Thorax Lab 2a

Lab 2A: Remove Heart and Lung

Learning objectives

Observe the orientation of the heart as it sits in the thorax.
Identify the great vessels of the pulmonary and systemic circulations.
Appreciate the bilateral symmetry of the left and right brachiocephalic veins relative to the asymmetry of the three first major branches of the aorta.
Understand the spatial relationships (i.e., the relative 3D positions) between the great vessels and the left and right vagus nerves.
Be able to distinguish between the right and left isolated lungs by their distinctive features. Identify the trachea and primary bronchi. Be able to list the structures contained within the hilum of the lung.

Great vessels of the heart

Reflect the manubrium

The first step will be to reflect the manubrium to allow better access to the aortic arch and its branches.

Push any remaining soft tissue away from the manubrium to isolate it so that you don’t accidentally cut any structures deep to the manubrium. Do the same with the clavicle, pushing soft tissue structures (like the subclavian artery and vein) away from the clavicle so that those structures are not inadvertently cut.

Now cut the isolated clavicles with nippers or a bone saw approximately at their mid-points, leaving the medial ends of the clavicles attached to the manubrium. You will also need to cut through the first ribs, which are articulated with the manubrium. Feel for the first ribs and isolate them before cutting through them with nippers or the bone saw. If you have any questions along the way ask a member of the teaching staff to guide you through this procedure.

Reflect the manubrium and the attached parts of the clavicles and first ribs superiorly, carefully freeing the bone from underlying connective tissue.

Great vessels

Clean any remaining connective tissue and adipose off the surface of the aortic arch and find the three large arteries that branch from the arch:

The first branch is the brachiocephalic a., which runs superiorly a short distance before splitting to form the right common carotid a. and the right subclavian a. In the axillary region (armpit), the subclavian artery changes its name to the axillary artery, When the axillary artery enters the brachium (arm), its name changes to the brachial artery.
The left common carotid a.
The left subclavian a.

Note that there is no brachiocephalic a. on the left side. The left common carotid a. and the left subclavian a. branch directly from the aortic arch. Other than the lack of a brachiocephalic a. on the left side, the course of the left common carotid and the left subclavian are the same on the left side as on the right.

The superior vena cava is formed by the union of the left and right brachiocephalic vv. Note that there are two brachiocephalic veins (left and right), but only one brachiocephalic artery. The brachiocephalic a. is on the right side, but since there is only one, it’s not referred to as the right brachiocephalic a. (because there is no left brachiocephalic a.).

The brachiocephalic vv. are formed by the union of the subclavian v. and the internal jugular v. on each side. The subclavian v. travels with the subclavian a., changing its name to the axillary v. and then the brachial v. as does the subclavian artery. The internal jugular v. runs in the carotid sheath with the common and internal carotid arteries. The origin of the internal jugular v. is at the base of the skull, where it drains all the venous blood from inside the skull.

You may also be able to identify the ligamentum arteriosum, the fibrosed arterial connection between the pulmonary trunk and the aortic arch (a remnant of fetal circulation, will be covered in lecture). Bluntly dissect between the bifurcation of the pulmonary trunk and the aorta to find the ligamentum arteriosum.

Helpful Hint

Given the direction of the flow of blood (away from the heart in arteries, toward the heart in veins) it may be helpful to think of the arteries as splitting apart as they move away from the heart, and the veins as coming together as they toward the heart.

Relationship of the great vessels and the vagus nerves

Locate the vagus nerves by removing the adipose (and remnants of the thymus gland) piecemeal from the superior mediastinum.

The left vagus nerve courses anterior to the aortic arch immediately lateral to the ligamentum arteriosum, and then passes posterior to the left pulmonary a. and the hilum of the left lung.

The right vagus nerve passes between the right subclavian a. and v., and then passes posterior to the hilum of the right lung.

Both vagus nn. give off branches that turn and go back up into the neck, where they eventually innervate the muscles of phonation in the larynx. These are the left and right recurrent laryngeal nn. The left recurrent laryngeal nerve hooks underneath the aortic arch, while the right recurrent laryngeal nerve hooks underneath the right subclavian a.

