Body cavities and serous membranes

Posterior (post- = behind) means toward the back. Anterior means toward the front, and lateral means away from the midline.

• Anterior:
• Correct: Posterior:
• Lateral:

An organ combines two or more kinds of tissue into one structure with a specific job. A group of similar cells is a tissue, and a group of organs is an organ system.

• A group of similar cells doing one job:
• Correct: A structure made of two or more kinds of tissue that does a specific job:
• A group of organs working on a shared job:

The transverse (horizontal) plane divides upper from lower. The frontal plane divides front from back, and the sagittal plane divides right from left.

• Correct: Transverse plane:
• Frontal plane:
• Sagittal plane:

The cranial cavity (crani/o = skull) is the space inside the skull, and it holds the brain. With the vertebral canal it forms the dorsal body cavity.

• Vertebral canal: The vertebral canal is the other part of the dorsal body cavity. It runs down through the backbone and holds the spinal cord, not the brain.
• Mediastinum: The mediastinum is the central compartment of the thoracic cavity. It holds the heart, the trachea and the esophagus.
• Correct: Cranial cavity: Correct. The brain sits in the cranial cavity, inside the skull.
• Pericardial cavity: The pericardial cavity is the thin fluid-filled space around the heart, in the mediastinum.

The heart lies in the mediastinum, the central compartment between the two lungs, running from the breastbone in front to the backbone behind. A blade that reaches the heart has entered the mediastinum.

• The left pleural cavity: The left pleural cavity is the thin space around the left lung, lateral to the heart. The heart itself is in the mediastinum.
• The abdominal cavity: The abdominal cavity lies below the diaphragm. The heart is above it.
• The vertebral canal: The vertebral canal is part of the dorsal body cavity, inside the backbone. It is posterior to the whole thoracic cavity.
• Correct: The mediastinum: Correct. The heart sits in the mediastinum, directly behind the breastbone.

The pleural cavity is only the thin, fluid-filled gap between the visceral layer on the lung and the parietal layer on the chest wall. The lung is covered by the visceral layer and lies outside that gap, like a fist in a balloon that never enters the air inside.

• Inside the pleural cavity, floating in serous fluid: This is the common error. The organ is never inside its serous cavity; only a thin film of serous fluid is.
• Correct: In the thoracic cavity, but outside the pleural cavity: Correct. In the thoracic cavity, yes; in the pleural cavity, no.
• In the mediastinum, with the pleural cavity below it: The mediastinum is the central compartment between the lungs. The lungs lie lateral to it, and the pleural cavities surround the lungs, not lie below them.
• In the abdominopelvic cavity, just above the diaphragm: Everything above the diaphragm is thoracic. The abdominopelvic cavity is below the diaphragm.

Normally a film of serous fluid lets the visceral layer on the lung glide over the parietal layer on the chest wall. When infection or injury makes the layers swollen and rough, they rub with each breath. The rubbing causes sharp pain and a grating sound.

• Fluid has leaked into her vertebral canal: The vertebral canal holds the spinal cord in the dorsal body cavity. It does not move with breathing and would not make a grating sound in the chest.
• Her lung has slipped into the pericardial cavity: The lung and the pericardial cavity are on different sides of the mediastinum. The lung cannot slip into the heart's serous space.
• Correct: The pleural layers are rough and rubbing: Correct. Rough layers rub instead of gliding, and each breath moves them.
• Her diaphragm has separated the two pleural cavities: The diaphragm lies below the lungs. The mediastinum, not the diaphragm, separates the right and left pleural cavities, and that separation is normal.

The urinary bladder lies in the pelvic cavity, the lower part of the abdominopelvic cavity, between your hip bones. When it fills, it rises into the abdominal cavity, because no wall separates the two.

• Correct: Pelvic cavity: Correct. The empty bladder sits low, in the pelvic cavity.
• Thoracic cavity: The thoracic cavity is above the diaphragm. It holds the heart and lungs.
• Abdominal cavity: The abdominal cavity is the upper part of the abdominopelvic cavity. A very full bladder can rise into it, but an empty one stays in the pelvis.
• Vertebral canal: The vertebral canal is inside the backbone and holds the spinal cord.

The spleen lies in the abdominal cavity, covered by the peritoneum. Blood from its surface collects in the peritoneal cavity, between the layer on the organs and the layer lining the abdominal wall.

• The left pleural cavity: The left pleural cavity surrounds the left lung, above the diaphragm. The spleen is below it.
• Correct: The peritoneal cavity: Correct. The spleen is an abdominal organ covered by peritoneum.
• The pericardial cavity: The pericardial cavity surrounds the heart, in the mediastinum.
• The mediastinum: The mediastinum is the central compartment of the thoracic cavity, above the diaphragm.

Below the diaphragm, the abdominal cavity holds the liver, spleen, stomach, intestines and, against its back wall, the kidneys. The pelvic cavity holds the urinary bladder.

• Correct: Liver: Correct. The liver sits in the upper right of the abdominal cavity, just under the diaphragm.
• Heart: The heart is in the mediastinum, above the diaphragm.
• Correct: Urinary bladder: Correct. The bladder is in the pelvic cavity.
• Correct: Kidneys: Correct. The kidneys lie against the back wall of the abdominal cavity, behind the peritoneum.
• Thymus: The thymus is in the upper mediastinum, above the heart.
• Correct: Spleen: Correct. The spleen is in the upper left of the abdominal cavity.