Human Biology: The Digestive System

• Chapter 11 is on The Digestive System, for Human Biology (BIOL-102), Health track

Learning Objectives

• List and describe the functional anatomy of the organs and accessory organs of the digestive system.
• Discuss the processes and controls of ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.
• Discuss the roles of the liver, pancreas, and gallbladder in digestion.
• Compare and contrast the digestion of the three macronutrients

Digestive System Functions

• Mechanical processing and movement are part of the digestive system's function.
• Secretion is a key function
• The system handles both mechanical and chemical digestion.
• Absorption of nutrients occurs.
• Elimination of waste happens through this system.
• Most digestion occurs in the small intestine.
• Nutrients are absorbed into the blood or lymph in the small intestine.

Interrelation with Other Body Systems

• The cardiovascular system provides oxygen and processed nutrients.
• The endocrine system provides hormones which help in regulating secretion.
• The integumentary system provides protection and synthesis of vitamin D.
• The lymphatic system defends against pathogens, and handles absorption and transport of lipids.
• The muscular system supports and protects abdominal organs.
• The nervous system regulates secretions and muscle contractions.

Digestive System Categories

• The alimentary canal is one category.
• Accessory digestive organs make up another category
• The main function of the alimentary canal's organs is to nourish the body.
• The alimentary canal includes the mouth, pharynx, esophagus, stomach, small intestine, large intestines, and anus.
• Accessory organs sprout from the developing gut's lining cells (mucosa) and accelerate chemical digestion.
• Accessory organs include salivary glands, gallbladder, liver, and pancreas.

Histology of the Alimentary Canal

• The alimentary canal has four basic tissue layers.
• Mucosa is a tissue layer that produces mucus.
• Mucosa consists of epithelium, a connective tissue layer (lamina propria), and the muscularis mucosa (thin smooth muscle).
• The lamina propria has numerous blood and lymphatic vessels transport nutrients absorbed through the alimentary canal.
• The submucosa consists of connective tissue and lies immediately beneath the mucosa.
• The submucosa connects the overlying mucosa to the underlying muscularis and includes blood and lymphatic vessels.
• Submucosa serve as a conduit for a dense branching network of nerves.
• Muscularis, also called the muscularis externa.
• Muscularis is made up of a double layer of smooth muscle in the small intestine (circular & longitudinal).
• Muscularis is made up of skeletal muscle in the most proximal and distal regions of the alimentary canal.
• Skeletal muscle regions include the mouth, pharynx, anterior esophagus, and external anal sphincter
• Serosa is the layer of visceral peritoneum overlying a layer of loose connective tissue.
• Serosa is superficial to the muscularis.
• The mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia, instead of a serosa
• Adventitia tissues serve to hold the alimentary canal in place.

Digestive Activities

• Digestion includes six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.
• Ingestion refers to the entry of food into the alimentary canal through the mouth.
• Chewing and mixing with saliva is part of ingestion
• As is breaking down carbohydrates with saliva
• Lipid digestion via lingual lipase starts in the mouth
• Propulsion refers to the movement of food through the digestive tract.
• The tongue and pharyngeal muscles propel food into the esophagus during propulsion
• Peristalsis is an involuntary process and part of propulsion
• Peristalsis involves sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which propel food along.
• Mechanical or Physical Digestion doesn't change the chemical nature of the food.
• Mastication, tongue movement, and mechanical churning in the stomach happens in this stage
• Segmentation occurs mainly in the small intestine
• Segmentation - localized contractions of circular muscle of the muscularis layer of the alimentary canal.
• During segmentation the content is moving back and forth while continuously subdividing, breaking up, and mixing the contents.
• Chemical Digestion involves digestive secretions that break down complex food molecules into their building blocks
• Absorption involves most nutrients are absorbed into the bloodstream through the epithelial cells that make up the mucosa.
• Absorption primarily happens in the small intestine (glucose and amino acids...)
• Lipids are absorbed into lacteals to lymphatic vessels to the bloodstream.
• Defecation is the removal of undigested material from the body as feces.

