Human Anatomy: Azygos Vein and GI Tract

Azygos Vein

• Drains venous blood from the thorax into the superior vena cava (SVC)
• Drains both parietal and visceral components of the thorax, excluding the heart
• Drains venous blood from structures supplied by the descending aorta: thoracic walls, pericardium, lungs, trachea, bronchi, esophagus, diaphragm
• Variation exists in how venous blood drains into the azygos vein
• Located posteriorly against the vertebral column, arching anteriorly over the right lung root before connecting to the SVC
• Leaves an impression on the medial surface of the right lung

GI Tract

• Abdomen is the cavity between the thorax and pelvis
• Abdominal wall is mainly composed of muscle and covering structures, not bone
• These structures attach to the thorax and pelvis

Anterior Abdominal Wall

• Bony Framework: Consists of bones of the thorax and pelvis, attaching to the inferior border of the thorax, iliac crest, anterior superior iliac spine (ASIS), pubic tubercle, and pubic crest.
• Lining/Covering Structures: Eight layers from superficial to deep: skin, superficial fascia (fat), paired muscles (external oblique, internal oblique, transversus abdominus), transversalis fascia, extra-peritoneal fascia, and peritoneum.

Lateral Flat Muscles

• Three paired layers: external oblique, internal oblique, and transversus abdominus
• Muscles are fleshy laterally, transitioning to aponeuroses medially, allowing for abdominal expansion
• Aponeuroses from each side meet and interdigitate to form the linea alba (raphe)

External Oblique

• Superficial layer with inferomedial fiber direction
• Medial attachment at linea alba; superior at the ribs; posterior at the thoracolumbar fascia with the posterior abdominal wall; and inferior, with lateral attachment to the iliac crest and ASIS, free central part, and medial attachment to the pubic tubercle and pubic crest.
• Forms the inguinal ligament, a thickened band formed by the free central part of the inferior border, which extends from the ASIS to the pubic tubercle.

Internal Oblique

• Intermediate layer with superomedial fiber direction (perpendicular to the external oblique).
• Medial attachment at linea alba; superior at the costal margin; posterior at the thoracolumbar fascia; and inferior, with lowermost fibers arising from the lateral 2/3 of the inguinal ligament, arching up, and inserting onto the pubic crest via a conjoint tendon.

Transversus Abdominus

• Deep layer with transverse fiber direction
• Medial attachment at linea alba; superior at the costal margin; posterior at the thoracolumbar fascia; and inferior, with lowermost fibers arising from the lateral 1/3 of the inguinal ligament, arching up, and inserting onto the pubic crest via a conjoint tendon.
• Conjoint tendon is composed of the internal oblique and transversus abdominus, inserting via a common tendon.
• Transversalis fascia resides deep to the transversus abdominus.

Medial Vertical Muscles

• Rectus Abdominus: Located on either side of the linea alba, the principle vertical muscle supporting the anterior abdominal wall, originating on the pubis and inserting on the ribcage, overlapping the costal margin. Contains three tendinous intersections for increased strength, enclosed in the rectus sheath formed by aponeuroses of the external oblique, internal oblique, and transversus abdominus.
• Pyramidalis Muscles: Small, pyramid-shaped muscles, not supporting the anterior abdominal wall.

Testicular Descent

• Testicles develop high in the posterior abdominal wall within extra-peritoneal fat.
• Descent to the scrotum is required for optimal spermatogenesis, as core body temperature inhibits the process.
• Descent involves navigating to the anterior abdominal wall and exiting the abdomen, connected to the scrotum by the gubernaculum (fibrous cord, also present in females).
• Testicular descent typically occurs around 7 months of gestation.

Inguinal Rings

• Holes in transversalis fascia and the external oblique aponeurosis facilitate testicular descent from extra-peritoneal fat.
• Deep inguinal ring is a circular hole in the transversalis fascia while the superficial inguinal ring is a triangular split in the external oblique aponeurosis at its inferior attachment.
• The pathway between these rings is oblique, forming the inguinal canal.

Inguinal Canal

• The path of testicular descent, traversing the layers of transversalis fascia, transversus abdominus, internal oblique, and external oblique (deep inguinal ring to superficial inguinal ring).
• Located superior to the inguinal ligament, with the inguinal ligament forming its floor, arching internal oblique and transversus abdominus fibers as the roof, external oblique aponeurosis and internal oblique fibers as the anterior wall, transversalis fascia and conjoint tendon as the posterior wall.
• Upon exiting the superficial inguinal ring, the pathway becomes the spermatic cord.
• Females possess inguinal canals but these are less prominent.

Inguinal Hernia

• Inguinal canal is a communication from the abdomen to the perineum.
• Increased abdominal pressure can cause abdominal contents to protrude through weak areas of the abdominal wall, leading to a hernia.
• Inguinal hernia is described as the protrusion of abdominal contents into the inguinal canal.

Cryptorchidism

• Undescended testicles.
• Can occur in the abdomen, inguinal region, or pre-scrotum.
• Requires surgical correction before 18 months-2 years to prevent infertility and increase the risk of testicular cancer.

Posterior Abdominal Wall

• Composed of muscles (and lumbar vertebrae)
• Attached to bony thorax, pelvis, and vertebrae (superiorly to the 12th rib, medially to lumbar vertebrae, laterally to thoracolumbar fascia, and inferiorly to the iliac crest).

Psoas Major

• Paired muscle located on either side of the lumbar vertebrae.
• Longitudinally oriented, arising from the lumbar vertebrae body and passing inferiorly under the inguinal ligament to insert on the lesser trochanter of the femur.
• Psoas Minor: smaller, lines the anterior surface of psoas major, not present in all individuals or on both sides.

Quadratus Lumborum

• Paired muscle located lateral to psoas major.
• Arises from the 12th rib and lumbar vertebrae transverse processes, inserting onto the iliac crest.
• Fills the space between the 12th rib and iliac crest.

Iliacus

• Paired muscle filling the iliacus fossa of the hipbone.
• Located inferior to quadratus lumborum and lateral to psoas major.

Thoracolumbar Fascia

• Muscles of the anterior and posterior abdominal walls along with back muscles are enclosed in thoracolumbar fascia.
• Fascia of the anterior and posterior abdominal walls meet lateral to the quadratus lumborum, creating a complete muscular wall.

