THE DIGESTIVE SYSTEM

 

All organisms require energy (ATP) to carryout various cellular and extracellular (physical) activities. The digestive system is the means whereby these goals are achieved. ATP is produced through the digestive system by digesting the food (mechanically and chemically) and absorbing the digested food molecules.

 

Digestion is divided into the following areas:

a.      Ingestion (intake and initial mechanical digestion)

b.      Mechanical (involves mastication of the food)

c.      Chemical (reactions with enzymes, gastric juice)

d.      Absorption (occurs mostly in the small intestines)

e.      Transport (food nutrients are transported to various parts of the body directly from the small intestines.

The processes and mechanisms of digestion are presented.

 

I.      THE ORGANS OF DIGESTION

 

1.  The Alimentary Canal

The alimentary canal is a tubular gastrointestinal tract (GI) and its accessory organs.

·       The GI starts from the mouth and ends in the anus. The organs of the GI include the mouth (oral cavity), pharynx, esophagus, stomach, small intestines and large intestines.

·       Accessory organs:

Teeth, tongue, salivary gland, liver, gallbladder and pancreas.

2.     Serous Membranes of the GI

Most of the organs of the GI are covered by serous membranes that line the body cavities and cover the organs that lie within these cavities. 

·       These membranes secrete fluid that moistens the cavity containing the abdominal viscera. There are two types of serous  membranes: the parietal portion lines the body wall and the visceral portion covers the internal organs.

·       Serous membranes of the abdominal cavity are called peritoneal membranes (peritoneum); they are composed of simple squamous epithelium, and connective tissues.  There are two types: parietal, visceral and

·       Parietal peritoneum:  lines the wall of the abdominal cavity. Sheets of the peritoneum combine to form a double-layered peritoneal fold called mesentery, which support the GI and allow the movement of small intestines during peristalsis. The mesentery also provides structures for passage of nerves, blood vessels. The specific portion of the mesentery that supports the large intestine is called mesocolon.

·       Visceral peritoneum:  is the membrane that covers the surfaces of the abdominal organs. The space – peritoneal cavity- is between the parietal and visceral peritoneum.

 

4.   Layers of the GI

The GI is composed of four layers: mucosa, submucosa, muscularis and serosa. These tissues perform specific function in digestive process.

·       Musosa: lines the lumen of the GI tract, provides absorptive surface and secretory layer. Composed of simple columnar epithelium supported by the lamina propria (thin layer of connective tissue). The lamina propria contains lymph nodes important in protecting against diseases. The smooth muscle layers above the lamina propria is called muscularis mucosa from which the folds of the small intestines (villi) are derived. The villi increases the absorptive surface area of the small intestines. The mucosa also contains numerous goblet cells, which secrete mucous.

·       Submucosa:  it is thick and highly vascularized layer of connective tissue.  Contains blood vessel, nerves and glands. The submucosal plexus (Meissner’s corposules) provide an ANS nerve supply to the muscularis mucosa.

·       Muscularis: responsible for intestinal contractions and peristaltic movements throughout the GI tract. It contains an inner circular and an outer longitudinal layer of smooth muscle. Contractions from these muscles churn food with digestive enzymes and move molecules along the GI tract. The myenteric plexus provides the major nerve supply to the GI. It includes fibers from the sympathetic and parasympathetic nerves from the ANS.

·       Serosa: The serosa is the outer membrane of the GI. It binds and protects the tissue beneath; it is composed of loose connective tissue and a layer of simple squamous epithelium of visceral origin.

 

5.        Innervation of the GI

GI tract receives nerve supply from the ANS (sympathetic and parasympathetic nerves).

·       Nerves from the parasympathetic innervate the stomach, esophagus, pancreas, gallbladder, small intestines and upper portion of the large intestine.

·       Parasympathetic nerves from the spinal nerves in the sacral region innervate the lower portion of the large intestines.

·       The submucosa plexus and mesentary are the sites where preganglionic fibers synapse with postganglionic fibers that innervate the smooth muscle of the GI. When the parasympathetic fibers are stimulated, it leads to increased secretion of fluids in the GI and also increases peristalsis.

·       Stimulation of the sympathetic nerves produce opposite effects. Stimulation of the sympathetic nerves leads to inhibition of peristalsis, decreases secretions and constricts sphincters along the GI tract.

 

6.        Mouth, Pharynx and associated structures

Mouth: It is the receptacle for food and also initial mastication, initial digestion of food and swallowing of the food.

·       The mouth is also involved in respiration (breathing). The cheeks form the lateral wall of the oral cavity (made of skin, subcutaneous fat, facial muscles and stratified squamous epithelium).

·       The lips are fleshy mobile organs made of mucous membrane called labial frenulum; they are mostly associated in humans with speech, sucking and manipulating food. The lips are attached to mucous membrane. The pharynx (throat) posterior to the mouth is lined with nonkeratinized stratified squamous epithelium; it is also the common gateway for digested food products and air.

 

7.    Tongue: 

The tongue moves food around in the mouth during mastication and to assist in swallowing food as well as in speech function.

