Anatomy Relevant to the Gastrointestinal System

by Adam Lawson BA, MSc and Terra Doucette Hiller BA, BSN, RN

The gastrointestinal system is important for the absorption of nutrients to provide for cells and growth of the body, as well as to eliminate wastes. The gastrointestinal system provides five functions: ingestion, motility, digestion, absorption and elimination.

Upper Gastrointestinal System

The four main structures of the upper gastrointestinal system include:

Mouth - performs the first steps of mechanical and chemical digestion.
- Secretions
  - Saliva - lubricates and binds food particles in preparation for swallowing. The medulla signals production in the exocrine salivary glands via cranial nerve VII and IX.
  - Lingual lipase - secreted by tongue glands and activated by the pH of the stomach to start the digestion of fat. It may ultimately digest up to 30% of the ingested dietary fat.
  - Salivary amylase - a component of saliva that breaks down starch.
  - IgA antibodies - provides defense against pathogens.
- Function and Motility
  - The teeth mechanically break down food into smaller particles. Muscles of mastication reposition the mandible:
    1. Click on the "muscles of mastication" link again.
    2. In the Systems tab, go to "Skeletal System" to add the bones of the cranium
    3. Rotate 90° and examine attachment of the muscle to the bones.
    4. Increase the skin opacity to 100%
  - The tongue repositions food within the mouth during mastication and helps convey the resultant bolus into the posterior pharynx. It is composed of several muscles:
  - The presence of food or fluid in the pharynx stimulates sensory receptors, and cranial nerve V signals the swallowing center in the medulla. The respiratory center is momentarily suppressed as signals are sent through cranial nerve IX and X to coordinate the movement of food into the esophagus.
    Damage to the sensory (CN V), motor (CN IX and CN X), or medulla can eliminate the ability to swallow or cause poorly coordinated swallowing; food or fluid can enter the nasopharynx, larynx, or follow other inappropriate pathways. Swallow studies following trauma are important for preventing aspiration pneumonia due to uncoordinated swallowing.
Esophagus
- Secretions
  - Mucus - lubricates the passage of the bolus and protects the esophageal wall.
- Function and Motility:
  - The esophagus contracts in rhythmic waves to transfer boluses from the mouth to the stomach. Primary peristalsis performs a single rhythmic wave toward the cardiac sphincter. If food or fluid gets caught in the esophagus, secondary peristalsis will continue to rhythmically force the food downward by relaxing the muscle below the distended area.
  - The contracted cardiac sphincter prevents food and acid from re-entering the esophagus.
    
    Increase the skin opacity to 80% and note the location of the cardiac sphincter.
    
    Move through several cross-sections (between 1365 & 1450) too see the wrinkling of the muscular esophageal mucosa.
    
    If the cardiac sphincter is malfunctioning (e.g., in "heart burn"), the symptoms are often mistaken to be signs of a heart attack; the esophagus is immediately posterior to the heart.
    
    Examine several cross-sections near cross-section 1445 and note how close the esophagus is to the heart.
Stomach
- Secretions
  - Gastrin - stimulates the production of hydrochloric acid and pepsinogen.
  - Hydrochloric acid (HCl) - denatures proteins
  - Intrinsic factor - allows absorption of vitamin B₁₂.
  - Pepsinogen - converts to pepsin in the presence of HCl to break down proteins into peptides.
  - Mucus - protects the stomach lining from autodigestion.
  Individuals in critical care are at risk of developing gastric or duodenal ulcers because stress causes an excess release of acid. A prophylaxis dose of an acid reducer drug (e.g., esomerperozole or famatodine) may be prescribed to prevent such ulcers from developing. Sucrafate may also be prescribed to maintain the integrity of the mucosal lining of the stomach.
- Function and Motility
  - Peristaltic contractions further mix the food and conduct the gastric contents toward the pylorus.
  - The pyloric sphincter prevents backflow of contents from the duodenum back into the stomach. Enzymes released in the duodenum can damage the lining of the mucosal wall of the stomach.
  In the cross-section window, look for folds in the area of the lower stomach and pylorus. These folds regulate the entry of chyme into the duodenum.
  
  Nasogastric Tube Placement
  (Click to view animation)
  
  A nasogastric tube (NGT) is often used to feed a patient when they are having difficulty swallowing, or to suction gastric contents when there is a back up of intestinal contents due to bowel obstruction or some other pathology.
  
