Body cavities are internal compartments that accommodate organs and allow them to move within surrounding structures. The major cavities include the cranial cavity, vertebral canal, thoracic cavity and abdominopelvic cavity. Serous membranes are thin coverings that line parts of the trunk and cover organs, providing lubricated surfaces where movement occurs.
What are the cranial cavity and vertebral canal?
The cranial cavity is the space enclosed by the skull that contains the brain. The vertebral canal is the passage within the vertebral column, or spine, that contains the spinal cord and spinal nerve roots. Nerve roots are bundles of nerve fibres connecting the spinal cord with spinal nerves.
These spaces are continuous through the foramen magnum, the large opening at the base of the skull. The spinal cord does not extend through the entire length of the vertebral canal; nerve roots continue below its lower end.
The brain and spinal cord are covered by meninges, protective membranes that are different from serous membranes. Cerebrospinal fluid, the fluid surrounding the brain and spinal cord, occupies the subarachnoid space between the arachnoid and pia mater, the middle and innermost meningeal layers. The cranial cavity and vertebral canal therefore should not be grouped with the pleural, pericardial and peritoneal serous cavities.
How is the thoracic cavity organised?
The thoracic cavity is the internal compartment of the chest. Its walls include the ribs, associated muscles, breastbone and thoracic vertebrae, the vertebrae in the chest region. Its lower boundary is the diaphragm, a dome-shaped sheet of muscle and tendon.
Each lung is covered by pleura, a serous membrane that also lines the surrounding chest wall. The pleural cavity is the narrow potential space between these coverings. A potential space is normally only a thin interval between closely opposed surfaces, rather than a large open chamber.
The mediastinum is the central compartment between the pleural regions. It extends from the breastbone to the thoracic vertebral column and from the upper opening of the chest to the diaphragm. It contains the heart, major blood vessels, the trachea or windpipe, the oesophagus or food passage, and other structures.
The heart is enclosed by the pericardium, the sac surrounding it. The pericardial cavity lies between the layers of the serous part of this sac. It is within the mediastinum, not within either pleural cavity.
How does the diaphragm divide the body cavities?
The diaphragm separates the thoracic cavity above from the abdominal cavity below. Its curved upper surface faces the lungs and heart, while its lower surface lies above abdominal organs, including the liver and stomach. Its muscular fibres converge on a central tendon, a strong sheet of connective tissue.
This boundary is not an unbroken seal. Openings allow structures such as the oesophagus, the aorta, the main artery leaving the heart, and the inferior vena cava, a major vein returning blood from the lower body, to pass between regions.
During breathing, contraction lowers the diaphragm and increases the space available within the thorax, or chest. Its movement changes the shape of the boundary without joining the thoracic and abdominal cavities into a single compartment.
What is the abdominopelvic cavity?
The abdominopelvic cavity extends from the diaphragm into the pelvis, the lower region of the trunk enclosed partly by the hip bones. It is described as having abdominal and pelvic parts, but no membrane or muscular sheet separates these parts. They are continuous across the pelvic inlet, the upper opening of the bony pelvis.
The abdominal part accommodates the liver, stomach, intestines, spleen and kidneys, among other structures. The pelvic part accommodates the urinary bladder, rectum and internal reproductive organs. The boundary is a regional division rather than a barrier, and an organ may extend across it.
The peritoneum is the serous membrane associated with the abdominopelvic cavity. It lines much of the cavity wall and covers many, but not all, organ surfaces. The abdominopelvic cavity is therefore not the same as the peritoneal cavity, which is the potential space between peritoneal layers.
The kidneys lie behind the wall-lining peritoneum, a position called retroperitoneal. The urinary bladder lies mainly below the peritoneum in the pelvis, with peritoneum covering its upper surface.
What are parietal and visceral serous layers?
A serous membrane consists of a thin surface layer of flattened cells, called mesothelium, supported by connective tissue. Its smooth surface produces a small amount of serous fluid, a watery lubricating fluid that reduces friction as adjacent surfaces move.
The layers are named according to what they cover:
- Parietal layer: lines the wall of the surrounding compartment.
- Visceral layer: covers the surface of an organ.
These are continuous portions of the same membrane, turning from the surrounding wall onto the organ at particular attachment regions. Between them is the serous cavity, normally containing a thin film of fluid.
An organ covered by a visceral layer does not occupy the serous cavity itself. For example, the lung lies against the visceral pleura, while the pleural cavity lies between visceral and parietal pleura. This distinction separates a regional compartment containing organs from a narrow lubricated space between membrane surfaces.
How do the pleura, pericardium and peritoneum differ?
The pleura is the serous membrane associated with each lung. Visceral pleura covers the lung surface and extends into its fissures, the clefts between its lobes. Parietal pleura lines the inner chest wall and covers the adjacent surfaces of the diaphragm and mediastinum. The pleural cavities are separate from one another.
The pericardium has an outer fibrous part, made of tough connective tissue, and an inner serous part. The parietal layer of serous pericardium lines the fibrous sac. Its visceral layer covers the heart and is also called the epicardium. The pericardial cavity lies between these serous layers, not between the fibrous sac and the parietal layer.
The peritoneum has parietal and visceral layers arranged around abdominal and pelvic structures. Organs largely covered by visceral peritoneum are described as intraperitoneal, although they do not lie inside the peritoneal fluid space. Many are attached to the body wall by a mesentery, a fold of peritoneum that carries blood vessels, nerves and lymphatic vessels, which transport tissue fluid.
What lies in each body cavity?
The table distinguishes broad compartments from serous spaces. The thoracic cavity includes the mediastinum and the pleural regions, whereas the pleural and pericardial cavities are specifically the intervals between membrane layers.
| Cavity or region | Main contents or relationships | Membrane relationship |
|---|---|---|
| Cranial cavity | Brain, its coverings, blood vessels and fluid spaces | Meninges; not a serous cavity |
| Vertebral canal | Spinal cord, nerve roots and their coverings | Meninges; not a serous cavity |
| Thoracic cavity | Lungs and mediastinal structures, including the heart | Includes pleural regions and the pericardial sac |
| Pleural cavities | Thin films of fluid beside the lungs, not the lungs themselves | Between parietal and visceral pleura |
| Pericardial cavity | Thin film of fluid around the heart, not the heart itself | Between parietal and visceral serous pericardium |
| Abdominal part of the abdominopelvic cavity | Liver, stomach, intestines, spleen and kidneys | Some organs are largely covered by peritoneum; others lie behind it |
| Pelvic part of the abdominopelvic cavity | Urinary bladder, rectum and internal reproductive organs | Peritoneum covers selected surfaces and forms folds between structures |
In the 3D model beside this article, rotate and zoom to explore spatial relationships, then select, hide or isolate parts. You can also fade parts, switch layers, cut a section along the sagittal, coronal or transverse plane, or pull parts apart.