Alimentary Canal: Appendix & Large Intestine

• The alimentary canal absorbs large quantities of water electrolytes and vitamins.
• Much of water electrolyte, and vitamin absorption occurs in the small intestine.
• A significant quantity of water electrolyte, and vitamin absorption occurs in the large intestine.

Appendicitis

• The small size and blind-ended structure of the appendix can lead to blockage, usually by fecal matter.
• Bacteria multiply within the feces in the appendix causing infection and inflammation.
• Abdominal pain occurs, with tenderness over the right lower quadrant.
• Rebound tenderness is exhibited, similar to peritonitis.
• Nausea and vomiting may occur.
• Immediate surgical removal of appendix is needed.
• If left untreated, the appendix can rupture and cause internal bleeding
• The untreated appendix leads to bacteria-filled fecal material spilling into the peritoneal cavity, causing peritonitis, and possibly becoming fatal if not well managed.

Large Intestine

• The large intestine runs along the abdominal cavity border, surrounding the small intestine.
• The large intestine is about 1.5 meters (5 feet) long; named for its large diameter, compared to the small intestine.
• The large intestine receives undigested/unabsorbed material from the small intestine.
• Feces or fecal matter exits the body through the large intestine.
• The large intestine is active in absorbing water and electrolytes important for maintaining fluid, electrolyte and acid-base homeostasis.
• The Large intestine secretes mucus, propels contents, and defecates.
• Bacteria lives in the large intestine, performing important jobs like synthesizing vitamins.

Gross Anatomy of the Large Intestine

• The cecum is the first portion of the large intestine, located in the right lower quadrant of the abdomen and is intraperitoneal
• The cecum is a blind pouch with a smaller blind-ended pouch extended from the posteroinferior end that is the vermiform appendix or simply called appendix.
• The appendix houses multiple lymphatic nodules and has a role in the immune system.
• The colon is the next longest portion of the large intestine and is divided into four portions: ascending, transverse, descending, and sigmoid colon
• The large intestine continues as the rectum after the sigmoid colon
• The rectum runs anterior to the sacrum.
• The rectum walls feature horizontal folds called rectal valves which allow the passage of flatus (gas) without risking the passage of feces.
• The rectum ends at the anal canal, the last portion of the large intestine.
• The terminal portion of the anal canal has two sphincters.
• The involuntary internal anal sphincter is a thickened circular layer of muscularis externa, and is supplied by parasympathetic motor neurons
• The voluntary external anal sphincter is skeletal muscle; this sphincter is voluntary and is supplied by somatic motor neurons controlled by the cerebral cortex

Large Intestine Histology

• Its mucosa lacks villi and its cells lack microvilli
• Structural adaptations reflect the fact that nutrient absorption is not the large intestine's primary function.
• The mucosa contains: goblet cells that secrete protective and lubricating mucus.

Large Intestine Bacteria

• Has more bacteria in the large intestine than human cells in the body
• As much as 60% of the dry mass of feces are bacteria
• Normal flora (gut flora) consist of about 500 different bacterial species
• Normal flora has a symbiotic (mutually beneficial) relationship with humans
• The location provides bacteria with the environment they need to survive.
• Bacteria perform several useful functions for humans including producing vitamins like vitamin K, which is necessary for blood clotting
• Bacteria metabolize undigested materials, such as carbohydrates (soluble fibers).
• The carbohydrates small intestine cannot digest; convert into fatty acids and other molecules that can be absorbed and used by the body
• Normal flora bacteria prevent the growth of pathogenic (disease-causing) microorganisms
• Normal flora bacteria does this by competing for nutrients and producing chemicals that kill certain harmful bacterial species
• During infancy, normal flora induces immune tolerance to their own antigens, simultaneously stimulating MALT development and antibody production

