What is the costodiaphragmatic recess?

The costodiaphragmatic recess (phrenicocostal sinus) is a narrow slit in the pleural cavity. The pleural cavity is the space between the outer (parietal) and inner (visceral) layers of the pleura, a double-layered membrane that covers each lung. The pleural cavity contains serous fluid and the lungs expand into the cavity when we breathe.

Not all parts of the pleural cavity are completely filled by the lungs, however. There are two recesses - the costodiaphragmatic recess and the costomediastinal recess - and in these recesses two layers of outer (parietal) pleura are only separated by fluid.

The costodiaphragmatic recess is the larger of the two recesses and is found between the costal (ribs) and diaphragmatic pleura. When viewed on a 2-dimensional x-ray, the costodiaphragmatic recesses are seen as the costophrenic angles because they are found at the acutely angled junction where the diaphragm (-phrenic) meets the ribs (costco-).

What is the function of the costodiaphragmatic recesses?

The costodiaphragmatic recesses are a clinically important space that provides a site for any fluid around the lungs to accumulate, including pleural effusions. They also provide a space for the lungs to expand into during forced inspiration.