Removal of the Heart and Lungs

Before you begin...

During this lab we will be removing the heart and lungs from the mediastinum as a single unit. We will then separate the heart from the lungs as well as separate the left and right lungs from one another.

For now, let’s focus on getting the heart and lung unit out of the thorax.

To begin the dissection, identify again the phrenic nerves and vagus nerves and attempt to preserve them by leaving them behind in the thorax as you remove the heart and lungs.

Mobilize the lungs by running your hands around and behind them, breaking any adhesions between the parietal and visceral pleurae with your fingers. Take care with sharp edges of the cut ribs. Healthy lungs are completely free to move around in the pleural cavities, but in older or ill individuals there may be extensive adhesions between the visceral and parietal pleurae (the result of inflammation of the pleura, a condition called pleurisy or pleuritis). If there is extensive pleural fluid in your cadaver's thorax, you may need to scoop some of it out to clear your field of view.

To preserve the phrenic nerves, pull them laterally and try to get them positioned lateral to the lungs. The phrenic nerves are located medial to the lungs in life, so this will require a combination of pulling laterally and tucking the lungs under them medially.

Any structures that you cut should first be isolated and identified, do not cut blindly! Transect the following structures, in the following locations:

The ascending aorta, about 1 cm from the heart.
The superior vena cava, 2cm superior to its union with the right atrium.
The inferior vena cava, as inferiorly as possible. Avoid cutting the diaphragm!
The trachea, superior to the split between the left and right primary bronchi.

Contents of the thorax

Use blunt dissection with your fingers, and scissor-spreading, to break the remaining connections between the heart/lung unit and the posterior mediastinum. This step can be challenging - take your time and ask for help from the teaching staff if necessary. You may need to cut portions of the pericardium that interfere with removal.

As you remove the heart/lungs you will see large branches of the left and right vagus nerves that provide parasympathetic innervation to the heart and lungs. These will have to be cut to remove the heart/lungs. You’ll find that there is a significant amount of connective tissue that seems to hold the heart and lungs to the posterior body wall. A lot of that tissue is actually the cardiopulmonary plexus, made up of branches of the vagus nerves and the post-ganglionic sympathetic neurons that go from the sympathetic chain ganglia to the heart and lungs.

You’ll also see the ligamentum arteriosum joining the pulmonary trunk to the arch of the aorta. You’ll have to cut this structure to remove the pulmonary vessels along with the heart and lungs, so make sure everyone gets a good look at it before it’s gone.

Remove the heart and lungs as a single unit from the thoracic cavity.

Separation of the Heart from the Lungs

Before you begin...

Now that the heart and lungs are detached from the mediastinum and removed from the thoracic cavity, it’s time to examine them as a single cardiopulmonary unit. Then we will separate the cardiac and pulmonary components of this unit.

Posterior view

Clean any adipose and connective tissue on the posterior surface of the cardiopulmonary unit to view the left and right inferior pulmonary vv. Scissor-spread along the veins a short distance into the lung tissue and you’ll see that the veins branch repeatedly. These branches are all tributaries to two of the four pulmonary veins that bring oxygenated blood from the lungs to the left atrium. The superior pulmonary veins are anterior to the bronchi and are easier to see from an anterior view.

Clean up the trachea and the left and right primary bronchi. The trachea is made up of multiple C-shaped cartilaginous arches that hold the airway open. Scissor-spread along the bronchi a short distance into the lung tissue. As with the pulmonary veins, you’ll see that the bronchi branch repeatedly as they move into the lung. The results of the first branching are the secondary bronchi (lobar bronchi), and secondary bronchi branch to form tertiary bronchi.

Posterior view of the cardiopulmonary unit

Airways

Look down into the trachea and you’ll see a structure at the split into left and right primary bronchi called the carina (“keel”). The carina is a cartilaginous ridge that projects into the trachea at the split between the left and right primary bronchi.

The carina is a cartilaginous structure at the point of the split of the trachea into the left and right primary bronchi.

