteriorly via their costal cartilages to the sternum. The 8th, 9th and 10th ribs articulate with their costal cartilages, each with the rib above. The 11th and 12th ribs remain free anteriorly and are known as ‘floating ribs’.



A typical rib comprises:






The first, 2nd, 10th, 11th and 12th ribs are atypical. Only the first and 12th are clinically important. The first rib is the shortest, flattest and most curved of the ribs. It is flattened from above downwards. The features of the first rib are shown in Fig. 11.4. The 12th rib is short, has no tubercle, and has only a single facet. There is no angle and no subcostal groove. Its only importance is in the loin approach to the kidney, where its relationship to the pleura is important. The pleura descends below the 12th rib at its medial extremity.



Clinical points






Sternum


This consists of three parts: the manubrium, the body and the xiphoid process.







DIAPHRAGM


The diaphragm (Fig. 11.6) is a dome-shaped septum separating the thorax from the abdomen. It is composed of a peripheral muscular part and a central tendon.



The muscular fibres arise from several sources: the crura, the arcuate ligaments, the ribs and the sternum. The right crus of the diaphragm arises from the front of the bodies of the first three lumbar vertebrae and the intervening intervertebral discs. The left crus arises from the first and second lumbar vertebrae and the intervening disc. The arcuate ligaments are a series of arches, the lateral being a condensation of the fascia overlying quadratus lumborum and the medial of the fascia overlying psoas major. The medial borders of the medial arcuate ligaments join anteriorly over the aorta as the median arcuate ligaments. The costal part of the diaphragm arises from the inner aspect of the lower six ribs and the sternal portion as two small slips from the posterior surface of the xiphisternum.


The central tendon is trefoil in shape and receives the insertion of the muscular fibres. Above, it fuses with the lower part of the pericardium.


There are three main openings in the diaphragm, although strictly speaking the aortic ‘opening’ is not in the diaphragm but lies behind it. The aortic ‘opening’ transmits the abdominal aorta, the thoracic duct, and often the azygos vein. The oesophageal opening lies in the right crus of the diaphragm and transmits the oesophagus, the vagus nerves, and branches of the left gastric artery and vein. The opening for the inferior vena cava lies in the central tendon of the diaphragm and transmits, in addition to the IVC, the right phrenic nerve.


The greater and lesser splanchnic nerves pierce the crura, and the sympathetic chain passes behind the medial arcuate ligament lying on psoas major.




ANATOMY OF RESPIRATION


There are two main mechanisms for increasing the volume of the thorax:









STRUCTURE OF THE RESPIRATORY TREE


The basic structure of the lower respiratory tree is shown in Fig. 11.7. The respiratory tree is designed to transport humidified air into the distal airways and alveoli where exchange between CO2 and oxygen takes place. The trachea is composed of C-shaped plates of cartilage with the curve of the C anteriorly; the ring is completed posteriorly with smooth muscle. The trachea contains mucous glands and is lined with ciliated epithelium. The trachea divides into bronchi and these contain discontinuous pieces of cartilage in their wall together with smooth muscle. They too are lined by columnar ciliated epithelium and contain mucous glands. The cilia beat rhythmically in a thin liquid layer and effectively transport the surface film of mucus and particles out of the lungs by way of the trachea.



The bronchi decrease in diameter and length with each successive branching. The cartilaginous support eventually disappears. In airways of about 1 mm the cartilage disappears completely. By convention all subsequent airways are called bronchioles. Bronchioles contain no cartilage or submucosal mucous glands. They contain cuboidal epithelium with ciliated cells as well as some additional cells which are thought to provide a watery secretion. The most distal air passages are the respiratory bronchioles, which are so-called because the first alveoli open directly into them. Respiratory bronchioles end in several alveolarducts, which are short channels which open into alveolar sacs which contain many alveoli. A single respiratory bronchiole, its alveoli and their blood supply are called a primary lobule or acinus. The alveoli are lined by flattened Type I pneumocytes together with some Type II pneumocytes. Type II cells secrete surfactant and replicate rapidly after injury to alveolar walls. These alveolar cells lie on a basement membrane together with an interstitial matrix including some elastin fibres which separate the air spaces from the pulmonary capillary walls. The structure of the alveolar-capillary membrane permits rapid and efficient diffusion of oxygen and carbon dioxide.



MEDIASTINUM


The mediastinum (Fig. 11.8) is the name given to the space between the two pleural cavities. It extends from the sternum in front to the thoracic vertebrae behind and from the thoracic inlet above to the diaphragm below.



For descriptive purposes it is divided into a superior and an inferior mediastinum, the latter being again subdivided into anterior, middle and posterior.








PHYSIOLOGY



CONTROL OF VENTILATION


The body succeeds in keeping arterial PO2 and PCO2 within remarkably narrow limits. This is made possible by highly developed negative feedback systems that consist of sensors, controllers and effectors. These are summarized in Table 11.1.


Table 11.1 Summary of the basic mechanisms controlling respiration



















Element Location Function
Controllers












Sensors







Effectors













Sensors


These are the central chemoreceptors, peripheral chemoreceptors, and several others. In the lung, sensors include primary stretch receptors, irritant receptors and J receptors. In the nose and upper airway there are sensitive irritant receptors, while joints and muscles utilise the gamma stretch receptor and associated reflex.