Abdominal Quadrants

• Median plane: vertically oriented, extending from the xiphoid process to the pubic symphysis.
• Transumbilical plane: horizontally oriented, crossing the umbilicus.
• Creates four quadrants: left upper, right upper, left lower, right lower, used to describe the positioning of abdominal viscera.

Peritoneum

• Innermost layer of the abdominal walls.
• Parietal peritoneum lines the abdominal wall.
• Visceral peritoneum lines the viscera.
• Peritoneal cavity is a potential space between the parietal and visceral peritoneum.
• Intraperitoneal viscus is suspended in the abdominal cavity, completely lined with visceral peritoneum.
• Retroperitoneal viscus sits against the posterior abdominal wall, external to the abdominal cavity, with the inner surface lined with parietal peritoneum. Examples include: kidney and adrenal glands, proximal ureters, abdominal aorta, inferior vena cava, pancreas, and duodenum.

Abdomen Structures

• The kidneys are located against the posterior abdominal wall and are lined with the parietal peritoneum on their anterior surface.
• The Mesentery is a double fold of peritoneum that provides a pathway for nerves and vessels, and contributes to gastrointestinal motility.
• Not all abdominal organs have a mesentery, for ex. retroperitoneal organs such as the kidneys do not have a mesentery.
• Intraperitoneal organs such as the jejunum and ileum possess a mesentery.

Oesophagus

• The oesophagus is a muscular tube that begins at C6 from the pharynx and descends down the midline.
• It passes through the diaphragm at T10, where it enters the stomach from the right side.
• The Oesophagus has three constrictions:
  • At the cervical constriction (C6) where it begins
  • At the thoracic constriction (T4/5) where it is compressed from the left side by the bifurcation of the trachea and aortic arch
  • At the diaphragmatic constriction (T10) where the oesophagus passes through the diaphragm
• The Oesophagogastric Junction is where the oesophageal mucosa transforms into the stomach mucosa.

Stomach

• The Stomach is a dilated, J-shaped, intraperitoneal hollow viscus located in the left upper quadrant of the abdomen.
• It has two openings: the cardiac orifice, where the oesophagus enters the stomach, and the pyloric orifice, where the duodenum begins.
• The Stomach has two curvatures: the lesser curvature (right side) and the greater curvature (left side).
• The Stomach also has two surfaces: the anterior and posterior surfaces.
• The Stomach consists of four parts:
  • The Cardia: the region near the cardiac orifice
  • The Fundus: the part above the cardia, which holds gas
  • The Body: the majority of the stomach
  • The Pyloric Part: the distal narrowing of the stomach, consisting of the pyloric antrum (wider) and the pyloric canal (narrower, more distal).
• The Pyloric Sphincter, also known as the Pylorus, is located at the end of the pyloric canal, controlling the passage of chyme into the duodenum.

Small Intestine

• The Small Intestine is a hollow viscus responsible for digestion and absorption.
• It consists of three parts: duodenum, jejunum, ileum.
• The wall of the small intestine comprises two complete muscle coats: the inner circular and outer longitudinal muscles.

Duodenum

• The Duodenum is the shortest and retroperitoneal part of the small intestine.
• It has a C shape and is located laterally to the right kidney and medially the head of the pancreas.
• The Duodenum has four parts:
  • The 1st part: the duodenal cap, which immediately follows the pylorus.
  • The 2nd part: the descending component, situated between the right kidney and pancreas. This part has the major duodenal papilla, which secretes pancreatic enzymes and bile into the duodenum, and the minor duodenal papilla, which lies superior to the major papilla and secretes products from the accessory pancreatic duct.
  • The 3rd part: the horizontal part that crosses the midline.
  • The 4th part: the part that hooks around and joins with the jejunum.

Jejunum

• The Jejunum is a large, intraperitoneal part of the small intestine, encompassing roughly 40% of its total length.
• It is located in the left upper quadrant and is attached to the abdominal wall by the mesentery.
• The Jejunum has a larger caliber, holding more gastric contents as the jejunum is responsible for most of the absorption process.
• It is thick walled for increased surface area, highly vascularised, and has less fat in the mesentery.

Ileum

• The Ileum is a large, intraperitoneal part of the small intestine, accounting for roughly 60% of its total length.
• It is located in the right lower quadrant (its location shifts during pregnancy), and is attached to the abdominal wall by the mesentery.
• The Ileum has a smaller caliber compared to the jejunum, holding less gastric contents as less absorption occurs within the ileum.
• It has fewer mucosal folds than the jejunum, is less vascularised, and has less fat in the mesentery.

Large Intestine

• The Large Intestine begins at the ileocaecal junction, where the ileum enters the large intestine.
• The Large Intestine comprises several parts:
  • The Ascending colon: retroperitoneal, begins as the large intestine ascends.
  • The Transverse colon: intraperitoneal, begins at the hepatic/right colic flexure near the liver, traverses the midline, and is the longest part of the large intestine.
  • The Descending colon: retroperitoneal, begins at the splenic flexure/left colic flexure near the spleen, and descends.
  • The Sigmoid colon: bends inferiorly and medially, is S-shaped and intraperitoneal, and ultimately connects to the rectum and anal canal.

Large Intestine Wall

• The large intestine has a complete layer of inner circular muscle and an outer longitudinal layer of muscle, which is incomplete and split into three bands called tenia coli.
• These tenia coli extend the entire length of the large intestine, creating a baggy appearance known as haustra due to their incomplete coverage.
• Fatty tags called omental appendices situated along the tenia coli.

Caecum

• The Caecum is an intraperitoneal blind pouch located inferior to the ileocaecal junction in the right lower quadrant.

Appendix

• The Appendix is an intraperitoneal structure situated inferior to the ileocaecal junction in the right lower quadrant.
• Its base is found where the three tenia coli bands meet, but its position can vary from person to person.

Rectum

• The Rectum is retroperitoneal, located after the sigmoid colon.
• Its tenia coli bands merge, thus forming a complete outer layer of longitudinal muscle.
• The Rectum doesn't have tenia coli, haustra, or omental appendices.

Liver

• The Liver is a solid, intraperitoneal viscus situated in the right upper quadrant under the right dome of the diaphragm.
• It is partially protected by the thoracic cage.
• The Liver fulfills functions such as bile production and nutrient metabolism.

Diaphragmatic Surface

• The Diaphragmatic surface of the Liver is anterior and superior, where it sits against the diaphragm.
• It is smooth and covered by peritoneum.
• It is partially protected by the ribcage.

