Body Cavity Definition
A body cavity is a space created in an organism which houses organs. It is lined with a layer of cells and is filled with fluid, to protect the organs from damage as the organism moves around. Body cavities form during development, as solid masses of tissue fold inward on themselves, creating pockets in which the organs develop. An example of a body cavity in humans would be the cranial cavity, which houses the brain.
A coelom is a special type of body cavity derived from the mesoderm, or middle layer of germ cells present in an embryo. Some organisms, like sponges, have no body cavities. Others, like segmented worms, have many body cavities, one present in each segment. Organisms with three distinct germ layers which form a body cavity are known as coelomates. Humans are coelomates, as we have a distinct coelom which forms during embryogenesis. The various body cavities and organs which occupy them are discussed below.
Body Cavities and Organs
The human body has two main body cavities. The first, the ventral cavity, is a large cavity which sits ventrally to the spine and includes all the organs from your pelvis to your throat. This cavity is the true coelom, as it forms during human embryogenesis from the mesoderm. At first it is a single cavity. It then gets subdivided several times, into smaller cavities separated by muscles, bones, and thin tissues. The first subdivision is the diaphragm muscle, which divides the abdominopelvic cavity from the thoracic cavity. This can be seen in the image below.
The abdominopelvic cavity is then further subdivided into the pelvic cavity and the abdominal cavity. The pelvic cavity holds the reproductive organs, bladder, and allows the intestines passage to the anus. The abdominal cavity is where the majority of the body’s organs lie. These are sometimes referred to as the “viscera”, and they include organs like the liver, stomach, spleen, pancreas, kidneys and others involved in digestion, metabolism, and filtering of the blood. A special membrane holds all of these organs in place and is called the peritoneum.
The thoracic cavity above is also subdivided into smaller section. In the human body, each lung is held within its own pleural cavity, which allows it to expand and avoid friction with the ribs and diaphragm as it reaches capacity. Another important feature of the separation of the pleural cavities is that if one lung fails or collapses, the other can go on functioning. The other division of the thoracic cavity is the mediastinum. This cavity surrounds the heart and associated veins and arteries. The heart is further protected by another layer of mesoderm which forms the pericardial cavity. The pericardium is similar to the peritoneum of the ventral body cavity, except it protects the heart. These different layers have fluids between them which act like lubricants, ensuring the heart pumps without friction.
The second main cavity, the dorsal cavity, includes both the cranial cavity and the vertebral cavity. Unlike the ventral body cavity, the dorsal cavity is not a coelom, as it was not derived from mesoderm. Instead, the nerve cord and its protective cavity form when the ectoderm of the embryo folds in on itself, creating a hollow tube. While this is similar to coelom formation, it happens on the outside of the organism, in a different germ layer. The cranial cavity is the most protective body cavity. It surrounds the brain in bone, soft tissue, and a protective layer of liquid which reduces the strain and damage from impacts. The vertebral cavity is similar, but has gaps were the nerves must enter or exit. Like the other body cavities, the dorsal cavity is surrounded in a thin layer of cells. In this case it is not called peritoneum, but meninges, as it was derived from a different germ layer. Its function however, is the same.
A body cavity in a non-human animal will be easy to recognize. It will be a membrane-bound, fluid-filled space containing organs. Not all animals have body cavities, and if they do, not all are as easy to recognize as the “true coelom” possessed by humans. Arthropods, for example, are considered to have a coelom, but it is hard to recognize as their open circulatory system is much different than our closed circulatory system. This requires a reframing of how a body cavity is viewed, but the same basic principles apply.
It should be noted that by study a body cavity across organisms, scientist have been able to hypothesize about the evolutionary history of life on Earth. For example, hagfish have a connection between the pericardial (heart) cavity and the rest of their coelom. In higher animals, this separation has become very distinct. This may represent the fact that the hagfish is more closely related to animals with a less developed heart, like the lancelet. This compares the fully separated body cavity in the lamprey, an animal very similar to the hagfish. This change represents a derived character in the vertebrate line, and allowed much more complex vertebrate to arise.