Alterations of Digestive Function

Structure and Accessory Organs of the Digestive System

The digestive system is a complex network of organs and glands responsible for the ingestion, digestion, and absorption of nutrients. The system includes the pharynx, tongue, larynx, trachea, esophagus, and stomach. The intestinal tract is composed of the small intestine (including the duodenum and ileum) and the large intestine, which consists of the transverse colon, hepatic flexure, ascending colon, cecum, vermiform appendix, splenic flexure, descending colon, sigmoid colon, rectum, and anal canal. Accessory organs that facilitate digestion include the liver, gallbladder, spleen, and pancreas. The liver connects to the system via the hepatic duct, which joins the cystic duct from the gallbladder to form the common bile duct. The pancreas features an accessory pancreatic duct and a main pancreatic duct that empties into the duodenum at the major duodenal papilla, also known as the Ampulla of Vater. The accessory papilla and the pyloric sphincter also play critical roles in regulating the flow of digestive fluids and chyme between compartments.

Hepatic Portal Circulation and Microscopic Anatomy of the Liver

The liver's blood supply is unique due to the hepatic portal circulation. Oxygenated blood enters via the hepatic artery, while nutrient-rich, deoxygenated blood from the stomach, spleen, small intestine, ascending colon, and descending colon enters via the hepatic portal vein, which is formed by the union of the superior mesenteric vein and the splenic vein (the latter receiving the inferior mesenteric vein). Blood leaves the liver through the hepatic veins and enters the inferior vena cava. Structurally, the liver is composed of plates of hepatocytes separated by sinusoids, which are lined with endothelial cells. Within the sinusoids, the liver houses the largest population of tissue macrophages in the body, known as Kupffer cells. These mononuclear phagocytes are bactericidal and remove foreign substances and bacteria from the blood. The space between the endothelial lining of the sinusoid and the hepatocytes is called the Disse space. Bile produced by hepatocytes is secreted into bile canaliculi, which are small channels adjacent to the cells that eventually empty into the bile ducts.

Liver Function: Metabolic, Hematologic, and Vascular Roles

The liver performs essential vascular and hematologic functions, serving as a reservoir for a large volume of blood, the storage of which depends on pressure relationships between arteries and veins. Its hemostatic functions include the synthesis of various clotting factors. Because bile salts are necessary for the reabsorption of fats, the absorption of fat-soluble vitamin K is entirely dependent on adequate bile production. Metabolically, the liver produces between $700\,ml/day$ and $1200\,ml/day$ of bile. Bile is required for the emulsification and intestinal absorption of fats. Liver fat metabolism involve the hydrolysis of triglycerides into glycerol and free fatty acids, which can then be converted into $ATP$ or lipoproteins. Primary bile acids, such as cholic acid and chenodeoxycholic (chenic) acid, are synthesized from cholesterol; secondary bile acids, including deoxycholic acid and lithocholic acid, are formed in the small intestine by intestinal bacteria. These bile acids are more water-soluble, restricting their diffusion from the duodenum and ileum.

Exocrine Pancreas: Structure and Enzymatic Secretions

The pancreas is a $20\,cm$ long organ tucked behind the stomach, with its head nestled in the duodenum and its tail touching the spleen. It is organized into spherical lobules called acini, composed of secretory cells that surround small ducts. The pancreatic secretions flow into the pancreatic duct and eventually through the duodenal papilla into the duodenum. Pancreatic juice is an aqueous secretion with electrolyte concentrations similar to plasma, though it contains high levels of bicarbonate ($HCO_3^-$), as well as magnesium and calcium. Its exocrine function is defined by the production of enzymes: amylases for carbohydrates, lipases for fats, and proteases for proteins. To protect the pancreas from autodigestion, protein-digesting enzymes such as trypsinogen, chymotrypsinogen, and procarboxypeptidase are secreted as inactive proenzymes. In the duodenum, trypsinogen is the first to be activated into trypsin, which then stimulates the conversion of chymotrypsinogen into chymotrypsin and procarboxypeptidase into carboxypeptidase.

Clinical Manifestations of Gastrointestinal Dysfunction

Gastrointestinal (GI) dysfunction presents through various manifestations. Nausea is a subjective experience often accompanied by hypersalivation and tachycardia. Retching involves vomiting movements without the expulsion of vomitus, while projectile vomiting is spontaneous and not preceded by nausea or retching. Vomiting itself is the forceful emptying of the stomach and intestinal contents through the mouth, stimulated by the area postrema in the medulla. Stimulation can be indirect via the cerebral cortex, thalamus (anxiety or pain), or the vestibular system through the $8^{th}$ cranial nerve (motion sickness). Antiemetic treatments include serotonin and neurokinin-1 antagonists, metoclopramide, domperidone, olanzapine, corticosteroids, and dopamine agonists like apomorphine, levodopa, or bromocriptine. Severe vomiting can lead to fluid and electrolyte imbalances, including hyponatremia, hypokalemia, hypochloremia, and metabolic alkalosis. Constipation is defined by infrequent or difficult defecation and is categorized as primary (normal transit, slow transit, or pelvic floor dysfunction) or secondary. Diarrhea is defined as three or more loose stools in $24$ hours and is classified as acute (<14 days), persistent ($14-30$ days), or chronic (>30\,days).