Visceral Surface

• The Visceral surface of the Liver is posterior and inferior, where it interfaces with other viscera.
• It is concave due to the impression left by adjacent structures and is covered by peritoneum, except the IVC and gall bladder.

Portal Hepatis

• The Portal Hepatis is the hilum of the liver, where vessels, nerves, lymphatics, and ducts reside.
• It is located at the visceral surface of the liver.
• The Portal Triad consists of the portal vein, hepatic artery, and bile duct.
• The Bile duct carries bile from the liver to the duodenum.
• The Hepatic artery is the principle arterial supply to the liver.
• The Portal vein carries venous blood from the GI tract, containing nutrients for metabolism in the liver.

Hepatic Veins

• The Hepatic Veins drain venous blood from the liver.
• They are not located in the hilum, but travel through grooves in the liver.
• These veins drain directly into the IVC.

Lobes

• The Diaphragmatic surface of the liver has a falciform ligament which divides the liver into two anatomical loves: a larger right lobe and a smaller left lobe.
• The Visceral surface also has a right and left anatomical lobe.
• The liver also has fissures and grooves where structures run through them, including:
  • Left sagittal fissure (ligaments run in)
  • Right sagittal fissure (IVC and gall bladder are located within)
• The Hilum separates the liver into two accessory lobes:
  • The Caudate lobe (above the hilum)
  • The Quadrate lobe (below the hilum)
  • These lobes are part of the right lobe.

Gall Bladder

• The Gall Bladder is located at the visceral surface of the liver.
• It is not covered in peritoneum.
• It is a hollow viscus situated in the right upper quadrant.
• The Gall Bladder has four parts:
  • The Fundus - the blind end.
  • The Body.
  • The Neck - narrowing.
  • The Cystic duct.
• The Gall Bladder functions in storing and concentrating bile, which is produced by the liver.
• Overconcentration of bile in the gall bladder can lead to gallstones.

Bile Pathway

• Bile produced by the liver travels through the hepatic ducts to the common hepatic duct.
• The common hepatic duct and the cystic duct merge to form the common bile duct.
• The common bile duct enters the duodenum via the major duodenal papilla.
• The major duodenal papilla has a functional sphincter.
• When this sphincter contracts, controlled by nerves, it prevents bile from entering the duodenum and results in the bile accumulating and filling the gall bladder.
• Contraction of the gall bladder forces bile through the cystic and bile ducts into the duodenum.

Pancreas

• The Pancreas is a retroperitoneal solid viscus located in the left upper quadrant.
• It has four parts:
  • The Head - surrounded by the "C" shaped duodenum.
  • The Neck.
  • The Body.
  • The Tail - located against the spleen.
• The Pancreas functions both exocrine and endocrine.

Exocrine Secretion

• The Pancreas secretes pancreatic enzymes into the duodenum via ducts:
  • The Main pancreatic duct: collects pancreatic enzymes from the tail, body, neck, and head, and is released into the duodenum through the major duodenal papilla.
  • The Accessory pancreatic duct: collects pancreatic enzymes from the head, and is released into the duodenum through the minor duodenal papilla.

Spleen

• The Spleen is an intraperitoneal, solid viscus located in the left upper quadrant, associated with organs of the GI tract.
• It is the largest lymphatic organ, rich in blood supply.
• It has two surfaces: diaphragmatic and visceral surfaces.
• The Hilum, located at the visceral surface, is where the splenic arteries and veins enter and exit the spleen.
• It is situated against the diaphragm and ribs 9, 10, and 11 on the left side.
• Fracture of these ribs can lead to damage to the spleen.
• The Spleen is soft and highly vascularized, encased in a delicate capsule.
• Sharp edges from a rib fracture can lacerate the capsule, resulting in profuse bleeding into the abdominal cavity.

Abdominal Aorta

• The Abdominal aorta is responsible for supplying blood to abdominal organs.
• It enters the abdominal cavity at T12, and bifurcates into the iliac arteries at L4.

Anterior Surface

• The anterior surface branches into three unpaired branches:
  • The Celiac trunk: supplies the foregut (abdominal oesophagus to the major duodenal papilla).
  • The Superior mesenteric artery: supplies the midgut (major duodenal papilla to the distal 1/3 of the transverse colon).
  • The Inferior mesenteric artery: supplies the hindgut (distal 1/3 of the transverse colon to halfway through the anal canal).
• The anterior surface also has three paired branches:
  • Suprarenal arteries: supply the adrenal glands.
  • Renal arteries: supply the kidneys.
  • Testicular/ovarian arteries: supply the gonads.

Posterior Surface

• The Posterior surface of the aorta includes paired arteries that supply abdominal wall structures:
  • Phrenic arteries: supply the diaphragm.
  • Parietal arteries: supply the anterior and posterior abdominal walls.

Venous Drainage

• All venous blood from abdominal organs drains into the IVC.
• Venous blood from paired viscera and abdominal walls drains directly into the IVC.
• Venous blood from unpaired viscera (except the liver) drains into the IVC through the portal vein.

Azygos Vein

• Drains venous blood from the thorax into the superior vena cava (SVC)
• Drains both parietal and visceral components of the thorax except the heart
• Specific organs drained include the thoracic wall, pericardium, lungs, trachea, bronchi, esophagus, and diaphragm
• Positioned posteriorly against the vertebral column, arching anteriorly over the right lung root before draining into the SVC
• Leaves an impression on the medial surface of the right lung

GI Tract

• Abdomen is the cavity located between the thorax and pelvis
• Primarily composed of muscle and covering structures, not bone
• Attached to the thorax and pelvis through its structures

Anterior Abdominal Wall

Bony Framework

• Composed of bones from the thorax (superiorly) and pelvis (inferiorly)
• Structures of the anterior abdominal wall attach to:
  • Inferior border of the thorax
  • Iliac crest
  • Anterior superior iliac spine (ASIS)
  • Pubic tubercle (lateral)
  • Pubic crest (medial)

Lining/Covering Structures

• 8 structures, from superficial to deep:
  • Skin
  • Superficial fascia (fat)
  • Paired muscles:
    • External Oblique
    • Internal Oblique
    • Transversus abdominus
    • Rectus Abdominus
    • Pyramidalis
  • Transversalis fascia
  • Extraperitoneal fascia
  • Peritoneum: lines both the abdominal wall (parietal peritoneum) and viscera (visceral peritoneum)