·       It is made up of mostly muscle and covered with connective tissue.  Two thirds of the tissue lies in the oral cavity and the remaining one-third lies in the pharynx attached to the hyoid bone.

·       Lingual tonsils are located on the surface of the base of the tongue. On the surface of the tongue are numerous elevations called papillae which gives the tongue its rough surface and taste buds that responds to sweet, sour, salty and bitter taste.

·       There are three types of papillae: filiform (sensitive to touch, lack taste buds and more numerous), fungiform (larger, round and scattered among filiform types) and vallate are arranged in ‘v’-shape on the posterior surface of the tongue.

 

Palate: forms the roof of the mouth and consist of bony hard palate (formed of palatine process of the maxillae and horizontal plates of palatine bone) anteriorly.

·       Transverse ridges called palatal ruggae which serve as friction ridges in food processing and tongue manipulation. 

·       The soft palate located posteriorly is a muscle covered with mucous membrane. The vulva is a cone-shaped suspension from the middle of the soft plate. The soft plate and vulva closes the nasopharynx preventing food and fluid from entering the nasal cavity.

 

8.   Teeth:

The teeth in humans function basically for cutting, piercing, and grinding. The incisors (four pairs) are used for cutting and shearing food. 

· Two pairs of canines or cuspids are for holding and tearing. Premolars or bicuspids and molar have two or three roots, rounded or irregular surfaces called dental cusps for grinding or crushing.

·  Two sets of teeth develop during a person’s life time: the twenty deciduous teeth (milk) begins at about 6months and last for about  2 1/2 years and are replaced by two-third of permanent teeth. By age 17, wisdom teeth replaces the molars.

· Each tooth consists of crown, roots, neck, alveoli and membrane. The bulk of the took consist of dentine, enamel, pulp cavity, root canal and apical forament.

 

9.        Salivary Gland

Produce saliva that aids in digestion. It also functions in cleansing the teeth and dissolving food molecules.

·       Saliva contains digestive enzyme called salivary amylase which initiates carbohydrate digestion, converting carbohydrates into disaccharides.

·       The salivary gland is divided into parotid gland, submandibular gland and sublingual glands. These glands are connected to ducts which transport the fluid secreted from these glands.

 

10.        Pharynx

This organ is the gateway that connects the oral cavity and the nasal cavity to the esophagus and the trachea.

·       It function in both respiratory and digestive systems. It is divided into nasopharynx, oropharynx and laryngopharynx.

 

11.        Esophagus

The portion of the GI that connects the pharynx to the stomach.

·       The supporting walls are made up of skeletal muscle and stratified squamous epithelium and mostly smooth muscle tissues in the middle and lower parts. 

·       Esophageal sphincter prevents regurgitation of fluid and food after they exit the esophagus.

 

12.        Stomach

The stomach is “J” shape and situated immediately below the diaphragm. There is no carbohydrate digestion in the stomach.

·       Gastric juice and HCl produced by the parietal cells mix with the food. HCl activates inactive trpsin and pepsinogen converting them to active pepsin and trpsin.

·       These latter enzymes act on specific bond on the proteins, thus cleaving the bonds produce dipeptides and tripeptides.

·       In additon to enzyme activation, the low pH also renders the food acidic, destroying pathogens. The food at this time is in a milky consistency called chyme and the pH of the entire food content is around pH 1.5-1.7 (approx. 2).

 

13.   Small Intestine

The beginning of the small intestine is the duodenum, which contains the pyloric sphincter valve.

·       This valve also prevents regurgitation of food from the small intestines into the esophagus.

·       the small intestines is a mass of long tube in the GI, which process the final part of digestion. It is supported by the mesentery that furnish blood vessels, nerves, connective tissue and ligaments.

·       the liver and gallbladder common bile duct as well as the pancreatic duct forms a hepatopancreatic duct which empties into the duodenum thru a duodenal papilla. It is thru this duct that pancreatic juice and bile enter the small intestines.

·       The duonenum histologically contains mucous secreting duodenal glands in the submucosa.

·       Plicae circulares: larger macroscopic folds of the mucosa

·       Villi: fingerlike projections provide absorptive surface for digested nutrients. Microvilli are microscopic projections. Both villi and microvilli are covered with columnar epithelia cells along with mucous secreting goblet cells. Lymphatic and blood vessels run thru the villi enabling absorbed nutrients (ie fats, proteins and carbs) to enter the blood vessel or lymphatic vessels.

·       Epithelia cells at the top of the villi contain intestinal crypts lumen which contain hydrolytic enzymes which act on disaccharides, polypeptides and other molecules.

·       Two types of movements: peristalsis and segmentation.

a.    Peristalsis: wavelike motion of the smooth muscles which move food and molecules along the SI.

b.   Segmentation: conscricted and contractile movements that occurs simultaneously along the lengths of the SI.

 

14.   Large Intestine

 

The large intestine is divided into cecum, colon, rectum and anal canal.

·       A specialized mesentery called mesocolon supports the large intestine. The cecum is the initial portion of the large intestines that contains the appendix.

