The Digestive System: an Overview

Chapter 24

The Digestive System: An Overview

▪Works in concert with respiratory, cardiovascular, and lymphatic systems to provide body tissues with appropriate nutrients and oxygen.

▪Consists of the digestive tract (gastrointestinal tract or alimentary canal) and accessory organs.

▪Functions of the Digestive System

▫Ingestion-

▫Mechanical processing-

▫Digestion-

▫Secretion-

▫Absorption-

▫Excretion-

▫Lining of the digestive tract also functions in protection of surrounding tissues.

▪The Digestive Organs and the Peritoneum

▫The peritoneal cavity is lined by a serous membrane that consists of a superficial mesothelium covering a layer of areolar tissue.

▫Mesenteries- double sheets of peritoneal membrane that suspend portions of the digestive tract, connects the parietal and visceral peritoneums.

-Lesser omentum-

-Falciform ligament-

-Greater omentum-

-Mesentery proper-

-Mesocolon-

▪Histological Organization of the Digestive Tract

▫The mucosa is the inner lining of the digestive tract, is a mucous membrane (epithelium and lamina propria)

▫The digestive epithelium is either simple or stratified, depending on location and common stresses.

▫The lamina propria consists of a layer of areolar tissue that contains blood vessels, sensory nerve endings, lymphatic vessels, smooth muscle cells, and scattered lymphoid tissue.

▫The submucosa is a layer of dense irregular CT that surrounds the muscularis mucosae.

▫The muscularis externa is a region dominated by smooth muscle cells, found outside the submucosa.

▫The serosa is a serous membrane that surrounds most portions of the digestive tract, it is basically the visceral peritoneum.

▪The Movement of Digestive Materials

▫Peristalsis-

▫Segmentation-

▪Control of the Digestive Function

▫ Local Mechanisms-

▫ Neural Mechanisms-

▫ Hormonal Mechanisms-

The Oral Cavity

▪The Tongue- manipulates materials inside the mouth and is occasionally used to bring foods into the oral cavity. The primary functions of the tongue include:

1) mechanical processing by compression, abrasion, and distortion

2) manipulation to assist in chewing and prepare for swallowing

3) sensory analysis in touch, temperature, and taste

4) secretions of mucins and lingual lipase

▫The tongue can be divided into the body and the root.

▫Epithelium covering the inferior surface of the tongue is thinner and more delicate than that of the dorsum.

-Lingual frenulum-

▫Tongue epithelium contains glands that secrete water, mucins, and lingual lipase

▫The tongue contains both extrinsic and intrinsic skeletal muscles.

-Extrinsic muscles-

-Intrinsic muscles-

-Both groups of muscles are innervated by the hypoglossal nerve.

▪Salivary Glands- three pairs of salivary glands secrete into the oral cavity

▫Parotid salivary glands- large glands that lie inferior to the zygomatic arch and under the skin that covers the mandible.

▫Sublingual salivary glands- located on the floor of the mouth, covered by mucous membrane

▫Submandibular salivary glands- located within the mandibular goove.

▫Saliva is 99.4% water, with the remaining 0.6% a mixture of electrolytes, buffers, glycoproteins, antibodies, enzymes, and waste products.

▫Control of salivary secretion is normally controlled by the ANS

▪The Teeth- Adults can have up to 32 teeth

▫Occlusal surfaces-

▫Dentin-

▫Pulp cavity-

▫Root canal-

▫Periodontal ligament-

▫Cementum-

▫Enamel-

▫Types of teeth- there are four types of teeth:

1. incisors

2. cuspids

3. bicuspids

4. molars

▫Dental succession-

▫Mastication-

The Pharynx

▪A common passageway for food and air, composed of nasopharynx, oropharynx, and laryngopharynx.

The Esophagus

▪The esophagus is a hollow muscular tube about 25 cm long, and a diameter of 2 cm.

▪Its primary function is to carry food and liquids to the stomach

▪The esophagus enters the abdominopelvic cavity through the esophageal hiatus, an opening in the diaphragm

▪Histology of the Esophagus

▫The wall of the esophagus contains mucosal, submucosal, and muscularis layers.

▪Swallowing, also called deglutition, is a complex process that starts voluntarily, then proceeds automatically. Divided into the buccal, pharyngeal, and esophageal phase.

▫Buccal phase-

▫Pharyngeal phase-

▫Esophageal phase-

The Stomach- the stomach has four major functions

▪Anatomy of the Stomach- is the shape of an expanded J

▫Lesser and greater curvature forms the shape of the stomach

▫The stomach can be divided into four regions; cardia, fundus, body, pylorus.

-Cardia-

-Fundus-

-Body-

-Pylorus-

▫While relaxed, the mucosa is thrown into prominent folds called rugae, allows for stomach to expand when food enters.

▫Contains circular, longitudinal, and oblique layers of smooth muscle

▫Simple columnar epithelium lines the entire stomach, provides a carpet of mucus that covers the interior surfaces, alkaline property protects from acids and enzymes.

