Human Anatomy Lesson 8
Lesson 8 - Mediastinum, Heart
This is lesson #8 in a semester-long undergraduate-level course in Human Anatomy. Now that we have covered the the thorax, body wall, pleurae, and lungs, we can turn our attention to the mediastinum and heart. We will start by focusing on the divisions of the mediastinum, and investigate the structures within the middle, superior, and inferior parts of the mediastinum. We will then turn our attention to the gross anatomy of the heart, cardiac chambers, coronary vasculature, and coronary innervation and function, and conclude with a few pointers on how to identify the position of the heart, great vessels, lungs, pleura, and a few other structures from surface anatomy. This is our final lesson on the thorax, and next lesson we will be turning our attention to the abdomen.
Learning Objectives - By the end of this lesson, you should be able to ...
- Identify the different parts of the mediastinum, and list the structures in each one
- Describe the different tissues that surround the heart
- Describe in a very elementary fashion the major arteries that supply the head, neck, and upper limbs
- Describe the gross anatomy of the heart and its four chambers
- Describe how blood flows from the body into the heart, out of the heart into the lungs, and from the heart to the rest of the body
- Describe the arterial supply and venous drainage of the heart itself
- Understand the sympathetic and parasympathetic innervation of the heart
Mediastinum and Pericardial Sacs
The heart, pericardial sac, "great" vessels (i.e., the gigantic arteries and veins sticking out of the superior part of the heart), thymus, and trachea lie in the mediastinum, which is a partition in the center of the thoracic cavity bordered anteriorly by the sternum, posteriorly by the vertebrae, to either side by the lungs and pleural cavity, superiorly by the superior thoracic aperture, and inferiorly by the diaphragm. A number of important nerves pass through the mediastinum from top to bottom on their way from the head and neck to the thorax, abdomen, pelvis, and perineum.
The mediastinum can be split into several regions: (1) the superior mediastinum, which runs from the superior thoracic aperture to the level of the sternal angle (angle between the manubrium and body of the sternum); (2) anterior mediastinum, which is the area below the superior mediastinum anterior to the pericardial sac; (3) middle mediastinum, the area below the superior mediastinum which includes the pericardial sac and its contents; and (4) posterior mediastinum, the area below the superior mediastinum posterior to the pericardial sac and diaphragm. The heart occupies the pericardial sac, and the great vessels which stem from the heart lie in the superior mediastinum. The anterior and posterior mediastinum are of lesser importance but have several important nerves and blood vessels running through them.
Let's start our investigation of the mediastinum with the middle mediastinum. The middle mediastinum contains the heart, origins of the great vessels, nerves, and smaller vessels. The heart is surrounded by pericardium, which is a fibrous sac made up of two layers: fibrous pericardium, made up of tough connective tissue, and serous pericardium, which is itself made up of an outer parietal layer that adheres to the fibrous pericardium and an inner visceral layer that adheres to the heart. The disposition of the parietal and visceral layers of serous pericardium is similar to the disposition of parietal and visceral layers of the pleurae, and in fact both sets of layers are from the same embryological precursors. Just as in the pleurae, the parietal layer adheres to the outer shell (in this case fibrous pericardium, the thoracic wall in the case of the lungs), and the visceral layer adheres to the organ itself (the lungs and the heart).
The fibrous and serous layers of pericardium are continuous with one another superior to the arteries (the left common carotid, brachiocephalic, and pulmonary arteries and the aorta) and surrounding the veins (including the superior and inferior venae cava and the pulmonary veins). The fibrous pericardium sits on the diaphragm and is attached to the posterior surface of the sternum via sternopericardial ligaments. Pericardiophrenic arteries and veins provide the blood supply and drainage for the fibrous pericardium. Innervation to the pericardium is from the vagus nerve (parasympathetic fibers), sympathetic trunks (sympathetic fibers), and the phrenic nerves (somatic sensory fibers from the parietal pericardium). Blood supply to the pericardium is via branches from the internal thoracic, pericardiacophrenic, musculophrenic, and inferior phrenic arteries, and the thoracic aorta.
Before moving to the heart, let's briefly review the structures in the superior, posterior, and anterior mediastinum.
The superior mediastinum is posterior to the manubrium of the sternum and the first four thoracic vertebrae, and continuous with the neck above and the other mediastina below. There are a number of important structures in the superior mediastinum, including the thymus; right and left brachiocephalic veins; left superior intercostal vein; superior vena cava; arch of aorta and its main branches, the brachiocephalic trunk, left common carotid, and left subclavian arteries; trachea; esophagus; phrenic and vagus nerves; left recurrent laryngeal branch of the left vagus nerve; and thoracic duct. In dissection, you will see other vessels and structures just superior to these (the right and left subclavian veins, right and left subclavian arteries, right and left internal jugular veins, and the right common carotid artery) that are just superior to the first rib and are therefore NOT part of the superior mediastinum. It is important that you understand which structures are part of the superior mediastinum and which are not.