Gastrointestinal Bleeding and Motility Disorders

Abdominal pain can be mechanical, inflammatory, or ischemic and is categorized as parietal (somatic), visceral, or referred pain. GI bleeding is split into upper GI bleeding (esophagus, stomach, duodenum) and lower GI bleeding (jejunum, ileum, colon, rectum). Common causes of upper GI bleeding include esophageal/gastric varices, Mallory-Weiss tears, cancer, and peptic ulcers. Lower GI bleeding is often caused by polyps, inflammatory bowel disease, diverticular disease, cancer, or hemorrhoids. Specific signs include hematemesis (bloody vomit), hematochezia (bright red blood in stools), melena (black, tarry stools), and occult bleeding, which is not visible but can result in anemia. Dysphagia, or difficulty swallowing, can result from mechanical obstructions (tumors, strictures, herniation) or functional disorders (neural or muscular atrophy). Upper esophageal obstructions cause discomfort $2-4$ seconds after swallowing, whereas lower obstructions cause discomfort $10-15$ seconds post-swallow. Symptoms across all types include retrosternal pain, regurgitation of undigested food, and weight loss.

Gastroesophageal Reflux Disease (GERD) and Hiatal Hernias

GERD involves the reflux of acid and pepsin from the stomach into the esophagus, leading to esophagitis. It is primarily caused by a low resting tone of the lower esophageal sphincter (LES), which may be transiently relaxed or weak. Conditions that increase intra-abdominal pressure, such as vomiting, coughing, lifting, bending, obesity, or pregnancy, contribute to GERD. Hiatal hernias are protrusions of the upper part of the stomach through the diaphragm into the thorax. There are three main types: Type I (sliding hiatal hernia), Type II (paraesophageal hiatal hernia), and Type III (mixed). While often asymptomatic, hiatal hernias can cause heartburn, regurgitation, dysphagia, and epigastric pain. Complications of GERD include esophageal ulcers and the potential for strictures.

Gastroparesis, Pyloric Obstruction, and Intestinal Obstruction

Gastroparesis is the delayed emptying of the stomach without mechanical obstruction, often associated with diabetes, vagotomy, or fundoplication. Pyloric obstruction is the narrowing or blocking of the opening between the stomach and the duodenum, which can be acquired (peptic ulcer disease near the pylorus or carcinoma) or congenital. Manifestations include epigastric fullness and vomiting that becomes more distressing later in the day as the stomach attempts to force chyme past the obstruction. Intestinal obstruction involves any condition that prevents the flow of chyme through the intestinal lumen. A simple obstruction involves a physical lesion (e.g., fibrous adhesions, the most common cause in the small intestine). Functional obstruction, or paralytic ileus, is a failure of motility, frequently occurring after surgery. Large bowel obstructions are commonly caused by colorectal cancer, volvulus (twisting), or diverticulitis-related strictures. Ogilvie syndrome, or acute colonic pseudo-obstruction, is a massive dilation of the large bowel seen in critically ill or immobilized older adults.

Gastritis and Peptic Ulcer Disease

Gastritis is an inflammatory disorder of the gastric mucosa that can be acute or chronic. Acute gastritis is often caused by injury to the mucosal barrier by drugs (NSAIDs that inhibit prostaglandin synthesis), chemicals, or H. pylori infection. Chronic gastritis is classified as Type A (fundal), which is more rare and involves the loss of T-cell tolerance, or Type B (antral), which is more common. Peptic ulcer disease (PUD) involves a break or ulceration in the protective mucosal lining of the lower esophagus, stomach, or duodenum. Duodenal ulcers are the most common type and are primarily caused by H. pylori infection and NSAID use. They are characterized by intermittent epigastric pain that is relieved by food ("pain-food-relief" pattern) and occurs when the stomach is empty. Gastric ulcers, which are less common, tend to develop in the antral region. The primary defect is increased mucosal permeability to hydrogen ions. Pain from gastric ulcers often occurs immediately after eating. Stress ulcers are acute forms of peptic ulcer that accompany severe illness or trauma and are classified into ischemic ulcers (following hemorrhage or sepsis), Curling ulcers (following burns), and Cushing ulcers (following head trauma or brain surgery).