▫Gastric Glands-

-Parietal cells- secrete intrinsic factor, facilitates the absorption of vitamin B12, also secrete HCl, which has four important functions:

-Chief Cells- secrete pepsinogen which is converted to pepsin by acids in the lumen of the stomach.

-Pyloric Glands- mainly produces mucous secretions, along with some endocrine hormones

▪Regulation of Gastric Activity

▫Can be controlled by the CNS, regulated by short reflexes of the enteric nervous system, or regulated by hormones of the digestive tract.

▫There are three phases of gastric control:

-Cephalic phase-

-Gastric phase- begins with the arrival of food in the stomach.

-Neural response-

-Hormonal response-

-Local response-

-Intestinal phase- begins when chyme starts to enter the small intestine.

-Neural response-

-Hormonal response-

▪Digestion and Absorption in the Stomach

▫Carbohydrates and lipids continue being digested in the stomach, until the pH goes below 4.5

▫Proteins begin being broken down by pepsin at and below pH of 2

▫Absorption does not take place because:

▫Alcohol and aspirin can be absorbed in the stomach

The Small Intestine and Associated Glandular Organs

▪The Small Intestine- plays a key role in the digestion and absorption of nutrients

▫90% of nutrient absorption occurs in the small intestine, the rest occurs in the large intestine

▫Averages about 6 meters (20 feet), consists of the duodenum, jejunum, and the ileum.

-Duodenum-

-Jejunum-

-Ileum-

▪Histology of the Small Intestine

▫Circular folds-

▫Intestinal Villi-

-Lacteal-

▫Intestinal Glands- goblet cells located between the columnar epithelial cells eject mucins onto the intestinal surfaces.

-Brush border enzymes-

-Duodenal glands-

▫Regional specializations-

▫Intestinal secretions-

▪Intestinal Movements

▫Short reflexes control weak peristaltic movements from the duodenum to the jejunum

▫Some smooth muscle cells contract periodically, even without stimulation, establishing a basic contractile rhythm that then spreads from cell to cell.

▫Stimulation of the parasympathetic system increases the sensitivity of the reflexes, and accelerates both local peristalsis and segmentation.

▫Gastroenteric reflex-

▫Gastroileal reflex-

▪The Pancreas- located posterior to the stomach, and laterally from the duodenum toward the spleen.

▫Histological Organization-The surface of the pancreas has a lumpy, lobular texture

-Primarily an exocrine organ, producing digestive enzymes and buffers

-Pancreatic duct-

-Accessory pancreatic duct-

-Pancreas is separated into lobules, contains pancreatic acini

-Pancreatic enzymes are produced by the acinar cells

▫Physiology of the Pancreas- main function is to produce and secrete pancreatic juice to aid in chemical digestion.

-Secretory activities are controlled primarily by hormones from the duodenum.

-When chyme enters the duodenum, secretin is released, secretin triggers a watery alkaline solution

-Cholecystokinin stimulates production and secretion of pancreatic enzymes.

-Pancreatic enzymes include:

-Pancreatic alpha-amylase-

-Pancreatic lipase-

-Nucleases-

-Proteolytic enzymes (proteases and peptidases)

-Proenzymes-

▪The Liver- the largest visceral organ, one of the most versatile organs in the body.

▫Anatomy of the liver- The liver is wrapped in a tough fibrous capsule, and is covered by a layer of visceral peritoneum

-Falciform ligament-

-Caudate lobe-

-Quadrate lobe-

-Porta hepatis-

-Hepatocytes-

▫Histological organization of the liver- each lobe of the liver is divided by CT into approximately 100,000 liver lobules, the basic functional unit of the liver.

-At the center of each lobule is a central vein, sinusoids empty their contents into the central vein.

-Kupffer cells-

-Portal areas-

-Blood travels from hepatic artery and hepatic portal vein, through the sinusoids, and into the central vein, which drains into the hepatic veins.

▫Bile duct system- bile first enters the bile canaliculi, then it enters the bile ductules. From there, it enters the right or left hepatic ducts, then on to the common hepatic duct.

▫Physiology of the liver- the liver has three general categories of function; metabolic regulation, hematological regulation, and bile production.

-Metabolic regulation- the liver is the primary organ involved in regulating the composition of circulating blood.

Carbohydrate metabolism:

Lipid metabolism:

Amino acid metabolism:

Removal of waste products:

Vitamin storage:

Mineral storage:

Drug inactivation:

-Hematological regulation- the liver is the largest blood reservoir in the body. As blood passes through it, it performs these functions:

Phagocytosis and antigen presentation:

Synthesis of plasma proteins:

Removal of circulating hormones:

Removal of antibodies:

Removal/ storage of toxins:

Synthesis and secretion of bile:

-Functions of Bile:

▪The Gallbladder- a hollow organ that stores concentrated bile prior to excretion into the small intestine.

▫Hepatopancreatic sphincter-

▫The gall bladder has two major functions:

▪The Coordination of Secretion and Absorption

▫Neural mechanisms involving the CNS deal with:

▫Intestinal hormones- all are peptide hormones with similar structures, hard to identify functions and target.

