USMLE First Aid Organ Systems GI

Primitive GI begins to form around which week in embryological development?
4th week

In the 4th week, the endoderm gives rise to
intestinal epithelium and glands

In the 4th week, mesoderm gives rise to
connective tissue, muscle, and wall of intestine

5th Week of Embryological Development
intestine elongates and midgut loop herniates through umbilicus.

Names of limbs of midgut loop and what each limb gives rise to
cranial limb becomes distal duodenum and proximal ileum
caudal limb becomes distal ileum to proximal 2/3rds of transverse colon

What is a key event in the 10th week of development?
midgut loop returns to abdominal cavity in its final fixed position after rotating a total of 270 degrees counterclockwise around axis of the superior mesenteric artery

Omphalocele
Failure of bowel to return to the abdomen following herniation of abdominal contents into base of umbilical cord during 5th week of embryonic life. Herniated intestine is covered by peritoneal membrane. Most commonly affects children of mothers of extremes of reproductive age.

Presentation of Omphalocele
2nd trimester ultrasound (95% of cases) or herniated sac found at birth

Associated anomalies of Omphalocele
Beckwith-Wiedemann syndrome: Gigantis, macroglossia, umbilical defect, and hypoclycemia
Pentalogy of Cantrell: Omphalocele, diaphragmatic hernia, cleft sternum, absent pericardium, and intracardiac defects

Diagnosis of Omphalocele
alpha-Fetoprotein (AFP) serum values are elevated in 70% of cases

Gastroschisis
Full-thickness abdominal wall defect caused by vascular injury during development allowing small or large bowel to escape abdominal cavity. No protective peritoneal membrane covers herniated intestine. Most common in children born to women younger than 20.

Presentation of Gastroschisis
Similar to omphalocele. Extruded abdominal contents are usually right of the abdominal midline

Treatment of Gastroschisis
Artificial covering may be used to minimize heatfluid loss and assist with temperature regulation (exposed bowel causes increased heat loss)

Ladd’s Bands
Aberrant peritoneal attachments that causes partial or complete obstruction of the duodenum due to malrotation during development

Midgut Volvulus
Malrotated intestine twists on axis of SMA, compromising intestinal blood flow

Presentation of Midgut Volvulus
May occur at any time during 1st year of life and presents with sudden onset of severe bilious emesis, abdominal pain and distention, and rectal bleeding

Bilious Emesis
presumed to be midgut volvulus until proven otherwise

Treatment of Malrotation
Ladd’s procedure

Duodenal Atresia
In weeks 6 and 7 of development, duodenum becomes completely obstructed by proliferating endoderm. Failure of duodenum to recanalize by week 10 results in duodenal atresia.

Presentation of Duodenal Atresia
Newborn may be small for gestational age. Vomiting (often bilious) and abdominal distention within 48 hours after birth.

Pyloric Stenosis
Idiopathic congenital elongation and thickening of pylorus, resulting in obstruction of gastric outlet.

Presentation of Pyloric Stenosis
Nonbiliious projectile vomiting at 3-6 weeks but notable absence of other signs of illness (fever, diarrhea)

Diagnosis of Pyloric Stenosis
Palpable mass size of an olive in epigastric region

Margins of Abdomen
Diaphragm superiorly, inlet of pelvis inferiorly

Surface Landmarks of Abdominal Wall
Surface Landmarks of Abdominal Wall

Linea Alba
Vertical fibrous band that extends from symphysis pubis to xiphoid process and lies in midline. Formed by fusion of aponeuroses of muscles of anterior abdominal wall that is represented by a median groove.

Linea Seminlunaris
Lateral edge of rectus abdominis muscle and crosses costal margin at tip of 9th costal cartilage

Inguinal Groove
Formed by rolled-under margin of aponeurosis of external oblique muscle

Transpyloric Plane
Cuts through pylorus horizontally and passes through tips of 9th costal cartilages on each side. Lies at L1.

What does the transpyloric plane identify?
Pylorus of stomach, duodenojejunal junction, neck of pancreas, and hila of kidneys

Subcostal Plane
Horizontal plane joining lowest point of costal margin on each side. Lies at level L3.

Intertubercular Plane
Horizontal plane joining tubercles on iliac crests. Lies at level L5.

McBurney’s Point
RLQ at 2/3rds distance from umbilicus to anterior superior iliac spine. Pain in this location indicates appendicitis.

