BIO 004 · Human Anatomy
Body Cavities & Regions
Block 1 · Module 1: Introduction to Anatomy
A reference for the Body Cavities video and your lab work. The body cavities enclose and organize the viscera, and the abdominopelvic regions and quadrants give every clinician one shared map for locating organs, pain, and masses.
How to use this sheet Toggle the toolbar above. Notes prints the full reference for review. Study prints as a fill-in-the-blank worksheet. Print it, then write each definition while you watch the video or read your book. Quiz me is on-screen typing practice: type the term, click Reveal to check yourself. The comparison grids respond to Study and Quiz too, with a Reveal button on each row.
The Foundations video gives you a complete foundational understanding of this topic, enough on its own for a foundational course. Learn it first, then move on to the Deep dive, which adds the majors-level material: the mediastinum, peritoneal relationships, and SADPUCKER.
- Distinguish the dorsal (posterior) and ventral (anterior) body cavities and name every subdivision of each.
- Define a serous membrane and differentiate its parietal and visceral layers in the pleural, pericardial, and peritoneal cavities.
- Map the boundaries and the four subdivisions of the mediastinum, and identify the major structures each contains.
- Classify abdominopelvic organs as intraperitoneal or retroperitoneal using SADPUCKER, and locate organs on the nine-region and four-quadrant grids.
Your pre-work
Work through these the evening before class. None of it is turned in. It is how you learn the material and build your spaced recall.
This is more than a checklist. Ticking these boxes is the start, not the finish. Committing this material to memory and being able to apply it takes considerable time and repeated effort. You are not done when the boxes are checked. Put in the real hours, and keep coming back for frequent recall and review until the material is genuinely yours.
Click any image to enlarge. Images: OpenStax Anatomy & Physiology 2e, CC BY 4.0.
Directional Terms
The vocabulary you use to describe where one structure sits relative to another. Always given from the patient's perspective in anatomical position.
Directional terms come in opposing pairs. Learn each pair together: knowing one term gives you its opposite.
| Term | Opposite | Axis or relationship |
|---|---|---|
| Superior | Inferior | Vertical axis. Superior is above, toward the head; inferior is below, toward the feet. |
| Anterior (ventral) | Posterior (dorsal) | Front-to-back axis. Anterior is toward the front of the body; posterior is toward the back. |
| Medial | Lateral | Relative to the midline. Medial is toward the midline; lateral is away from it. |
| Proximal | Distal | Along a limb. Proximal is closer to the trunk or point of attachment; distal is farther from it. |
| Superficial | Deep | Relative to the body surface. Superficial is closer to the surface; deep is farther in. |
| Cranial (cephalad) | Caudal | Along the long axis of the trunk. Cranial is toward the head; caudal is toward the tailbone. |
| Ipsilateral | Contralateral | Side comparison. Ipsilateral is the same side of the body; contralateral is the opposite side. |
The Body Cavities
Dorsal (Posterior) Cavity
- Cranial cavityformed by the cranial bones, encloses the brain
- Vertebral cavityruns through the vertebral foramina, encloses the spinal cord
- One continuous spacethe cranial and vertebral cavities connect, both lined by the meninges
Ventral (Anterior) Cavity
- Thoracic cavitysuperior to the diaphragm, walled by the rib cage
- Pleural cavities (2)one on each side, each surrounds a lung
- Mediastinummedian partition between the pleural cavities, see the section below
- Pericardial cavitysurrounds the heart, sits within the mediastinum
- Abdominopelvic cavityinferior to the diaphragm, the largest cavity of the body
- Abdominal cavitysuperior portion, holds the stomach, intestines, liver, spleen, pancreas, kidneys
- Pelvic cavityinferior portion, cradled by the bony pelvis, holds the bladder, rectum, internal reproductive organs
- No dividing wallthe abdominal and pelvic cavities are continuous, named regions of one space
The Boundary
- Diaphragmmusculotendinous partition, the floor of the thoracic cavity and the boundary with the abdominopelvic cavity
Serous Membranes
What a serous membrane is
- Serous membranea thin, double-layered membrane lining a ventral body cavity
- Mesotheliumthe simple squamous epithelium of the membrane, resting on areolar connective tissue
- Parietal layerlines the cavity wall
- Visceral layercovers the outer surface of the organ
- Serous cavitythe narrow, fluid-filled space between the parietal and visceral layers
- Serous fluidslippery secretion of the mesothelium, lets the organ glide against the wall
The three named pairs
Each serous membrane has the same two-layer plan. Compare them side by side: the parietal layer always lines the wall, the visceral layer always covers the organ.