Lateral Flat Muscles

• 3 paired layers:
  • External Oblique (EO)
  • Internal Oblique (IO)
  • Transversus Abdominus (TA)
• Fleshy laterally, transitioning to aponeuroses medially, allowing abdominal expansion
• Aponeuroses from each side meet and interdigitate, forming the linea alba (raphe)

External Oblique (EO) Muscles

• Superficial layer
• Fibres run inferomedially
• Medial attachment: linea alba
• Superior attachment: ribs
• Posterior attachment: thoracolumbar fascia
• Inferior attachment:
  • Lateral portion attaches to the iliac crest and ASIS
  • Central portion remains free
  • Medial portion attaches to the pubic tubercle and pubic crest via a fibrous split
• Inguinal ligament:
  • Formed by the free, central portion of the EO's inferior border
  • Thickened as it curves under, extending from ASIS to pubic tubercle

Internal Oblique (IO) Muscles

• Intermediate layer
• Fibres run superomedially, perpendicular to EO
• Medial attachment: linea alba
• Superior attachment: costal margin
• Posterior attachment: thoracolumbar fascia
• Inferior attachment:
  • Lower IO fibres originate from the lateral 2/3 of the inguinal ligament, arch upwards, and descend to insert onto the pubic crest via a conjoint tendon

Transversus Abdominus (TA) Muscles

• Deep layer
• Fibres run transversely
• Medial attachment: linea alba
• Superior attachment: costal margin
• Posterior attachment: thoracolumbar fascia
• Inferior attachment:
  • Lower TA fibres originate from the lateral 1/3 of the inguinal ligament, arch upwards, and descend to insert onto the pubic crest via a conjoint tendon
• Conjoint tendon: forms from a shared insertion point of both the IO and TA muscles.
• Lies deep to the TA
• Transversalis fascia

Medial Vertical Muscles

• Rectus Abdominus:

  • Located on either side of the linea alba
  • Main vertical muscle supporting the anterior abdominal wall
  • Originates on the pubis and inserts onto the ribcage (overlapping the costal margin)
  • 3 tendinous intersections increase muscle strength (functioning as 4 short muscles instead of a single long muscle)
  • Enclosed within the rectus sheath:
    • Formed by aponeuroses of the EO, IO, and TA, which pass either in front of or behind the rectus abdominus
    • EO aponeurosis: front
    • IO aponeurosis: splits, passing both in front and behind
    • TA aponeurosis: behind
    • Creates a fibrous compartment for the rectus abdominus

• Pyramidalis Muscle:

  • Pyramid shaped
  • Small
  • Does not support the anterior abdominal wall

Testicular Descent

• Testes develop high in the posterior abdominal wall within extraperitoneal fat
• Descend to the scrotum for proper function, since spermatogenesis is hindered at core body temperature, requiring a 2-3 degree Celsius reduction
• Involve moving to the anterior abdominal wall, and eventually leaving the abdomen
• Connect to the scrotum through the gubernaculum (fibrous cord), a structure also present in females
• Descent typically occurs around 7 months of gestation

Inguinal Rings

• Testes must create an opening through transversalis fascia and the EO aponeurosis to leave the abdomen and enter the scrotum
• Deep inguinal ring: circular hole in transversalis fascia
• Superficial inguinal ring: triangular split in EO aponeurosis at its inferior attachment point
• Path between rings is oblique

Inguinal Canal

• The pathway testes travel to descend into the scrotum

• Passes through the transversalis fascia (deep inguinal ring), TA, IO, and EO (superficial inguinal ring) layers

• Located superior to the inguinal ligament

• Boundaries:

  • Floor: inguinal ligament
  • Roof: arching IO and TA fibres
  • Anterior wall: EO aponeurosis and IO fibres (laterally)
  • Posterior wall: transversalis fascia and conjoint tendon (medially)

• Becomes the spermatic cord after emerging from the superficial inguinal ring

• Females have less prominent inguinal canals

Inguinal Hernia

• Inguinal canal allows communication between the abdomen and perineum
• Increased abdominal pressure can cause abdominal contents to protrude through weak points in the abdominal wall, leading to hernias
• Inguinal hernia: protrusion of abdominal contents into the inguinal canal

Cryptorchidism

• Undescended testes
• Can be abdominal, inguinal, or pre-scrotal
• Require surgical correction before 18 months to 2 years
• Increases risk of infertility and testicular cancer

Posterior Abdominal Wall

• Composed of muscle (and lumbar vertebrae)
• Attached to the bony thorax, pelvis, and vertebrae
  • Superior attachment: 12th rib
  • Medial attachment: lumbar vertebrae
  • Lateral attachment: thoracolumbar fascia
  • Inferior attachment: iliac crest

Psoas Major

• Paired muscle on either side of lumbar vertebrae
• Longitudinally oriented
• Arises from the lumbar vertebral body
• Passes inferiorly and under the inguinal ligament
• Inserts onto the lesser trochanter of the femur
• Psoas minor:
  • Lines the anterior surface of psoas major
  • Smaller than psoas major
  • Not present in all individuals or on both sides

Quadratus Lumborum

• Lateral to psoas major
• Paired muscle on either side of psoas major
• Arises from the 12th rib and tips of the lumbar vertebral transverse processes.
• Inserts onto iliac crest
• Fills the space between the 12th rib and iliac crest

Iliacus

• Paired muscle
• Fills the space in the iliacus fossa of the hipbone
• Located inferior to quadratus lumborum and lateral to psoas major

Thoracolumbar Fascia

• Encloses muscles of the anterior and posterior abdominal walls (and back muscles)
• Fascia of the anterior and posterior abdominal walls meets lateral to quadratus lumborum, creating a complete muscular wall

Abdominal Quadrants

• Median plane: vertically oriented, from the xiphoid process to the pubic symphysis
• Transumbilical plane: horizontally oriented, crossing the umbilicus
• Creates 4 quadrants: left upper, right upper, left lower, right lower
• Used to describe the positioning of abdominal viscera

Peritoneum

• Innermost layer of the abdominal walls
• Parietal peritoneum: lines the abdominal wall
• Visceral peritoneum: lines the viscera
• Peritoneal cavity: potential space between parietal and visceral peritoneum
• Intraperitoneal viscus: suspended within the abdominal cavity, completely lined with visceral peritoneum
• Retroperitoneal viscus:
  • Lies against the posterior abdominal wall
  • Located outside the abdominal cavity
  • Inner surface lined with parietal peritoneum
  • Examples: kidneys, ureters, adrenal glands, aorta, inferior vena cava, pancreas (except for tail), and parts of the duodenum

