Each air sac consists of 2 chambers. Atrium and Posterior chamber.
1. Atrium or small chamber: The small chamber of the air sac is called the atrium. It is covered by a thin wall consisting of two layers. The outer layer is called tunica externa and the inner layer is called tunica interna. The tunica externa is composed of connective tissue and the tunica interna is composed of smooth muscle. There is a gas gland of red color in its inner wall. This gas gland is called reti mirabili. The air sacs are filled with gases released from the reti mirabili. Most of the gases in the air are oxygen. However, there is a small amount of carbon dioxide and nitrogen. Siddique Publications
2. Posterior chamber or large chamber: The large chamber of the air sac is called the posterior chamber. It is covered by a thin wall consisting of two layers. The outer layer is called tunica externa and the inner layer is called tunica interna. The tunica externa is composed of connective tissue and the tunica interna is composed of smooth muscle. There is a gas gland of red color in its inner wall. This gas gland is called reti mirabili. Reti Mirabili absorbs gas.
The air sacs of roe fish contain tiny bones. These bones are called Becherian bones.