Central chemoreceptors


The most important receptors involved in respiratory control, they are situated on the ventral surface of the medulla. Here the extracellular fluid (ECF) of the brain surrounds the central chemoreceptors. Changes in hydrogen ion concentration of the ECF cause these receptors to respond. As hydrogen ion concentration increases, so ventilation increases, and the opposite is true. The pH of the ECF is most affected by the cerebrospinal fluid (CSF) hydrogen ion content. CSF is separated from the circulation by the blood-brain barrier. Whilst ions such as hydrogen and bicarbonate do not easily cross the blood-brain barrier, CO2 can cross readily. Thus as arterial PCO2 (PaCO2) rises, CSF PCO2 goes up, liberating hydrogen ions, which in turn lower the pH of the ECF. This stimulates central chemoreceptors to effect increased ventilation. Increasing ventilation decreases arterial PaCO2. An increased PaCO2 also causes cerebral vasodilatation, facilitating more rapid diffusion of CO2 into the CSF.


A feature of CSF is that it has much lower buffering capacity than blood. Small changes in PCO2 effect larger changes in CSF pH than in blood pH. However, bicarbonate can only diffuse slowly across the blood-brain barrier, and so changes in pH of CSF are eventually compensated for by a rise or fall in bicarbonate. This process takes about 36 h to complete, so that if PaCO2 is elevated for a prolonged period the chemoreceptors will ‘reset’, e.g. chronic lung disease, where patients may have an elevated PaCO2 with a normal CSF pH and normal respiratory rates. (See acid-base section.)






How do these mechanisms combine to control ventilation?


Consider the three main chemical factors which affect respiration:










MECHANICS OF BREATHING


The major function of the lungs is the transport of gas in and out of the alveoli and the exchange of respiratory gases. This is achieved not just by the lungs, but by the surrounding tissues, bones and muscles.




Elastic properties of the lung


As the thoracic volume increases during inspiration the lung tissues become stretched; the greater the degree of chest expansion, the greater the degree of stretching of the lungs. However this relationship is not entirely linear. Figure 11.11 shows the relationship between the volume of the lung and the negative pressure surrounding it. As the negative pressure increases, so the lung volume increases, up to a point where further negative pressure does not increase lung volume. When the pressure around the lung decreases, the lung volume also decreases, but it does not follow the same curve. This is called hysteresis. The lung volume at any given pressure during deflation is larger than that during inflation.



The slope of the volume pressure curve (the volume change per unit pressure) is known as the compliance. The usual compliance of the human lung is about 200 mL/cmH2O pressure, but from the slope of the line it can be seen that compliance decreases at higher lung volumes as the lungs become stiffer. Lung compliance can also be reduced, for example, in pulmonary venous engorgement or in alveolar oedema. The compliance of the lung falls if the lung remains unventilated for a long period. This may occur, for example, following anaesthesia, resulting in atelectasis. Lung compliance is decreased by fibrosis of the lung and certain diseases. Age and emphysema increase it.


Because the change in lung volume per unit change in pressure will be larger in a large lung and smaller in a small lung, the compliance per unit volume of lung is often quoted. This is known as the specific compliance.


The elastic properties of the lung are in part due to the elastic tissue clearly visible in histological section. The arrangement of the elastin fibres is also important to the compliance of the tissue, and this has been likened to that of the filaments in nylon stocking.


For all its elastic properties the compliance of the lung would be greatly reduced without the presence of surfactant.


The following is a summary of the major factors affecting compliance:











Surface tension


Surface tension is defined as the force acting along an imaginary line drawn on the surface of a liquid. This force exists because of the strong cohesive forces between molecules along the surface. Its importance in the lung can be demonstrated by comparing the pressure volume behaviour in isolated lungs inflated with either water or air. Lungs inflated with air have a much greater compliance and so are easier to distend than lungs filled with water (Fig. 11.11).


Uninhibited, the surface tension within the alveoli would significantly decrease the compliance of the lungs, perhaps by as much as 50%. However, specialised cells within the alveolar epithelium secrete surfactant, a lecithin-rich, detergent-like substance that significantly decreases surface tension. Although these cells are plentiful in adult life they are not productive until a late stage of fetal maturity. Premature babies are very prone to develop respiratory distress, characterised by stiff lungs, atelectasis and pulmonary oedema.


Surfactant promotes several important properties within the lung, as follows:







Regional differences in ventilation


Ventilation of the lung does not occur uniformly. Indeed dependent regions of the lung are much better ventilated than non-dependent regions of the lung. There are two main reasons for this:





This situation can be changed dramatically when the lung is ventilating at low volumes. Under these circumstances the lung tissue at the base becomes compressed after full expiration. The intrapleural pressures are now positive at the lung base and much less negative at the apex. When the lung expands, the non-dependent region is in the most advantageous part of the compliance curve, so that its volume will increase rapidly, whilst the dependent lung cannot increase its volume at all until the intrapleural pressures become subatmospheric. This situation can occur during anaesthesia in a spontaneously breathing patient.





Sites of airway resistance


As the airways penetrate toward the periphery of the lungs they become narrower, but more numerous. However, although the radii of these terminal bronchi are very small, they do not account for a great deal of resistance. The major site of resistance is in the medium-sized bronchi, and the very small bronchioles contribute very little. Most of the pressure drop across the airways occurs up to the seventh generation of bronchi and less than 20% beyond this point.


Because the peripheral airways contribute so little to resistance, the detection of lung disease here is made much more difficult.


The major factors affecting airway resistance are the following.


Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Respiratory system
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