Lecturio: Study for Medical School, USMLE, MCAT & MBBS

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The posterior abdominal wall is a complex musculoskeletal structure that houses the abdominal aorta, the inferior vena cava, as well as important retroperitoneal organs, like the kidneys, renal glands, pancreas, and duodenum. This vital anatomical structure consists of the posterior abdominal muscles, their respective fascia, lumbar vertebrae, and the pelvic girdle. The structure is supported by 12th thoracic rib, lumbar vertebrae, and pelvic rim.

Anterior Abdominal Wall

The anterior abdominal wall is anatomically delineated as a hexagonal area defined superiorly by the xiphoid process, laterally by the midaxillary lines, and inferiorly by the pubic symphysis. From the superficial to deep order, the anterior abdominal wall consists of the skin, subcutaneous tissue, muscle, transversalis fascia, and peritoneum. The lateral abdominal muscles include the external and internal obliques and the transversus abdominis. Anterior abdominal muscles include the rectus abdominis and pyramidalis muscles. The abdominal wall is primarily supplied by epigastric arteries and innervated by thoracoabdominal nerves.

Urinary Tract

The urinary tract is located in the abdomen and pelvis and consists of the kidneys, ureters, urinary bladder, and urethra. The structures permit the excretion of urine from the body. Urine flows from the kidneys through the ureters to the urinary bladder and out through the urethra. The bladder acts as a reservoir for urine until micturition is appropriate. Regulation of micturition relies on the CNS and the spinal cord.

Prostate, Seminal, and Bulbourethral Glands: Anatomy

The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. The prostate consists of multiple lobes and is made up of glandular and fibromuscular tissue. The glandular tissue has ducts that empty into the prostatic portion of the urethra, and the fibromuscular tissue encircles the urethra.

Inguinal Canal and Hernias

The inguinal region, or the groin, is located in the RLQ and LLQ of the anterior abdominal wall, bordered by the thigh inferiorly, the pubis medially, and the iliac crest superolaterally. The inguinal canal is a tubular structure that runs in a straight line from the anterior superior iliac spine to the pubic tubercle. The canal contains the spermatic cord in men and the round ligament in women. This region is clinically relevant, as it is the site for the most common type of hernias, such as indirect and direct inguinal hernias. Patients with hernias will present with a unilateral bulge in the groin that may be associated with pain. In symptomatic or high-risk cases, hernias can be repaired surgically.

Kidneys: Anatomy

The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. The kidneys also play a major role in homeostatic processes, including electrolyte concentration, blood pressure, and acid–base regulation. Grossly, they consist of an outer cortex and inner medulla. Microscopic functional units known as nephrons filter the blood through a structure called the glomerulus, and this filtrate is then modified and concentrated as it moves through a complex tubular system. The renal arteries supply the kidneys via a central opening, known as the renal hilum, on its medial side; large renal veins empty directly into the vena cava.

Spleen

The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th‒11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. The spleen can also activate immune responses, produce antibodies, and function as a reservoir for platelet storage. There are 2 primary types of splenic tissue: red pulp, which contains dense fibrovascular networks for filtering the blood, and white pulp, which is primarily made up of lymphoid tissue surrounding the larger vessels. The spleen has a relatively weak capsule; thus, it can rupture more easily than other abdominal organs and lead to life-threatening hemorrhage.

Peritoneum and Retroperitoneum

The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). The peritoneum supports and suspends the organs within the abdominal cavity and provides an important conduit for the neurovasculature supplying these organs. There are several peritoneal folds, known as mesenteries, omenta, and ligaments. The greater and lesser omenta divide the peritoneal cavity into greater and lesser sacs, which are important anatomic spaces within the cavity. Organs located behind the posterior parietal peritoneum are known as retroperitoneal, while organs that protrude into the cavity and are fully covered by visceral peritoneum are known as intraperitoneal.

Rectum and Anal Canal

The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal incontinence can occur if this function is disturbed. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Peristaltic waves within the rectal muscularis, involuntary relaxation of the internal anal sphincter (controlled by the ANS), and voluntary relaxation of the external anal sphincter (controlled by the cerebral cortex) are essential for defecation to occur. The rich plexus of veins surrounding the anal canal can develop into hemorrhoids if dilated.