- Veins: Let's start with veinous drainage in the superior mediastinum. The right and left brachiocephalic veins are covered, at least partially, by the thymus. On each side, they form at the junction of the subclavian veins, which receive deoxygenated blood from the neck, axillae, and arms, and the internal jugular veins, which receive blood from the head. The right brachiocephalic vein begins posterior to the medial end of the right clavicle and is oriented more-or-less in a vertical direction, whereas the left brachiocephalic vein begins posterior to the medial end of the left clavicle and runs a longer oblique course to the right to meet up with its mate on the right side. The two brachiocephalic veins form the superior vena cava, which drains into the right atrium of the heart. About half of the SVC is in the superior mediastinum and half is in the middle mediastinum. SVC receives the azygos vein and, potentially, other veins before entering the pericardial sac.
- Aorta and its branches: The ascending aorta arises from the heart, at which point it immediately curve toward the left side and begins to descend inferiorly in the thorax on the left side of the vertebral bodies. It makes an upside-down letter "J," and the curve of the "J" is known as the arch of aorta, or aortic arch. The part that runs along the vertebral bodies is known as the thoracic, or descending, aorta. Only the arch of the aorta is in the superior mediastinum. The aortic arch gives off the (1) brachiocephalic trunk, which gives rise to the right subclavian artery and right common carotid artery at the level of the upper edge of the right sternoclavicular joint; (2) left common carotid artery, which runs parallel to the trachea as it ascends through the neck to the head; and (3) left subclavian artery, which ascends through the superior mediastinum to the left of the trachea. In summary, there are 2 subclavians and 2 common carotids that come off the aortic arch. Students sometimes become confused because the "left" arteries come directly off the arch, but the "right" arteries come off the brachiocephalic trunk first, which itself attaches to the aortic arch. You should be sure to take a minute or two to understand and memorize this distinction.
- Trachea and esophagus: The trachea is clearly evident, and in fact palpable, in the jugular notch as it enters the mediastinum. The trachea divides into right and left main bronchi around the T4/T5 vertebral level. The esophagus is positioned just posterior to the trachea and immediately anterior to the vertebral bodies in the midline, and passes into the posterior mediastinum below the T4 level.
- Nerves: The vagus nerves pass through the superior mediastinum and posterior mediastinum on their way to the abdomen, providing parasympathetic innervation to the thoracic viscera they encounter along the way. The right vagus nerve lies between the right brachiocephalic vein and brachiocephalic trunk in the superior mediastinum, descends in a posterior direction toward the trachea, passes posterior to the root of the right lung, and descends through the thorax on the right lateral side of the esophagus. The left vagus nerve enters the superior mediastinum posterior to the left brachiocephalic vein between the left common carotid and left subclavian arteries. Initially, it lies deep to the parietal pleura and crosses the left side of the aortic arch, at which point it descends posterior to the root of the left lung to reach the esophagus in the posterior mediastinum. The left vagus nerve gives off the left recurrent laryngeal nerve at the inferior margin of the aortic arch. This nerve loops under the aortic arch and ascends along the medial surface of the aorta, enters a groove between the trachea and esophagus, and continues superiorly to enter the larynx in the neck.
- Phrenic nerves: The phrenic nerves arise from anterior rami of 3rd, 4th, and 5th spinal nerves, and descend through the thorax to supply sensory innervation to the mediastinal pleurae, fibrous pericardium, and parietal layer of serous pericardium in the thorax and motor and sensory innervation to the diaphragm. The right phrenic nerve enters the superior mediastinum lateral to the right vagus nerve and lateral and slightly posterior to the beginning of the right brachiocephalic vein. It runs in close approximation to this vein and the SVC, and descends through the thorax within the fibrous pericardium on the right side of the pericardial sac anterior to the root of the right lung, accompanied by the pericardiacophrenic artery and vein, and finally passes through the diaphragm with the inferior vena cava. The left phrenic nerve enters the superior mediastinum lateral to the left vagus nerve and lateral and slightly posterior to the beginning of the left brachiocephalic vein, descends along the lateral side of the aortic arch, and descends through the thorax within the fibrous pericardium on the left side of the pericardial sac anterior to the root of the left lung, accompanied by the pericardiacophrenic artery and vein. It pierces the diaphragm near the apex of the heart.
- Thoracic duct: The thoracic duct, which is the major lymphatic vessel in the body, passes through the posterior part of the superior mediastinum. We will discuss this structure when we cover the neck.
