Body Cavities and Serous membranes

 The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments (Figure 1).Function of these cavities is to contain and protect delicate internal organs.

Subdivisions of the Posterior (Dorsal) and Anterior (Ventral) Cavities

Ventral cavity:

allows changes in the size and shape of the organs as they perform their functions.The anterior (ventral) cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity (see Figure 1). 

1: The thoracic cavity is superior subdivision of the anterior cavity.

• Enclosed by the rib cage. 
• Contains the lungs and the heart, which is located in the mediastinum. 
• Diaphragm forms the floor of the thoracic cavity and separates it from the more inferior abdominopelvic cavity. The diaphragm divides the thoracic and the abdominal cavities. 
• The two pleural cavities are situated on both sides of the body, anterior to the spine and lateral to the breastbone. 
• The superior mediastinum is a wedge-shaped cavity located between the superior regions of the two thoracic cavities.
• The pericardial cavity within the mediastinum is located at the center of the chest below the superior mediastinum. It roughly outlines the shape of the heart. 

2:The abdominopelvic cavity is the largest cavity in the body. he abdominal and pelvic cavity can be referred to as the abdominopelvic cavity.

• no membrane physically divides the abdominopelvic cavity, 2 further sudivided cavities
• the abdominal cavity, the division that houses the digestive organs, occupies the entire lower half of the trunk, anterior to the spine.
• the pelvic cavity, the division that houses the organs of reproductions funnel shaped and is located inferior and anterior to the abdominal cavity. 

Dorsal body cavity: 

consists of cranial cavity and vertebral cavity.

1. Cranial cavity is a large, bean-shaped cavity where the brain is located.  
2. The vertebral cavity is a very narrow, thread-like cavity running from the cranial cavity down the entire length of the spinal cord. 

• The brain and spinal cord make up a continuous, uninterrupted structure, the cranial and spinal cavities are also continuous. 
• The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the posterior (dorsal) cavity.

Figure 2: Ventral Body Cavities The ventral cavity includes the thoracic and abdominopelvic cavities and their subdivisions. 

Figure 3: The dorsal cavity includes the cranial and spinal cavities.

Abdominal Regions and Quadrants

To promote clear communication,  health care providers typically divide up the cavity into either nine regions or four quadrants.

                                                                           Figure 4 :4 Quadrants

 Figure 4 shows four abdominopelvic quadrants. The simpler quadrants approach, which is more commonly used in medicine, subdivides the cavity with one horizontal and one vertical line that intersect at the patient’s umbilicus (navel).

• The right upper quadrant (RUQ) includes the lower right ribs, right side of the liver, and right side of the transverse colon.
• The left upper quadrant (LUQ) includes the lower left ribs, stomach, and upper left area of the transverse colon. 
• The right lower quadrant (RLQ) includes the right half of the small intestines, ascending colon, right pelvic bone and upper right area of the bladder. 
• The left lower quadrant (LLQ) contains the left half of the small intestine and left pelvic bone.

                                                                           Figure 5 : 9 Regions

The more detailed regional approach subdivides the cavity with one horizontal line immediately inferior to the ribs and one immediately superior to the pelvis, and two vertical lines drawn as if dropped from the midpoint of each clavicle (collarbone). There are nine resulting regions. These regions divide the abdomen into nine squares. 

• The upper right square is the right hypochondriac region and contains the base of the right ribs. 
• The upper left square is the left hypochondriac region and contains the base of the left ribs. 
• The epigastric region is the upper central square and contains the bottom edge of the liver as well as the upper areas of the stomach. The diaphragm curves like an upside down U over these three regions. 
• The central right region is called the right lumbar region and contains the ascending colon and the right edge of the small intestines. 
• The central square or the umbilical reigon contains the transverse colon and the upper regions of the small intestines. 
• The left lumbar region contains the left edge of the transverse colon and the left edge of the small intestine. 
• The lower right square is the right iliac region and contains the right pelvic bones and the ascending colon. 
• The lower left square is the left iliac region and contains the left pelvic bone and the lower left regions of the small intestine. 
• The lower central square is the hypogastric region contains the bottom of the pubic bones, upper regions of the bladder and the lower region of the small intestine.

Membranes of the Anterior (Ventral) Body Cavity

A serous membrane (also referred to a serosa) is one of the thin membranes that cover the walls and organs in the thoracic and abdominopelvic cavities. The serous membranes form fluid-filled sacs, or cavities, that are meant to cushion and reduce friction on internal organs when they move, such as when the lungs inflate or the heart beats. Both the parietal and visceral serosa secrete the thin, slippery serous fluid located within the serous cavities.

• The parietal layers: line the walls of the body cavity
• The visceral layer: covers the organs (the viscera). 
• Between the parietal and visceral layers is a very thin, fluid-filled serous space, or cavity, provide additional protection to the viscera they enclose by reducing friction that could lead to inflammation of the organs.

                                                                       Figure 6: Serous membranes

The pericardium is a two-layered sac that surrounds the entire heart except where the blood vessels emerge on the heart’s superior side. The pericardium has two layers because it folds over itself in the shape of the letter U.

• The inner layer that borders the heart is the visceral pericardium.
• The outer layer is the parietal pericardium. 
• The space between the two layers is called the pericardial cavity. 

There are three serous cavities and their associated membranes:

• The pleura is the serous membrane that encloses the pleural cavity; the pleural cavity surrounds the lungs. The pleural cavity reduces friction between the lungs and the body wall. Likewise, the pericardial cavity reduces friction between the heart and the wall of the pericardium. 
• The pericardium is the serous membrane that encloses the pericardial cavity; the pericardial cavity surrounds the heart. 
• The peritoneum is the serous membrane that encloses the peritoneal cavity; the peritoneal cavity surrounds several organs in the abdominopelvic cavity. The peritoneal cavity reduces friction between the abdominal and pelvic organs and the body wall.

Video explanation on: 

https://youtu.be/yVUMFdmhLxE

References( in APA style):

https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.

• Moore, K. L., & Dalley, A. F. (1999). Clinically oriented anatomy. Philadelphia: Lippincott Williams & Wilkins.