BIO 226 Chap 21

The Large Intestine

• Mass movements involve powerful peristaltic contractions.
• Mass movements occur a few times each day due to stomach and duodenum distension.
• Mass movements start at the transverse colon and push materials along the distal large intestine.

Large Intestine Wall

• The large intestine wall lacks villi.
• The large intestine wall contains distinctive intestinal glands.
• Intestinal glands are dominated by mucin-secreting goblet cells.
• Mucus helps lubricate the feces as they become drier and more compact.
• The mucosa of the large intestine does not produce enzymes.

Rectum Anatomy

• The anal canal is the distal part of the rectum.
• The anal canal has small longitudinal folds called anal columns.
• The epithelium transitions from columnar to stratified squamous epithelium at the anal canal.
• The lamina propria and submucosa have a network of veins.
• Increased venous pressure can distend these veins, leading to hemorrhoids.
• Internal anal sphincter: inner circular smooth muscle layer that is not under voluntary control.
• External anal sphincter: outer skeletal muscle layer that is under voluntary control.
• The epidermis becomes keratinized at the anus, as it exits the anal canal.

Absorption in the Large Intestine

• Absorption in the large intestine accounts for less than 10% of all nutrient absorption in the digestive tract.
• The large intestine absorbs vitamin K, biotin, and vitamin B5.
• These vitamins are produced by normal bacteria in the colon.
• Water reabsorption in the large intestine prevents dehydration.
• Approximately 1500 mL of material enters the colon, with over 1 L of water being reabsorbed through osmosis and around 200 mL of feces being ejected.

Composition of Feces

• About 200 mL of feces are ejected daily.
• Feces: 75% water, ~20% indigestible material/inorganic matter/epithelial remains, 5% bacteria.
• Bacterial compounds contribute to the odor of feces, e.g., ammonia, indole/skatole, and hydrogen sulfide.
• Hydrogen sulfide is responsible for the "rotten-egg" odor.

Defecation Reflex

• Defecation reflex begins when the rectal wall is distended after feces arrive.
• Stretch receptors in the rectal wall trigger it.
• The defecation reflex involves two positive feedback loops; long and short reflexes.
• Long Reflex: coordinated by the sacral parasympathetic system and pushes feces from descending/sigmoid colon.
• Short Reflex: stimulates the myenteric plexus in the sigmoid colon and rectum.

Accessory Digestive Organs

• Salivary glands produce saliva containing mucins and enzymes.
• The gallbladder stores and concentrates bile secreted by the liver.
• The pancreas has exocrine cells that secrete buffers and digestive enzymes, and endocrine cells that secrete hormones.
• The liver has close to 200 known functions.
• Salivary glands, pancreas, and liver have vital metabolic and endocrine functions as well.

Digestive and Metabolic Liver Functions

• Synthesizing and secreting bile is a key liver function.
• The liver stores glycogen and lipids.
• Normal blood concentrations of glucose, amino acids, and fatty acids are maintained by the liver.
• The liver synthesizes and interconverts nutrient types (carbohydrates to lipids).
• Cholesterol, bound to transport proteins, is synthesized and released.
• Toxins are inactivated.
• Iron and fat-soluble vitamins are stored.
• Plasma proteins and clotting factors are also synthesized.
• Damaged red blood cells are phagocytized by stellate macrophages.
• The liver stores blood and metabolizes hormones and immunoglobulins.
• The liver also absorbs and inactivates lipid-soluble drugs.

Salivary Glands

• There are three pairs of salivary glands.
• Saliva is secreted into the oral cavity through ducts.
• Saliva from each pair has slightly different properties.
• Salivary reflex stimulates via trigeminal nerve (V) and taste buds innervated by VII, IX, or X.
• Salivary secretion increases with parasympathetic stimulation.

Sublingual Salivary Glands

• Lies under either side of the tongue
• Secretes into numerous sublingual ducts and opens along either side of the lingual frenulum
• Responsible for ~5% of total saliva secretion.
• Produces a mucous secretion that acts as a lubricant and buffer.

