Respiratory and Digestive Systems: Structures, Pathology, and Clinical Terms

Overview

Medical billers and coders must read clinical documentation accurately before assigning any code. That means understanding what structures are being described, what has gone wrong with them, and how clinicians name those problems. This lesson covers the two body systems most commonly documented together in outpatient and inpatient records: the respiratory system and the digestive system.

The Respiratory System

The respiratory system moves air into and out of the body and exchanges oxygen and carbon dioxide across thin lung membranes. It is divided into the upper respiratory tract and the lower respiratory tract.

Upper respiratory tract structures include the nose and nasal cavity, the paranasal sinuses (frontal, maxillary, ethmoid, and sphenoid), the pharynx (nasopharynx, oropharynx, and laryngopharynx), and the larynx. The larynx contains the vocal cords and the epiglottis, a flap that covers the airway during swallowing.

Lower respiratory tract structures include the trachea, which branches at the carina into the right and left main bronchi. The bronchi further divide into lobar bronchi, segmental bronchi, and then bronchioles, finally ending in the alveoli—the microscopic air sacs where gas exchange occurs. Each lung is enclosed in a pleural membrane; the space between the two layers is the pleural cavity. The right lung has three lobes; the left lung has two.

Common respiratory pathologies you will see in documentation:

• Pneumonia – infection and inflammation of the alveoli, often with consolidation visible on imaging
• Chronic obstructive pulmonary disease (COPD) – a group of diseases (including emphysema and chronic bronchitis) that obstruct airflow; in emphysema the alveolar walls are destroyed, in chronic bronchitis the bronchial mucosa is chronically inflamed
• Asthma – reversible bronchospasm and airway inflammation
• Pulmonary embolism (PE) – a clot (usually from a deep vein) lodged in a pulmonary artery, blocking blood flow to lung tissue
• Pleural effusion – abnormal accumulation of fluid in the pleural cavity

Key procedural terms in respiratory documentation include bronchoscopy (endoscopic visualization of the bronchi), spirometry (lung function testing), thoracentesis (needle drainage of the pleural cavity), and intubation (placement of an endotracheal tube).

The Digestive System

The digestive (gastrointestinal, or GI) system breaks down food, absorbs nutrients, and eliminates waste. It runs from the mouth to the anus and includes several accessory organs that contribute digestive secretions.

The GI tract in order: mouth → pharynx → esophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (cecum, ascending colon, transverse colon, descending colon, sigmoid colon) → rectum → anus.

The stomach produces hydrochloric acid and pepsin. The small intestine is the primary site of chemical digestion and nutrient absorption; its inner wall has folds, villi, and microvilli that vastly increase surface area. The large intestine absorbs water and compacts waste.

Accessory organs:

• Liver – produces bile (stored in the gallbladder), metabolizes drugs and nutrients, synthesizes clotting factors
• Gallbladder – concentrates and releases bile through the common bile duct into the duodenum
• Pancreas – exocrine function: secretes digestive enzymes and bicarbonate into the duodenum; endocrine function: secretes insulin and glucagon into the bloodstream

Common digestive pathologies you will see in documentation:

• Gastroesophageal reflux disease (GERD) – stomach acid refluxes into the esophagus, causing mucosal irritation
• Peptic ulcer disease (PUD) – erosion of the gastric or duodenal mucosa, often associated with Helicobacter pylori infection or NSAID use
• Cholelithiasis – gallstones (calculi in the gallbladder); cholecystitis is inflammation of the gallbladder, often caused by an obstructing stone
• Crohn's disease and ulcerative colitis – inflammatory bowel diseases; Crohn's can affect any GI segment (often the ileum), while ulcerative colitis is limited to the large intestine and rectum
• Colorectal polyps and colorectal cancer – abnormal growths arising from the colonic mucosa

Key procedural terms in GI documentation include upper endoscopy (esophagogastroduodenoscopy, EGD), colonoscopy, sigmoidoscopy, cholecystectomy (surgical removal of the gallbladder), and Whipple procedure (pancreaticoduodenectomy).

Translating Clinical Notes into Anatomical Descriptions

When you read a clinical note, your job is to identify every anatomical structure mentioned and understand its location and role before you look up a code. A note that reads "patient presents with dyspnea and decreased breath sounds at the left base, consistent with pleural effusion" tells you the problem is in the pleural cavity of the left lung, at its inferior aspect. A note describing "right upper quadrant pain radiating to the right shoulder, positive Murphy's sign" points to the gallbladder and biliary system.

Always look up unfamiliar terms in a current medical dictionary before assigning codes. Mis-identifying a structure can lead to the wrong ICD-10-CM or CPT code, resulting in a claim denial or—more seriously—incorrect documentation that could affect patient care decisions.

Safety Note

Coders do not diagnose patients or correct physician documentation without going through the proper query process. If a clinical note is unclear about which structure is involved or what condition is present, the correct action is to submit a formal physician query—never assume or guess. Inaccurate coding can delay care, generate compliance risks, and harm patients indirectly by corrupting health data.