Lesson 7 - Body Wall, Thorax, Lungs
Welcome! Now that we have covered the anatomy of the back, we can move to the other side of the body and open up the thorax, so to speak. For teaching purposes, the thorax can be broken down into a few component regions: the anterior external surface of the rib cage, which is called the pectoral region; the thoracic wall, which constitutes the rib cage and spaces between ribs; the abdominal diaphragm, which borders the thorax inferiorly and separates it from the abdominal cavity and viscera; the pleural cavities, which contain the lungs and supporting structures; and the mediastinum, which contains the heart, esophagus, trachea, nerves, and blood vessels. We will investigate the anatomy of the pectoral region, thoracic wall, abdominal diaphragm, and pleural cavities in this lesson, and save the mediastinum and heart for lesson 8.
Learning Objectives - By the end of this lesson, you should be able to ...
- Identify the major regions of the thorax and list the structures in each region
- List and describe the muscles on the outer aspect of the thorax
- List and describe the muscles, nerves, arteries, veins, and lymphatic vessels of the thoracic wall and intercostal spaces
- Understand how the diaphragm and accessory muscles of respiration change the volume of the thorax during breathing
- Describe the gross anatomical structure of the lungs
- Understand the organization of the lungs, pleural cavities, and bronchial tree
- List and describe the innervation and blood supply of the lungs, pleural cavities, and bronchial tree
The thorax is a cylinder bordered by the vertebrae and intervertebral discs, rib cage, and sternum posteriorly, laterally, and anteriorly. The 1st and 2nd thoracic vertebrae, first ribs, and manubrium of the sternum border the superior thoracic aperture, which is open to the neck, and the 11th and 12th ribs, costal cartilages, and xiphoid process make up the inferior thoracic aperture, which is sealed off from the abdomen by the diaphragm. The thorax serves several functions, including housing the lungs (the organs related to breathing) and related "machinery" such as the diaphragm, intercostal muscles, and ribs; protecting vital organs such as the heart, lungs, aorta, and even the stomach, spleen, and kidneys; and forming a conduit between the head and neck and the body below the diaphragm.
- Breasts: There are a few important structures in the pectoral region (the breasts, superficial fascia, and skin) which are situated in a superficial compartment, but the pectoral muscles and supporting structures are located in a deep compartment. The breasts are made up of mammary glands, which are modified sweat glands in the superficial fascia anterior to the pectoral muscles. The suspensory ligaments of the breast are continuous with the dermis and serve to support each breast. The base of each breast runs from ribs 2 to 6, and from the sternum laterally to as far as the midaxillary line. Each breast is innervated by anterior and lateral cutaneous branches of the 2nd to 6th intercostal nerves, and the nipple is innervated by the 4th intercostal nerve. Blood is supplied to the breast via perforating branches off the internal thoracic artery, which you will learn more about later in this lesson. About 75% of lymphatic drainage of the breast is through axillary nodes, and the rest is into parasternal nodes associated with the internal thoracic artery.
- Muscles: Pectoralis major is a large muscle separated from the breast by a layer of deep fascia, and originates from two heads: a clavicular head, which originates from the medial 1/2 of the clavicle, and the sternocostal head, which originates from the sternum and ribs down to the 7th sternochondral junction. From its origins, the pectoralis major passes laterally along the 7th costal cartilage onto the aponeurosis of the external abdominal oblique, and inserts into the lateral aspect of the intertubercular sulcus of the humerus. The clavicular and sternocostal heads of the pectoralis major come from different embryological precursors, and they are innervated by different nerves. The clavicular head is innervated by the lateral pectoral nerve, which is a branch off the lateral cord of the brachial plexus. The sternocostal head is innervated by the medial pectoral nerve, which is a branch off the medial cord of the brachial plexus. Note that the lateral and medial pectoral nerves are named for their areas of origin from the brachial plexus, NOT for the position they take relative to one another. In fact, the lateral pectoral nerve is situated almost directly superior relative to the medial pectoral nerve. Pectoralis major AD-ducts, flexes, and medially rotates the arm. Two muscles, subclavius and pectoralis minor, lie deep to pectoralis major. Subclavius runs from the anterior and medial part of rib 1 to insert onto the inferior surface of the clavicle, and is innervated by nerve to subclavius, which is a branch of the anterior ramus of C5. Pectoralis minor runs from the anterior surface of ribs 3 to 5 to insert onto the coracoid process of the scapula, and is innervated by the medial pectoral nerve, just like the sternocostal portion of pectoralis major. Both muscles depress the tip of the shoulder. These three muscles together form the anterior wall of the axilla ("armpit"), which is the region through which major nerves, arteries, and veins running from the neck to the upper limb pass. Pectoralis minor in particular is an important landmark during dissection that is used to identify the positions of structures in the axilla. We will revisit this muscle in the section of the course focused on the upper limb. All three muscles are migrants from the upper limb that have swung around to the anterior side of the chest in the adult.
