The thoracic cavity is divided into the right and left pleural cavity which encloses the corresponding right and left lungs. The mediastinum which includes the heart, blood vessel passing through and fro the heart, the structure passing from the neck into the abdomen.
PLEURA
The pleura are serous invaginating sacs made up of an inner visceral layer and outer parietal layer and a connecting part or root.
The visceral pleura is also known as the pulmonary pleura because if encloses tightly the lungs, it encloses all the surface of the lung even extending into its fissures, though it is deficient at the hilum of the lung. The parietal pleura is the outer layer that lines the thoracic cage. It is made up of four parts, they are:
· costal pleura
· Diaphragmatic pleura
· Mediasternal pleura
· Cervical pleura
The parietal pleura is held into the thoracic cage by a loose areolar tissue known as the endothoracic fascia.
THE COSTAL PLEURA
This is the part of the pleura that line the thoracic wall it overlies the internal surface of the sternum, the transversus thoracis muscle, the ribs and its costal cartilages, the innermost intercostalis muscles and the bodies of the vertebrae and the sides of the vertebrae bodies.
THE DIAPHRAGMATIC PLUERA
This is the part that lines the thoracic surface or the upper surface of the diaphragm.
MEDIASTERNAL PLEURA
This is the part that lines the lateral boundary of the mediastinum.
THE CERVICAL PLEURA
It is the continuation of the costal pleura over the apex of the lung. It ascends from the medial 1/3rd of the clavicle up to the apex of the lungs, to reach the lateral part of the trachea and here it continues as the mediastinal pleura. The cervical pleura is overlaid by a tough membrane known as the suprapleural membrane.
THE CONNECTION OR ROOT
This is present at the medial aspect of the lung where the mediastinal pleura continues with the visceral pleura. Its upper half is traversed by the root of the lungs while the lower half is empty but for the presence of lymphatic vessels this part is known as the pulmonary ligament.
LINE OF PLEURA REFLECTION
The costal pleural is continuous with the mediastinal pleura anteriorly behind the sternum and posteriorly in front of the vertebral column. The costal pleura is also continues with the diaphragmatic pleura along the costal arches this constitute the sternal and costal reflections. The knowledge of the limit of the sternal and costal reflections is of high clinical importance because it marks the limit of the pleura cavity which will easily be endangered in surgery involving the upper abdominal compact.
To plot the extent of the pleura in the living body, the different even numbers of ribs are taken into consideration or act as a guide this include the 2nd, 4th, 6th, 8th, 10th, and the 12th ribs. The pleura reflection from the apex of the lung the cervical pleura descends anteriorly passing behind the sternoclavicular joint as the sternal reflection now both pleura will descend behind the manubnum to meet each other at the manubrosternal joint which corresponds to the point of articulation of the 2nd costal cartilage. Both sternal reflections will continue downward close to each other up to the level of the 4th costal cartilage, here the left sternal reflection will deviate laterally while the right will continue downward up to the back of the xiphoid process.
The left sternal reflection will now descends downward at about 2- 25mm to the lateral edge of the sternum. It will continue its descent up to the 6th costal cartilage it now starts deviating obliquely crossing the 8th rib at the midclavicular line, the 10th rib at the mid axillary line and the 12th rib at its neck.
From the back of the xiphoid process the right sternal reflection crosses the 7th costal cartilage, cross the 8th rib at the midclavicular and thereafter it crosses the 10th rib at the midaxillary line, similar to the left costal reflection.
The pleura is exposed where it lies beyond the thoracic cage. This is at:
1. xiphiscostal angle
2. Right costovertebral angle
3. Left costverebral angle.
These are the areas where the pleura are in danger of laceration in cases of upper abdominal surgery.
PLEURA CAVITY
This is the potential space lying between the visceral and parietal pleura. The left pleura cavity is smaller than the right this is because of the position of the heart towards the left thoracic cavity. Inside the pleural cavity is a thin layer of fluid known as the pleural fluid which helps in lubrication, allowing the visceral pleura to glide smoothly and soundless over the parietal pleura. It also helps in presenting a high surface tension between the visceral and parietal pleura as a result of this the lungs which adheres to the viscera also adheres to the parietal layer which is also adherent to the thoracic wall, as a result of this an intra thoracic pressure is established and the lungs is perpetually inflated against its elasticity.
The pleura cavity though encloses the lungs but the lung does not fill all the spaces within the pleura cavity, the space within the pleura cavity that is devoid of lungs is known as the pleural recesses.
The size of the pleura recesses is determined by the respiratory activity of the lung. It increases during expiratory process and decreases in inspiratory process.
The three areas where the lung does not fully extend into include:
1. Left costomediasternal recess
2. Right costodiaphragmatic recess
3. Left costodiaphragmatic recess
The left costomediasternal recess lies anterior to the 4th and 5th intercostal space at this point the left lung is deficient where it forms a varied cardiac notch as a result of the development of the heart. Here the heart and the pericardium are covered by a double layer of parietal pleura. This marks the area of the superficial cardiac dullness.
The right and left costodiaphramatic recess lies along the corresponding side of the costal arches. At this point the diaphragmatic pleura comes in contact with the costal pleura.
APPLIED ANATOMY
1. The visceral pleura always glide over the parietal pleura without any sound or pain. In situation where pain or sounds are experienced. It will be as a result of inflation of the pleura.
2. The relation of the pleura to the 12th rib is of importance during renal surgery this is because the medial border of the kidney lies above it. The pleura crosses the 12th rib at the lateral border of the muscle rectus spinae in situation where it does not extends beyond the lateral border of rectus spinae the 11th rib might be mistaken for the 12th rib and when incision is carried along this 11th rib it will lead to laceration of the pleura cavity unknowingly. To avoid this, the ribs should be counted from the sternal angle where the 2nd rib is most prominent.
3. The intra thoracic pressure and the surface tension created by the pleural fluid maintain the lungs in an inflated position. Anything that disrupt the intra thoracic pressure and the surface tension will automatically lead to the collapse of the lungs. The condition may be as a result of the movement of air or fluid due to pathological conditions or when artificially air is introduced in order to rest the lungs after surgery. In cases where air enters the pleural cavity such condition is known as pneumothorax and it could be of two types:
a. Sucking pneumothorax: This condition happens when air enters and leaves the pleural cavity through a gapping wound in the thoracic wall with each respiration. This condition leads to hyperexpansion of the normal side and a condition known as the mediasternal flutter where the content of the mediastinum will shift to the normal side during inspiration and abnormal or injured side during expiration.
b. Tension pneumothorax: In this condition air moves into the pleura cavity with inspiration but could not leave during expiration. This condition arises when the penetrating wood act like a valve allowing one way flow of air. In this condition there is hyperexpansion of the thoracic cavity with the mediastinum shifting towards to the normal side. Both conditions do affect the vital capacity of the normal lung thereby affecting the ventilation of the lungs. Pneumothorax can be induced artificially this is normally done to relax the lungs after surgery of the lungs.
4. A condition where there is presence of blood in the pleural cavity is known as hemothorax this could be as a result of injury to the large and small vessels of the lungs. A condition where there is fluid in the pleural cavity as a result of direct or indirect pathological conditions like congestive heart failure, inflammation of the pleura such condition is referred to as hydrothorax. When it becomes infected it will result in pus formation within the pleura cavity a condition known as pyothrorax. When there is accumulation of lymph within the pleura cavity as a result of rapture to the thoracic duct such condition is known as pylothorax. Accumulation of these substances in the pleural cavity reduces the vital capacity of the lungs.
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