digestion

Digestive tract

A: Functions - The digestive tract functions in the following ways.

  1. Ingestion of food into the mouth
  2. Movement of food along the tract propulsion.
  3. Mechanical breakup of food Must be small enough to be absorbed hydrolysis.
  4. Secretion of digestive juices
  5. Chemical digestion of food Hydrolysis eat protein, amino acids to absorb.
  6. Absorption of digested nutrients into the blood and lymphatics Small intestine, mostly.
  7. Elimination of undigestible substances and waste products

B: General anatomy - The digestive tract is a long tube to which a series of accessory glands are attached.

Regions - The digestive tube has five major regions which are as follows.

  1. a. Mouth - salivary glands
  2. b. Esophagus pharynx. Stomach
  3. d. Small intestine – Liver, pancreas. e. Large intestine
  4. Anatomy of the tract wall - The great bulk of the functions of the digestive
    tract are carried out by the wall of the tract. Beginning at the esophagus and continuing throughout the rest of the tract, the wall of the tube has the same basic structure. The wall consists of four layers with nervous plexuses connecting the layers. The layers are as follows.

Tunica mucosa – membrane lines track, epithelium mostly simple columnar, some stratified squamous, layers areolar tissue, smooth muscle, function secretion absorption of end products protect from infection.

Tunica submucosa – thick layer, dense or areolar tissue contains blood vessels, lymphatics, nerves glands, nerve groups, sub mucosal, part of ANS, function carries away absorbed nutrients.

Tunica muscularis – two layers, circular and longitudinal, double smooth muscle, inner layer fibers arranged in circle, outer longitudinal. inner sometime forms sphincters - function mix and propel food.

Tunica serosa – connective tissue, strong, esophagus is called visceral peritoneum

Intrinsic nerve plexuses- complex, between tunics (layers)

3. Accessory glands - Although the wall of the tract contains a large number of glands, there are three d glands that lie outside of the tract and dump their secretions into the tract via ducts.

a. Salivary glands – produces saliva, several functions; cleans teeth, dissolves food, enzyme breakdown starch in mouth, two secretions serous cells watery mucus viscus cont. (mucin)

(1) Parotids – 3 pairs beneath the ears, largest of 3 pairs, inflammation of mumps
(2) Submandibular (submaxillary) –these lie medial to mandible
(3) Sublingual’s – floor of mouth beneath tongue

b. Pancreas - This large gland is located beneath the stomach. It produces pancreatic juice which contains many digestive enzymes. Acimi cells produce enzymes

c. Liver - This, the largest organ in the body, is divided into a left and right lobe. Cords and cells that radiate outward has capillary network, sinusoids, houses a falciform ligament which separates lobes Hepatocytes, function part of liver production bile.

(1) produces bile – Aids and breakdown of fat, helps get rid of metabolic waste then secreted by hepatic duct.

(2) The liver has many more functions, over 200 at last count. Some
of the major ones include the following.

(a) produces albumin other plasma proteins
(b) manufacturers thrombin and other clotting factors
(c) synthesizes and stores glycogen
(d) regulates carbs proteins, lipid metabolism
(e) produce waste product like urea ketone body
(f) stores iron, vitamin A, D, E and K
(g) detoxes most mater toxic to cells NH3 (ammonia) into urea
(h) major role in body defense phagocytizing microorganisms via macrophages (Kupffer cells)

C: Movements of materials through the tract - There are two basic types of contraction movement found in the digestive tract.

1: Segmentation –stationary control but mixes materials

2: Peristalsis – wave like, moves through tract

D: Control of flow - valves - The directional movement of materials throughout the tract is regulated by a series of valves that are often referred to as sphincters.

E: Digestive process - In examining the process of digestion we will look at each of the five major regions of the tract, beginning with the mouth, and detail the major processes that occur in each region.

  1. Mouth and pharynx - The mouth and oral cavity are the first parts of the
    tract. They extend from the lips to the oropharynx. The digestive activities of the mouth include the following.

a. Analysis/ingestion. receptors analyze food materials before swallowing.
b. Mastication chewing breaking of food, greater surface area for enzymes to act on

-tongue - moves food, contains glands, containing lingual frenulum , breaks down lipids, papillary. projections on tongue aids licking, houses taste buds and,

-intrinsic muscles – intrinsic muscle allow change shape

-extrinsic muscles – extrinsic moves back and forth,

-lingual frenulum - lingual squamous mucus tissue secure tongue to mouth

c. Salivation – mix saliva with food particles to form moist bolus
(1) Composition of saliva – 97% water mucin, salivary amylase enzyme, hydrolyzes starch and electrolytes

(2) Functions of saliva-2 types of secretion: serous mucus, salivary amylase dissolves food cleans teeth, aids in speech and some bacteria.

d. Swallowing - The processes involved here are as follows: The food bolus is forced into the pharynx by the tongue, series of reflexes close off other openings into pharynx, esophagus opens, food bolus forced in, peristaltic waves move the bolus down the esophagus to the stomach.