- Ribs and joints: The thoracic wall is made up of the thoracic vertebrae posteriorly, the ribs posteriorly, laterally, and anteriorly, and the sternum anteriorly. Each rib articulates with two vertebra at demifacets (superior and inferior costal facets - see lesson 3), swings posteriorly and laterally to articulate with transverse processes, and continue in a lateral and inferior direction to articulate with a costal cartilage that either articulates directly with the sternum (ribs 1 to 7, "true" ribs), with costal cartilage that indirectly joins the sternum (ribs 8-10), or "floats" in the body wall and does not articulate with the sternum (ribs 11 and 12). Each rib therefore has a costovertebral joint where it articulates with the bodies of adjacent vertebrae and a costotransverse joint where its tubercle articulates with the transverse process. The costotransverse joint is reinforced by the costotransverse, lateral costotransverse, and superior costotransverse ligaments. The sternocostal joints and other joints between the costal cartilages and ribs within the sternum allow a small but important amount of movement in the rib cage.
- Intercostal spaces: There are 11 intercostal spaces on each side. Intercostal spaces 1 to 6 end in the sternum medially, and are sometimes referred to as long intercostal spaces. Intercostal spaces 7,8, and 9 are short and bounded medially by costal cartilage, whereas 10 and 11 are short and "open." There is, from superior to inferior, a vein, artery, and nerve (VAN) that runs in each intercostal space directly inferior to the rib in the costal groove between 2 inner layers of muscle in the thoracic wall.
- Arterial supply: The main arterial supply to the intercostal spaces is via intercostal arteries, which branch off the aorta and subclavian artery. The first two posterior intercostal arteries branch off the costocervical trunk, which is a branch of the subclavian artery, but the other posterior intercostal arteries branch directly from the thoracic aorta. As noted above, a posterior intercostal artery runs with a nerve and vein in the costal groove on the inferior side of each rib, and meets an anterior intercostal artery which is running from medial to lateral on the anterior side of the thorax. Anterior intercostal arteries arise either from the internal thoracic artery (above the level of T6) or the musculophrenic artery (a branch of the internal thoracic artery) below that level. Veinous drainage largely parallels arterial supply, occurring mainly through the azygos system, which brackets the aorta, and the internal thoracic veins, which parallel the course of the internal thoracic arteries. Lymphatic drainage in the thoracic region occurs via parasternal, intercostal, and diaphragmatic nodes.
- Layers of tissue in the thoracic wall: from external to internal - (1) skin and superficial fascia; (2) outer layer of muscle which differs in different areas of the thorax - including serratus posterior superior and serratus posterior inferior muscles that we already talked about on the posterior wall, serratus anterior on the anterolateral wall, and pectoralis major and minor on the anterior wall; (3) external intercostal muscle; (4) internal intercostal muscle; (5) the VAN; (6) innermost intercostal muscle or transversus thoracis; (7) endothoracic fascia; (8) parietal pleura; (9) visceral pleura; (10) lung.
- Intercostal muscles: The 3 intercostal muscles are innervated by the T1 through T11 intercostal nerves. They are layered like plywood, with fibers of the external intercostals running in an anteroinferior direction (the direction of your fingers when you put your hands in your pockets) in the anterior intercostal spaces, and the internal and innermost intercostals running in an anterosuperior direction. They provide structural support for the intercostal spaces during breathing and move the ribs slightly during inspiration (external intercostals) and expiration (internal intercostals). The transversus thoracis muscle attaches to the lower ribs in lieu of the innermost intercostals near the sternum.
The diaphragm is a thin musculotendinous structure that attaches to the inferior ribs, vertebrae, costal cartilages, and xiphoid process, and separates the thoracic cavity from the abdominal cavity. It is pierced by a number of structures that run in a longitudinal direction between the thorax and lower parts of the body, including the inferior vena cava (at ~T8), esophagus (T10), vagus nerves, aorta (T12), thoracic duct, and azygos and hemiazygos veins. The diaphragm is innervated by C3 to C5 fibers in the phrenic nerves, supplied by pericardiaphrenic, musculophrenic, and superior and inferior phrenic arteries, and drained by the azygos system and other surrounding veins.
During breathing, the dimensions of the thoracic cavity change as air is inspired and expired. The diaphragm is the major muscle that accomplishes these dimensional changes. Contraction of the diaphragm at the inferior boundary of the thorax depresses the diaphragm, thereby increasing the volume of the thoracic cavity to make more room for inspired air in the lungs. Relaxation of the diaphragm elevates it, thereby decreasing the volume of the thorax to expel air from the lungs.