Abdominal Regions and Contents
Abdominal Regions and Contents

Right Hypochondriac Region
Liver and gallbladder
(Kidney) and suprarenal gland
Colon, hepatic flexure

Epigastric Region
Liver
(Transverse colon)
Abdominal aorta and vena cava
Pylorus and duodenum (first part)

Left Hypochondriac Region
Stomach
Spleen
(Kidney) and suprarenal gland
Colon, splenic flexure

Right Lumbar Region
Kidney
Colon, ascending
Small intestine
Duodenum (first part)

Umbilical Region
(Transverse colon)
Duodenum and pancreas
Abdominal aorta and vena cava
Small intestine
Iliac vessels

Left Lumbar Region
Kidney
Colon, descending
Pancreas
Small intestine (jejunum)

Right Iliac region
Cecum
Appendix
Small intestine (ileum)

Hypocastric Region
Distensible organs of pelvis (bladder/uterus)
Small intestine
Iliac vessels
Spermatic cords

Left Iliac Region
Sigmoid colon
Small intestine

Layers of Anterolateral Abdominal Wall
Skin
Superficial fascia (fatty Camper’s and membraneous Scarpa’s)
Deep fascia
Aponeurosis
External oblique muscle – lateral wall
Internal oblique muscle – lateral wall
Transversus abdominis muscle – lateral wall
Transversalis fascia
Extraperitoneal fat
Parietal peritoneum

Abdominal Quadrants and Contents
Abdominal Quadrants and Contents

Inguinal Canal
Site of inguinal hernias

Boundaries of inguinal canal
Deep inguinal ring
Superficial inguinal ring
Anterior wall
Posterior wall
Roof
Floor

Patent communication between abdominal cavity and scrotal sac
Failure of processus vaginalis to obliterate

Boundaries of Femoral Triangle
Superiorly: Inguinal ligament
Laterally: Sartorius muscle
Medially: Adductor longus muscle

Femoral Canal
Femoral Canal

Third Spacing
Non-intravascular space where fluid and edema can accumulate

Common causes of ascites
Liver failure, right heart failure, ovarian cancer

Pneumoperitoneum
air or other gas in peritoneal cavity (intestinal/stomach perforation, or intentional insufflation for laparoscopy)

Parietal Peritoneum
Outer membranes that lines deep surface of abdominal walls and inferior surface of diaphragm

Nerve Supply of Parietal Peritoneum
Nerves of surrounding abdominal muscles and skin
Intercostal and phrenic nerves in abdominal region
Obturator nerve in pelvic region

Visceral Peritoneum
Membrane that directly covers abdominal organs. No somatic nerve supply.

Intraperitoneal Viscera
Stomach
First part of duodenum
Jejunum, ileum, cecum, and appendix
Transverse and sigmoid colon
Proximal rectum
Liver and gallbladder
Tail of pancreas
Spleen

Extraperitoneal Viscera
Parts 2, 3, and 4 of duodenum
Ascending and descending colon
Distal rectum
Head, neck, and body of pancreas
Abdominal aorta
Inferior vena cava
Kidneys, ureters, and adrenal glands

Lesser Sac (Omental Bursa) of Peritoneal Cavity
Posterior to stomach, liver, and lesser omentum

Epiploic Foramen (Winslow’s Foramen)
Enables communication between greater and lesser sac of peritoneal cavity

Greater Sac
Subdivided by transverse mesocolon into supracolic compartment and infracolic/pelvic compartments below mesocolon

Supracolic Compartment
Divided by falciform ligament

Subphrenic Recess
Area to right of falciform ligament between liver and kidney

Mesentary
Double layer of peritoneum that wraps around abdominal organs, includes blood vessels, and attaches organ to its major blood supply

Gradual occlusion of abdominal aorta can result in
Claudication and impotence

3 Main Branches of Celiac Trunk
Supply structures derived from foregut:
Left gastric
Splenic
Common hepatic

Abdominal Aorta and Its Branches
Abdominal Aorta and Its Branches

Celiac Trunk
Celiac Trunk

Superior Mesenteric Artery
Supplies derivatives of midgut

Inferior Mesenteric Artery
Supplies hindgut derivatives, distal 3rd of transverse colon, descending colon, sigmoid colon, superior portion of rectum

Leave a Reply

Your email address will not be published. Required fields are marked *