Mesentery

• A double fold of peritoneum that provides support to the intestines
• Contains nerves and vessels
• Enables intestinal motility
• Not all viscera possess a mesentery

Oesophagus

• Begins at the level of C6, extending from the pharynx
• Descends vertically through the mediastinum
• Passes through the diaphragm at T10
• Enters the stomach through the cardiac orifice on the right side
• Composed of two muscle layers: inner circular and outer longitudinal
• Has several constrictions along its length, including:
  • Cervical constriction at C6, where the oesophagus begins
  • Thoracic constriction at T4/5, caused by compression from the aortic arch and trachea
  • Diaphragmatic constriction at T10, as the oesophagus passes through the diaphragm

Stomach

• Located in the left upper quadrant
• J-shaped, dilated organ
• Intraperitoneal
• Two openings:
  • Cardiac orifice: Proximal opening where the oesophagus connects
  • Pyloric orifice: Distal opening connecting to the duodenum
• Two curvatures:
  • Lesser curvature: Located on the right side
  • Greater curvature: Located on the left side
• Four distinct parts:
  • Cardia: Region near the cardiac orifice
  • Fundus: Superior portion above the cardiac orifice, holding gas
  • Body: Majority of the stomach
  • Pyloric part: Distal narrowing consisting of:
    • Pyloric antrum: Wider portion of the pyloric part
    • Pyloric canal: Narrower, more distal portion of the pyloric part
• Pyloric sphincter (pylorus):
  • Anatomical sphincter located at the end of the pyloric canal
  • Regulates the flow of chyme into the duodenum
  • Rugae (gastric folds) line the internal surface of the stomach

Small Intestine

• Hollow viscus responsible for digestion and absorption
• Comprises of three parts: duodenum, jejunum, ileum
• Contains two muscle coats: inner circular and outer longitudinal

Duodenum

• The shortest part of the small intestine
• Retroperitoneal
• C-shaped
• Located adjacent to the right kidney (laterally) and the head of the pancreas (medially)
• Contains four distinct parts:
  • 1st part (duodenal cap): Immediately after the pylorus
  • 2nd part:
    • Descending component
    • Located between the right kidney and pancreas
    • Contains the major and minor duodenal papillae
  • 3rd part: Horizontal, crosses the midline
  • 4th part: Hooks around and joins the jejunum

Jejunum

• Approximately 40% of the small intestine
• Intraperitoneal
• Attaches to the abdominal wall via the mesentery
• Found in the left upper quadrant
• Larger calibre, holding more gastric contents due to increased absorption in the jejunum
• Mucosal folds increase surface area
• Thick-walled and highly vascularized
• Less fat in the mesentery, making vessels and nerves more visible

Ileum

• Approximately 60% of the small intestine
• Intraperitoneal
• Connects to the abdominal wall via the mesentery
• Located in the right lower quadrant, repositioned during pregnancy
• Smaller calibre, holding less gastric contents due to less absorption in the ileum
• Fewer mucosal folds
• Less vascularized
• Less fat in the mesentery, making vessels and nerves less visible

Large Intestine

• Ileocecal junction: Where the ileum connects to the large intestine
• Ascending colon:
  • Retroperitoneal
  • Ascending portion of the large intestine
• Transverse colon:
  • Intraperitoneal
  • Begins at the right colic flexure near the liver
  • Traverses the midline
  • The longest part of the large intestine
• Descending colon:
  • Retroperitoneal
  • Begins at the left colic flexure near the spleen
  • Descends vertically
• Sigmoid colon:
  • Intraperitoneal
  • Bends inferiorly and medially, forming an S-shaped structure
• Sigmoid colon transitions into the rectum and anal canal

Large Intestine Wall

• Complete layer of inner circular muscle
• Outer longitudinal layer of muscles is incomplete, split into 3 bands called tenia coli
• Tenia coli extend along the entire length of the large intestine, creating a baggy appearance (haustra)
• Omental appendices are fatty tags located along the tenia coli

Caecum

• Intraperitoneal
• Blind pouch
• Located inferior to the ileocecal junction
• Situated in the right lower quadrant

Appendix

• Intraperitoneal
• Located in the right lower quadrant
• Inferior to the ileocecal junction
• Base of the appendix is located where the three tenia coli bands meet
• Position varies, but the base remains constant

Rectum

• Retroperitoneal
• Tenia coli bands merge to form a complete outer layer of longitudinal muscles
• Lack of tenia coli, haustra, and omental appendices

Liver

• Located in the right upper quadrant
• Situated beneath the right dome of the diaphragm
• Solid viscus
• Intraperitoneal
• Partially protected by the thoracic cage
• Responsible for bile production and nutrient metabolism

Diaphragmatic Surface

• Anterior and superior surface
• Where the liver sits against the diaphragm
• Smooth surface
• Partially protected by the ribcage
• Covered by peritoneum

Visceral Surface

• Posterior and inferior surface
• Interface between the liver and other visceral organs
• Concaved due to the impression of adjacent structures including the IVC and gallbladder
• Covered by peritoneum, except for the IVC and gallbladder

Portal Hepatis

• Hilum of the liver containing vessels, nerves, lymphatics, and ducts
• Portal triad: Portal vein, hepatic artery, and bile duct
• Bile duct: Carries bile from the liver to the duodenum
• Hepatic artery: Main arterial supply to the liver
• Portal vein: Carries venous blood from the GI tract, containing nutrients for liver metabolism

Hepatic Veins

• Drain venous blood from the liver
• Not located in the hilum
• Run through grooves
• Drain directly into the IVC

Lobes

• Diaphragmatic surface: Falciform ligament divides the liver into two anatomical lobes (larger right lobe and smaller left lobe)
• Visceral surface:
  • Right and left anatomical lobes
  • Fissures and grooves contain various structures:
    • Left sagittal fissure (contains ligaments)
    • Right sagittal fissure (contains the IVC and gallbladder)
  • Hilum separates the liver into two accessory lobes between the fissures: - Caudate lobe: Located above the hilum - Quadrate lobe: Located below the hilum
• Accessory lobes are actually part of the right lobe

Gallbladder

• Located on the visceral surface of the liver
• Not covered in peritoneum
• Hollow viscus
• Situated in the right upper quadrant
• Composed of four parts:
  • Fundus: Blind end
  • Body
  • Neck: Narrowing portion
  • Cystic duct
• Functions to store and concentrate bile produced by the liver
• Overconcentration of bile can lead to gallstones

