Comprehensive Notes: Body Cavities, Membranes, Planes, and Abdominal Regions
Big Picture: organization of the human body
Anatomy as one of the oldest natural disciplines; tracing back to ancient Greece and even earlier among Egyptian priests performing autopsies.
The human body is described as a hollow vessel with walls that enclose internal organs.
Two principal parts of the body:
2 parts: soma and viscera.
Soma = body walls (skin, bones, and skeletal muscles). Skeletal muscles are part of the soma and are under somatic (voluntary) nervous system control.
Viscera = internal organs. Internal organs contain muscles that can be smooth or cardiac, which are under the autonomic division also known as involuntary
The big picture: soma (body walls) + viscera (internal organs) together form the functional anatomy of the human body.
Foundational idea: differentiation between somatic (voluntary) nervous system vs visceral/autonomic (involuntary) nervous system.
Cavities and compartments of the body
There are two principal body cavities:
2 principal cavities: dorsal body cavity and ventral body cavity.
On the posterior Dorsal body cavity contains the central nervous system (brain and spinal cord).
On the anterior Ventral body cavity contains all the thoracic and abdominopelvic contents.
Dorsal body cavity anatomy:
Cranial cavity (brain).
Spinal (vertebral) cavity (spinal cord).
These two are connected along the CNS axis.
Ventral body cavity anatomy:
Thoracic cavity (chest) and abdominopelvic cavity (abdomen + pelvis).
The diaphragm separates the thoracic cavity from the abdominopelvic cavity.
Thoracic cavity contents: two pleural cavities (left and right) each containing a lung, and the mediastinum in between, which houses the heart within its own pericardial cavity.
Abdominopelvic cavity contents: abdominal cavity with digestive organs and pelvic cavity with pelvic organs (urinary bladder, reproductive organs, rectum, uterus, etc.).
Central nervous system (CNS) components: brain and spinal cord are within the dorsal cavity.
Membranes lining the dorsal and ventral cavities
Dorsal cavity membranes (meninges):
The meninges are a trio: pia mater, arachnoid mater, and dura mater (innermost to outermost).
Pia mater: innermost, very thin and gentle; lines the surface of brain and spinal cord.
Arachnoid mater: resembles a spider web in appearance.
Dura mater: the toughest, outermost membrane.
Spaces around the meninges (important for CSF circulation and protection):
Epidural space: located outside the dura mater (e.g., around brain and spinal cord).
Subdural space: between dura mater and arachnoid mater.
Subarachnoid space: between arachnoid and pia mater; contains cerebrospinal fluid (CSF).
Ventral (serous) membranes and their cavities:
Serous membranes line the ventral body cavities and form closed, double-layered sacs with a slit-like potential space containing serous fluid for lubrication.
Three main serous membranes:
Pleura: around the lungs; consists of two layers—visceral pleura (stuck to the lung) and parietal pleura (lines the chest wall). The pleural cavity (between these layers) contains serous fluid.
Pericardium: around the heart; visceral pericardium (epicardium) lining the heart surface and parietal pericardium forming the outer sac around the heart. The pericardial cavity contains serous fluid.
Peritoneum: lines the abdominal cavity and covers abdominal organs; contains a potential space with serous fluid to reduce friction.
Function and significance of serous membranes:
They minimize friction as organs move (e.g., lungs expanding/deflating, heart beating, and intestines shifting with movement).
Visceral membranes adhere to the organs; parietal membranes line the body walls.
Pleura, pericardium, and peritoneum: details and implications
Pleura:
Visceral pleura covers the surface of each lung.
Parietal pleura lines the internal thoracic wall and the lateral aspects of the mediastinum.
Pleural space contains serous fluid to reduce friction during respiratory movements.
Pericardium:
Visceral pericardium (epicardium) adheres to the heart surface.
Parietal pericardium forms the outer layer of the pericardial sac.
Pericardial cavity contains serous fluid for lubrication during heartbeats.
Peritoneum:
Serous membrane lining the abdominal cavity and covering abdominal organs.
Contains serous fluid to cushion and allow smooth movement of abdominal viscera.
Diaphragm and the major body cavities in cross-section
Diaphragm:
Dome-shaped muscular partition between the thoracic cavity (above) and the abdominal cavity (below).
Not a perfect dome; a curved, dome-like muscle essential for respiration and separation of cavities.
Borders and contents:
Thoracic cavity contains the lungs (via pleural cavities), heart (via mediastinum and pericardial cavity), and great vessels.
Abdominopelvic cavity contains abdominal organs (liver, stomach, intestines, gallbladder, etc.) and pelvic organs (urinary bladder, reproductive organs, rectum).
Directional terms and their use
Anatomical position: the reference position for reading the body map.
Standing upright, facing the observer, with palms forward.
Superior vs inferior: toward the head vs toward the feet; synonyms: cranial vs caudal.
Anterior vs posterior: toward the front vs toward the back; synonyms: ventral vs dorsal.
Medial vs lateral: closer to the midline vs farther from the midline; lateral means to the side.
Median smack on the middle
Ipsilateral vs contralateral: same side vs opposite side.
Collateral: parallel, going in the same direction (used in anatomy for certain structures).
