Part 2: Lungs and Bronchi

Within the thoracic cavity, identify the left lung. Each lung (right and left) is divided into lobes based on the branching pattern of its principal (primary) bronchus into lobar bronchi. (See DG Fig. 3-13). The left lung is divided into two lobes, a cranial lobe and a caudal lobe. (See DG Figs. 3-10 and 3-11)

• Identify the cranial lobe of the left lung.

  • • The left cranial lobe is further divided into cranial and caudal parts. Identify these parts on your specimen.

• Identify the caudal lobe of the left lung.

  • • Comment: You may notice a groove/impression in the caudal lobe due to it pressing up against the aorta, this is called the aortic impression.

      Related Learning Objectives

      • D4.5
      • D4.7

       

Identify the right lung which is divided into four lobes: cranial, middle, caudal, and accessory. You should also note that the right lung is larger than the left lung. (See DG Fig. 3-12)

• Identify the cranial lobe of the right lung.

  • • Identify the cardiac notch between the cranial and middle lobes of the right lung (ventrally) at the level of the fourth and fifth intercostal spaces. This is where the right ventricle of the heart can be accessed for cardiac puncture.

• Identify the middle lobe of the right lung.

• Identify the caudal lobe of the right lung.

• Reflect the right caudal lobe to identify the accessory lobe of the right lung.

  • • Dissection Note: A portion of the accessory lobe wraps around the caudal vena cava and can also be seen from the left side.

Transect the root of the LEFT lung near the hilus and remove the lung as a single unit – not separate lobes! Do this by cutting the left principal (primary) bronchus (before the branching of the lobar bronchi) and transecting all of the pulmonary vessels entering the lung. However, be sure you are very careful to avoid cutting vagal nerves that course across the heart near/just dorsal to the hilus of the lung!

• Dissection Note: Be careful not to cut through the phrenic nerve when completing this transection. In cats especially, the phrenic nerve sometimes runs quite close to the hilus region.

From the RIGHT side, slip the accessory lobe of the right lung over the caudal vena cava. (Be careful not to tear the accessory lobe as you do this; it can be tricky to free it from around the caudal vena cava.)

Transect the entire root of the RIGHT lung near the hilus (similar to the left lung) and remove it as a single unit – not separate lobes! Again, be very careful to avoid cutting vagal nerves that course across the heart near the hilus of the lung!

• Dissection Note: Be careful not to cut through the phrenic nerve when completing this transection. In cats especially, the phrenic nerve sometimes runs quite close to the hilus region.

After removal of both lungs, within the thorax, identify the trachea. Trace the trachea distally to the point where it branches into the principal (primary) bronchi (left and right). (Terminology Note: ‘bronchi’ is plural, ‘bronchus’ is singular.)

• Identify the principal (primary) bronchi and, if possible, note the point of bifurcation where the carina would be located.

  • • Comment: The carina is the internal ridge formed between the primary bronchi at their origin from the trachea.) (See DG. Fig. 3-13)

Attempt to locate and identify tracheobronchial lymph nodes in your specimen. (These may be attached to the isolated lungs or still within the thorax – usually near the bifurcation of the trachea.) (See DG Fig. 3-20)

• Dissection Note: If you do not see any tracheobronchial lymph nodes in your specimen be sure to look at other animals and/or the demonstration animals to see examples of what they look like and where they are located.

On the now isolated lungs, identify the lobar bronchi entering each of the lung lobes. (Note that the bronchi will have cartilaginous rings, which will help differentiate them from arteries and veins.) (See DG Figs. 3-10, and 3-13)

• On the isolated LEFT lung, trace the main lobar bronchi into the lung tissue of the cranial and caudal lung lobes, using blunt dissection. (Do this without damaging the major arteries and veins also present in the lung tissue.)

  Related Learning Objective

  • D4.6

   

While still working with the isolated lungs, identify the pulmonary arteries and pulmonary veins (cut surfaces) entering/leaving the lungs. (See DG Fig. 3-10)

• Observe the main pulmonary artery entering each lung and note that the main artery then branches into smaller lobar pulmonary arteries. In latex injected specimens, these arteries may appear blue (if the injection reached the lungs).

  • • On the left lung with the dissected lobar bronchi, trace the pulmonary artery and note the orientation of the pulmonary artery relative to the bronchi. (The pulmonary arteries are dorsal and lateral relative to the bronchi.)

• Observe the pulmonary veins leaving the lungs. Usually you will see a single pulmonary vein from each lobe (lobar pulmonary vv.). In latex injected specimens, these veins may appear red (if the injection reached the lungs).

  • • On the left lung with the dissected lobar bronchi, trace the pulmonary veins and note the orientation of the pulmonary veins relative to the bronchi. (The pulmonary veins are ventral and medial relative to the bronchi.)

• Note that in latex injected specimens, the coloration seen in the pulmonary arteries and veins is opposite of what you would expect based on the coloration of all other arteries and veins in the injected specimens. Be sure you are able to explain why this is the case!

On the isolated lungs, review the lung lobes (and associated terms) previously described when you saw the lungs in situ (in place within the body).