Bile Pathway

• Bile travels from the liver through hepatic ducts to the common hepatic duct
• The common hepatic duct merges with the cystic duct to form the common bile duct
• The common bile duct enters the duodenum through the major duodenal papilla
• The major duodenal papilla contains a functional sphincter
• Sphincter contraction (controlled by nerves) closes, causing bile to accumulate in the gallbladder
• Gallbladder contracts, pushing bile through the cystic duct and common bile duct into the duodenum

Pancreas

• Located in the left upper quadrant
• Retroperitoneal
• Solid viscus
• Composed of four parts:
  • Head: Surrounded by the C-shaped duodenum
  • Neck
  • Body
  • Tail: Located against the spleen
• Possesses exocrine and endocrine functions
• Exocrine secretion into the duodenum:
  • Main pancreatic duct:
    • Collects pancreatic enzymes from the tail, body, neck, and head
    • Secretes into the duodenum through the major duodenal papilla
  • Accessory pancreatic duct:
    • Collects pancreatic enzymes from the head
    • Secretes into the duodenum through the minor duodenal papilla

Spleen

• Located in the left upper quadrant
• Solid viscus
• Intraperitoneal
• Associated with GI tract organs
• Largest lymphatic organ
• Rich blood supply
• Two surfaces: diaphragmatic and visceral surfaces
• Hilum is located on the visceral surface, where splenic arteries/veins enter/exit
• Situated against the diaphragm and ribs 9, 10, and 11 on the left side
• Fracture of these ribs can lead to spleen damage
• Soft and highly vascularized, enclosed by a delicate capsule
• Sharp edges of fractured ribs can lacerate the capsule, causing internal bleeding

Abdominal Aorta

• Supplies abdominal organs
• Enters the abdomen at T12
• Bifurcates into the iliac arteries at L4

Anterior Surface

• Three unpaired branches:
  • Supply unpaired viscera (GI tube)
  • Celiac trunk: Supplies the foregut (abdominal esophagus to major duodenal papilla)
  • Superior mesenteric artery: Supplies the midgut (major duodenal papilla to distal 1/3 of the transverse colon)
  • Inferior mesenteric artery: Supplies the hindgut (distal 1/3 of the transverse colon to halfway through the anal canal)
• Three paired branches:
  • Supply paired viscera
  • Suprarenal arteries: Supply the adrenal glands
  • Renal arteries: Supply the kidneys
  • Testicular/ovarian arteries: Supply the gonads

Posterior Surface

• Paired arteries supply abdominal wall structures
• Phrenic arteries: Supply the diaphragm
• Parietal arteries: Supply the anterior and posterior abdominal walls

Venous Drainage

• All abdominal venous blood drains into the IVC
• Venous blood from paired viscera and abdominal walls drains directly into the IVC
• Venous blood from unpaired viscera (excluding the liver) drains into the IVC via the portal vein

Azygos Vein

• Drains venous blood from the thorax into the superior vena cava (SVC)
• Drains both parietal and visceral components of the thorax, except for the heart
• Drains venous blood from organs supplied by the descending aorta: thoracic walls, pericardium, lungs, trachea, bronchi, esophagus, and diaphragm
• Arches over the right lung root before draining into the SVC
• Leaves an impression on the medial surface of the right lung

Abdominal Cavity

• Cavity between the thorax and pelvis
• Predominantly consists of muscle and covering structures, not bone
• Structures attach to the thorax and pelvis

Anterior Abdominal Wall

• Consists of bony framework, lining structures, and paired muscles
• Bony framework is formed by the thorax superiorly and the pelvis inferiorly, with anterior abdominal wall structures attaching to the inferior border of the thorax, iliac crest, anterior superior iliac spine (ASIS), pubic tubercle and pubic crest
• Lining structures consists of 8 layers (superior to deep): skin, superficial fascia (fat), three paired muscles (external oblique, internal oblique, transversus abdominus), transversalis fascia, extra-peritoneal fascia, and peritoneum

Lateral Flat Muscles

• Three paired layers: external oblique, internal oblique, and transversus abdominus
• Muscles are fleshy laterally and replaced by aponeuroses medially allowing for abdominal expansion
• Aponeuroses from each side meet and interdigitate forming the linea alba

External Oblique (EO) Muscle

• Superficial layer with inferomedial fiber direction
• Attaches medially to the linea alba, superiorly to the ribs, posteriorly to the thoracolumbar fascia, and inferiorly to the iliac crest and ASIS laterally, with a free central portion and medial attachment to the pubic tubercle and pubic crest
• The free, inferior portion forms the inguinal ligament, a thickened band curving under and extending between the ASIS and the pubic tubercle

Internal Oblique (IO) Muscles

• Intermediate layer with superomedial fiber direction (perpendicular to EO)
• Attaches medially to the linea alba, superiorly to the costal margin, posteriorly to the thoracolumbar fascia, and inferiorly to the lateral two thirds of the inguinal ligament, arching upward before inserting on the pubic crest via the conjoint tendon

Transversus Abdominus (TA) Muscles

• Deepest layer with transverse fiber direction
• Attaches medially to the linea alba, superiorly to the costal margin, posteriorly to the thoracolumbar fascia, and inferiorly to the lateral one third of the inguinal ligament, arching upward before inserting on the pubic crest via the conjoint tendon
• The conjoint tendon comprises two structures (IO and TA) inserting via a common tendon

Medial Vertical Muscles

• Rectus abdominus and pyramidalis muscles

Rectus Abdominus

• Located on either side of the linea alba, the primary vertical muscle supporting the anterior abdominal wall
• Originates on the pubis and inserts on the ribcage, overlapping the costal margin
• Contains three tendinous intersections which strengthen the muscle
• Enclosed by the rectus sheath, formed by aponeuroses of EO, IO, and TA muscles
  • EO aponeurosis: anterior
  • IO aponeurosis: splits and passes anterior/posterior
  • TA aponeurosis: posterior
  • Creates a fibrous compartment for the rectus abdominus

Pyramidalis Muscles

• Pyramid shaped
• Small, not involved in supporting the anterior abdominal wall

Testicular Descent

• Testes develop high in the posterior abdominal wall within extra-peritoneal fat
• Must enter the scrotum to function properly since spermatogenesis is inhibited at core body temperature (2-3 degrees Celsius lower is ideal)
• Descend to the anterior abdominal wall and leave the abdomen via the inguinal canal
• Connected to the scrotum by the gubernaculum (fibrous cord, also present in females)
• Occurs around 7 months of gestation