Malabsorption Syndromes

Malabsorption syndromes result from the failure of the intestinal mucosa to absorb digested nutrients. Pancreatic insufficiency is the deficient production of lipase, amylase, trypsin, or chymotrypsin, leading to fat maldigestion as the primary problem; signs include steatorrhea (fatty stools) and weight loss. Lactase deficiency is the inability to break down the milk sugar lactose into monosaccharides, causing osmotic diarrhea, gas, and abdominal cramping. Bile salt deficiency results from conditions that interfere with bile secretion (liver disease, bile duct obstruction) or intestinal resection. Without bile salts, micelles cannot form, leading to fat malabsorption and deficiencies in fat-soluble vitamins: Vitamin A deficiency causes night blindness; Vitamin D deficiency results in decreased calcium absorption, bone pain, and osteoporosis; Vitamin K deficiency leads to prolonged prothrombin time and bleeding/bruising; and Vitamin E deficiency may cause testicular atrophy and neurological defects.

Inflammatory Bowel Diseases: Ulcerative Colitis and Crohn’s Disease

Inflammatory bowel diseases (IBD) are chronic, relapsing disorders with no known specific origin, though genetics, environment, and altered immune reactions to flora play roles. Ulcerative Colitis (UC) is a chronic inflammatory disease causing ulceration of the colonic mucosa, specifically the sigmoid colon and rectum. Lesions are continuous without "skip lesions" and are limited to the mucosa. Clinical manifestations include diarrhea (10-20 stools per day), urgency, bloody stools, and cramping. Crohn’s Disease (CD) involves any part of the GI tract from the mouth to the anus, but most commonly the distal small intestine and proximal large colon. It is characterized by "skip lesions" (inflamed areas mixed with uninflamed areas), transmural inflammation (affecting all layers of the wall), noncaseating granulomas, and the potential for fistulas and deep penetrating ulcers. Anemia can develop due to malabsorption of Vitamin B12 and folic acid. Treatment for both involves anti-inflammatory drugs, corticosteroids, and immunomodulatory agents.

Irritable Bowel Syndrome and Diverticular Disease

Irritable Bowel Syndrome (IBS) is a functional GI disorder without structural or biochemical alterations. Pathophysiology involves visceral hypersensitivity, abnormal permeability, and altered gut microbiota. Clinical manifestations include lower abdominal pain, gas, bloating, and diarrhea-predominant, constipation-predominant, or alternating habits. Treatment involves laxatives, fiber, gluten restriction, antidiarrheals, and visceral analgesics. Diverticular disease includes diverticula (herniations or outpouchings of the mucosa through the muscle layers, primarily in the sigmoid colon), diverticulosis (asymptomatic disease), and diverticulitis (inflammation). Diverticulitis presents with fever, elevated WBC count, and tenderness in the Left Lower Quadrant (LLQ). It is treated with analgesia and antibiotics like rifaximin.

Portal Hypertension and Related Complications

Portal hypertension is abnormally high blood pressure in the portal venous system, defined as a pressure increase to at least $10\,mmHg$ (normal is $3-5\,mmHg$). It is caused by obstruction of blood flow in any component of the portal system. Prehepatic causes include portal vein thrombosis; intrahepatic causes include cirrhosis, inflammation, or vascular remodeling; and posthepatic causes include hepatic vein thrombosis or right-sided heart failure. The most common cause is fibrosis from cirrhosis. Complications include varices (distended, collateral veins in the esophagus, stomach, rectum, or abdominal wall), which can rupture and cause life-threatening hemorrhage. Splenomegaly is also common due to increased pressure in the splenic vein, leading to the sequestration of blood cells. Other complications include hepatopulmonary syndrome (HPS) and portopulmonary hypertension (PPH), the latter involving the thickening of pulmonary artery walls.

Ascites: Pathophysiology and Clinical Management

Ascites is the accumulation of fluid in the peritoneal cavity, with portal hypertension being the most common cause. Pathophysiologically, cirrhosis leads to portal hypertension and decreased albumin synthesis by the liver. The decreased capillary oncotic pressure and increased capillary hydrostatic pressure allow plasma to transude into the peritoneum. Splanchnic arterial vasodilation and increased lymph production also contribute. This reduces the effective plasma volume, triggering the renin-angiotensin-aldosterone system ($RAAS$) and antidiuretic hormone ($ADH$) release, which leads to sodium and water retention by the kidneys, further exacerbating the fluid accumulation. Clinical signs include abdominal distention and displacement of the diaphragm, which can impair breathing. Management includes dietary salt restriction, potassium-sparing diuretics (like spironolactone), strong diuretics (furosemide), and the possible administration of albumin.

