Dissector Answers - Superior Mediastinum & Lungs

Learning Objectives:

Upon completion of this session, the student will be able to:

1. Identify the contents of the superior mediastinum.
2. Identify and trace the tributaries to each of the brachiocephalic veins and the formation of the superior vena cava.
3. Identify and trace all the branches of the arch of the aorta in the region and the relationship of the vagus and phrenic nerves to them.
4. Identify the thoracic part of the trachea and describe its blood supply and innervation.
5. Identify and describe the location of the lungs in the thoracic cavity.
6. Identify the relations of the pulmonary artery, pulmonary veins, and the bronchi at the hilum of each lung.
7. Define a bronchopulmonary segment and discuss its general organization.
8. Name the bronchopulmonary segments and give their approximate location in reference to the lobes of the lungs.
9. Trace the lymphatic drainage of the lungs and respiratory tract.

Learning Objectives and Explanations:

1. Identify the contents of the superior mediastinum. (W&B 370, M 138, 142-150, N193, N206, N209, N211, N212, N232, N240, N262, TG4-18, TG4-35A, TG4-35B, TG4-36A, TG4-36B)

• Vessels:
  • Arch of the aorta and roots of major branches: begins at the level of the sternal angle and arches superoposteriorly (up and back) and to the left. The arch usually gives off three major branches (below) supplying the head, neck, and upper extremities. (Greek, aorta = to lift)
    • Brachiocephalic trunk: first branch of the aortic arch. The brachiocephalic trunk rises superolaterally and splits into the right subclavian and the right common carotid arteries. (Latin, bracchium = arm, Latin/Greek, cephalicus/kephalikos = head)
    • Left common carotid artery: second branch. It ascends into the neck to supply the left side of the head. (Greek, kara = head)
    • Left subclavian artery: third branch. It ascends lateral to the left common carotid into the neck, but does not give off any branches in the mediastinum. ("clavian" refers to the clavicle)
  • Brachiocephalic veins: formed by the joining of internal jugular and subclavian veins. The brachiocephalic veins drain the head and upper extremities, uniting to form the superior vena cava, which dumps into the right atrium. (Latin, bracchium = arm, Latin/Greek, cephalicus/kephalikos = head)
  • Superior vena cava (upper portion): returns blood to the heart from the head, neck, upper extremities, and thorax (except the heart and lungs). The terminal SVC lies in the middle mediastinum.
  • Thoracic duct: drains all of the lymph from all of the body below the diaphragm and the left half of the body above the diaphragm. In the superior mediastinum it can be found behind the aortic arch, on the left side of the esophagus.
• Nerves:
  • Right vagus nerve (CN X): enters thorax anterior to the right subclavian artery, giving off the right recurrent laryngeal nerve (which hooks back up into the neck to supply the larynx). The right vagus nerve then travels in a posteroinferior direction, following the right side of the trachea. It contributes to the cardiac, pulmonary, and esophageal plexuses. (Latin, vagus = wandering, plectere = to braid)
  • Left vagus nerve (CN X): descends into the mediastinum between the left common carotid artery and the left subclavian artery. It gives off the left recurrent laryngeal nerve, which is distinctive because it wraps around the aortic arch before it travels up into the neck to supply the larynx. The left vagus then travels posteroinferiorly, contributing to the cardiac, pulmonary, and esophageal plexuses. (Latin, vagus = wandering, plectere = to braid)
  • Phrenic nerves: provide somatic motor and sensory innervation to the diaphragm. These nerves pass anterior to the roots of the lungs, which separates them from the vagus nerves which pass posterior to the roots of the lungs. Contribution to the phrenic nerves include the 3rd, 4th, and 5th cervical spinal nerves - "C3, 4, and 5 keep the diaphragm alive."
• Viscera/Fascia:
  • Thymus: a primary lymphoid organ, Important in the immune system, the thymus is the site of T-cell differentiation. (Greek, thymus = warty excrescence)
  • Trachea: lies anterior to the esophagus, just a bit to the right of the median plane. The bifurcation into right and left main bronchi occurs at the level of the sternal angle. (Greek, trachys = rough)
  • Esophagus: enters the mediastinum a little to the right of the median plane, posterior to the trachea. It passes to the middle of the body, then to the left and anteriorly as it travels down to the stomach. (See "Extra Explanation" in Posterior Mediastinum Answers.) (Greek, oisein = to carry + phagein = to eat)
  • Ligamentum arteriosum: remnant of fetal ductus arteriosus. The ligament attaches between the inferior surface of the aortic arch and the root of the left pulmonary artery. (Latin, ligare = band, tie)

