Atelectasis is composed of 78% nitrogen and 21% oxygen. Since oxygen is exchanged at the alveoli-capillary membrane, nitrogen is a major component for the alveoli's state of inflation. If a large volume of nitrogen in the lungs is replaced with oxygen, the oxygen may subsequently be absorbed into the blood, reducing the volume of the alveoli, resulting in a form of alveolar collapse known as absorption atelectasis. It is usually associated with accumulation of blood, fluid, or air within the pleural cavity, which mechanically collapses the lung. This is a frequent occurrence with pleural effusion, caused by congestive heart failure (CHF).
Ultimopharyngeal body Ultimopharyngeal body The ultimopharyngeal body or ultimobranchial body or ultimobranchial gland is a small organ found in the neck region of many animals. In humans, the ultimobranchial body is an embryological structure that gives rise to the calcitonin-producing cells—also called parafollicular cells or C cells—of the thyroid gland. In humans, this body is a derivative of the ventral recess of the fourth pharyngeal pouch (technically fifth, but that one is rudimentary and merges with the fourth). The cells that give rise to the parafollicular cells are actually derivatives of the endoderm. These cells migrate and associate with the ultimopharyngeal body