Anatomy of the airways

Clinical correlation: Intubation and Aspirated Bodies

When you put in an endotracheal tube you need to know where the end of the tube is relative to the carina. If the tube is distal to the carina you’re only ventilating one lung (that’s bad). You should also note that the passage from the trachea into the right primary bronchus is a fairly straight shot, while the left primary bronchus is at more of an angle to the trachea (because it has to go over the heart). One of the results of these differing angles is that an aspirated body tends to go into the right primary bronchus.

Anterior view

Clean the pulmonary trunk on the anterior surface of the cardiopulmonary unit and follow its split into the left and right pulmonary arteries. As you did with the veins and airways, follow the pulmonary arteries into the lung tissue to see that they split repeatedly as they enter the lungs. Look for the left and right superior pulmonary veins slightly inferior and posterior to the pulmonary arteries.

We’ve completed the examination of the heart and lungs as a single unit. Now it’s time to separate the heart from the lungs.

Cut along the dotted lines in the figure below to separate the heart and lungs. Cut the pulmonary arteries and veins approximately mid-way along their lengths, so that they can be identified at both the heart and the lungs.

You may keep the trachea intact and the two lungs attached to one another by the trachea, or divide the trachea slightly off the midline to get a better view of the carina.

Anterior view of the cardiopulmonary unit. The dashed lines indicate the approximate location of cuts made to separate the heart from the lungs.

External Lateral Features of the Lungs

Now that we have the lungs isolated, let’s identify some external landmarks that are visible on the lateral surfaces of the lungs.

Apex – the pointed “tip” of the lung. The apex of the lung is part of the superior (upper) lobe.
Base – the concave “bottom” of the lung. This is the part of the lung that sits on top of the diaphragm. The base of the lung is part of the inferior (lower) lobe of the lung.
Oblique fissure – divides the upper and lower lobes of the left lung, and the middle and lower lobes of the right lung
Horizontal fissure – divides the upper and middle lobes of the right lung.

Once you’ve had a chance to familiarize yourself with the lateral exterior of the lungs, flip them over to explore the medial surface. First, we will look at the left lung.

External Medial Features of the Left Lung

Note the apex and base of the lung, the superior and inferior lobes (separated by the oblique fissure), and the anterior and posterior borders. Find the following landmarks on the medial surface of the left lung:

Two distinct contact impressions on the medial surface of the left lung: the groove for the aorta (arch and descending aorta) and the cardiac impression.
The hilum of the lung is the region that connects the lung to the structures in the mediastinum:
- The bronchi have cartilage in their walls and are easy to distinguish from blood vessels.
- The pulmonary arteries are typically the most superior vessels of the hilum and are slightly thicker-walled.
- The pulmonary veins, are typically inferior and anterior to the bronchi and thinner walled.
- The bronchopulmonary lymph nodes are typically firm black nodules and can be quite large.

Realize that the most superficial tissue of the hilum is the pleura. At the hilum the visceral pleura of the lung is continuous with the parietal pleura of the mediastinum. The pulmonary ligament is an extension of the hilum that contains no airways or blood vessels. At the pulmonary ligament, the visceral and parietal pleura are also continuous.

The bronchial vessels, which supply the tissue of the lung, also enter the lung at the hilum.

Now that we’ve viewed the left lung, let’s move on to the right lung.

External Medial Features of the Right Lung

Note the apex and base, the superior, inferior and middle lobes (separated by the oblique and horizontal fissures), and the anterior and posterior borders. Find the following landmarks on the medial surface of the right lung:

The contact impressions of the right lung are typically not as regular and clear as those of the left, though you may see grooves for the superior vena cava (SVC) and the azygous vein.
The hilum of the right lung contains the same structures as the left:
- Bronchi
- Pulmonary arteries
- Pulmonary veins
- Bronchopulmonary lymph nodes
- Pulmonary ligament
- Bronchial vessels

RALS - The position of the pulmonary artery relative to the airway (airway is easy to find because cartilage). In the Right lung, the artery is Anterior to the airway. In the Left lung, the artery is Superior to the airway.

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