The Mouth

• Includes the lips, tongue, palate, gums, and teeth.
• Cheeks, tongue, and palate frame the mouth, which is sometimes called the oral cavity.
• The Palate forms the roof of the mouth
• Hard palate is bony while the soft palate is mainly skeletal muscle

The Tongue

• The tongue is attached to the mandible.
• The tongue consists of different skeletal muscles that perform three tasks: placing food for chewing, gathering food into a bolus (rounded mass), and positioning food to be swallowed.
• The tongue performs chemical digestion via the lingual lipase.
• The tongue allows sensation (of taste, texture, and temperature) and vocalization

The Salivary Glands

• Major salivary glands include the submandibular, sublingual, and parotid glands.
• Submandibular glands are located on the floor of the mouth, and secrete saliva that contains amylase and mucus thickens the liquid.
• Sublingual glands lie below the tongue, and secrete the thickest saliva with the least amount of salivary amylase.
• Parotid glands lie between the skin near the ears, and secrete a watery solution that contains salivary amylase.
• Each gland releases saliva within a specific duct.
• Saliva is mainly water and includes ions, glycoproteins, enzymes, growth factors, and waste products.
• Salivary amylase initiates the breakdown of carbohydrates.
• Bicarbonate and phosphate ions function as chemical buffers, maintaining saliva at a pH between 6.35 and 6.85.
• Salivary mucus helps lubricate food, facilitating movement in the mouth, bolus formation, and swallowing.
• Immunoglobulin A prevents microbes from penetrating the epithelium.
• Lysozyme in saliva makes it antimicrobial

The Teeth

• Normally 32 permanent teeth.
• 8 incisors (four top and four bottom): sharp front teeth used for biting into food.
• 4 canines: for tearing food.
• 8 premolars (or bicuspids): for mashing foods.
• 12 molars: most posterior and largest teeth used to crush food

Anatomy of the Tooth

• Gingivae (gums) are soft tissues around the teeth
• The two main parts of a tooth are the crown and root.
• The crown is the portion projecting above the gum line.
• The root is embedded within the maxilla and mandible.
• Both parts contain an inner pulp cavity, containing loose connective tissue through which run nerves and blood vessels.
• The root canal is the region of the pulp cavity that runs through the root of the tooth.
• Dentin is a bone-like tissue that surrounds the pulp cavity.
• Cementum is a harder bone-like layer covering the dentin within the root.
• Enamel is the hardest substance in the body that covers the dentin in the crown.
• Dental caries (cavities) develops when colonies of bacteria feeding on sugars in the mouth release acids that cause soft tissue inflammation and degradation of the calcium crystals of the enamel.

The Pharynx

• The pharynx (throat) is a short tube of skeletal muscle involved in both food swallowing and respiration.
• The pharynx has three subdivisions: nasopharynx, oropharynx, and laryngopharynx.
• The Nasopharynx (most superior), is involved only in breathing and speech.
• The Oropharynx begins inferior to the nasopharynx and is continuous below with the laryngopharynx.
• The Laryngopharynx connects to the esophagus, whereas the anterior portion connects to the larynx.
• Both Oropharynx and Laryngopharynx are used for both breathing and food swallowing

The Esophagus

• The esophagus is a muscular tube that connects the pharynx to the stomach.
• The esophagus is about 25.4 cm (10 in) in length, and located posterior to the trachea.
• The esophagus penetrates the diaphragm through an opening called the esophageal hiatus.
• The upper two-thirds of the esophagus consists of both smooth and skeletal muscle fibers.

The Esophagial Sphincters

• Sphincters are muscles that surround tubes and serve as valves, closing the tube when the sphincters contract and opening it when they relax.
• The upper esophageal sphincter is continuous with the inferior pharyngeal constrictor.
• The upper esophageal sphincter controls the movement of food from the pharynx into the esophagus.
• The lower esophageal sphincter, also called the gastroesophageal or cardiac sphincter, allows food passes into the stomach
• The lower esophageal sphincter relaxes to let food pass into the stomach and contracts to prevent stomach acids from backing up into the esophagus

Acid Reflux

• Acid reflux happens when the lower esophageal sphincter doesn't completely close, the stomach's contents can reflux (back up) into the esophagus.
• This causes heartburn or gastroesophageal reflux disease (GERD).

Deglutition

• Deglutition, also known as swallowing, is the movement of food from the mouth to the stomach.
• Deglutition involves the tongue's skeletal muscle and the pharynx/esophagus muscles
• Mucus and saliva helps with deglutition
• Deglutition has three stages: voluntary, pharyngeal, and esophageal.
• In the voluntary phase, the tongue moves upward and backward against the palate, pushing the bolus to the back of the oral cavity and into the oropharynx.
• Pharyngeal phase (involuntary phase) involves stimulation of receptors in the oropharynx which sends impulses to the deglutition center in the medulla oblongata.
• in phase the soft palate and uvula rise to close off the entrance to the nasopharynx.
• In phase the larynx is pulled superiorly and the cartilaginous epiglottis folds inferiorly, covering the glottis.
• The closure of the glottis blocks access to the trachea and bronchi, deglutition apnea takes place.
• In phase, the upper esophageal sphincter then relaxes to allow food to enter the esophagus.
• Esophageal phase (involuntary ): food enters the esophagus marks, initiating peristalsis
• Peristalsis propels bolus toward stomach
• As bolus nears the stomach, distention initiates a short reflex relaxation of lower esophageal sphincter
• Sphincter allows bolus pass into stomach.
• During esophageal phase, esophageal glands secrete mucus to lubricate the bolus and minimizes friction.

