Pleura - visceral and parietal, structure, borders of pleura, pleural dome and recesses (draw scheme), innervation

Overview:

The pleurae are serous membranes lining the lungs and the thoracic cavity, facilitating smooth and efficient respiration. There are two pleurae, one for each lung.

Structure of the Pleurae:

The pleurae consist of two parts:

1. Visceral Pleura:

   - Covers the surface of the lungs, including the interlobar fissures.

   - It is tightly adhered to the lungs and extends into the fissures between lobes.

   - It is continuous with the *parietal pleura* at the *hilum* of each lung, where the structures enter and exit the lung.

2. Parietal Pleura:

   - Covers the internal surface of the thoracic cavity and is thicker than the visceral pleura.

   - The parietal pleura can be subdivided based on its relation to surrounding structures:

     - *Mediastinal pleura:* Covers the lateral aspects of the mediastinum.

     - *Cervical pleura:* Extends into the neck, covering the pleural dome.

     - *Costal pleura:* Covers the inner aspect of the ribs and intercostal spaces.

     - *Diaphragmatic pleura:* Covers the superior surface of the diaphragm.

Pleural Cavity:

- The *pleural cavity* is a potential space between the *parietal* and *visceral pleurae*, containing a small volume of serous fluid.

- *Serous fluid* serves two main functions:

  1. Lubricates the pleurae, allowing them to slide smoothly during respiration.

  2. Produces surface tension, pulling the two pleurae together. This ensures that when the thoracic cavity expands, the lungs also expand.

- If air enters the pleural cavity (as in a pneumothorax), the surface tension is lost, causing lung collapse.

Pleural Recesses:

The lungs do not fully occupy the pleural cavity, particularly in certain regions, creating *pleural recesses*:

- Costodiaphragmatic recess: Located between the *costal pleura* and the *diaphragmatic pleura*. This is the most clinically significant recess, often used for thoracocentesis.

- Costomediastinal recess: Found between the *costal pleura* and the *mediastinal pleura*, located posterior to the sternum. It is larger on the left due to the cardiac notch.

Pleural Dome (Cervical Pleura):

- The pleural dome, also called the cupula, is part of the cervical pleura*.

- It extends into the neck, about 2-3 cm above the level of the first rib.

- Vulnerable to injuries in the neck region.

Borders of the Pleura:

- *Superior Border:* Reaches the base of the neck, above the clavicle.

- *Inferior Border:* Extends down to the diaphragm, reflecting onto its surface.

- *Medial Border:* In contact with the mediastinum.

- *Lateral Border:* Adjacent to the inner surface of the thoracic wall.

Projection of Pleura onto Thoracic Wall:

- *Apex (Cervical Pleura):* Projects 2-3 cm above the clavicle.

- *Anterior Border:* Projects close to the midline, behind the sternum, especially at the costomediastinal recess.

- *Inferior Border:*

  - *Midclavicular line*: 6th rib.

  - *Midaxillary line*: 8th rib.

  - *Paravertebral line*: 10th rib.

Neurovasculature:

- Parietal Pleura:

  - *Sensory innervation:* Sensitive to pain, pressure, and temperature.

  - Innervated by:

    - Phrenic nerve (mediastinal and diaphragmatic pleura).

    - *ntercostal nerves (costal pleura).

  - Blood supply: Via the *intercostal arteries*.

- Visceral Pleura:

  - Autonomic innervation via the *pulmonary plexus* (sympathetic and parasympathetic fibers).

  -Blood supply: From the *bronchial arteries*.

  - *Less sensitive to pain* than the parietal pleura, but it detects stretch.

Schematic of Pleura and Recesses:

Imagine a schematic drawing showing the lungs and the pleura within the thoracic cavity:

- The costodiaphragmatic recess forms a downward curve between the ribs and diaphragm.

- The costomediastinal recess lies in the anterior midline, near the sternum.

- The pleural dome projects above the first rib, in the cervical region.

Summary:

The pleurae form a critical component of the respiratory system, providing a smooth interface for lung expansion during breathing. Their anatomy and innervation are essential for understanding pleural diseases, thoracocentesis, and conditions like pneumothorax.