Submandibular Salivary Glands

• Located along the inner surface of the mandible (in the mandibular groove)
• Secretes into a submandibular duct and opens on each side of the anterior margin of the lingual frenulum
• They Provide ~70% of total saliva secretion.
• Secrete a mixture of buffers, mucins, and salivary amylase.
• Amylase breaks down starches.
• Cells transport IgA antibodies into the saliva of submandibular glands. This provides protection against pathogens.

Parotid Salivary Glands

• Its location is inferior to the zygomatic arch, deep to the skin, covering the mandible.
• It secretes into a parotid duct.
• The secretion empties into the vestibule of the mouth near the second upper molar.
• It provides ~25 percent of total saliva secretion.
• This gland produces serous secretion containing large amounts of salivary amylase.

Cells of Salivary Glands

• Duct cells assist in the secretion of buffers and antibodies.
• Mucous cells secrete mucins, water, and buffers.
• Serous cells secrete salivary amylase and lysozyme (an antibacterial enzyme).
• Interstitial fluid antibodies are transported into saliva by serous cells.

Saliva Composition and Function

• Saliva is a mixture of glandular secretions produced at 1.0-1.5 L a day.
• 99.4% of Saliva consists of water.
• Saliva constantly flushes oral surfaces.
• Buffers in Saliva keep pH of mouth around 7.0 and prevent the buildup of acids produced by bacteria.
• Saliva contains antibodies (IgA) and lysozyme to help control oral bacteria populations.
• Food is mixed with saliva to form bolus making it easy to be swallowed.

Liver Anatomy

• The liver is the largest visceral organ and weighs about 1.5 kg (3.3 lb).
• The liver has a tough fibrous capsule and is covered by visceral peritoneum.
• The liver is composed of four lobes: right, left, caudate, and quadrate.

Liver Lobes

• The liver has four lobes: left, right, caudate, and quadrate.
• The right lobe is separated from the left by the falciform ligament.
• The coronary ligament is an extension of the falciform ligament.
• The bare area is where the liver contacts the diaphragm with no peritoneal covering.
• The round ligament is a thickening in the posterior margin of the falciform ligament, marking the path of the fetal umbilical vein.
• The caudate lobe is on the posterior surface of the liver, separated from the right lobe by an indentation left by the inferior vena cava.
• The quadrate lobe is located between the left lobe and the gallbladder.

Associated Structures

• The gallbladder temporarily stores bile produced by the liver.
• The bile duct carries bile from the liver and gallbladder to the duodenum by way of the porta hepatis.

Liver Lobules

• The liver has basic functional units called liver lobules
• The liver contains close to 100,000 liver lobules with each roughly 1 mm in diameter.
• Adjacent lobules are separated by an interlobular septum and are hexagonal.
• Liver lobules are surrounded by 6 portal areas (one at each corner of the lobule).

Portal Area

• The portal area is also referred to as the portal triad.
• The portal triad contains three structures: a branch of the hepatic portal vein, a branch of the hepatic artery proper, and a bile duct.
• A branch of the hepatic portal vein receives blood from the hepatic portal system, which brings blood from abdominal viscera.
• Branches from the arteries and veins of each portal area deliver blood to liver (hepatic) sinusoids of adjacent liver lobules.

Liver Cells and Liver Sinusoids

• Liver lobules contain hepatocytes (liver cells).
• Hepatocytes form a series of irregular plates arranged like the spokes of a wheel.
• Plates of hepatocytes are only one cell thick and their exposed surfaces are covered with short microvilli.
• Plates of hepatocytes are separated by liver sinusoids, which are delicate blood vessels.
• Liver sinusoids lack a basement membrane and resemble large fenestrated capillaries.