The Stomach

• The stomach links the esophagus to the first part of the small intestine (the duodenum).
• Mechanical Digestion involves type of peristalsis that mixes food with gastric juice forming chyme.
• Gastric emptying occurs as rhythmic mixing waves force about 3 mL of chyme at a time through the pyloric sphincter and into the duodenum.
• Chemical digestion occurs through HCL and gastric enzymes (pepsin & lingual lipase).
• The parts of the stomach are cardia, fundus, body, and pylorus
• The cardia connects to the esophagus
• The fundus is dome shaped and inferior to the diaphragm
• The body is the main part
• The Pylorus connects the stomach to the duodenum
• The stomach wall has the same four layers as the alimentary canal but with adaptations to the mucosa and muscularis for the organ's unique functions

Histology of the Stomach

• The stomach has mucosa, submucosa, muscularis extena, and serosa layers
• The stomach mucosa’s epithelial lining has alkaline mucus
• Mucus creates a Mucosal barrier
• Gastric glands secrete gastric juice.
• The pyloric antrum secretes mucus and the stimulatory hormone, gastrin.
• Larger glands of the fundus and body of the stomach are the site of most chemical digestion, and produce most of the gastric secretions.
• Secretory cells of the stomach include parietal cells, chief cells, mucous neck cells, and enteroendocrine cells
• Parietal cells produce hydrochloric acid (HCl) and intrinsic factor.
• Chief cells secrete pepsinogen, the inactive proenzyme form of pepsin.
• Mucous neck cells secrete thin, acidic mucus
• Enteroendocrine cells secrete hormones like gastrin

The Small Intestine

• Small intestine is the longest part of the alimentary canal where most digestion and absorption occur
• The small intestine is subdivided into three regions: duodenum, jejunum, and ileum.
• The duodenum is the shortest region which begins at the pyloric sphincter
• The hepatopancreatic ampulla (ampulla of Vater) is a portion of the of the duodenum, where the main pancreatic duct and the common bile duct join
• The jejunum is about 0.9 meters long and runs from the duodenum to the ileum with a diameter more than that of the ileum.
• Wall is thicker and more vascular in the jejunum
• The ileum is the longest part.
• the ileum joins the cecum at the ileocecal sphincter
• Circular folds, villi and microvilli are unique features that increase surface area

Histology of the Small Intestine

• Villi
• Villi are small (0.5–1 mm long) hair like vascularized projections to give a furry texture
• Each villus contains a capillary bed with one arteriole and one venule, as well as a lymphatic capillary called a lacteal.
• Microvilli ( Brush border) are much smaller than villi
• Microvilli consist of cylindrical apical surface extensions of the plasma membrane of the mucosa's epithelial cells

Intestinal Glands

• Intestinal glands include Brunner's glands and Crypts of Lieberkühn
• The mucous glands located in the wall of the first centimeters of the duodenum.
• Mucous glands secrete alkaline mucus protect the duodenal wall from digestion by acidic gastric juice
• Crypts of Lieberkühn are small pits over the surface
• The villi and crypts surfaces are covered by an epithelium composed of goblet cells and enterocytes
• Goblet cells secrete mucus that lubricates and protects.
• Enterocytes secrete water and electrolytes
• Mechanical Digestion includes segmentation and peristalsis
• Segmentation combines the chyme with digestive juices and pushes food particles against the mucosa to be absorbed
• Proteins and carbohydrates are completed digested here
• Pancreas and intestinal glands assist

The Large Intestine

• Functions include further break food, absorbs most residual water, electrolytes, and vitamins and propels feces toward rectum for defecation.
• The large intestine is subdivided into four main regions: cecum, colon, rectum, and anal canal.
• The cecum is 6 cm that receives the contents of the ileum, and continues the absorption of water and salts, includes the appendix (or vermiform appendix)
• The colon is subdivided into 4 sections: ascending, transverse, descending and sigmoid colon
• The rectum receives food from residue leaving the sigmoid in pelvis
• The anal canal long structure that opens to the at the anus.