The thoracic wall also has a limited ability to expand. Since the first ten ribs are attached to the vertebrae posteriorly and sternal and costal cartilages anteriorly and run in a semicircle, they function like a bucket handle. Elevation of the ribs expands the volume of the thorax slightly. The muscles that attach to the ribs and accomplish this movement can therefore perform as "accessory muscles" of respiration.
It is important to understand that the diaphragm is by far the most important muscle of respiration, and the so-called accessory muscles have little impact on thoracic volume during breathing. It is true that the intercostal muscles fire during inspiration and expiration, but their effect is minimal and their pattern of recruitment complicated. These muscles seem to be more important for structural than functional purposes.
The two pleural cavities occupy the space just deep to the intercostal muscles of the ribs and superior to the diaphragm on either side of the thoracic cavity between the heart and mediastinum. There is a thin layer of endothoracic fascia that separates the innermost intercostal muscles from the parietal pleura and visceral pleura. The parietal pleura is associated with the thoracic walls, whereas the visceral pleura is adherent to the lungs. The two pleurae are separated from each other by a thin layer of serous fluid, and, in normal function, the lungs and visceral pleura freely slide against the parietal pleura and thoracic wall. On the medial surfaces of the lungs, the parietal pleura doubles back on itself to form a sleeve-like covering for vessels, nerves, bronchi, and arteries at the root of the lung, where the visceral and parietal pleura are continuous with one another.
Parietal pleura receives somatic sensory innervation from fibers that run in intercostal nerves and phrenic nerves. Visceral pleura receives no sensory innervation.
Bronchi and Lungs
The root of each lung is called the hilum, and a mainstem bronchus, pulmonary artery carrying deoxygenated blood, pulmonary vein containing oxygenated blood, nerves, and lymphatics run in each hilum. The two lungs are the primary organs of respiration, and expand and contract as air enters and leaves through the main bronchi. The airway extends from the trachea -> mainstem bronchi -> lobar bronchi -> segmental bronchi -> bronchioles. The right lung has 3 lobes, and 3 corresponding lobar bronchi, whereas the left lung has 2 lobes and 2 lobar bronchi. In the right lung, the horizontal fissure separates the superior lobe from the middle lobe, and an oblique fissure separates the superior lobe and middle lobe from the inferior lobe. In the left lung, an oblique fissure separates the superior and inferior lobes.
There are two pulmonary arteries, a right and a left. They both originate with the pulmonary trunk and carry deoxygenated blood from the right ventricle of the heart to the lungs to become oxygenated. Be sure to pay special attention to the fact that this is the one set of arteries that carries deoxygenated blood. There are four pulmonary veins, two superiorly and inferiorly on each side, which carry oxygenated blood from the lungs to the left atrium of the heart. Once again, be sure to note that these veins carry oxygenated blood back to the heart. Students will sometimes become confused about the vasculature linking the heart to the lungs because they create a mental shortcut where arteries carry oxygenated blood and veins deoxygenated blood. This is not necessarily true. Arteries and veins are defined as such based on whether a blood vessels is carrying something away from or toward the heart. Veins throughout the body bring deoxygenated blood back to the heart. The pulmonary veins are likewise bringing blood back to the heart, but in this case the blood has been passed through the lungs and become oxygenated. The pulmonary arteries are moving blood away from the heart, but in this case the blood is deoxygenated blood from the rest of the body that has been through the heart and is now being moved to the lungs to become oxygenated. So, for these two blood vessels, the type of blood carried in arteries and veins is reversed even though the flow of blood in relation to the heart is the same.
The trachea and bronchi have smooth muscle in their walls, and are therefore under autonomic control. Visceral motor and sensory innervation to the bronchi and lungs come from pulmonary plexuses which lie anterior and posterior to the trachea at its bifurcation into mainstem bronchi. The two plexuses are made up of branches off the sympathetic trunks and vagus nerves. Parasympathetic innervation from the vagus nerve constricts bronchioles, and sympathetic innervation from the sympathetic trunk dilates bronchioles.
Preview of Upcoming Lessons
- Human Anatomy Lesson 9
This lesson covers the anatomy of the abdominal wall and the contents of the abdominal cavity - the organs, viscera, blood vessels, and nerves related to digestion and elimination.
- Human Anatomy Lesson 10
This lesson covers abdominal vasculature and innervation and the posterior abdominal region.
- Human Anatomy Lesson 11
This lesson covers the basic anatomy of the pelvis and perineum, focusing on osteology of the pelvis and the muscles of the pelvic floor.
- Human Anatomy Lesson 12
This lesson covers anatomy of the organs in the pelvic cavity, including the rectum, anal canal, bladder, ureters, urethra, and components of the male and female reproductive systems.
© 2014 Robert McCarthy