Gastrointestinal Anatomy

The salivary glands are exocrine glands positioned in and around the oral cavity. These glands are responsible for secreting saliva into the mouth, which aids in digestion. Saliva helps keep the oral mucosa lubricated, and it provides antimicrobial protection. There are 3 major paired salivary glands: the sublingual, submandibular, and parotid glands. There are also hundreds of minor salivary glands found in patches around the oral cavity.

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.

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.

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.

The palate is the structure that forms the roof of the mouth and floor of the nasal cavity. This structure is divided into soft and hard palates. The palate is formed between weeks 7 and 10 of gestation, and deformities of this structure (cleft palate) are usually relevant because of its role in feeding, especially in infants.

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.

The lips are the soft and movable most external parts of the oral cavity. The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. Together, these structures play an important role in each of these vital processes. The blood supply of the lips and tongue originates from the external carotid artery, and the innervation is through cranial nerves.

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.

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.

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.

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.

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.

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.

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.