The Digestive System
The digestive system breaks food down into molecules small enough to be absorbed into the bloodstream, then eliminates what the body cannot use. As a medical assistant, you will take dietary histories, prepare patients for GI procedures, collect stool specimens, and educate patients on conditions like GERD, IBS, and celiac disease. Knowing how this system works makes those tasks more meaningful and your documentation more accurate.
The GI Tract: Mouth to Anus
The gastrointestinal (GI) tract is a continuous muscular tube approximately 30 feet long. Food moves through it by peristalsis, wave-like muscle contractions.
Mouth (Oral Cavity)
Digestion begins here. Teeth mechanically break down food. Salivary glands (parotid, submandibular, sublingual) secrete saliva containing amylase, which begins carbohydrate digestion. The tongue mixes food into a bolus and initiates swallowing.
Esophagus
A muscular tube about 10 inches long connecting the pharynx to the stomach. Carries food by peristalsis. The lower esophageal sphincter (LES) at the bottom prevents stomach acid from refluxing upward. When the LES is weak or relaxes inappropriately, GERD develops.
Stomach
A J-shaped muscular organ in the left upper quadrant. Stores food, churns it with gastric juice (hydrochloric acid and pepsin), and produces chyme (a semi-liquid mixture). The stomach secretes intrinsic factor, needed for vitamin B12 absorption. The pyloric sphincter controls release of chyme into the small intestine.
Small Intestine
The primary site of digestion and nutrient absorption. About 20 feet long, divided into three sections:
- Duodenum: First 12 inches. Receives chyme from the stomach plus bile from the gallbladder and digestive enzymes from the pancreas. Most chemical digestion happens here.
- Jejunum: Middle section. Main site of nutrient absorption via villi and microvilli (brush border) that dramatically increase surface area.
- Ileum: Final section. Absorbs vitamin B12 and bile salts. Connects to the large intestine at the ileocecal valve.
Large Intestine (Colon)
About 5 feet long. Absorbs water and electrolytes, and forms and stores feces. Sections: cecum (with appendix), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. Normal bowel flora here produce some B vitamins and vitamin K.
Accessory Organs
These organs are not part of the GI tube but contribute secretions essential for digestion.
Liver
The largest internal organ. Located in the right upper quadrant. Functions include producing bile (emulsifies fats), metabolizing drugs and toxins, synthesizing clotting factors and plasma proteins, storing glycogen, and producing cholesterol. Bile is stored in the gallbladder and released into the duodenum when fat is present.
Gallbladder
Small pear-shaped sac on the underside of the liver. Concentrates and stores bile. Releases bile through the common bile duct into the duodenum. Gallstones (cholelithiasis) form when bile becomes supersaturated with cholesterol or bilirubin.
Pancreas
Dual function organ. Exocrine function: secretes digestive enzymes (lipase for fats, amylase for carbohydrates, proteases for proteins) into the duodenum via the pancreatic duct. Endocrine function: islets of Langerhans secrete insulin and glucagon directly into the bloodstream.
Nutrient Absorption Summary
- Carbohydrates: Broken into glucose and simple sugars; absorbed in the small intestine.
- Proteins: Broken into amino acids; absorbed in the small intestine.
- Fats: Emulsified by bile, broken down by lipase into fatty acids and glycerol; absorbed via lacteals (lymphatic vessels in villi).
- Water: Primarily absorbed in the large intestine.
- Vitamins and minerals: Absorbed throughout the GI tract; fat-soluble vitamins (A, D, E, K) require bile; B12 requires intrinsic factor from the stomach.
Common Digestive Conditions
GERD (Gastroesophageal Reflux Disease)
Chronic acid reflux due to LES dysfunction. Symptoms: heartburn (burning in the chest or throat), regurgitation, dysphagia, chronic cough. Untreated, can progress to Barrett esophagus (precancerous changes). Treatment: antacids, H2 blockers, PPIs (proton pump inhibitors), lifestyle changes (elevate head of bed, avoid late meals, lose weight).
Peptic Ulcers
Erosions in the stomach (gastric ulcer) or duodenum (duodenal ulcer). Most caused by H. pylori infection or chronic NSAID use. Symptoms: burning epigastric pain, often worse with an empty stomach (duodenal) or after eating (gastric). Complications: bleeding (melena/coffee-ground emesis), perforation.
IBS (Irritable Bowel Syndrome)
Functional GI disorder with no structural damage. Symptoms: abdominal cramping, bloating, alternating diarrhea and constipation. Triggers include stress, certain foods, and hormonal changes. Diagnosed by exclusion after ruling out organic pathology. Treatment: dietary changes, fiber, antispasmodics, stress management.
Diverticulitis
Diverticula are small pouches that form in the colon wall. When they become inflamed or infected, it is diverticulitis. Symptoms: left lower quadrant pain, fever, change in bowel habits. Treatment: antibiotics, liquid diet, sometimes surgery. Low-fiber diet is a risk factor for diverticulosis (having diverticula without inflammation).
Celiac Disease
Autoimmune condition triggered by gluten (found in wheat, barley, rye). Gluten exposure causes immune damage to intestinal villi, impairing absorption. Symptoms: diarrhea, bloating, weight loss, fatigue, and nutritional deficiencies. Diagnosed with blood tests (tissue transglutaminase antibodies) and small bowel biopsy. Treatment: strict gluten-free diet.
Know the order of GI tract structures from mouth to anus, the three accessory organs and their functions, and the key conditions: GERD, peptic ulcers, IBS, diverticulitis, and celiac disease. Understand the role of bile and where nutrients are absorbed. The difference between gastric and duodenal ulcer pain patterns is a frequent question.
Practice Questions
Question 1: A patient presents with epigastric pain that improves after eating. Which type of ulcer is most likely, and why?
Answer: Duodenal ulcer. Duodenal ulcers typically improve with eating because food buffers stomach acid temporarily. Gastric ulcers usually worsen with eating because eating stimulates more acid secretion. Both are commonly caused by H. pylori infection or NSAID use.
Question 2: Which organ produces intrinsic factor, and why does this matter for patient nutrition?
Answer: The stomach. Intrinsic factor is required for vitamin B12 absorption in the ileum. Patients who have had gastric surgery (gastrectomy) or who have pernicious anemia (autoimmune destruction of parietal cells) cannot produce intrinsic factor and will develop B12 deficiency, leading to megaloblastic anemia and neurological symptoms.
Question 3: A patient is diagnosed with celiac disease. What dietary education should the MA reinforce?
Answer: The patient must follow a strict gluten-free diet for life. Gluten is found in wheat, barley, rye, and often contaminated oats. The patient should read all food labels, avoid cross-contamination in food preparation, and be aware that gluten can hide in sauces, soups, medications, and processed foods. Refer to a registered dietitian for comprehensive counseling.