  The tube is inserted through the nose and advanced down into the stomach, but it is often inadvertently advanced into the lungs instead. It is important to obtain an x-ray before using the tube for giving fluids or suctioning. Refer to the animation to see the correct and incorrect NGT placement.
Duodenum
- Secretions
  - Pancreatic enzymes - further breakdown proteins and carbohydrates; degrades fats into fatty acid chains and monoglycerides.
  - Bile - emulsifies fat to allow efficient use of the lipase enzyme.
- Function and Motility
  - Chyme distends the duodenum and stimulates peristalsis.
  - The hepatopancreatic ampulla empties into the duodenum (this will be discussed in more detail later).

Lower Gastrointestinal System

The lower gastrointestinal system comprises the small intestine, large intestine, and anus. It can be subdivided into 5 sections that absorb different nutrients and excrete the waste.

Jejunum
- Absorption: Amino acids, small peptides, vitamins (B₁, B₂, niacin, folic Acid, B₆, pantothenic acid), and most glucose and fructose.
- Function and Motility: Segmentation contractions of the jejunum further mix digestive enzymes with the chyme and help to expose all molecules to the intestinal mucosa.
Segments can be best seen in the cross-section. The segments have the appearance of "linked sausage."
Ileum
- Absorption: Vitamins (C and B₁₂), bile salts, fatty acids and any nutrients which were not absorbed by the jejunum.
- Function and Motility
  - Final stages of digestion and absorption of proteins and carbohydrates.
  - Peristaltic contractions of longitudinal smooth muscle convey the chyme toward the colon.
Jejunum vs. Ileum

Look in the cross-section. Finger-like projections called plicae line the walls of the jejunum and ileum. There are longer plicae in the jejunum to provide greater surface area for nutrient absorption. As chyme travels toward the colon, less nutrients are absorbed; the plicae become smaller and fewer in number.
Colon
- Absorption
  - Water and potassium are absorbed from chyme in the large intestine.
  - Diarrhea reduces absorption time, limiting potassium and water reabsorption; hypokalemia and dehydration may result from severe cases.
- Function and Motility
  - The colon contains essential colonies of bacteria which develop over time. Some species produce vitamin K and/or B vitamins. Some species also produce ammonia, which is then absorbed into the bloodstream and converted by the liver into uric acid. An ammonia build-up, due to a dysfunctional liver, may lead to hepatic encephalopathy.
Note how the sigmoid colon descends toward the rectum on the left side of the body. Laying a patient on their right side will allow gravity to work on this section of the colon and may help induce bowel movement.
Rectum
- Function and Motility
  - Filling of the rectum triggers the defecation reflex by stimulating stretch receptors in the rectal wall. Afferent nerve fibers send impulses to the lower spinal cord. Reflex signals stimulate somatic motor neurons that innervate the skeletal muscle of the external anal sphincter. Signals are also sent through parasympathetic motor fibers which innervate the sigmoid colon, rectum and internal sphincter.
Anus
- Function and Motility: The external anal sphincter, internal anal sphincter and the puborectalis act as a unit to maintain fecal continence.
"Neurogenic bowel" is used to describe colon dysfunction resulting from lack of nervous control. This can occur in individuals with spinal cord injuries, amyotrophic lateral sclerosis, multiple sclerosis, spina bifida and neuropathy. Loss of nervous control of the bowels can lead to fecal incontinence; the internal sphincter will remain tight, and only the local stretch reflex will trigger a bowel movement. Digital stimulation is often performed to ensure that bowel movement occurs at a socially acceptable time.

In digital stimulation, a finger is moved in a circular motion around the edges of the anus in an attempt to relax the external sphincter and expel any feces present in the rectal vault. It is important not to advance the finger too far within the rectum because the circular motion has the potential to damage the external sphincter.
The next link will display the visible human female database because the visible human male does not have an appendix.
The appendix is located in the right lower quadrant.
- Function: Immunological
  - The appendix has been found to be rich in lymphoid cells and plays a role in supporting beneficial bacteria in the colon.
Increase the skin opacity to about 80% to see how the appendix is related to the skin surface.