2. Identify and trace the tributaries to each of the brachiocephalic veins and the formation of the superior vena cava. (W&B 384, N74, N238, TG4-37B)
3. Identify and trace all the branches of the arch of the aorta in the region and the relationship of the vagus and phrenic nerves to them. (W&B 383-384, N211, N212, N232, N237, N240, TG4-38)

As discussed above, the arch of the aorta gives off the brachiocephalic artery (which splits into right common carotid and right subclavian arteries), the left common carotid artery, and the left subclavian artery. Also consider the veins of this region, which as a rule of thumb, lie anterior to the "analogous" artery.

Most laterally, we have the phrenic nerves. They lie on the anterior aspects of the anterior scalene muscles on both sides. These muscles separate the respective subclavian veins (anterior) from the subclavian arteries (posterior). So, the phrenic nerves cross between subclavian vein and subclavian artery as they descend into the thorax.

The vagus nerves are more medial, descending into the thorax just lateral to the common carotid arteries. They are also mostly anterior to the branches and the arch proper as they descend. The right vagus nerve gives off the right recurrent laryngeal nerve, which loops around to the posterior aspect of the right subclavian artery and travels back up into the neck. The left vagus nerve gives off the left recurrent laryngeal nerve, which loops around the arch proper, next to the ligamentum arteriosum, and ascends back into the neck between the trachea and the esophagus.

4. Identify the thoracic part of the trachea and describe its blood supply and innervation. (W&B 234-235, N33, N74, N126, N207, N209, N240, TG4-33, TG4-38)

The trachea begins at the level of C6 and terminates at the level of the sternal angle, where it divides into the two main bronchi. The trachea receives arterial supply from the inferior thyroid arteries, internal thoracic arteries, and branches of the bronchial arteries. The blood drained via tracheal veins ends up in the inferior thyroid veins. Parasympathetic innervation to the smooth muscle and glands is via the vagus nerve (CN X), either directly or by the recurrent laryngeal nerves. Sympathetic innervation comes directly from the sympathetic trunk.

5. Identify and describe the location of the lungs in the thoracic cavity. (W&B 401-404, N196, N197, N199, N206, TG4-13A, TG4-13B, TG4-15, TG4-16, TG4-17, TG4-18)

The lungs occupy the large space inside the rib cage, on either side of the mediastinum, superior to the diaphragm. These spatial demarcations also define the three surfaces of each lung, namely the costal, mediastinal, and diaphragmatic surfaces. (The pleura of the same name covers the lung over that surface.) The lungs extend out of the thoracic cavity into the root of the neck at the apex, which is covered by the cervical pleura.

Each lung has a hilum on its medial aspect. The hilum of the lung is the point of entry for the root of the lung, which includes the bronchi, the pulmonary arteries, and the pulmonary veins. A pleural sleeve is created around these structures, where the pleura reflects, changing from visceral to parietal. Inferior to the hilum on each lung is the pulmonary ligament, a continuation of that pleural reflection.

6. Identify the relations of the pulmonary artery, pulmonary veins, and the bronchi at the hilum of each lung. (W&B 400, N199, TG4-31AB, TG4-31CD, TG4-34A)

On both sides, the bronchus is posterior and the pulmonary vein(s) are anterior and inferior. The difference between the two involves the pulmonary artery(ies). On the right side, the artery(ies) lie more or less anterior to the bronchus. On the left side, however, the artery(ies) are the most superior structure, even superior to the bronchus.

7. Define a bronchopulmonary segment and discuss its general organization. (W&B 402, 404-407, N201, N203, N204, N205, TG4-32AB, TG4-32CD, TG4-33A)

A bronchopulmonary segment:

• is pyramid shaped, with the apex at the lung root;
• is the largest subdivision of a lobe;
• is surrounded by connective tissue;
• has separate arterial supply from other segments and receives its own segmental (tertiary) bronchus;
• is drained by intersegmental veins that lie in the connective tissue around the segment; and
• can be removed surgically without affecting the function of other segments.