There are numerous structures in the posterior mediastinum that pass from the thoracic cavity to the abdominopelvic cavity through the diaphragm. These include the esophagus, thoracic aorta, azygos system of veins (including the large azygos vein on the right and hemiazgos and accessory hemiazygos veins on the left), thoracic duct, the sympathetic chain and accompanying white and gray rami communicans, and branches coming off the ganglia, including the greater splanchnic nerve, lesser splanchnic nerve, and least splanchnic nerve. Note that the splanchnic nerves originate from ganglionic structures in the thorax, then descend into the abdominopelvic cavity through the diaphragm.
There is a potential space just anterior to the pericardial sac and posterior to the body of the sternum, which contains the thymus, fat, connective tissue, lymph nodes, mediastinal branches of internal thoracic vessels, and the sternopericardial ligaments, which partially suspend the fibrous pericardium surrounding the heart from the deep surface of the sternum.
Gross Anatomy of the Heart
As noted above, the heart rests on top of the abdominal diaphragm (on its diaphragmatic surface) and its apex points to the left. The base of heart is posterior, and the right and left pulmonary surfaces point laterally, toward the lungs. The heart is made up of four chambers: two atria and two ventricles. The two atria, one on the right and one on the left, are "waiting areas" that receive blood (from either the body or the lung) and pump that blood to the ventricles, which pump the blood out of the heart (to either the lungs or to the rest of the body). The right side of the heart is part of the pulmonary circuit, since it receives deoxygenated blood from the body and moves it to the lungs to become oxygenated. The left side of the heart is part of the aortic circuit, since it receives oxygenated blood from the lungs and moves that blood to the rest of the body. We will cover the route of blood through the heart in more detail in the next module.
The two atria are more lateral than superior to the two ventricles. The four chambers are not arranged in a simple fashion. The right atrium is on the right anterior side of the heart, and the left atrium takes up most of the base of the heart on its posterior surface. The right ventricle makes up most of the anterior surface of the heart, although a bit of the left ventricle is visible on the left side. The left ventricle makes up most of the posterior surface of the heart to the left of the left atrium.
As noted above, the atria receive blood from the body or lungs and pump it into the ventricles, which then pump blood out to the lungs or to the rest of the body.
- Right atrium: receives blood from the body via the superior vena cava (SVC) and inferior vena cava (IVC). The SVC drains the head and neck and receives tributaries from the right and left brachiocephalic veins in the superior mediastinum, and the IVC drains the entire body below the heart and diaphragm. The right atrium also receives blood from the tissue of the heart itself, which drains into the coronary sinus via an opening that is very close to the opening for the IVC. We will cover vasculature of the heart itself in the next section. The outer wall of the right atrium is largely made up of pectinate muscle, but the wall that borders the left atrium is made up of smooth muscle. There is a weak, membranous portion of this wall called the fossa ovalis that represents the embryonic position of the foramen ovale, which in embryos provides a clear line of bloodflow between the right and left atria. The opening for the tricuspid valve is in the left part of the right atrium. Blood flows into the right atrium from the SVC, IVC, and coronary sinus and flows out of the right atrium into the right ventricle through the tricuspid valve.
- Right ventricle: Deoxygenated blood flows from the right atrium into the right ventricle through the tricuspid valve and makes a 90 degree turn, flowing up through the semilunar cusps of the pulmonary valve into the pulmonary trunk and into the right and left pulmonary arteries, which run into the hila of the lungs. The three cusps of the tricuspid valve are attached, via chordae tendinae, to papillary muscles in the inferior part of the right ventricle. Once blood passes into the right ventricle, the papillary muscles relax so that the tricuspid valve can close, preventing blood from rushing back into the right atrium. The inferior part of the right ventricle is otherwise largely filled with rough muscle called trabeculae carnae. The upper part of the right ventricle, called the conus arteriosus, is largely made up of smooth muscle. Once blood passes through the semilunar cusps of the pulmonary valve, gravity pushes some of it back down, but the semilunar cusps fill up and close the valve, preventing significant backflow into the right ventricle after pulmonary contraction.
- Left atrium: blood that is pushed back into the heart from the lungs enters the left atrium through the pulmonary veins. There are two pulmonary veins on each side of the left atrium. The walls of the left atrium are largely made up of smooth muscle, and the remnant of the foramen ovale can be clearly seen on the wall that is shared with the right atrium. The left border of the left atrium empties into the left ventricle through the mitral valve.