Esophagus – 10 inches long, upper more skeletal muscle for swallowing, lower smooth muscle cardiac orifice where meet stomach also called Z line, epithelium changes to cuboidal

  1. Stomach – when empty long folds called rugae,

a. Functions
(1) It stores food and slowly releases it to the intestine.
(2) It mixes food with the stomach (gastric) secretions which results in liquification.
(3) Protein digestion begins in the stomach.
(4) Production of intrinsic factor, a glycoprotein which must be present for the absorption of vitamin B12

b. Gastric secretions - The stomach produces several different substances
which are collectively termed gastric juice. The epithelium that lines the stomach is a simple columnar one made up entirely of goblet cells that secrete an alkaline mucous that coats and protects the stomach from digestion. Gastric juice is produced by the gastric glands. These glands are made up of four types of cells.

(1) Mucous neck cells – found and neck of gland acidic mucus
(2) Parietal cells – produce HCL and intrinsic factor
(a) speeds of breakage decrease of pepsinogen HCL does four things
(b) provides PH for pepsin
(c) kills microorganisms
(d) stimulates release of pancreatic juice

(3) Chief (zymogenic) cells - Produce pepsinogen active form of pepsin hydrolyzes
(4) Enteroendocrine cells – produces several hormones to , (rennin) enzyme secreted by gastric glands of infants, milk protein casein

c. Control of gastric secretions - The stomach produces up to three liters of gastric juice per day. Secretion is under both neuronal and hormonal control.
(1) Phases of secretion (first 2 are gastric)
(a) Cephalic phase - This is stimulated by vagal activity to the
stomach. The trigger can be the sight, smell, or taste of food.
(b) Gastric phase - The trigger for this is distention of the
stomach, increase in gastric pH, and the presence of proteins
in the stomach.
(c) Intestinal phase - This occurs when acid chyme (liquified food)
from the stomach begins to distend the duodenum (first
section of the small intestine).

  1. Small intestine
    a. Functions
    (1) Digestion – bulk of chemical digestion takes place here via hydrolysis
    (2) Absorption – most absorption occurs here into blood, nutrients, vitamins, minerals, water are absorbed

b. Regions - The entire small intestine is about six meters long and is divided into three regions based upon its microscopic structure.
(1) Duodenum small C shaped section, receive secretions from liver, pancreas, thick and immovable, short, has Brunner’s glands produced alkaline mucus acts on acidic chyme from stomach
(2) Jejunum – second section
(3) ileum - final section contains piers patches prevents bacteria from entering blood

c. Digestive juices - The small intestine receives three major digestive juices. Collectively these three juices contain all of the enzymes and other substances which are necessary for the breakdown and absorption of all food categories.

(1) Pancreatic juice - Approximately 750 ml of this juice are produced per day.
(a) Composition
/1/ Digestive enzymes - proteases, lipases, and carbohydrase.
/2/ Water
/3/ Bicarbonate – increase PH (bicarbonate) 8 PH keeps small intestine in optimal range neutralizes acidic chyme

(b) Control of secretion - Both neural and hormonal mechanisms are involved.
/1/ Neural - There is a vagal reflex which increase enzyme concentration and pancreatic juices
/2/ Hormonal
/a/ Secretin - release bicarbonate
/b/ Cholecystokinin – stimulates enzyme release, response to chemical release of chyme, protein, fats, increases pancreatic juices.

-carbs - breaks down starch and lactase
-proteins broken up by triplon from pancreas carboxypeptidase
-lipids occurs in small intestine sole site of lipid digestion by light phase
-nucleic acids hydrolyzed by pancreatic nucleic

(2) Bile - About 600 to 1000 ml of bile are produced per day. About 40 to 70 ml of this amount is stored in the gall bladder at any one time. Bile is released into the duodenum via the common bile duct.
(a) Composition - The principal components of liver bile are water, electrolytes, bile salts, bile pigments, bilirubin Verde, cholesterol, phospholipids

(b) Function - Liver bile emulsifies fat, assist in absorption of fat-soluble vitamins, assists calcium absorption, eliminates hemoglobin and cholesterol

(c) Control of secretion - There are three levels of control for the secretion of bile, chemical, neural, and hormonal.
/1/ Chemical - Bile salts in the plasma stimulate bile flow. Common bile duct
/2/ Neural - Vagal stimulation increases bile secretion.
/3/ Hormonal - Cholecystokinin stimulates the contraction of the gall bladder as well as the relaxation of the Sphincter of Odi which regulates bile flow through the common bile duct.