Functional Anatomy of a Liver Lobule

• Blood enters liver sinusoids from branches of the hepatic portal vein and hepatic artery proper.
• About 1/3 of blood supply is arterial, remainder is hepatic portal vein blood supply.
• Hepatocytes adjacent to the sinusoids regulate solute and nutrient levels by absorption and secretion.
• Stellate macrophages (Kupffer cells) in the sinusoidal lining engulf pathogens, cell debris, and damaged blood cells.
• Stellate macrophages also store iron/lipids/heavy metals absorbed from the digestive tract.
• Sinusoids all drain into a central vein. Central veins of all lobules merge to form the hepatic veins
• Hepatic veins empty into the inferior vena cava.
• Hepatocytes secrete bile into a network of narrow channels called bile canaliculi.
• Bile canaliculi merge to form bile ductules.
• Bile ductules carry bile to bile ducts in the nearest portal area.
• Bile plays a role in the digestion of fats.

Liver Diseases and Conditions

• Liver diseases can lead to degenerative changes in liver tissue and constriction of blood flow.
• Examples of liver diseases include viral hepatitis and alcoholism.
• Constricted blood flow from a clot or damaged tissue causes portal hypertension.
• Increased pressure in the hepatic portal system results in distended peripheral veins and capillaries.
• The rupture of those vessels leads to potentially fatal bleeding.
• Fluid is forced into the peritoneal cavity, producing ascites.

Gallbladder Anatomy

• The gallbladder is a hollow, pear-shaped organ.
• It is located in the depression on the posterior surface of the liver's right lobe.
• The gallbladder is divided into three regions: the fundus, the body, and the neck.
• The gallbladder stores and concentrates bile secreted from the liver.
• Bile salts break lipid droplets apart (emulsification), increasing available surface area for enzymes.

Path of Bile

• Right and left hepatic ducts collect bile from the liver bile ducts.
• Hepatic ducts unite to form the common hepatic duct.
• Bile flows from the common hepatic duct into either the bile duct (to the duodenum) or the cystic duct (to the gallbladder for storage.)
• Contraction of the gallbladder forces bile back along the cystic duct to the common bile duct when needed.
• The common bile duct penetrates the duodenal wall and meets the pancreatic duct at the duodenal ampulla.
• The duodenal papilla contains the chamber that the common bile duct penetrates into.
• The hepatopancreatic sphincter encircles the lumens of the ducts where they enter the duodenum.
• The hepatopancreatic sphincter prevents bile from flowing into the duodenum except at mealtime.

Bile Storage and Ejection

• The liver produces about 1 liter of bile per day.
• It flows into the gallbladder for storage unless the hepatopancreatic sphincter is open.
• Release of CCK by the duodenum triggers both dilation of the hepatopancreatic sphincter and contraction of the gallbladder.
• Bile is ejected into the duodenum.
• Bile salts break apart lipid droplets through emulsification.

The Pancreas

• The pancreas is located posterior to the stomach.
• The pancreas has three regions: head, body, and tail.
• The pancreatic duct delivers exocrine secretions to the duodenum.
• Pancreatic juice is a combination of water and ions secreted by epithelial cells lining the duct and exocrine secretions (enzymes and buffers).
• About 1000 mL (1 qt) of pancreatic juice are produced each day.

Pancreatic Anatomy

• The accessory pancreatic duct (Santorini duct) branches from the pancreatic duct and empties separately into the duodenum.
• It occurs in 3-10% of the population.
• Pancreatic tissue is dominated by pancreatic acini.
• Pancreatic acini contains pancreatic acinar cells that produce digestive enzymes and buffers.
• Pancreatic islets contain endocrine cells.

Major Pancreatic Enzymes

• Pancreatic alpha-amylase is a carbohydrase that breaks down certain starches. It is almost identical to salivary amylase.
• Pancreatic lipase breaks down complex lipids, releasing products that can be easily absorbed.
• Nucleases break down RNA or DNA.
• Proteolytic enzymes break proteins into dipeptides, tripeptides, and amino acids.
• Proteolytic enzymes are secreted as inactive proenzymes and activated in the duodenum.
• Active forms of proteolytic enzymes include trypsin, chymotrypsin, carboxypeptidase, and elastase.