·        The entire large intestine is divided into ascending, transverse and descending colon, ending with the anal canal.

·       The sympathetic nerve innervation originate from the mesenteric plexus and the parasympathetic nerves originate from the splanichnic and vagus nerves.

·       The large intestines is primarily responsible for fluid and electrolyte absorption. Water is extraction as a result of aldosterone action promote water absorption (20%) into small intestines and 90% into the large intestines.

·       Mineral transport occur as a result of mineral gradient or Na/K ATPase pump. In addition, it stores, and expected unwanted materials in form of fecal waste.

·       Most of the bulk of unbroken  or partially food materials are broken down by bacteria.

15.   Liver and Gallbladder

The liver is the largest organ in the body consisting of two major and two minor lobes. These lobules consist of hepatic plates of only one or two-cell layers thick.

·       The lobules consists of cells called hepatocytes. The hepatic plates are separated from each other by large capillary spaces called sinusoid and phagocytic Kupffer cells line the sinusoids. 

·       Hepatic portal vein and artery open into the sinusoid. The hepatic portal system describes the circulation from capillaries to vein to capillaries. 

·       The liver has many functions. They include secretion of bile, detoxification of blood (removal of molecules from blood and excretion into bile, conversion of metabolic toxins into bilirubin and uric acid, conjugation of active compound into inactive products for urinary excretion). Other include secretion of glucose, triglycerides, ketone bodies, (processes of glycogenesis; lipogenesis and glycogenesis and gluco- neogenesis and ketogenesis) production of plasma proteins  (about 70% of plasma albumin).

 

16.    Gallbadder: secrete bile. Bile a greenish yellowish solution derived from bile salts emulsifies fat (a process similar to soap).

 

17.        Pancreas:

produces pancreatic juice which contain water, bicarbonate and a wide variety of digestive enzymes. The enzymes include pancreatic amylases which digests starches; trypsin which digest protein and lipase which digest lipids or triglycerides.

 

II.  DIGESTION AND ABSORPTION

 

The caloric content of food nutrients:

Carbohydrates:   4 kcals/g

Proteins              4 kcals/g

Fats                    9 kcals/g

Alcohol              7 kcals/g

The pH of the partially digested food mixture ( ie chyme) entering the duodenum (ie beginning of the small intestines) is changed from pH 2.0 to pH 7.2-7.4) by the action of pancreatic juices which contains bicarbonate. The bicarb neutralizes the acidity and raises the pH to physiological level.

 

Absorption of glucose, proteins and lipids

·       Carbohydrates which enter the SI as disaccharides (ie lactose, maltose and sucrose) are acted upon by pancreatic amylases or ptyalin (lactase, maltase and sucrase). These enzymes converts disaccharides into glucose as the end product.

·       Proteins enter the SI as dipeptides and tripeptides. These molecules are acted upon by pancreatic proteseas (chymotrypsin, carboxypeptidases, aminopeptidases) converting the peptides into monomers of amino acids.

·       Lipids or triglycerides initially undergo emulsification in the duodenum by the action of bile, producing HDL, chylomicrons and LDL. Pancreatic Lipases act on the lipid molecules converting them into fatty acids and glycerol.

 

Vitamins and Minerals and Water

These molecules are not converted into any form.

 

Absorption of Food Nutrients

·       The end products of carbohydrates, proteins, lipids, vitamins, minerals and water are absorbed across the wall of the small intestines.

·       Small molecules including lipids, vitamins, minerals are transported across the intestinal wall by diffusion.

·       Water is transported passively (ie osmosis) and larger molecules by facilitated, targeted, or carrier-mediated transport.

·       Transport of monovalent (sodium or potassium ions) or divalent cations (zinc, copper, mg, etc) against their concentration gradient may occur by active transport.

·       Large molecules of lipids directly enter the lymphatic vessels for transport.

 

Transport of food nutrients

·       Once absorbed into the blood vessels, the molecules are transported into the tissues for cell utilization and generation of ATP via cellular respiration (aerobic) and glycolysis (anaerobic).

·        This are may be expanded when we discuss metabolism (how these food products are used to generate energy in various metabolic pathways

 

Vitamins and Minerals

Vitamins: They are essential key constituents of food and one of six classes of nutrients. They do not contain any calories therefore are rapidly absorbed and metabolized or incorporated into tissues.

a.          There are two classes of vitamins: water soluble (vitamin, B1-B12, Vit. C) and fat-soluble vitamins.

Water-soluble vitamin fall in three categories: promote energy metabolism, coenzymes, DNA and blood-forming, tissue-building/antioxidants.

b.         Fat soluble Vitamin (ADEK): mostly fat soluble; aids in vision improves, tissue building, calcium absorption, blood clotting mechanism and antioxidant activities.

 

Minerals: Minerals are one of the classes of nutrients. There are about 92 minerals divided into macrominerals (Mg, Fe, S, N,  and trace elements (zn, cu, I, Md, Va, Sel). Both macro and trace elements function as cofactors and participate in tissue remodeling or as structural components of tissues.

 

The Eeeeend.