During the fifth week, the developing lung buds begin to invaginate into these canals, creating a pair of enlarging cavities that encroach into the surrounding somites and further displace the transverse septum caudally - namely the pleural cavities. The two cavities communicate via a slim pair of remnant coeloms adjacent to the upper foregut called the pericardioperitoneal canal. The caudal portions of the coeloms fuse later below the umbilical vein to become the larger peritoneal cavity, separated from the pericardial cavity by the transverse septum. The cranial end of the intraembryonic coeloms fuse early to form a single cavity, which rotates invertedly and apparently descends in front of the thorax, and is later encroached by the growing primordial heart as the pericardial cavity. The dehiscence of these two layers creates a fluid-filled cavity on each side, and with the ventral infolding and the subsequent midline fusion of the trilaminar disc, forms a pair of intraembryonic coeloms anterolaterally around the gut tube during the fourth week, with the splanchnopleure on the inner cavity wall and the somatopleure on the outer cavity wall. The dorsal layer joins the overlying somites and ectoderm to form the somatopleure and the ventral layer joins the underlying endoderm to form the splanchnopleure. Only the parietal pleurae contain somatosensory nerves and are capable of perceiving pain.ĭuring the third week of embryogenesis, each lateral mesoderm splits into two layers. which branches off the C3-C5 cervcial cord. The mediastinal pleurae and central portions of the diaphragmatic pleurae are innervated by the phrenic nerves. The costal pleurae (including the portion that bulges above the thoracic inlet) and the periphery of the diaphragmatic pleurae are innervated by the intercostal nerves from the enclosing rib cage, which branches off from the T1-T12 thoracic spinal cord. The parietal pleurae however, like their blood supplies, receive nerve supplies from different sources. The visceral pleurae are innervated by splanchnic nerves from the pulmonary plexus, which also innervates the lungs and bronchi. The parietal pleura receives its blood supply from whatever structures underlying it, which can be branched from the aorta ( intercostal, superior phrenic and inferior phrenic arteries), the internal thoracic ( pericardiacophrenic, anterior intercostal and musculophrenic branches), or their anastomosis. The visceral pleura receives its blood supply from the parenchymal capillaries of the underlying lung, which have input from both the pulmonary and the bronchial circulation. Therefore, in cases of a unilateral pneumothorax, the contralateral lung will remain functioning normally unless there is a tension pneumothorax, which may shift the mediastinum and the trachea, kink the great vessels and eventually collapse the contralateral cardiopulmonary circulation. In humans, the left and right lungs are completely separated by the mediastinum, and there is no communication between their pleural cavities.
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