The motility of the Large Intestine and Defecation

• Proximal ascending and transverse colon are one functional part of the large intestine
• The large intestine's functional segments are distal which include the descending and sigmoid colon, rectum, and anal canal
• The Proximal large intestine's primary jobs are water, electrolyte absorption, and bacterial activity; exhibits two main motility types
• Segmentation (churning) aids in water and electrolyte absorption.
• Mass movement (mass peristalsis) propels contents towards the distal large intestine
• Mass movements occur three to four times per day and are triggered by food consumption
• Any change that affects large intestine motility affects the amount of water present in feces.
• Increased motility means the large intestine has less time to absorb water from the fecal material and results in watery feces known as diarrhea
• Colon irritation due to bacterial or viral infections and drugs stimulates the parasympathetic nervous system and increases motility
• Decreased motility means the large intestine absorbs too much water and the fecal material becomes hard causing constipation
• Medications such as opiate narcotics, or drugs that block acetylcholine slow the large intestine's motility

The Pancreas, Liver, and Gallbladder

• The pancreas, liver, and gallbladder are accessory organs
• Like salivary glands, pancreas, liver, and gallbladder are exocrine glands that secrete their product through a duct outside of the body
• These exocrine glands secrete products into the alimentary canal which are considered outside to external environment
• The pancreas is a gland with both endocrine and exocrine jobs
• Endocrine secretions have many cells in the body affected – hormones are secreted such as insulin and glucagon
• Clusters of acinar cells primarily secrete exocrine enzymes that are released into the ducts of the pancreas which empty into the alimentary canal
• The pancreas extends from the duodenum to the spleen in the left upper quadrant of the abdomen and is retroperitoneal
• The pancreas has three regions and head has a wide section in contact with the duodenum
• The body separates the head from the thinner tail and tapers off toward the spleen
• Secretions from the acinar cells receive and travel down the middle of the main pancreatic duct

Pancreatic Juice

• Pancreatic juice is the collective secretions of pancreatic acinar and duct cells; consisting of water, multiple digestive enzymes, and other proteins
• Duct cells secrete bicarbonate ions, and pancreatic juice becomes alkaline
• Alkaline juice helps neutralize acidic chyme entering the duodenum from the stomach protecting the duodenum from damage by acid
• Digestive enzymes secreted by acinar cells are crucial in chemical digestion and catalyze reactions that digest carbohydrates, proteins, lipids, and nucleic acids
• The pancreas secretes at a basal rate between meals, rising during eating due to parasympathetic and hormonal stimulation
• CCK, produced by duodenal enteroendocrine cells in response to fats and partially digested proteins in the duodenum, stimulates digestive enzyme secretion from acinar cells
• Secretin is released by duodenal cells in response to acid and fats in the Duodenum and primarily triggers duct cells to secrete bicarbonate ions

Liver and Gallbladder

• The pyramid-shaped liver is located in the right upper quadrant just inferior to the diaphragm surface.
• The gallbladder is a small sac that sits on the posterior side of the liver and have both close anatomical and functional relationships
• The liver is covered by a thin connective tissue capsule and visceral peritoneum and is composed of four lobes
• There are large right and left lobes on the right and left sides of the liver which are separated by a fold of visceral peritoneum called the falciform ligament which also anchors the liver to the anterior abdominal wall
• The liver releases both endocrine and exocrine secretions, and converts harmful chemicals into nontoxic substances that the body can eliminate
• The liver's main digestive job is to produce bile which is a liquid with water, electrolytes, and organic compounds
• Bile serves two critical functions being required for the digestion and absorption of lipids and the mechanism by which the liver excretes wastes and other substances that kidneys cannot
• Hepatocytes secrete bilirubin from the breakdown of hemoglobin (by spleen) into bile.
• Normal flora in the large intestine converts bilirubin to urobilinogen then to stercobilin which is responsible for the characteristic brown color of feces
• Some urobilinogen by the large intestine is reabsorbed, and ends up back in the hepatic portal system so most is re-secreted into bile.
• Some of the remaining urobilinogen in the blood is excreted by the kidneys and the yellow urobilinogen is largely responsible for the color of urine
• The liver has many functions including nutrient metabolism processes nutrients from diet
• It detoxifies substances that our bodies produce and substances that are eaten or drank, some of which are harmful.
• The liber excretes bilirubin in bile along with other processed liver substances, particularly certain antibiotics and modifies substances so kidneys can excrete them
• The gallbladder receives most bile from the common hepatic duct.
• The gallbladder stores and concentrates bile by removing water, releasing it when stimulated.
• CCK stimulates bile release and triggers contraction of smooth muscle in the gallbladder wall, causing it to release bile into the cystic duct
• The cystic duct joins the common hepatic duct to form the common bile duct which joins the main pancreatic duct near the duodenum to form the hepatopancreatic ampulla.
• An ampulla ring of smooth muscle surrounds the ampulla the hepatopancreatic sphincter which controls bile and pancreatic fluid emptying in the duodenum
• Hepatopancreatic ampulla contents empty into the duodenum at the major duodenal papilla.
• Bile includes calcium salts and cholesterol with the possibility of both precipitating and forming hard lumps called gallstones.

