Embryological development

The primitive gut forms during the fourth week of gestation from the dorsal section of the yolk sac. It begins as a hollow tube arising from the endoderm, which then forms the foregut, midgut, and hindgut. The foregut develops into the esophagus, stomach, upper portion of the duodenum (bile duct entrance), liver, biliary system, and pancreas. It is perfused by the celiac artery. The midgut develops into the distal duodenum and the remainder of the small intestine, cecum, appendix, the ascending colon, and most of the proximal portion of the transverse colon and is perfused by the superior mesenteric artery. The hindgut develops into the remaining transverse colon, the descending colon, the sigmoid colon, the rectum, and the superior portion of the anal canal and is perfused by the inferior mesenteric artery.

By the end of the sixth week of gestation, the gut herniates outside of the abdominal cavity, where it rotates 90 degrees counterclockwise and continues to elongate. By the tenth week of gestation, the gut returns to the abdominal cavity and rotates another 180 degrees counterclockwise. With the normal intestinal rotation, the stomach and pancreas rotate into the left upper quadrant and are pressed against the dorsal abdominal wall to fuse into position.

The pancreas arises from ectodermal cells from the most caudal part of the foregut and develops into dorsal and ventral buds. The dorsal bud is larger and becomes the major portion of the pancreas. The dorsal and ventral buds fuse to form the main pancreatic duct. Secretion of insulin begins around the twentieth week of gestation. Up until the fourteenth week, the spleen is only a hematopoietic organ. Between weeks 15 and 18, the spleen then loses its hematopoietic function and transforms into an organ of the immune system.

The liver begins as a bud that develops on the distal part of the foregut and grows into the septum transversum, where it divides into two parts. The larger part develops into the right and left lobes of the liver, and the second smaller division of the hepatic bud develops into the biliary system. Hematopoiesis begins at the sixth week of gestation and is responsible for the large size of the liver. It is approximately 10% of the total weight of the fetus. Bile begins to form at 16 weeks of gestation, giving meconium the dark green color.

Development of the kidney begins with a primitive, transitory structure called the pronephros, or forekidney, which arises near the segments of the spinal cord. These segments appear early in the fourth week of gestation on either side of the nephrogenic cord. The pronephros itself soon degenerates but leaves behind its ducts for the next kidney formation, the mesonephros, or midkidney, to utilize. In the fifth week, the metanephros, or hindkidney, begins to develop and becomes the permanent kidney. By the eighth week, the hindkidney begins to produce urine and continues to do so throughout the fetal period.

The adrenal glands develop from the medulla, which originates from the neuroectoderm. At the seventh week of gestation, the medulla attaches to the fetal cortex, which develops from the mesoderm, and by the eighth week, the fetal cortex begins to encapsulate the medulla. The fetal adrenal gland is 20 times larger than the adult adrenal and is large compared to the kidneys. However, the adrenals rapidly decrease in size as the fetal cortex regresses and completely disappear by 4 years of age and are replaced by the adult cortex.

Anatomy and physiology

The abdomen is the area of the torso from the diaphragm to the pelvic floor and is lined by the peritoneum, a serous membrane covering the abdominal viscera (Figure 14-1). The membrane of the peritoneum creates a smooth, moist surface that allows the abdominal viscera to glide freely within the confines of the abdominal wall.

The liver lies immediately below the right diaphragm and is the largest and heaviest organ in the body. It is composed of the right and left hepatic lobes and is an extremely vascular organ. The liver is perfused by the hepatic artery, which arises from the abdominal aorta, and the portal vein, which delivers blood from the spleen, pancreas, and intestines. The liver is responsible for metabolizing carbohydrates, fats, and proteins. It also breaks down toxic substances and drugs; stores vitamins and iron; produces antibodies, bile, prothrombin, and fibrinogen for coagulation; and excretes waste products. The hepatic veins then return blood to the vena cava. Within the inferior surface of the liver lies the gallbladder, a saclike organ. The liver excretes bile into the hepatic duct, which is then collected and stored in the gallbladder. Next bile is secreted into the duodenum via the cystic duct and the common bile duct to aid in the digestion of fats.

Below the left diaphragm, from posterior to anterior respectively, lie the spleen, pancreas, and stomach. The spleen is a concave organ made mostly of lymphoid tissue that lies around the posterior fundus of the stomach. The spleen filters and breaks down red blood cells and produces white blood cells (lymphocytes and monocytes). It also stores blood that can be released into the vascular system during an acute blood loss. The pancreas is nestled between the spleen and stomach and crosses the midline over the major vessels. The pancreatic head extends to the duodenum and the tail reaches almost to the spleen. It is responsible for production of enzymes needed for the metabolism of proteins, fats, and carbohydrates; these enzymes are excreted into the duodenum via the pancreatic duct. The pancreas also produces insulin and glucagon, which are secreted directly into the bloodstream to help regulate blood glucose levels. The stomach is the most anterior organ in the left upper quadrant of the abdomen. It is connected proximally to the esophagus, which enters through the diaphragm at the esophageal hiatus. The stomach receives food from the esophagus through the lower esophageal sphincter. It secretes hydrochloric acid and digestive enzymes used to metabolize proteins and fats. When the stomach is distended, it is stimulated to contract and expel its contents through the pyloric sphincter into the duodenum, the first portion of the small intestine.