Secretin: released when chyme inters the duodenum.

-Origin-

-Target-

-Effects-

Cholecystokinin (CCK): released when chyme containing lipids and partially digested proteins enter the duodenum.

-Origin-

-Target-

-Effects-

Gastric Inhibitory Peptide (GIP): released when chyme containing large quantities of fats and glucose enter the small intestine.

-Origin-

-Target-

-Effects-

Vasoactive Intestinal Peptide (VIP): released when chyme enters the duodenum

-Origin-

-Target-

-Effects-

Gastrin: released in response to vagus nerve stimulation of arrival of food in the stomach, and arrival of chyme containing large quantities of undigested protein.

-Origin-

-Target-

-Effects-.

Enterocrinin: released when chyme enters the duodenum

-Origin-

-Target-

-Effects-

▫Intestinal Absorption- usually takes about 5 hours for materials to pass from the duodenum to the end of the ileum

-The movement of the small intestine along with the plicae, villi, and microvilli cause the intestinal contents to change constantly.

The Large Intestine

▪The Cecum- expanded pouch at the beginning of the large intestine

▫Ileocecal valve-

▫The cecum collects and stores materials from the ileum and begins the process of compaction

▫Appendix- slender hollow organ attached to the posteromedial surface of the cecum.

-The mucosa and submucosa are dominated by lymphoid nodules

▪The Colon- has a larger diameter and a thinner wall than the small intestine. Distinctive features include:

▫Wall of the colon forms haustra (pouches), allow for expansion

▫Taeniae coli-

▫Omental appendices- numerous teardrop-shaped sacs of fat.

▫The colon can be subdivided into four regions:

-Ascending colon-

-Transverse colon-

-Descending colon-

-Sigmoid colon-

▪The Rectum- forms the last 15 cm of the digestive tract

▫Expandable for the temporary storage of fecal material

▫The movement of fecal materials into the rectum triggers the urge to defecate

▫Anal canal-

-The distal portion of these columns is where the columnar epithelium changes to keratinized stratified squamous epithelium

▫Anus-

-Internal anal sphincter-

-External anal sphincter-

▪Histology of the Large Intestine- thinner wall than the small intestine

▫The colon lacks villi, but has an abundance of goblet cells and distinctive intestinal glands.

-The intestinal glands are dominated by goblet cells, which produce high quantities of mucus, which provide lubrication for fecal material.

-Mucus is secreted in response to friction or exposure to harsh chemicals

▪Physiology of the Large Intestine- no digestive enzymes are released by cells in the large intestine. If any are present, they came from the small intestine, or the normal microbiota of the large intestine.

▫Less than 10% of nutrient absorption takes place in the colon

▫Reabsorption of water is the primary function of the large intestine

▫The large intestine also reabsorbs bile salts, vitamins, and organic waste products

▫Bacteria in the large intestine generate three vitamins that supplement our dietary supply:

-Vitamin K-

-Biotin-

-Vitamin B5-

▫Organic wastes absorbed by the large intestine include:

-Bilirubin-

-Peptides that remain in the feces are broken down by bacteria and this generates ammonia (NH4+), indole and skatole, and hydrogen sulfide (H2S).

-Ammonia and other toxins are absorbed and converted in the liver to less toxic substances that can be released into the blood, or excreted at the kidneys.

▫Movements of the large intestine- gastroileal and gastroenteric reflexes move materials into the cecum.

-Movement from the cecum to the transverse colon is slow, allowing for water reabsorption

-Peristaltic waves move along the length of the colon, and haustral churning mix the contents of adjacent haustra.

-Mass movements occur a few times a day, are powerful peristaltic contractions that move material from the transverse colon through the rest of the large intestine

-Defecation reflex- distension of the rectal wall triggers this reflex, involves two positive feedback loops.

-Rectal stretch receptors also trigger two reflexes important in the voluntary control of defecation. Both are long reflexes.

-The first triggers the relaxation of the internal sphincter

-The second triggers the contraction of the external sphincter

Digestion and Absorption

▪The Processing and Absorption of Nutrients

▫Food contains large organic molecules, many of them insoluble

▫These molecules have to be broken down to be able to be absorbed.

▫The smaller products of digestion will either be used to provide energy or used to synthesize carbohydrates, proteins, and lipids.

▫Digestive enzymes break the bonds between these large macromolecules using a process called hydrolysis (adding a water to a molecule to break it into smaller parts)

-These enzymes come from saliva, tongue, stomach, pancreas, and the brush border.

▪Carbohydrate Digestion and Absorption- proceeds in two steps:

▫Salivary and pancreatic enzymes- break complex carbohydrates into disaccharides and trisaccharides

-Salivary amylase-

-Pancreatic alpha-amylase-

▫Brush Border Enzymes-

▫Intestinal epithelium absorbs monosaccharides by facilitated diffusion and cotransport.

▪Lipid Digestion and Absorption- involves lingual lipase and pancreatic lipase