| Membrane | Where it sits | Parietal layer | Visceral layer |
|---|---|---|---|
| Pleura | around each lung, within the thoracic cavity | parietal pleura lines the thoracic wall, mediastinum, and diaphragm | visceral pleura clings to the outer surface of the lung |
| Pericardium | around the heart, within the middle mediastinum | parietal layer of the serous pericardium, fused to the inner surface of the fibrous pericardium | visceral layer, also called the epicardium, the outer surface of the heart |
| Peritoneum | around the abdominopelvic organs | parietal peritoneum lines the wall of the abdominopelvic cavity | visceral peritoneum covers the outer surface of the organs |
The Mediastinum
The median partition of the thoracic cavity, set between the two pleural sacs. In the clinic you locate a mass, a sound, or a bleed by the compartment it occupies.
Boundaries
- Superior boundarythe superior thoracic aperture, also called the thoracic inlet
- Inferior boundarythe diaphragm
- Anterior boundarythe sternum
- Posterior boundarythe bodies of thoracic vertebrae T1 through T12
- Lateral boundariesthe mediastinal pleura, the medial wall of each pleural cavity
The four compartments
The transverse thoracic plane, running from the sternal angle to the disc between T4 and T5, splits a superior mediastinum from an inferior mediastinum. The inferior mediastinum is then divided into anterior, middle, and posterior compartments. Locate a mass, a sound, or a bleed by the compartment it occupies.
| Compartment | Location | Key contents |
|---|---|---|
| Superior mediastinum | above the transverse thoracic plane | arch of the aorta and its three branches, brachiocephalic veins, upper superior vena cava, trachea, esophagus, thoracic duct, thymus, phrenic and vagus nerves |
| Anterior mediastinum | inferior mediastinum, between the body of the sternum and the pericardium | thymus or its fatty remnant, lymph nodes, fat, connective tissue |
| Middle mediastinum | inferior mediastinum, the pericardium and the structures within it | heart within the pericardium, ascending aorta, pulmonary trunk, lower superior vena cava, main bronchi, phrenic nerves |
| Posterior mediastinum | inferior mediastinum, between the pericardium and the thoracic vertebral column | thoracic (descending) aorta, esophagus, thoracic duct, azygos and hemiazygos veins, sympathetic trunks, vagus nerves |
Mediastinum subdivisions
Add a labeled sagittal view showing the superior mediastinum and the anterior, middle, and posterior compartments. Drop the image and matching alt text in here.
Mediastinal mass localization by compartment
A short reference for which masses are classically anterior, middle, or posterior. We will write this one in your voice on the next pass.
Peritoneum and Peritoneal Relationships
The peritoneum is the serous membrane of the abdominopelvic cavity. Whether an organ lies inside that membrane or behind it changes how you reach it surgically and how fluid and infection travel.
The peritoneum
- Parietal peritoneumlines the wall of the abdominopelvic cavity
- Visceral peritoneumwraps the surface of the organs
- Peritoneal cavitythe potential space between the two layers, holding a thin film of serous fluid
- Mesenterya double fold of peritoneum that slings an organ from the body wall and carries its vessels and nerves
- Greater omentumthe apron of peritoneum hanging from the greater curvature of the stomach
- Lesser omentumthe fold linking the lesser curvature of the stomach and the duodenum to the liver
- Mesocolonthe peritoneal fold anchoring the transverse and sigmoid colon
- Peritoneal recesseslow points of the peritoneal cavity where fluid, blood, or pus can collect, the reason body position matters when you look for free fluid
Intraperitoneal versus retroperitoneal
Every abdominopelvic organ has one of these two relationships to the peritoneum. The relationship sets how mobile the organ is and how a surgeon reaches it.