-Gall bladder SAC stores bile sphincter of odi controlled output

-Obstructive jaundice bile is major route for cholesterol excretion, if bile salts are inadequate (lesofin) is there to break it down, may have crystals form (gallstones) causes yellow pigment in skin

(3) Intestinal juice - This juice is produced by the cells of the mucosa (crypts of Lieberkühn, intestinal glands) and Brunner's glands (submucosal glands) which are located in the submucosa of the duodenum.

d. Absorption - This occurs across the epithelial cells that line the mucosa of the small intestine. The mucosa is highly folded, with the folds being termed plicae. This effectively increases the surface area.
(1) Intestinal villi - In addition to the plicae, there is another mechanism for increasing available surface area. These are the microscopic villi, finger-like projections that cover the mucosa.
The bulk of absorption occurs in the Jejunum, and this is where plicae and Villi are best developed. As the ileum is approached the plicae and villi decrease, and by the end of the ileum there are no plicae at all. There are located here nodules of lymphoid tissue known as Peyer's patches, which protect the small intestine against bacteria from the large intestine.

(2) Mechanisms of absorption - Materials move into capillary and lactiles and villi by usual diffusion ATP osmosis, most nutrients go through capillary lipids usually entering lactiles
(a) Active transport - Most organic nutrients, electrolytes absorption this way. mostly small intestine except two electrolytes in duodenum iron and calcium
(b) Osmosis – water absorbed active transport nutrient or electrolyte creates osmatic radiance so water can actively enter

carbs – across villi via protein carriers
proteins – coupled with sodium yielding amino
fatty acids - acids associated (micelles) through epithelium by diffusion then resynthesized through triglycerides then combine with protein forming chylomicron through to lymph’s

pentose sugars – through endothelium via ATP
vitamins – most are fused with dietary fats B12 must bind to intrinsic factor typical day 5 to 10 liters of water enters 5% will reach large intestine

  1. Large intestine
    a. Anatomy – The large intestine is about 1.5 meters long. It extends from the ileocecal valve to the anus. There are six different regions.

(1) Cecum -This is a blind pouch which receives the ileum. The vermiform appendix extends downward from the cecum. The remaining regions of the large intestine are referred to as the colon.
(2) Ascending colon – extends up from cecum
(3) Transverse colon –across two descending
(4) Descending colon –towards sigmoid: towards mid line of b
(5) Sigmoid colon –
(6) Rectum –straight portion

b. Functions
(1) Reabsorption of water – retain as much as possible
(2) Synthesis of vitamins – certain bacteria and: B&K vitamins
(3) Elimination –: the colon compact stores and eliminates food residue

c. Movements – The colon undergoes several different types of movements.
(1) Sluggish peristalsis –very slow wave like, some water
(2) Mass peristalsis –forces material through rectum through distention then defecation
(3) Defecation reflex – This reflex stimulates peristaltic contractions of the descending colon, sigmoid colon, and rectum.

F: pathologies.

  1. Vomiting –complex reflex coordinated by vomiting center in medulla which reverses peristalsis eject in stomach leads to esophagus.
  2. Diarrhea -fecal water due to over secretion of fluid or intestinal infection
  3. Constipation –reduce motility of large intestine fecal matter remains for extended period of time which leads to excess water reabsorption
  4. Peptic ulcer – This is an erosion of the mucosa of the stomach or small intestine by acid and digestive enzymes. Excessive gastric juice secretion or insufficient mucous secretion is normally the immediate cause. Erosion may continue into the blood vessels causing a bleeding ulcer. Complete penetration through the tract wall is termed a perforated ulcer. Recent evidence indicates that a bacterial infection may be a contributing factor to ulcers, and most ulcer treatment now involves administration of antibiotics as well as the traditional treatments.

G: Effects of aging – There is a decrease in the secretion and a loss of motility with age. As a result the digestive process is not as efficient as it was at an earlier age. Digestion takes much longer and food remains in the tract for extended periods. Various pathologies such as ulcer and cancer also increase with age although they are not necessarily a direct consequence of aging.

hepatitis - inflammation of liver, virus
cirrhosis - chronic inflammation due to excess alcohol or hepatitis causes liver to shrink

exam question stomach
mucus cell produce acidic, needed to activate pepsin (partial cells) produce ACL and intrinsic factor speeds up BD of activating pepsinogen chief cells by injuring to HCL ( chlorine ) (parietal cells) leads to activate enzyme pepsin helps (neck cells)