Superficial vs deep: toward the surface vs away from the surface.
Proximal vs distal: closer to the point of attachment vs farther from the point of attachment; especially used for limbs and tubular structures.
Tubular structures and direction of flow:
Arteries: blood flow generally from the heart toward the periphery; proximal end is near the heart, distal end is further away.
GI tract (tube): flow from mouth to anus; proximal to distal along the length of the tract.
Practical reminder on orientation:
When describing regions on a patient, remember that the observer’s right is the patient’s left and vice versa.
Mislabeling sides has led to critical clinical errors (e.g., amputating the wrong leg or removing the wrong kidney).
Planes of section and 3D thinking
Three primary planes of section (perpendicular to each other) used to visualize anatomy in 3D:
Sagittal plane: divides the body into left and right parts. Midsagittal if it splits the body into equal halves; parasagittal if it divides into unequal parts. Spelling: sagittal (s-a-g-i-t-t-a-l).
Transverse (horizontal) plane: divides the body into superior (upper) and inferior (lower) parts. Used in CT scans and cross-sectional views.
Coronal (frontal) plane: divides the body into anterior (front) and posterior (back) parts.
Purpose:
By comparing sagittal, transverse, and coronal views of the same organ (e.g., brain), you obtain a 3D mental image of structure and relationships.
Key takeaway:
Developing a three-dimensional way of thinking is essential for anatomy mastery; these planes help visualize and communicate complex spatial relationships.
Abdominal regions and quadrants
Abdominal region mapping (nine-region plan): achieved by four lines:
Two vertical lines: midclavicular lines (through the midpoint of the clavicles).
Two horizontal lines: one through the inferior border of the ribs (subcostal plane) and one through a level near the pelvis (pelvic line).
Nine regions (centers around the umbilicus):
Epigastric region (upper middle).
Hypochondriac regions: right hypochondriac and left hypochondriac (upper right and upper left, near the lower ribs).
Umbilical region: centered around the navel.
Lumbar (lateral) regions: right lumbar and left lumbar (middle right and middle left).
Iliac (inguinal) regions: right iliac and left iliac (lower sides, near the groin).
Hypogastric (pubic) region: center below the umbilical region.
Practical notes:
The liver spans the right hypochondriac and epigastric regions and extends into the right lumbar region.
The ilioinguinal distinction: iliac is above the groin (inguinal); sometimes the two terms are blended into ilioinguinal in clinical descriptions.
Naming body regions uses adjectives (not nouns) to describe regions.
Example adjectives: frontal region (from France? actually from frontal), axillary region (armpit), thoracic region (chest).
Suffixes to form adjectives from nouns: -ich, -ac, -al, -ere (e.g., frontal region, axillary region).
Abdominal quadrants (four-quadrant map):
Draw a vertical line and a horizontal line through the umbilicus, creating: Right Upper Quadrant (RUQ), Left Upper Quadrant (LUQ), Right Lower Quadrant (RLQ), Left Lower Quadrant (LLQ).
Note on orientation: the image viewer’s right is the patient’s left and vice versa; clinicians describe the patient’s right/left, which may differ from the observer’s perspective.
Practical importance:
Regions vs quadrants help localize organs and plan clinical assessments, imaging, and surgical approaches.
Be aware that organ locations can cross region boundaries (e.g., the liver in multiple regions).
Summary of key concepts and terms (quick reference)
Soma: body walls (skin, bones, skeletal muscles). Innervated by somatic (voluntary) nervous system.
Viscera: internal organs. Innervated by visceral (autonomic/involuntary) nervous system.
Dorsal body cavity: cranial cavity + spinal/vertebral canal; CNS centers reside here; lined by meninges.
Ventral body cavity: thoracic + abdominopelvic cavities; separated by the diaphragm; contains serous membranes.
Meninges (dorsal): pia mater, arachnoid mater, dura mater (innermost to outermost).
Spaces around meninges: epidural, subdural, subarachnoid (CSF in subarachnoid space).
CSF: cerebrospinal fluid found in the subarachnoid space.
Serous membranes (ventral): pleura, pericardium, peritoneum; have visceral and parietal layers with serous fluid in the separating space.
Pleura: around the lungs; pleural cavity between visceral and parietal pleura.
Pericardium: around the heart; pericardial cavity.
Peritoneum: around abdominal organs; intraperitoneal and extraperitoneal spaces exist depending on organ relations.
Diaphragm: dome-shaped muscular divide between thoracic and abdominal cavities.
Anatomical position: reference posture for orientation; sagittal, transverse, and coronal planes to describe sections.
Planes of section: sagittal (left-right), transverse/horizontal (superior-inferior), coronal/frontal (anterior-posterior).
Abdominal nine-region map: epigastric, umbilical, hypogastric; right/left hypochondriac, right/left lumbar, right/left iliac (inguinal).
Abdominal quadrant map: RUQ, LUQ, RLQ, LLQ; orientation depends on patient’s perspective.
Proximal vs distal: relation to the point of attachment; used mainly for limbs and tubular structures.
Etymology tip: naming body regions uses adjectives derived from nouns (e.g., frontal region, axillary region).
Practical caution: avoid misidentifying sides in clinical settings; always align with the patient’s orientation.