"Detox" Products

• Certain manufacturers claim that dietary supplements and health products can remove disease-causing toxins from our bodies
• "Detoxification" is a big part of the multibillion-dollar alternative health industry's profits
• Liver helps converts harmful chemicals (toxins) into nonharmful substances that can be excreted in bile or urine

Overview of Digestion and Absorption

• Food breaks down into smaller and smaller pieces until only the individual nutrient molecules remain in the process
• Mechanical digestion is when food is physically broken into smaller parts.
• Chemical digestion is when chemical bonds between food molecules are broken
• Mastication in the mouth, churning in the stomach, and segmentation in the small intestine are mechanical digestion mechanisms
• Chemical digestion varies for each nutrient because enzymes recognize one bond within one type of nutrient
• Digestive enzymes catalyze hydrolysis reactions which use a water molecule to break a bond between two molecules
• Water is secreted gastric and pancreatic juices with water molecules being key reactants breaking down food.
• Enzymes speed up this by the speed up these reactions of enzymatic hydrolysis

Understanding Absorption in Alimentary Canal

• Molecules must cross physical barriers to travel from the alimentary canal lumen to the blood.
• Water, as a small molecule, may cross between enterocytes.
• The apical side of the plasma membrane is the most significant barrier to the absorption of ingested molecules.

Digestion and Absorption of Carbohydrates

• Digestion begins in the mouth with salivary amylase from salivary glands.
• Salivary amylase catalyzes reactions that break polysaccharides into shorter oligosaccharides.
• Actual digestion doesn't occur in the mouth because food goes to fast.
• Salivary amylase digestion that continues in the stomach is inactivated as amylase is generally inactivated by stomach acid
• Chemical digestion of carbohydrates resumes in the small intestine.
• Pancreatic amylase resumes and polysaccharides and oligosaccharides are encountered
• Oligosaccharide digestion is completed by reactions catalyzed by enterocytes in the small intestine, including lactase, maltase, and sucrase
• Lactase catalyzes the digestion of lactose (in milk-based products) into glucose and galactose
• Maltase and sucrase catalyze reactions that break oligosaccharides into disaccharides and monosaccharides
• Monosaccharides like glucose, fructose, and galactose are the end result of enzymes
• Once carbs turn to monosaccharides, they are able to be absorbed
• The absorbed glucose and galactose are transported across enterocyte's apical membrane the same mechanism
• Concentrations of glucose and galactose are higher in the enterocyte's cytosol than in the small intestine's lumen

Lactose Intolerance

• Milk sugar cannot be digested by adults that have a lactase, as lactose digests milk sugar called lactose intolerance
• Lactase production declines as we age
• Up to 75% of adults exhibit decreased lactase activity
• Percentage of lactose intolerance differs, with 5% of northern Europe afflicted and 90% of population in certain regions of Asia
• Without lactase, disaccharides such as lactose remains in the small intestine and is eventually delivered to large intestine
• As lactose acts as an osmotic particle in the large instestine, it creates a gradient to draw water into feces as lactose leads to cramping and diarrhea is consumed.
• Lactose-containing foods can be avoided or lactase supplements can be taken for management

Digestion and Absorption of Proteins

• Chemical digestion of proteins does not begin until they reach the stomach, where they are encounter pepsin
• Chief cells of gastric glands produce pepsinogen
• Pepsinogen must require a pH of about 2 to become pepsin
• Pepsin becomes completely inactive at pH 7
• The majority of protein digestion takes place in the small intestine with pancreatic enzymes and brush border enzymes.
• Enzymes digest proteins, all released as inactive precursors with five pancreatic enzymes
• Inactive precursors protect the pancreas from autodigestion (digestion of its cells by its own enzymes).
• Trypsinogen is converted active enzyme trypsin, as is becomes after encountering enzymes on the intestinal brush border