| Relationship | Definition | Organs |
|---|---|---|
| Intraperitoneal | almost completely wrapped in visceral peritoneum and slung from the body wall by a mesentery, so they are relatively mobile | stomach, liver, gallbladder, spleen, first part of the duodenum, jejunum, ileum, transverse colon, sigmoid colon, cecum and appendix, tail of the pancreas |
| Retroperitoneal | lie against the posterior abdominal wall behind the parietal peritoneum, with peritoneum covering only their anterior surface | the structures that sit behind the peritoneum, remembered with SADPUCKER below |
SADPUCKER, the retroperitoneal checklist
A memory aid for the structures that sit behind the peritoneum. Each letter is one retroperitoneal organ or vessel.
SADPUCKER
| Letter | Structure | What lies retroperitoneal |
|---|---|---|
| S | Suprarenal glands | the adrenal glands, capping each kidney |
| A | Aorta and IVC | the abdominal aorta and the inferior vena cava |
| D | Duodenum | the second, third, and fourth parts; the first part is intraperitoneal |
| P | Pancreas | the head, neck, and body; the tail is intraperitoneal |
| U | Ureters | both ureters, running down the posterior wall |
| C | Colon | the ascending and descending colon; the transverse and sigmoid colon are intraperitoneal |
| K | Kidneys | both kidneys, set in the perirenal fat |
| E | Esophagus | the short abdominal segment below the diaphragm |
| R | Rectum | the lower portion, below the peritoneal reflection |
Retroperitoneal organs (SADPUCKER)
Add a labeled transverse or coronal view showing the SADPUCKER organs against the posterior abdominal wall. Drop the image and matching alt text in here.
Abdominopelvic Regions (Nine)
The four planes that draw the grid
- Midclavicular planes (2)two vertical planes dropped from the midpoint of each clavicle
- Subcostal planehorizontal, across the inferior margins of the rib cage
- Transtubercular planehorizontal, across the tubercles of the iliac crests
The nine regions
The four planes divide the abdomen into a three-by-three grid. Read it like a tic-tac-toe board, top row to bottom row.
| Region | Position | Key organs |
|---|---|---|
| Right hypochondriac | upper right | gallbladder, right lobe of the liver |
| Epigastric | upper center | stomach, duodenum, pancreas |
| Left hypochondriac | upper left | spleen, tail of the pancreas |
| Right lumbar | middle right | ascending colon, right kidney |
| Umbilical | center | transverse colon, coils of small intestine |
| Left lumbar | middle left | descending colon, left kidney |
| Right iliac (inguinal) | lower right | cecum, appendix |
| Hypogastric (pubic) | lower center | urinary bladder, sigmoid colon |
| Left iliac (inguinal) | lower left | sigmoid colon |
Four Quadrants
The two planes
- Median planethe vertical midsagittal plane, divides right from left
- Transumbilical planethe horizontal plane through the umbilicus, divides upper from lower
The quadrants
| Quadrant | Key organs | Classic pain pattern |
|---|---|---|
| RUQ, right upper | liver, gallbladder, right kidney, hepatic flexure, head of the pancreas | gallbladder disease, hepatitis |
| LUQ, left upper | stomach, spleen, left kidney, splenic flexure, tail of the pancreas | splenic injury, gastric pain |
| RLQ, right lower | cecum, appendix, right ovary | classic site of appendicitis pain |
| LLQ, left lower | sigmoid colon, left ovary | classic site of diverticulitis pain |
Study questions
Work on answering these in writing, in your own words. They are the questions to bring to class, and good practice for the reasoning the exams ask for.
- Distinguish the dorsal and ventral body cavities and name every subdivision of each.
- Explain the two-layer plan of a serous membrane, using the pleura as your example.
- Map the four compartments of the mediastinum and name one structure in each.
- Classify three abdominopelvic organs as intraperitoneal or retroperitoneal and explain how you decided.
Step 2 . Retrieval check
Now explain it back, in your own words.
In 60 words or more, pull together what the video just taught you. Include the key concepts. This is the point where the learning actually sticks. After you submit, your spaced-recall cards for this topic unlock.