Abdominal Quadrant Auscultation
(Click to view animation)

Accessory Digestion Organs

There are four organs known as accessory digestion organs:

Salivary glands - include the parotid glands, submandibular glands, and sublingual gland.
1. Click on the 'salivary glands' link and rotate to 90°
2. Add the 'Bones of Cranium' under the Systems tab to see where the glands are in relation to the jaw bone.
3. Increase the skin opacity to 80% to see where the glands are underneath the skin.
- Function: Saliva produced by these glands enters the mouth via the parotid ducts.
Liver
Rotate the man to 165° to see the liver from a posterolateral angle. Right-click-dissect the right 10th, 11th, and 12th false ribs.
- Function
  - Bile, which is produced by the liver, collects from the vessels in both lobes and flows into the left and right hepatic ducts.
  - The right and left hepatic ducts converge to form the common hepatic duct.
  - The common hepatic duct divides into two different ducts:
    - Cystic duct - sends bile to the gallbladder for storage
    - Bile duct - sends bile to the hepatopancreatic ampulla to drains into the duodenum.
    The bile duct can be best seen from the back, this is the view you are currently looking at.
    
    Look at the cross-section below of the hepatopancreatic ampulla draining into the duodenum. Scrolling up and down in the cross-sections may help.
Gallbladder
- Function
  - The gallbladder releases stored bile when signaled by the duodenum
  - Bile releases into the cystic duct, travels to the bile duct, and drains into the duodenum via the hepatopancreatic ampulla.
Pancreas
- Function
  - The pancreas makes and secretes the enzymes: trypsinogen, chymotrypsin, amylase, and lipase.
  - Enzymes flow through the pancreatic duct converges with the bile duct, and drains into the duodenum via the hepatopancreatic ampulla
    Note that the man has a pancreatic divisum in his duct, which occurs in 4% of the population. In these cases, pancreatic drainage flows through a relatively narrow and minor duct. Individuals with this division are more likely to develop pancreatitis.
    
    Click here to view the ducts as they relate to the skin surface.
    
    When an individual has a blocked duct, pain is often felt in the right quadrant under the ribs, and the pain radiates to the back. Because of the location of the bile ducts to the back, it can be seen why some pain with a gallstone may be felt in the back.

Circulation of the Gastrointestinal System

The blood supply to the gastrointestinal system arises from branches of the thoracic aorta and the abdominal aortic artery. The gastrointestinal system's blood supply is larger than any other organ system.

Esophagus
- The esophageal artery branches off the thoracic aorta and provides blood supply to the esophagus.
- Blood is mostly drained from the esophagus through the esophageal veins; drainage continues through the azygos and hemiazygos veins into the inferior and superior vena cava.
- The interior esophagus is drained by the superficial veins into the left gastric vein before reaching the portal vein.
The portal vein typically carries 1500 mL of blood per minute at a pressure of 9mmHg. When portal pressure rises above 12mmHg, this results in an increased pressure gradient between the portal vein and the superior vena cava (i.e., portal hypertension). Blood from the portal vein is redirected to areas of less pressure such as: the lower esophagus, abdominal wall, stomach and rectum. The distention of these veins in these areas, called "varicies," results from the lack of structural support for such pressures.

The causes of portal hypertension include: cirrohsis, veno-occlusive disease, Budd-Chiari syndrome, or alcohol-related liver disease.

Esophageal varicies are the distentions of superficial veins around the interior esophagus. They are responsible for 5-11% of upper GI bleeding. (Azer 2006). Bleeding can be brisk and result in quick blood loss; they need to be treated quickly to prevent rupture or, during rupture, to prevent hemorrhage.
Celiac Trunk Blood Supply
- The celiac trunk consists of the left gastric artery, the common hepatic artery and the splenic artery. These arteries perfuse the liver, spleen, gallbladder, duodenum, stomach, and the lower esophagus.
- Branches from the common hepatic artery form capillary beds which drain into the portal vein. Blood continues to the hepatic vein followed by the inferior vena cava.
The liver and spleen are both very vascular organs and, because of their location, are also vulnerable to trauma. Note their proximity to the rib cage; when blunt abdominal trauma occurs, it is important to monitor these organs for lacerations. These injuries can result in a quick loss of blood.
Mesenteric Arteries
- The superior mesenteric artery branches off the abdominal aorta and perfuses the small intestine.
- The inferior mesenteric artery branches off the abdominal aorta and perfuses the descending colon, sigmoid colon, and rectum.
Venous circulation for the stomach and intestines does not directly enter the inferior vena cava, but rather first passes through the portal vein. The portal vein forms behind the spleen when the superior mesenteric vein and splenic vein converge. This blood, rich in both nutrients and toxins, then flows into the liver. Gastric and intestinal drainage passes through the liver first so that the liver can absorb nutrients, such as a vitamins, and filter out toxins.

The liver receives a blood supply both the venous and arterial systems. It has been observed that the liver will not necrose if the hepatic artery is cut while the portal vein remains intact.