The pleural cavity is the thin fluid-filled space between the two pulmonary pleurae (known as visceral and parietal) of each lung. A pleura is a serous membrane which folds back onto itself to form a two-layered membranouspleural sac. The outer pleura (parietal pleura) is attached to the chest wall, but is separated from it by the endothoracic fascia. The inner pleura (visceral pleura) covers the lungs and adjoining structures, including blood vessels, bronchi and nerves. The pleural cavity can be viewed as a potential space because the two pleurae adhere to each other (through the thin film of serous liquid) under all normal conditions.
All cells come from preexisting cells. A cell divides its DNA in a process known as mitosis and its cytoplasmic contents in a process known as cytokinesis, producing two new daughter cells while all life begins as a single cell, the human body is not just one cell type. Cells that can divide without becoming a specific cell type (i.e. skin cells) are known as stem cells. When stem cells undergo mitosis and divide, they produce two cells. One cell remains a stem cell and the other is a partially specialized cell known as a progenitor cell. Cells undergo a process of specialization or determination called differentiation to become neurons, muscle cells, or skin cells. Stem cells are the source of all different cell types.