The duodenum is C-shaped and curls around the head of the pancreas. The pancreatic and bile ducts empty into the upper portion of the duodenum. The duodenum then transitions to the jejunum, which is responsible for the majority of the absorption of water, proteins, carbohydrates, and vitamins. The ileum composes the last and longest part of the small intestine and absorbs bile salts, vitamins C and B₁₂, and chloride. The intestinal contents leave the ileum through the ileocecal valve and empty into the cecum, located in the right lower quadrant of the abdomen, which is the beginning of the large intestine. The appendix, a long, narrow tubular structure, arises from the base of the cecum. The large intestine lies anteriorly over the small intestine, ascends along the right anterior abdominal wall and forms the ascending colon, traverses across the abdomen to the splenic flexure forming the transverse colon, and descends along the left lateral abdomen wall as the descending colon (Figure 14-2). At the level of the iliac crest, the colon becomes the S-shaped sigmoid colon. It descends into the pelvic cavity and turns medially to form a loop at the level of the midsacrum. The sigmoid colon connects to the rectum, which lies behind the bladder in males and the uterus in females. It stores feces until it is expelled through the anal canal and out the anus, which is located within a ring of nerves and muscle fibers midway between the tip of the coccyx and the scrotum or vaginal fourchette. The anal canal and anus remain closed involuntarily by way of a ring of smooth muscle, the internal anal sphincter, and voluntarily by a ring of skeletal muscle, the external anal sphincter.

The kidneys lie on either side of the vertebral column in the retroperitoneal space below the liver and spleen. The right kidney tends to be lower than the left because it lies below the right lobe of the liver. Kidneys have a lobulated appearance at birth, which disappears with the development of the glomeruli and tubules in the first year of life. The kidneys are perfused by the renal arteries and filter and reabsorb water, electrolytes, glucose, and some proteins. They regulate blood pressure, electrolytes, and the acid-base composition of blood and other body fluids; actively excrete metabolic waste products; and produce urine. The kidneys are capped by the adrenal glands, pyramid-shaped organs that synthesize, store, and secrete epinephrine and norepinephrine in response to stress. The adrenals also produce the corticosteroids, which affect glucose metabolism, electrolyte and fluid balance, and immune system function.

Urine is excreted from the kidney into the ureters, long, thin muscular tubules that transport urine to the bladder. The ureters connect to the superior pole of the renal pelvis. They descend posteriorly to the peritoneum and slightly medially in front of the psoas major muscle into the pelvic cavity and implant into the superior posterior wall of the urinary bladder. The oblique insertion of the ureters through the bladder wall creates a one-way valvular mechanism that prevents the reflux of urine. The urinary bladder lies anterior to the uterus in females and anterior to the rectum in males. When filled to its capacity, the bladder then contracts and releases urine through the bladder neck and out the urethra. The urethra is normally located at the tip of the penis in males and between the clitoris and vagina in females. In nonpregnant females, the reproductive organs lie within the pelvis between the bladder (anterior pelvis) and the rectum (posterior pelvis). They include the ovaries, uterine or fallopian tubes, and uterus. These organs descend into the pelvic cavity during normal growth and development and ascend into the abdominal cavity during pregnancy, or with ovarian cysts or other abnormalities of the female reproductive system.

Finally, a layer of fascia and then muscle cover the anterior abdomen. The rectus abdominis muscle extends the entire length of the front of the abdomen and is separated by the linea alba in the midline. The transverse abdominis and internal and external oblique muscles cover the lateral abdomen. The umbilicus lies in the midline usually below the midpoint of the abdomen (Figure 14-3).

Physiological variations

Many aspects of the development of the organs and structures in the abdomen continue into the first few years of life. The muscle tone of the lower esophageal sphincter is not fully developed until 1 month of age, and may remain slightly weak for the first year. The stomach is round until approximately 2 years of age and then elongates into its adult shape and position by about 7 years of age. Stomach capacity is also smaller in the infant whereas emptying time is faster, which results in the pattern of small and frequent feedings. The small bowel grows from approximately 270 cm at birth to up to 550 cm by 4 years of age. The liver constitutes 5% of the term neonate’s body weight, versus 2% in the adult. The kidneys also remain relatively large for the size of the abdomen until adolescence.

System-specific history

Information gathering for the assessment of the abdomen should include questions regarding diet, elimination, medications, environmental exposures and a thorough psychosocial history (see Information Gathering table). An assessment of the menstrual cycle in the female and history of sexual activity in adolescents is also essential (see Chapter 17).

When a complaint of abdominal pain is reported, the examiner must elicit a detailed history of the pain. Information regarding the character and severity of the pain, onset and duration, location or radiation, position of comfort, things that alleviate or worsen the pain, history of trauma, and any associated symptoms of fever, vomiting, anorexia, constipation, or diarrhea is important in narrowing the scope of the differential diagnosis. A detailed history can help in determining whether the abdominal pain is acute or chronic. Remember that abdominal pain can be referred from an extra-abdominal source or can be a condition associated with systemic disease. For example, abdominal pain is common in children with beta-streptococcal pharyngitis, lower lobe pneumonia, sickle-cell anemia, cystic fibrosis, Henoch-Schönlein purpura, and many other conditions. See Box 14-2 for differential diagnosis of symptoms or conditions related to different abdominal regions.