Inguinal Rings

• Testes leave the abdominal extra-peritoneal fat through holes in transversalis fascia and the EO aponeurosis
• Deep inguinal ring: circular hole in transversalis fascia
• Superficial inguinal ring: triangular split in the EO aponeurosis
• Path between the deep and superficial inguinal rings is oblique

Inguinal Canal

• Path the testes take to descend into the scrotum
• Passes through transversalis fascia (deep inguinal ring), TA, IO, and EO (superficial inguinal ring) layers
• Located superior to the inguinal ligament
• Boundaries: floor (inguinal ligament), roof (arching IO and TA fibers), anterior wall (EO aponeurosis and IO fibers laterally), and posterior wall (transversalis fascia and conjoint tendon medially)
• Becomes the spermatic cord after exiting the superficial inguinal ring
• Present in females (but less prominent)

Inguinal Hernia

• The inguinal canal connects the abdomen to the perineum
• Increased abdominal pressure can lead to abdominal contents protruding through the abdominal wall at points of weakness (hernia)
• Inguinal hernia is a protrusion of abdominal contents into the inguinal canal

Cryptorchidism

• Undescended testes, occurring abdominally, inguinally, or pre-scrotally
• Requires surgical correction before 18 months to 2 years old
• Can lead to infertility and increases the risk of testicular cancer

Posterior Abdominal Wall

• Predominantly composed of muscle (and lumbar vertebrae)
• Attaches to the bony thorax, pelvis, and vertebrae
  • Superiorly: 12th rib
  • Medially: lumbar vertebrae
  • Laterally: thoracolumbar fascia
  • Inferiorly: iliac crest

Psoas Major

• Paired muscle on either side of the lumbar vertebrae
• Longitudinally oriented, arising from the lumbar vertebra body
• Passes inferiorly under the inguinal ligament and inserts on the lesser trochanter of the femur
• Psoas minor: smaller, lines the surface of psoas major anteriorly, not present in all individuals or on both sides

Quadratus Lumborum

• Lateral to psoas major, paired muscle on either side of psoas major
• Arises from the 12th rib and lumbar vertebrae transverse processes, inserting on the iliac crest
• Fills the space between the 12th rib and the iliac crest

Iliacus

• Paired muscle
• Occupies the space in the iliacus fossa of the hip bone, inferior to quadratus lumborum and lateral to psoas major

Thoracolumbar Fascia

• Encloses muscles of the anterior and posterior abdominal walls and back muscles
• Fascia of the anterior and posterior abdominal walls meet lateral to quadratus lumborum
• Posterior abdominal wall muscles end where anterior abdominal wall muscles begin
• Creates a complete muscular wall

Abdominal Quadrants

• Median plane: vertical from the xiphoid process to the pubic symphysis
• Transumbilical plane: horizontal plane crossing the umbilicus
• Forms four quadrants: left upper, right upper, left lower, and right lower
• Used to describe the position of the abdominal viscera

Peritoneum

• Innermost layer of the abdominal walls
• Parietal peritoneum lines the abdominal wall
• Visceral peritoneum lines viscera
• Peritoneal cavity: potential space between the parietal and visceral peritoneum
• Intraperitoneal viscus: suspended in the abdominal cavity, completely lined with visceral peritoneum
• Retroperitoneal viscus: sits up against the posterior abdominal wall, outside the abdominal cavity, with the inner surface lined by parietal peritoneum (e.g. kidneys)

Peritoneum

• Double fold of peritoneum that provides a pathway for nerves and vessels
• Provides motility for viscera
• Not all viscera in the abdomen have a mesentery:
  • Intraperitoneal viscera have a mesentery
  • Retroperitoneal viscera do not have a mesentery

Oesophagus

• Begins at C6 from the pharynx
• Descends down the midline
• Passes through the diaphragm at T10
• Enters the stomach from the right side
• Muscular tube with inner circular and outer longitudinal muscles
• Three constrictions:
  • Cervical constriction:
    • Where the oesophagus begins
    • Anatomical sphincter
  • Thoracic constriction:
    • Bifurcation of the trachea and aortic arch compress the oesophagus from the left side
  • Diaphragmatic constriction:
    • Oesophagus passes through the diaphragm, which creates a functional sphincter

Oesophagogastric Junction

• Where oesophageal mucosa changes to stomach mucosa
• Sudden change
• Z-line

Stomach

• Located in the left upper quadrant
• Dilated J shaped, hollow viscus
• Intraperitoneal
• Two openings:
  • Cardiac orifice: proximal, where the oesophagus enters the stomach
  • Pyloric orifice: distal, where the duodenum starts
• Two curvatures:
  • Lesser curvature: right side
  • Greater curvature: left side
• Two surfaces: anterior and posterior surfaces
• Four parts:
  • Cardia: region near the cardiac orifice
  • Fundus: part above the cardiac orifice, holds gas
  • Body: majority of the stomach
  • Pyloric part: distal narrowing
    • Pyloric antrum: wider
    • Pyloric canal: narrower, more distal
• Pyloric sphincter/pylorus:
  • Anatomical sphincter
  • Located at the end of the pyloric canal
  • Controls the amount of chyme entering the duodenum
• Rugae: gastric folds/mucosa on the internal surface

Small Intestine

• Hollow viscus
• Three parts: duodenum, jejunum, ileum
• Function in digestion and absorption
• Wall comprises two complete muscle coats (inner circular and outer longitudinal muscles)

Duodenum

• Short, retroperitoneal, C shaped
• Laterally is the right kidney, medially is the head of the pancreas
• Four parts:
  • First part (duodenal cap): immediately after the pylorus
  • Second part:
    • Descending component
    • Between the right kidney and the pancreas
    • Papilla on medial wall secretes pancreatic enzymes and bile into the duodenum
      • Major duodenal papilla: secretes products from the bile duct and main pancreatic duct
      • Minor duodenal papilla: superior to major, secretes products from the accessory pancreatic duct
  • Third part: horizontal, crosses midline
  • Fourth part: hooks around and joins with the jejunum

