Body cavities are protected spaces that hold the organs. Thin membranes line each cavity and cover the organs inside it.
Dorsal cavity
The cranial cavity holds the brain and the spinal canal holds the spinal cord. Both are wrapped in the meninges and bathed in cerebrospinal fluid.
Thoracic cavity
The chest holds two pleural cavities, one around each lung, and the mediastinum in the middle with the heart, great vessels, trachea and esophagus. The diaphragm separates it from the abdomen.
Abdominopelvic cavity
The abdomen holds the stomach, intestines, liver, spleen and other digestive organs, and the pelvis below holds the bladder, rectum and internal reproductive organs. The kidneys sit behind the peritoneum (retroperitoneal).
Serous membranes
Each serous membrane has two layers, a parietal layer lining the cavity wall and a visceral layer covering the organ, with a thin film of fluid between them. They are the pleura around the lungs, the pericardium around the heart and the peritoneum in the abdomen.
| Membrane | Surrounds | When fluid or air collects |
|---|---|---|
| Pleura | Lungs | Pleural effusion, pneumothorax |
| Pericardium | Heart | Pericardial effusion, tamponade |
| Peritoneum | Abdominal organs | Ascites, peritonitis |
At the bedside
Fluid or air in these spaces changes the exam: reduced breath sounds with an effusion, muffled heart sounds with tamponade and shifting dullness with ascites. A FAST ultrasound looks for blood in these same spaces after trauma.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).