  Bronchi (gas exchange): Each segment is supplied by one tertiary bronchus (see below). This "conducting" bronchus branches about 15 times, the result of which is small "terminal" bronchioles. These terminal bronchioles branch several more times, becoming "respiratory" bronchioles, which have alveoli along them, allowing for some gas exchange. The terminal bronchioles terminate in several alveolar sacs, where most of the gas exchange takes place. (Latin, alveus = cavity, hollow)

  Pulmonary arteries (deoxygenated blood): Leaving the right ventricle and traveling through the pulmonary arteries, the blood eventually reaches intrasegmental arteries. These arteries follow the branches of the bronchi and eventually end in capillary beds in the alveolar sacs. It is here that oxygen from the air crosses alveolar walls and enters the blood, while carbon dioxide from the blood goes the opposite way, to be exhaled from the lungs. It is important that you do not confuse pulmonary intrasegmental arteries with intrasegmental bronchial arteries. The former carry the deoxygenated blood from the right side of the heart in order to pick up oxygen. The latter are systemic, carrying oxygenated blood, like all systemic arteries, to supply the tissues of the bronchi.

  Pulmonary veins (oxygenated blood): The pulmonary veins arise at the alveolar capillary beds and travel within the intersegmental septa to carry newly oxygenated blood back to the heart. These veins, as they begin to accept tributaries, will effectively drain both segments separated by the septa within which they run.

8. Name the bronchopulmonary segments and give their approximate location in reference to the lobes of the lungs. (W&B 402, N201, N203, TG4-32AB, TG4-32CD, TG4-33A)

Right Lung:*
Superior Lobe: a three-piece lobe - a posterior and an anterior segment next to one another with an apical segment on top
Middle Lobe: a flat, two-piece lobe that sits on the anterior aspect of the lung - a medial and a lateral segment next to one another
Inferior Lobe: a five-piece, "square-based-pyramid" - four basal segments that point to the four "sides" of the body (anterior basal, medial basal, lateral basal, and posterior basal), and a superior segment on top.
Left Lung:*
Superior Lobe: similar to the right superior lobe, except that the apical and posterior fuse to form an apicoposterior segment; there is also an anterior segment like on the right side. However, the left also has a "lingual" portion, with a superior and an inferior segment.
Inferior Lobe: a five-piece, "square-based-pyramid" - four basal segments that point to the four "sides" of the body (anterior basal, medial basal, lateral basal, and posterior basal), and a superior segment on top.

9. Trace the lymphatic drainage of the lungs and respiratory tract. (W&B 234-235, N208, TG4-42)

Lymph from the lungs drains first to pulmonary nodes, which are small nodes within the lung tissue and also beneath the visceral pleura. The numerous small pulmonary nodes drain into larger bronchopulmonary nodes located the hilum (also called hilar nodes). The bronchopulmonary nodes drain into tracheobronchial nodes located around the tracheal bifurcation. These nodes may be separated into three large groups. The large cluster of nodes below the tracheal bifurcation are called inferior tracheobronchial nodes (or carinal nodes) and they receive lymph from the lower lobes primarily. Beside the tracheal bifurcation on either side are the fairly large superior tracheobronchial nodes. Oddly, most of the lymph from the inferior tracheobronchial nodes drains up into the right superior tracheobronchial nodes, which means that lymph from the lower lobe of the left lung crosses to the right here, for the most part. The lymph drainage ascend beside the trachea, with several modest sized paratracheal nodes intercepting some of this lymph on either side. The lymph channels coalesce as they ascend to form bronchomediastinal lymph trunks on either side. The left bronchomediastinal lymph trunk drains into the termination of the thoracic duct in the root of the neck, and the right bronchomediastinal trunk usually unites with the right subclavian and right jugular lymph trunks to form a short right lymphatic duct, which drains, similar to the thoracic duct, into the junction of the subclavian and internal jugular veins on the right.

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Cultural enrichment: Check out these sections from the 1918 version of Gray's Anatomy of the Human Body! Some of the terms are (of course) out-of-date, but the illustrations are timeless.

The Trachea and Bronchi - The Lungs - The Pulmonary Veins - The Thoracic Duct - The Vagus Nerve - Surface Anatomy of the Thorax - Surface Markings of the Thorax

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Questions and Answers:

8. Are there plexuses of nerves on the pulmonary arteries? (N209, TG4-45, TG8-14, TG8-15)

The pulmonary plexuses lie around the roots of the lungs, and the pulmonary artery is one of the structures of the lung roots. Therefore, there are plexuses on the arteries.

9. Determine the relation of the left superior intercostal vein to the aortic arch, the phrenic nerve, and vagus nerves. (N231, TG4-36B)

The left superior intercostal vein runs between the vagus and phrenic nerves. It crosses the aortic arch horizontally passing from the heads of the ribs anteriorly to end in the left brachiocephalic vein.

