Chapter 4: Apply knowledge of body cavities

Human Anatomy and Physiology is fundamental for Nutrition and Dietetics professionals to understand how the human body functions in health and disease. Knowledge of body cavities is essential because these spaces house vital organs involved in digestion, absorption, metabolism, and excretion, processes central to nutrition. In Kenya, professionals working in clinical nutrition at hospitals or community health settings benefit from understanding body cavities to assess nutritional status and manage conditions affecting organ systems properly. This chapter explores the definition, types, and functions of body cavities with a focus on their relevance to nutrition and dietetics practice.

4.1 Body cavities

The body contains several enclosed spaces that hold the internal organs. These spaces are called body cavities. Knowing which organs lie in which cavity helps a dietitian to understand where food is digested and absorbed, where the liver and kidneys sit, and why a swelling, fluid collection or injury in one cavity changes what a client can eat. During nutrition assessment, for example, a swollen abdomen may come from fluid inside the abdominal cavity and not from weight gain, and this changes how the client's weight is interpreted. This topic defines body cavities and names their types and contents. What the cavities do for the organs is taught in the topic Functions of body cavities.

4.1.1 Definition of body cavities

A body cavity is a space within the body, enclosed by bone, muscle or connective tissue, that contains one or more organs. The spaces are not empty holes: they are lined by thin membranes and, in most cases, hold only a film of fluid between the organ and the wall of the cavity. Some cavities are closed spaces, such as the pleural cavity around a lung. Others, such as the oral and nasal cavities, open to the outside.

Characteristics of body cavities

• Enclosed: each cavity is surrounded by bones, muscles or connective tissue, such as the skull around the cranial cavity and the ribcage around the thoracic cavity.
• Membrane lined: most cavities are lined by thin membranes. The pleura, pericardium and peritoneum are serous membranes that produce a small amount of fluid, and the cranial and spinal cavities are lined by the meninges.
• Divided into compartments: each cavity is separated from the next by a wall or muscle, such as the diaphragm between the chest and the abdomen, which keeps the organs of one compartment apart from those of the next.
• Variable in volume: some cavities change size, for example the thoracic cavity enlarges during breathing in and the abdominal cavity bulges after a large meal.
• Organised in two groups: the dorsal cavity lies at the back of the body and the ventral cavity lies in front.

4.1.2 Types of body cavities

The two main groups are the dorsal cavity and the ventral cavity. Each is divided further into smaller cavities, as shown below.

4.1.2.1 Content of dorsal cavities

The dorsal cavity runs along the back of the body, from the head to the lower back. It is made up of two continuous parts. The cranial cavity lies inside the skull and holds the brain. The vertebral cavity (also called the spinal canal) runs through the stacked vertebrae of the backbone and holds the spinal cord and the roots of the spinal nerves. Three protective membranes, the meninges, cover the brain and spinal cord, and a clear fluid called cerebrospinal fluid circulates between them.

Although the dorsal cavity contains no digestive organs, the nervous tissue inside it controls appetite, swallowing, the movement of the gut and the secretion of digestive juices. An injury to the spinal cord can change bladder and bowel control and the ability to feed oneself, so feeding plans for such clients are adapted.

4.1.2.2 Content of ventral cavity

The ventral cavity lies in front of the dorsal cavity and is much larger. The diaphragm divides it into an upper thoracic cavity and a lower abdominopelvic cavity.

The thoracic cavity (chest cavity) is surrounded by the ribcage. It contains the two pleural cavities and the mediastinum, and the pericardial cavity lies within the mediastinum. The two pleural cavities each hold one lung, and a thin film of pleural fluid surrounds each lung. The pericardial cavity holds the heart inside its sac, the pericardium. The space between the two lungs is the mediastinum, which contains the heart, the great blood vessels, the trachea, the oesophagus, the thymus gland and the thoracic duct, the main lymphatic vessel, which returns lymph from most of the body, including the fat absorbed from the gut, to the blood.

The abdominopelvic cavity has no bony wall at the front and is protected by muscle. It is divided into two connected parts. The abdominal cavity lies below the diaphragm and contains the stomach, liver, gall bladder, pancreas, spleen, small intestine and most of the large intestine. The kidneys and adrenal glands lie at the back of the abdomen, behind the lining membrane, which is called the peritoneum. The pelvic cavity lies within the basin formed by the hip bones and contains the urinary bladder, the rectum and the internal reproductive organs. The peritoneum lines the abdominal cavity and covers most of its organs. A fluid collection in this space (ascites) can occur in clients with severe liver disease, and it makes the abdomen swell and the client feel full after small meals.

Regions and quadrants of the abdominopelvic cavity

For examination, the abdominopelvic cavity is divided into four quadrants by one vertical and one horizontal line that cross at the navel: right upper, left upper, right lower and left lower. The liver and gall bladder lie mainly in the right upper quadrant, and the stomach and spleen lie mainly in the left upper quadrant. A finer map divides the abdomen into nine regions, three across the top (right hypochondriac, epigastric, left hypochondriac), three in the middle (right lumbar, umbilical, left lumbar) and three at the bottom (right iliac, hypogastric, left iliac). Health workers use these names to say where a client feels pain or where a swelling is found.

4.1.2.3 Contents of body cavities

This section brings the contents together from the organ's point of view, so that a dietitian can find the cavity in which a given organ lies.

Several smaller cavities in the head and face are also described. The oral cavity (mouth) holds the tongue and teeth and is where food is chewed and mixed with saliva. The nasal cavity lies above it and carries air to the throat. The orbital cavities hold the eyeballs, and the middle ear cavities hold the small bones of hearing. Of these, the oral cavity is the first part of the digestive tract and bears directly on food intake.

Chapter Summary

This chapter explored the concept of body cavities, defining them as hollow spaces within the human body that house and protect vital organs. It detailed the main types of body cavities, distinguishing between the dorsal and ventral cavities based on their location and contents. The dorsal cavity was described in terms of its protection of the brain and spinal cord, while the ventral cavity was examined for its role in containing major organs such as those in the thoracic and abdominal regions. Each body cavity’s specific contents were outlined to emphasize their functional importance. The chapter concluded with an analysis of the functions of body cavities, highlighting their roles in safeguarding organs, allowing organ movement, and providing structural support. Understanding these cavities is essential for appreciating how the body maintains internal organization and protects its critical systems.