Hepatic Encephalopathy and Jaundice

Hepatic encephalopathy (portosystemic encephalopathy) is a complex neurological syndrome characterized by impaired cognitive, behavioral, and motor function. It is caused by the liver's inability to eliminate toxins, particularly ammonia, which is produced by the intestinal breakdown of amino acids. Symptoms range from personality changes, confusion, and sleep disturbances to flapping tremors (asterixis), stupor, and coma. Jaundice (icterus) is a yellow or greenish pigmentation of the skin caused by hyperbilirubinemia, defined as plasma bilirubin concentrations exceeding $2.5-3\,mg/dL$. Jaundice is classified as extrahepatic (gallstones obstructing bile flow), intrahepatic (hepatocellular disease like cirrhosis), or prehepatic (excessive hemolysis of RBCs). Clinical signs include dark urine (due to bilirubin excretion), clay-colored stools (due to lack of bile in the GI tract), and yellowing of the sclera and skin.

Hepatorenal Syndrome and Acute Liver Failure

Hepatorenal syndrome (HRS) is functional renal failure that occurs during late-stage liver disease, characterized by sudden oliguria and low systolic blood pressure. Acute liver failure is the severe impairment or necrosis of liver cells without preexisting disease, with acetaminophen overdose being the leading cause. The pathophysiology involves edematous hepatocytes and patchy areas of irreversible hepatic necrosis. Clinical manifestations include anorexia, vomiting, abdominal pain, and progressive jaundice. Treatment for acetaminophen-induced failure involves N-acetylcysteine, while other cases may require antiviral therapy, blood ammonia lowering, or liver transplantation.

Autoimmune and Viral Hepatitis

Autoimmune hepatitis is a rare, chronic, and progressive inflammatory liver disease often treated with immunosuppressive therapy like corticosteroids and azathioprine. Viral hepatitis is a systemic disease primarily affecting the liver, with five main types: A, B, C, D, and E. Manifestations range from asymptomatic to fulminant hepatitis with rapid liver failure. Hepatitis A and E are generally acute, while B and C can become chronic. Hepatitis C ($HCV$) and chronic alcohol abuse are the leading causes of cirrhosis.

Cirrhosis of the Liver

Cirrhosis is an irreversible, inflammatory, fibrotic disease that disrupts liver function and structure. Fibrotic tissue (nodules) replaces normal parenchyma, obstructing biliary channels and causing portal hypertension. This shunts blood away from the liver, causing hypoxic necrosis and further failure. Nonalcoholic fatty liver disease ($NAFLD$) involves the infiltration of hepatocytes with triglycerides in the absence of alcohol intake. Biliary cirrhosis begins in the bile canaliculi and ducts and can be primary (autoimmune destruction of small ducts) or secondary (obstruction of the common bile duct). Clinical manifestations include pain, fever, jaundice, ascites, edema, bleeding tendencies (due to decreased Vitamin K absorption), and hormone metabolism changes leading to gynecomastia, loss of body hair, and spider angiomas. Biochemical markers include elevated $AST$, $ALT$, and bilirubin levels, and low serum albumin.

Disorders of the Gallbladder: Cholelithiasis and Cholecystitis

Cholelithiasis is the formation of gallstones. These are classified as cholesterol stones (formed in bile that is supersaturated with cholesterol) or pigmented stones. Black pigmented stones form in the gallbladder and are associated with liver or hemolytic disease, consisting of calcium bilirubinate. Brown stones are associated with bacterial infections of the bile ducts and biliary stasis. Biliary colic occurs when a stone lodges in the cystic or common duct, causing pain in the epigastric and right hypochondrium areas. Cholecystitis is inflammation of the gallbladder or duct, often caused by a gallstone lodged in the cystic duct. It presents with fever, leukocytosis, and abdominal tenderness.

Pancreatitis: Acute and Chronic Mechanisms

Pancreatitis is inflammation of the pancreas, usually associated with alcohol intake or cholelithiasis. Pancreatic enzymes leak into the surrounding tissue, causing autodigestion and damaging blood vessels. Acute pancreatitis is typically mild and resolves spontaneously, presenting with epigastric pain, nausea, and fever. The primary diagnostic marker is elevated serum lipase. Severe cases can lead to systemic inflammatory response syndrome ($SIRS$), shock, or sepsis. Chronic pancreatitis involves repeated exacerbations that destroy acinar cells and the Islets of Langerhans, replacing them with fibrous tissue, calcification, and cysts. Chronic alcohol abuse and smoking are the most common causes, and chronic pancreatitis is a known risk factor for pancreatic cancer.

Questions and Discussion

1. Question: Which of the following removes foreign substances and traps bacteria in the liver sinusoids?
   Response: Kupffer cells.

2. Question: Which finding is typical for Crohn's disease?
   Response: Noncaseating granulomas.

3. Question: Which intervention is most appropriate for a person with portal hypertension?
   Response: Monitor for hematemesis.

4. Question: A person has alcoholic liver disease. What is the sequence for the development of this disease?
   Response: Steatosis, steatohepatitis, and fibrosis.", "title": "Alterations of Digestive Function Study Guide"}