Jejunum

• Large (proximal 40% of small intestine)
• Intraperitoneal
• Mesentery attaches the jejunum to the abdominal wall
• Left upper quadrant
• Larger caliber, holding more gastric contents as more absorption occurs in the jejunum
• Mucosal folds increase surface area
• Thick walled, highly vascularised
• Less fat in the mesentery (vessels and nerves more visible)

Ileum

• Large (distal 60% of the small intestine)
• Intraperitoneal
• Mesentery connects the ileum to the abdominal wall
• Right lower quadrant (changes during pregnancy)
• Lower caliber, holds less gastric contents as less absorption occurs in the ileum
• Less mucosal folds
• Less vascularised
• Less fat in the mesentery (vessels and nerves less visible)

Large Intestine

• Begins at the ileocaecal junction
• Four parts:
  • Ascending colon: retroperitoneal
  • Transverse colon: intraperitoneal, longest part
    • Begins at the hepatic/right colic flexure near the liver
    • Traverses the midline
  • Descending colon: retroperitoneal
    • Begins at the splenic flexure/left colic flexure near the spleen
  • Sigmoid colon: bends inferiorly and medially, intraperitoneal
    • Becomes the rectum and anal canal
• Large intestine wall:
  • Complete layer of inner circular muscle
  • Outer longitudinal layer of muscles is incomplete and split into three bands (tenia coli)
    • Bands don't completely cover the large intestine, giving it a baggy appearance (haustra)
  • Omental appendices: fatty tags along the tenia coli

Caecum

• Intraperitoneal, blind pouch
• Inferior to the ileocaecal junction
• Right lower quadrant

Appendix

• Intraperitoneal
• Right lower quadrant
• Inferior to the ileocaecal junction
• Base of the appendix is located where the three tenia coli bands meet
• Position varies, but the base is always the same

Rectum

• Retroperitoneal
• Tenia coli bands merge to form a complete outer layer of longitudinal muscles
• No tenia coli, no haustra, no omental appendices

Liver

• Right upper quadrant, sits under the right dome of the diaphragm
• Solid viscus, intraperitoneal
• Partially protected by the thoracic cage
• Function in bile production and nutrient metabolism
• Two surfaces:
  • Diaphragmatic surface: anterior and superior, smooth, partially protected by the ribcage, covered by peritoneum
  • Visceral surface: posterior and inferior, concaved, covered by peritoneum (except IVC and gall bladder)
• Portal Hepatis: hilum (holds vessels, nerves, lymphatics and ducts)
  • Portal triad: portal vein, hepatic artery, bile duct
    • Bile duct: carries bile from the liver to the duodenum
    • Hepatic artery: principle arterial supply to the liver
    • Portal vein: carries venous blood from the GI tract
• Hepatic veins: drain venous blood from the liver, not in the hilum, run through grooves, drain directly into the IVC
• Lobes:
  • Diaphragmatic surface: falciform ligament separates the liver into two anatomical lobes
    • Right lobe (larger)
    • Left lobe (smaller)
  • Visceral surface:
    • Right and left anatomical lobes
    • Fissures and grooves contain structures:
      • Left sagittal fissure: ligaments run in
      • Right sagittal fissure: IVC and gall bladder located in
• Accessory lobes:
  • Caudate lobe: above the hilum
  • Quadrate lobe: below the hilum
  • Located in the right lobe

Gall Bladder

• Located at the visceral surface of the liver, not covered in peritoneum
• Hollow viscus in the right upper quadrant
• Four parts:
  • Fundus: blind end
  • Body
  • Neck: narrowing
  • Cystic duct
• Function to store and concentrate bile
• Overconcentration of bile can lead to gallstones
• Bile Pathway:
  • Liver to hepatic ducts to common hepatic duct
  • Common hepatic duct and cystic duct merge to form the common bile duct
  • Common bile duct enters the duodenum through the major duodenal papilla
    • Major duodenal papilla has a functional sphincter
  • Contraction causes the sphincter to close, causing bile to fill and collect in the gall bladder
  • Gall bladder contracts, pushing bile through the cystic and bile ducts into the duodenum

Pancreas

• Left upper quadrant
• Retroperitoneal solid viscus
• Four parts:
  • Head: surrounded by the duodenum
  • Neck
  • Body
  • Tail: located against the spleen
• Exocrine and endocrine functions
• Exocrine secretion into the duodenum:
  • Main pancreatic duct: collects pancreatic enzymes, secretes into the duodenum via the major duodenal papilla
  • Accessory pancreatic duct: collects pancreatic enzymes from the head, secretes into the duodenum via the minor duodenal papilla

Spleen

• Left upper quadrant
• Solid viscus, intraperitoneal
• Largest lymphatic organ
• Rich blood supply
• Two surfaces: diaphragmatic and visceral surfaces
• Hilum is located at the visceral surface (where splenic arteries/veins enter/exit the spleen)
• Located against the diaphragm and ribs 9, 10 and 11 on the left side
• Fracture of these ribs can cause damage to the spleen
• Spleen is soft and highly vascularised with a delicate capsule
• Sharp edges of fractured ribs can lacerate the capsule, causing profuse bleeding into the abdominal cavity

Abdominal Aorta

• Abdominal organs are supplied by the abdominal aorta
• Enters the abdomen at T12
• Bifurcates into the iliac arteries at L4
• Anterior Surface:
  • Three Unpaired Branches:
    • Supply unpaired viscera (GI tube)
    • Celiac trunk: foregut (abdominal oesophagus to the major duodenal papilla)
    • Superior mesenteric artery: midgut (major duodenal papilla to distal 1/3 of the transverse colon)
    • Inferior mesenteric artery: hindgut (distal 1/3 of the transverse colon to ½ way through the anal canal)
  • Three Paired Branches:
    • Supply paired viscera
    • Suprarenal arteries: adrenal glands
    • Renal arteries: kidneys
    • Testicular/ovarian arteries: gonads
• Posterior Surface:
  • Paired arteries supply the abdominal wall structures
    • Phrenic arteries: diaphragm
    • Parietal arteries: anterior and posterior abdominal walls

Venous Drainage

• All abdominal venous blood drains into the IVC
• Venous blood from paired viscera and abdominal walls drains directly into the IVC
• Venous blood from unpaired viscera (except the liver) drains into the IVC via the portal vein

This quiz covers the anatomy related to the azygos vein and the gastrointestinal (GI) tract. It explores the drainage of venous blood in the thorax, the structure of the abdominal cavity, and the anterior abdominal wall framework. Enhance your knowledge of human anatomy through this engaging quiz.