Lungs

When the lungs are removed from the thoracic cavity, the parietal pleura, a serous membrane, is observed peeled back from the posterior body wall. Each lung is enveloped by a tighter layer called the visceral pleura, which adheres directly to the lung surface. Between these two pleural layers lies the pleural cavity, which contains a thin layer of serous fluid. This fluid reduces friction, allowing the lungs to glide smoothly against the thoracic wall during respiration, and its surface tension helps to keep the two pleural membranes, and thus the lung, adhered to the chest wall.

Differences Between Left and Right Lungs
Right Lung

The right lung is typically larger and composed of three distinct lobes:

  1. Superior Lobe

  2. Middle Lobe

  3. Inferior Lobe
    These lobes are separated by two fissures:

  • Oblique Fissure: Separates the middle and inferior lobes from the superior lobe posteriorly.

  • Horizontal (or Transverse) Fissure: Separates the superior lobe from the middle lobe anteriorly.

Left Lung

The left lung is smaller, accommodating the heart which projects into the left side of the chest cavity. It has only two lobes:

  1. Superior Lobe

  2. Inferior Lobe
    These two lobes are separated by a single fissure:

  • Oblique Fissure: Separates the superior and inferior lobes.
    A unique feature of the left lung's superior lobe is the lingula, a small, tongue-like projection of tissue located anteriorly and inferiorly. The lingula is considered the anatomical equivalent of the right lung's middle lobe, albeit not a separate lobe itself. The indentation on the medial surface of the left lung where the heart resides is known as the cardiac notch.

The Hilum of the Lung

Both lungs feature a region on their medial surface called the hilum. This is the entry and exit point for the structures that form the root of the lung, which connect the lung to the mediastinum. These major structures include:

  • Primary Bronchus: These are the main airways branching from the trachea. Upon entering the hilum, the primary bronchi divide into secondary (lobar) and then tertiary (segmental) bronchi. The bronchus is characterized by its very rigid, cartilage-reinforced walls, making it easily identifiable by touch, especially its medial position within the hilum.

  • Pulmonary Artery: Typically positioned superior to the bronchus, the pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation. It branches extensively within the lung tissue.

  • Pulmonary Veins: Usually located inferior to the bronchus, the pulmonary veins carry oxygenated blood from the lungs back to the left atrium of the heart, to be distributed to the rest of the body. There are typically two pulmonary veins per lung (superior and inferior).

Trachea and Bronchi

Within the chest cavity, the trachea is the main airway, easily identified by its prominent C-shaped cartilage rings that maintain its patency. It branches into the right primary bronchus and left primary bronchus, each entering their respective lung at the hilum. These primary bronchi also possess cartilage rings, similar to the trachea, ensuring they remain open for air passage. The right primary bronchus is generally shorter, wider, and more vertical than the left, making it a more common site for inhaled foreign objects to lodge unlawfully.