Pleura of the lungs
The pleural sac consists of 2 membranes that enclose the lungs. The visceral pleura adheres to the surface of the lung itself and the parietal pleura lines the pulmonary cavity. The visceral pleura cannot be removed from the lung during a cadaveric dissection. The parietal pleura is thicker than the visceral pleura and consists of three distinct parts. Three parts of the parietal pleura are the costal part, the mediastinal part, and diaphragmatic part. The pleural cavity is the space between the visceral pleura and the parietal pleura. The pleural cavity consists of a capillary layer of serous pleural fluid that allows smooth movement during respiration. This fluid also keeps the lungs adhered to the surface of the thoracic cavity.
Pleurisy is the inflammation of the lining of the lungs. The distinguishing characteristic of pleurisy is the sound of friction in the lungs with a stethoscope. The sound can be described as something similar to a "hair clump being rolled between the fingers." Patients with pleurisy often describe the condition as a stabbing, sharp pain during respiration. Pains often become more intense with physical exertion such as climbing stairs.
Nerves of the Lungs and Pleurae
The lungs and visceral pleura are innervated by the pulmonary plexus. These nerves contain parasympathetic, sympathetic, and visceral afferent fibers. The parasympathetic fibers are derived from presynaptic fibers of the vagus nerve. These fibers provide motor innervation to the smooth muscle, inhibitory action to the pulmonary vessels, and secretory to the glands of the lungs. The sympathetic fibers are postsynaptic neurons. They function in opposition to the parasympathetic fibers and this includes inhibitory to the brochial muscle, motor to the pulmonary vessels, and inhibitory to the glands of the bronchial tree. The visceral innervation to the lungs is characterized as being either reflexive or nociceptive. The reflexive fibers accompany the parasympathetic fibers of the bronchial tree. The nociceptive fibers accompany the sympathetic fibers of the bronchial tree. The parietal pleura innervation is provided by the intercostal nerves and the phrenic nerves.
The visceral pleura receives no innervation by nociceptors, so no pain sensation is felt in this pleura. However, the parietal pleura is extremely sensitive to pain because of the numerous intercostal nerves and phrenic nerves that innervate this region. The pain sensation felt in the parietal pleura is labeled referred pain. Irritation to the diaphragmatic regions of the parietal pleura produce referred pain in the thoracic and abdominal walls. The referred pain sends signals to the dermatones in different areas that produce pain in a specified area of the body as shown in the figure below.
The pericardium is housed within the middle mediastinum along with the heart and the root of its great vessels. The pericardium is a two layer closed sac. The toughest layer is the fibrous pericardium, and it is continuous with the central tendon of the diaphragm. The serous pericardium is mostly mesothelium. The mesothelium around the heart is composed of a thin layer of flat cells that line the internal surface of the fibrous pericardium and the external surface of the heart. The arterial supply to the pericardium is the pericardiacophrenic artery. This artery branches off of the internal thoracic artery. The venous blood drainage of the pericardium is the pericardiacophrenic veins and they branch off of the brachiocephalic veins. Nerve innervation for the pericardium is the phrenic nerve (C3-C5).
In order to remove excess fluid that has accumulated around the heart, medical professionals often perform a pericardiocentesis. This consist of inserting a bore needle in to the 5th or 6th intercostal near the sternum. The cardiac notch in the left lung and the shallower notch in the left pleural sac is partially exposed to allow the need to pierce the pericardium to relieve excess fluid. There is an alternate route to perform a pericardiocentesis by going toward the infrasternal angle. This process requires more precision due to the fact that the lungs and the pleura must be avoided and the internal thoracic artery is easily punctured in this area. The figure below is presenting the alternate method for performing a pericardiocentesis by going toward the infrasternal angle.