Protein Digestion and Absorption

• Proteins from nutrition has to be broken down into small oligopeptides and free amino acids to be absorbable in small intestine There is the ability to absorb small amounts of whole, undigested proteins involved in immunity by process of endocytosis
• Specialized cells overlie Peyer's patches in small intestine endocytose and deliver these proteins to lymphatic tissue in patches

Digestion and Absorption of Lipids

• The process of lipid digestion proceeds as follows: lipids are broken apart by stomach churning and by gastric lipase
• Lipids enter the small intestine and are emulsified by bile salts.
• Pancreatic lipase catalyzes reactions that digest lipids into free fatty acids and monoglycerides
• Bile salts remain associated with digested lipids to form micelles.
• Lipids are nonpolar, creating water-based barriers which are deter them from entering cytosol in enterocytes such as mucus lining of small intestine and polar phosphate heads of enterocytes plasma membranes
• Lipids are escorted to enterocyte plasma membranes by micelles
• Lipids pass through the phospholipid bilayer and are reassembled into triglycerides and packaged into chylomicrons.
• Chylomicrons are released into interstitial fluid by exocytosis and then enter the lacteal.

Digestion and Absorption of Nucleic Acids

• Nucleic acids in food start chemical digestion in the small intestine using pancreatic enzymes called nucleases which that break nucleic acids into individual nucleotides
• Further digestion using brush border enzymes removes phosphate group and sugar from the nucleotide to leave phosphate ion, ribose or deoxyribose, and a nitrogenous base
• Absorption occurs using the primary and secondary active transport mechanisms into capillaries in villi

Absorption of Water, Electrolytes, and Vitamins

• Over 9 liters of water enter the small intestine each day
• About 2 liters of water are ingested, with 7 liters secreted into the alimentary canal from the alimentary canal itself and accessory organs
• About 8 of these 9 liters are absorbed into enterocytes of the small intestine
• Most of the remaining water is absorbed into enterocytes of the large intestine, leaving about 0.1 liter of water to be excreted in feces
• Water absorption occurs exclusively by osmosis and requires a concentration gradient
• The cytosol of enterocytes and extracellular fluid in villi must be more concentrated than fluid in the lumen to drive water movement:
• A principle called Gradients Core Principle
• Solutes that remain in the lumen of the intestines results in excess water which will prevent reabsorption
• Electrolytes are taken in from diet and are present in secretions from digestive organs
• The small intestine absorbs most electrolytes, however large intestine absorbs significant amounts as well
• A mechanism for absorption of sodium ions revolves around the cotransport of monosaccharides and amino acids
• Sodium ion absorption is key in the absorption of anions, like chloride and bicarbonate ions to enter enterocytes as electrical gradients are created
• Iron and magnesium ions use active transport mechanisms in order to absorb the low concentrations of certain electrolytes
• Chemicals in diet generally which are involved in metabolic reactions are called vitamins
• Vitamins are divided into Water-soluble (polar molecules) and Fat-soluble vitamins (lipid-based, nonpolar)
• Most water-soluble vitamins are absorbed in the small intestine via diffusing through the enterocytes' plasma membranes
• Vitamin B12 has to be bound to intrinsic factor, an molecule produced to be absorbed in the ileum within parietal cells of the stomach
• Micelles packaged with fats and other lipids include include the A, D, E, and K fat-soluble vitamins that are then absorbed.

Intrinsic Factor and Vitamin B12 Deficiency

• Vitamin B12 helps with metabolic functions within erythrocytes as well as normal functionality of the nervous system
• Absorption has to have an intrinsic factor (IF) and and enables via enterocyte plasma membrane the IF-B12 complex it its plasma membrane
• Multiple conditions and mechanisms that can impair production of the intrinsic factor include:
• Autoimmune condition that attacks the stomach
• Ulcers and infections cause atrophy of gastric mucosa
• People who had surgical removal of stomach from weight loss or cancer treatment
• Vitamin B12 deficiency results from intrinsic factor, even with adequate B12 and leads to pernicious anemia and nervous system problems, is only treated by B12 injections