10. Observe the formation of the superior vena cava (confluence of both brachiocephalic veins), and relate its position to the manubrium and other great vessels. (N N196, N206, N212, TG4-37B, TG4-15, TG4-19)

The superior vena cava lies immediately to the right of the manubrium. It is overlapped on its left margin by the ascending aorta. Posteriorly, it lies against the right pulmonary artery and the right superior pulmonary vein.

11. Are there cardiac nerves arising from the left vagus? (N209, N240, TG4-45, TG8-15)

The left vagus normally gives parasympathetic branches to the cardiac plexus. They are most easily seen where the vagus crosses the aortic arch.

12. Do you find a vertebral artery from the arch of the aorta? (N33)

Sometimes the left vertebral artery will branch directly from the aortic arch.

13. Pull the aortic arch toward the left and observe the thoracic portion of the trachea. What innervates it and how? (N209, N240, TG4-35, TG4-45)

The recurrent laryngeal nerves innervate the upper trachea and esophagus as they pass superiorly in the tracheoesophageal groove to reach the larynx, which they also innervate. Recurrent laryngeal nerves provide skeletal motor innervation to the voluntary muscle of upper esophagus and larynx and parasympathetic motor innervation to the smooth muscle of the trachea. They also provide parasympathetic motor innervation to the mucous glands of all of these structures.

14. What is the blood supply of the trachea? What structure does it lie upon? (N33, N207, TG7-14, Objective 4)

For the blood supply, see Objective #4 above. The trachea lies upon the esophagus.

15. Locate the right and left bronchial arteries. What is their source? How do the two sides differ in number? (N207, TG4-39)

The two left bronchial arteries are branches from the highest part of descending aorta. The single right bronchial artery is either a branch of one of the left bronchial arteries or it may branch from the right 3rd posterior intercostal artery, in which case the common trunk is called the intercostobronchial trunk.

16. What is the difference between the "root" of the lung and the "hilum"? (N199, TG4-31AB, TG4-31CD)

The root of the lung is the collection of structures that connect the lung to the mediastinum. This includes the pulmonary arteries and veins, the primary bronchi and bronchial arteries, and the pulmonary nerve plexuses and lymphatics. The hilum is the place on the lung where these structures enter and leave the lung.

17. How do the cardiac and pulmonary plexuses differ? Where do they distribute? (N209, N240, TG4-45)

The cardiac and pulmonary plexuses are continuous at the tracheal bifurcation. The heart receives the cardiac plexus and lungs the pulmonary plexus.

18. Where and what is the cardiac notch? (N199, TG4-30AB, TG4-31AB)

The cardiac notch is a lateral deviation in the anterior border of the left lung, caused by the position of the heart. The lingula is an anterior projection of the left superior lobe below the cardiac notch.

19. What is the average projection of each lung and its fissures to the rib cage? (N196, N 197, N200, TG4-15, TG4-16, TG4-30C)

The anterior borders of each lung generally follow the sternal margin, although the cardiac notch causes a lateral deviation to the left anterior border between the 4th and 6th ribs. The apex of each lung projects to the level of T1, which means, due to the declination of the first rib anteriorly, that the apex rises above the first rib. The inferior border of each lung is located at rib 6 in the midclavicular line, rib 8 at midaxillary line, and rib 10 in the paravertebral line. (Recall that the pleura goes 8, 10, 12.) Both oblique fissures begin posteriorly at the spine of T3, passing obliquely (like the name implies) to reach the 6th costal cartilage near the midclavicular line. The horizontal fissure of the right lung begins at rib 5 in the midaxillary line, then reaches rib 4 to follow it anteriorly to the sternal margin.

20. What is the lingular bronchus? (N202, N 203, TG4-33A)

The left superior lobe bronchus has two divisions: superior and lingular. The lingular division or lingular bronchus has two segmental bronchi: superior lingular and inferior lingular.

21. What is its significance of the superior segmental branch of the right inferior lobar bronchus? (N203, TG4-33A)

Due to its larger caliber and more vertical orientation, the right primary bronchus is more likely to receive foreign bodies that enter the airway. The superior segmental bronchus branches posteriorly off of the intermediate bronchus or the inferior lobe bronchus, so it is the most likely segmental bronchus to receive those foreign bodies that enter the right bronchus. This is especially true if the patient is in a supine position.