Pharynx

The pharynx is a component of the digestive system that lies posterior to the nasal cavity, oral cavity, and larynx. The pharynx can be divided into the oropharynx, nasopharynx, and laryngopharynx. Pharyngeal muscles play an integral role in vital processes such as breathing, swallowing, and speaking. The muscles of the pharynx receive innervation from the vagus and glossopharyngeal nerve to propel food from the oral cavity into the esophagus.

Penis

The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis, which are the extremities of the corpora cavernosa. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum, which sits beneath the corpora cavernosa and through which the urethra passes. The glans is ensheathed by the prepuce or foreskin. The penis is homologous with the clitoris in females.

Pancreas

The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. The pancreas is covered with a very thin connective tissue capsule that extends inward as septa, partitioning the gland into lobules. This organ has both exocrine and endocrine tissue. The exocrine portion is organized in grape-like clusters of acini, which are small sacs surrounding the terminal ends of pancreatic ducts. The cells lining the acini and ducts secrete products that make up pancreatic juices, which play a major role in digestion. The endocrine portion of the gland consists of circular islets interspersed between acini, which secrete glucagon, insulin, and somatostatin.

Adrenal Glands

The adrenal glands are a pair of retroperitoneal endocrine glands located above the kidneys. The outer parenchyma is called the adrenal cortex and has 3 distinct zones, each with its own secretory products. The outer zona glomerulosa secretes mineralocorticoids (primarily aldosterone); the middle zona fasciculata secretes glucocorticoids (primarily cortisol); and the innermost zona reticularis secretes androgens. Beneath the cortex lies the adrenal medulla, which secretes catecholamines involved in the fight-or-flight response. The medullary cells function more like neurosecretory postsynaptic neurons rather than traditional endocrine cells. The adrenal glands have a rich vascular supply and complex relationships with their surrounding organs.

Stomach

The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. On a microscopic level, the stomach wall has several layers, including a mucosa, submucosa, muscularis, and serosa. The stomach is filled with glands that secrete a variety of substances involved in the digestive process. The arterial supply to the stomach is primarily from vessels originating from the celiac trunk.

Small Intestine

The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. The small intestine is divided into 3 segments: the duodenum, the jejunum, and the ileum. Like the entire GI tract, the walls of the small intestine have several layers: an inner absorptive mucosal layer (which is made up of an epithelium, lamina propria, and muscularis mucosa) and submucosal, muscular, and serosal layers. The arterial supply to the small intestine is via branches of the superior mesenteric artery, and veins drain into the hepatic portal system. The small intestine is innervated by the ANS.

Gallbladder and Biliary Tract

The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Bile is secreted by hepatocytes into thin channels called canaliculi. These canaliculi lead into slightly larger interlobular bile ductules, which are part of the portal triads at the “corners” of hepatic lobules. The bile leaves the liver via the right and left hepatic ducts, which join together to form the common hepatic duct. The common hepatic duct joins with the cystic duct to form the common bile duct, which empties into the small intestine. If the sphincters leading into the intestines are closed, bile will travel via the cystic duct into the gallbladder for storage.

Esophagus

The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. The wall of the esophagus is made up of 4 primary layers: mucosa (lined with squamous epithelium), submucosa, a thicker muscularis layer, and an outer layer of connective tissue. The esophagus also has a sphincter at each end, which allows it to help control the passage of food into the stomach.

Colon, Cecum, and Appendix

The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. The colon also contains many mucus-secreting glands to lubricate the stool passing through it. The colon receives its blood supply from colic branches of the superior and inferior mesenteric arteries, which form an important anastomosis along the transverse colon. The colon is regulated by the ANS and receives both sympathetic (inhibitory) and parasympathetic (stimulatory) input.

Liver

The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism (carbohydrates, proteins, lipids, hormones), nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. The liver can be divided into 4 lobes or 8 segments. Microscopically, it is divided into hepatic lobules. Its main neurovascular bundle is found within the transverse fissure of the liver, also called the porta hepatis.

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Abdomen

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Your Educators of course Abdomen

James Pickering, PhD

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