The term viscera refers to the large, complex internal organs contained within the main cavities of the body. These organs are responsible for all life-sustaining functions, from circulating blood to processing nutrients and eliminating waste. These structures include the heart, lungs, and the entire digestive, excretory, and reproductive machinery.
Anatomical Definition and Location
Viscera are housed within the ventral body cavities. The most superior of these is the thoracic cavity, situated above the diaphragm, which contains the heart and the lungs. The heart is centrally located within the mediastinum, the space between the lungs, while the lungs themselves reside within the two pleural cavities.
Below the muscular diaphragm is the abdominopelvic cavity. The superior portion, the abdominal cavity, is home to the majority of the digestive system organs, such as the stomach, liver, pancreas, spleen, and the intestines. These organs are situated deep to the abdominal wall muscles and the lower ribs.
The most inferior region is the pelvic cavity, which is partially encased by the bones of the pelvis. This area contains organs involved in excretion and reproduction, including the urinary bladder and the internal reproductive organs.
Structural Classification
Anatomists commonly classify viscera based on their physical structure: hollow and solid organs. This classification is particularly relevant for the organs found in the abdominopelvic cavity. Hollow viscera are tube-like or pouch-like organs that are primarily designed for the storage, transport, and expulsion of substances.
Examples of hollow viscera include the stomach, small and large intestines, gallbladder, and urinary bladder. These organs feature a lumen, which is a central open space surrounded by muscular walls that contract to move contents through the system. The stomach uses its muscular walls to churn food, while the intestines rely on rhythmic contractions to propel digested material along the tract.
In contrast, solid viscera possess a compact, firm internal structure and are dedicated to metabolic, filtration, or synthesizing functions. These organs do not have a large, empty lumen for transport but are instead packed with specialized functional tissue called parenchyma. The liver and the kidneys, which filter blood and produce urine, are prime examples of solid viscera. Other solid organs include the pancreas, spleen, and the adrenal glands.
The Protective Layer (Serous Membranes)
The movement of the viscera within the body cavities, such as the expansion of the lungs or the churning of the stomach, creates friction. This protection is provided by specialized tissues known as serous membranes. These membranes are smooth, thin layers of epithelial tissue that line both the walls of the body cavities and the surfaces of the organs they contain.
Each serous membrane secretes a small amount of serous fluid into the space between the organ and the cavity wall. This fluid acts as a lubricant, enabling the organs to glide smoothly against the cavity walls and against each other with minimal friction. The specific serous membranes are named for their location: the pleura surrounds the lungs, the pericardium envelops the heart, and the peritoneum lines the abdominopelvic cavity and covers many of the digestive organs.
The peritoneum is the most extensive of these membranes, holding multiple abdominal organs in place while allowing for the great mobility required for digestion. Chronic friction could lead to inflammation and damage, impairing the function of the viscera.