Disorders of the Digestive System

Oral Cavity

• Periodontal disease is the most common cause for loss of teeth.
• It occurs when dental plaque forms between gums and teeth.
• Resulting bacterial activity may cause gingivitis (inflammation of the gums), tooth decay, and eventual breakdown of periodontal ligaments and surrounding bone.

Salivary Glands

• Mumps are an infection of the salivary glands caused by the mumps virus.
• Affects mostly the parotid salivary gland and typically occurs at 5–9 years of age.
• Mumps may cause sterility in postadolescent males due to infection of the testes.
• An effective vaccine is available as part of the MMR (measles, mumps, rubella) vaccine, given to infants after age 15 months.

Esophagus

• Esophagitis is an inflammation of the esophagus.
• It usually results from stomach acids leaking through a weakened or permanently relaxed lower esophageal sphincter.
• Gastro-esophageal reflux is the backflow of acidic stomach contents into the esophagus.
• Gastro-esophageal reflux commonly results in symptoms called heartburn.

Liver

• Hepatitis is inflammation of the liver.
• Causes can include alcohol abuse, drugs, or infection.
• Cirrhosis is characterized by degeneration of liver cells and replacement with scar tissue.
• Viral hepatitis (A, B, and C) causes inflammation, tender liver, high fever, and destroys liver cells.
• Jaundice is the buildup of bilirubin causes yellowing of the skin and eyes.

Gallbladder

• Gallstones occur with crystals of insoluble minerals and salts form when bile becomes too concentrated and may be flushed through bile ducts with no problems if small.
• Cholecystitis is inflammation of the gallbladder. It is also the irritation and damage to the gallbladder wall caused by gallstones too large to pass.
• Gallstones may block cystic duct or common bile duct.
• Treatment for Cholecystitis may involve surgery of the gallbladder but surgery has no effect on bile production.

Stomach

• Gastritis is the inflammation of the mucous membrane lining the stomach.
• Caused by ingesting drugs (e.g., aspirin, alcohol), severe stress, bacterial infection, or ingestion of strong chemicals.
• A peptic ulcer forms when gastric enzymes and acids erode through the stomach or duodenal lining.
• A gastric ulcer is a peptic ulcer in the stomach wall.
• A duodenal ulcer is a peptic ulcer in the duodenal wall.
• Bacterial infection from Helicobacter pylori is responsible for over 80 percent of peptic ulcers.
• Treatment includes Cimetidine (Tagamet), which inhibits acid production and antibiotics for H. pylori if present.

Pancreas

• Pancreatitis is inflammation of the pancreas, Can be due to duct blockage, viral infection, or toxic drugs, including alcohol. Injury to pancreatic cells causes lysosomes to activates and those digestive enzymes are activated and released with the cell. In about 1/8 of cases, death occus when the digestive process does not stop as the lysosomal enzymes destroy the pancreas.

Small Intestine

• Enteritis is inflammation of the intestine (usually the small intestine), causing watery bowel movements (diarrhea).
• One cause of enteritis is infection by Giardia lamblia.
• Dysentery is inflammation of the small and large intestine, producing diarrhea containing blood and mucus.
• Gastroenteritis is inflammation of the stomach and intestines, due to pathogenic infection
• Gastroenteritis is more common in areas with poor sanitation and low water quality.

Large Intestine

• Colitis is inflammation of the colon, with diarrhea or constipation.
• Diarrhea results from too much fluid or from compromised absorption capabilities.
• Constipation is infrequent bowel movement (usually with dry, hard feces) resulting from excess water reabsorption due to slow-moving feces.
• Colorectal cancer is the third most common cancer in the U.S., affecting both men and women.
• Most common in those over age 50 and risk factors include diet rich in animal fat and low in fiber. Inherited disorders mai also promote the formation of epithelial tumors.
• Colorectal cancer begins as small, localized tumors (polyps). Greatly improved prognosis if polyps are removed before metastasis.