Mechanical and Chemical Digestion in the Large Intestine

• Peristalsis forces chyme into the cecum under control of ileocecal sphincter.
• Contractions of the ileocecal sphincter get stronger when the cecum is distended with chyme.
• Chemical digestion occurs because of bacteria in the colon with saccharolytic fermentation.
• This has discharge of hydrogen, carbon dioxide, and methane gases.

Defecation

• In Defecation Passing of the feces occurs through the rectum to the anus.
• As feces enter the rectum, a neural reflex is initiated
• Reflex causes - rectum to contract and expel feces, the internal anal sphincter is relaxed, and external anal sphincter to relax
• If defecation is delayed, the feces get firmer and potentially become constipation, and diarreah if defication goes too fast.

Accessory Organs

• Chemical digestion in the small intestine relies on the activities of three accessory digestive organs: liver, gallbladder and pancreas.
• The liver produces bile salts, which emulsify lipids, aiding their digestion and absorption.
• The gallbladder stores, concentrates, and releases bile.
• The pancreas produces digestive enzymes and bicarbonate.

The Liver

• The liver is the largest gland in the body (~1360 g in an adult) situated in the right upper quadrant of the abdomen.
• The liver has two primary lobes: a large right lobe and a smaller left lobe
• The liver regulates metabolism, processes food/toxins, and produces bile.
• The porta hepatis ("gate to the liver") is where the hepatic artery and hepatic portal vein enter the liver.
• The hepatic artery carries oxygenated blood from the heart to the liver.
• The hepatic portal vein carries partially deoxygenated blood containing nutrients, toxins and drugs absorbed from the small intestine.
• Post-processing, the liver releases need nutrients back into blood, drains to central vein, then through hepatic vein to inferior vena cava.

The Bile

• Bile is yellow-brown or yellow-green alkaline solution (pH 7.6 to 8.6), whichproduced by hepatocytes to accomplish the emulsification of lipids in the small intestine.
• Bile is a mixture of water, bile salts, bile pigments (Billuribin), phospholipids (such as lecithin), electrolytes, cholesterol, and triglycerides.
• Bile salts and phospholipids are critical to emulsification
• Emulsification is the process by which large fat globules are broken down into smaller ones

The Gallbladder

• The gallbladder is 8–10 cm (~3–4 in) in is nested on the posterior aspect of the right lobe.
• This muscular sac collects and concentrates before propelling bile in common bile.
• The gallbladder is divided into three regions: fundus, body, and neck.
• The fundus is widest and tapers medially and then narrows to become the neck.
• the neck angles upward slightly towards the hepatic duct
• the cystic duct is 1–2 cm long and goes behind the reck as it bridges the neck and hepatic duct.

The Pancreas

• The pancreas produces over a liter of pancreatic juice each day.
• Pancreatic juice is mostly water with some salts, sodium bicarbonate, and several digestive enzymes.
• Sodium bicarbonate is responsible for the alkalinity of pancreatic juice (pH 7.1 to 8.2).
• Sodium bicarbonate buffers the stomach juice.
• Juice inactivates pepsin from the stomach, and optimal setting for the activity of sensitive enzymes.
• Pancreatic juice contains digestive enzymes, the active (lipase, amylase and nuclease) and inactive (trypsinogen, chymotrypsinogen.
• Pancreatic enzymes help in in digesting sugars, proteins, and fats.
• The pancreas has exocrine (secreting digestive enzymes) and endocrine glands.
• Regulation of pancreatic juice excretion requires secretion.
• Acidic chyme stimulates the secretion of secretin and triggers the secretion of bicarbonates.
• The entry of fats and proteins in duodenum activates the release of cholecystokinin (CCK)
• CCK stimulates the exocrine cells to release digestive-rich liquid

Carbohydrate, Protein, and Lipid Digestion

• Chemical digestion of starch begins in mouth by salivary amylase.
• The brush border enzyme α-dextrinase starts working on α-dextrin to break off a glucose unit.
• Lactose is broken down by the brush border enzyme lactase.
• Sucrase cleaves the disaccharide sucrose into Glucose and fructose and Maltose is hydrolyzed by intestinal maltase
• Proteins start in the stomach, where HCl and pepsin break smaller.
• Cells of the brush border secret enzymes such as aminopeptidase. and dipeptidase to peptide breaks acids.
• Dietary lipids are triglycerides glycerol three acid chain,
• The lipases responsible for lipid digestion are lingual, gastric and pancreatic.
• The fatty acids include chain (less than 10 to 12 carbons) and long- acids.
• Two fluids of
• Nuclease acids Deoxrib nuclease two brush.