- Left ventricle: The mitral valve is set up in a similar fashion as the tricuspid valve, with chordae tendinae that tie each valve down to papillary muscles. The only difference is that, on the left side, the mitral valve has two valves (and so it is sometimes called the bicuspid valve) instead of three. Oxygenated blood rushes into the left ventricle from the left atrium and makes a sharp turn to pass through the semilunar cusps of the aortic valve, into the ascending aorta, aortic arch, and then either through the vessels in the superior mediastinum that supply the head, neck, and upper limbs, or into the descending aorta, which runs through the thorax and abdomen on its way to supplying the entire body below the heart. The strength of the muscular contraction necessary to push blood to the fingers and toes is much greater than that necessary to push blood to the lungs, so the wall of the left ventricle is much thicker than the wall of the right ventricle. The aortic valve is set up the same way as the pulmonary valve, with semilunar cusps that prevent blood from backflowing into the atrium.
There are a number of arteries and veins which supply and drain blood to and from the heart. Most of these vessels run in the coronary sulcus between the atria and ventricles or the interventricular grooves between the two ventricles. The arteries are, of course, from the aorta, and arise at the level where the ascending aorta exits from the left ventricle. For the most part, the veins drain blood into the coronary sinus, the largest vein of the heart that empties directly into the right atrium near the opening of the IVC (see above).
Since I've already introduced the coronary sinus, let's go over the veins first. The coronary sinus occupies a large portion of the posterior aspect of the coronary sulcus. The great cardiac vein runs superiorly in the anterior interventricular groove until it reaches the coronary sulcus, at which point it makes a 90 degree turn and enters the coronary sinus. The middle cardiac vein runs superiorly in the posterior interventricular groove and drains into the coronary sinus directly. The small cardiac vein begins on the anterior surface of the right ventricle, and runs superiorly until it reaches the right side of the coronary sulcus, at which point it makes a 90 degree turn and runs for a short distance in the coronary sulcus around the right side of the heart before draining into the coronary sinus. The coronary sinus, therefore, has veins draining into it from the left side (great cardiac vein), right side (small cardiac vein), and inferior side (middle cardiac vein). There are numerous other less important veins, but they are more variable in distribution and need not concern us here.
Just like the veins, arteries run in the coronary sulcus and anterior and posterior interventricular grooves. On the right, the right coronary artery runs in the coronary sulcus from the anterior to the posterior side of the heart, and gives off a marginal branch along the right side of the right ventricle before taking a 90 degree turn on the posterior side of the heart to run in the posterior interventricular groove. The posterior interventricular artery is therefore a branch of the right coronary artery, and is properly referred to as the posterior interventricular branch of the right coronary artery. The left coronary artery branches off the ascending aorta on the posterior side of the heart, in close approximation to the left atrium. It runs posterior to the pulmonary trunk to terminate in two branches: the circumflex branch of left coronary artery, which runs back around to the posterior side of the heart in the coronary sinus; and the anterior interventricular artery, more properly called the anterior interventricular branch of the left coronary artery. As with the veins, there are numerous smaller, less important arterial branches, but their distribution across the heart is more variable than those we have covered above.
Cardiac Conduction and Innervation
Cardiac Conduction: The cardiac conduction system initiates and coordinates contraction of the heart muscles. The four basic components of the conduction system are the sinu-atrial (SA) node, atrioventricular (AV) node, AV bundle with right and left bundle branches, and Purkinje fibers.
Impulses begin at the SA node, which is considered to be the cardiac pacemaker. The SA node is located in the superior part of the wall of the right atrium just posterior to the SVC. The signal generated by the SA node causes both atria to contract simultaneously. This stimulates the AV node, which is located in the interventricular septum close to the opening of the coronary sinus and the tricuspid valve in the right atrium. The excitatory impulse runs from the AV node through the AV bundle, which sends a right bundle branch to the right side of the interventricular septum and a left bundle branch to the left side. These branches further divide to become continuous with Purkinje fibers, a plexus of specialized ventricular conduction cells that invest the ventricular musculature including papillary muscles.
Cardiac Innervation: cardiac muscle in the heart receives innervation from the autonomic division of the PNS, which regulates heart rate and the strength of each contraction. Parasympathetic innervation slows heart rate, reduces contractile forces, and constricts the coronary arteries. The right and left vagus nerves provide parasympathetic innervation of the heart via vagal cardiac branches. Sympathetic innervation increases heart rate and contractile force. Sympathetic fibers reach the heart through cardiac nerves (a type of "organ" nerve) from the sympathetic chain of paravertebral ganglia. Parasympathetic and sympathetic fibers from the vagus nerve and cardiac nerves form superficial and deep cardiac plexuses around the aortic arch and bifurcation of the trachea, respectively. Preganglionic fibers from both of these types of nerve synapse either in these plexuses or else in the walls of the atria.
This completes our look at the thorax! Next, we are moving on to the abdomen, pelvis, and perineum.
© 2014 Robert McCarthy