Chapter 24

Digestive System

Digestive System Anatomy• Digestive tract

– Alimentary tract or canal– GI tract

• Accessory organs– Primarily glands

• Regions– Mouth or oral cavity– Pharynx– Esophagus– Stomach– Small intestine– Large intestine– Anus

The function of digestive system: Mechanically breakdown the food stuff and enzymatically digest them. Absorb necessary nutrients including ions. Lubrication, emulsification, mixing, transport - Deglutition, bolus, peristalisis, etc.Release of wastes from the body

Functions• Ingestion: Introduction of food into stomach• Mastication: Chewing• Propulsion

– Deglutition: Swallowing

– Peristalsis: Moves material through digestive tract

Functions

• Mixing: Segmental contraction that occurs in small intestine• Secretion: Lubricate, liquefy, digest• Digestion: Mechanical and chemical• Absorption: Movement from tract into circulation or lymph• Elimination: Waste products removed from body

Digestive Tract Histology

Digestive System Regulation

• Nervous regulation– Involves enteric

nervous system• Types of neurons:

sensory, motor, interneurons

– Coordinates peristalsis and regulates local reflexes

• Chemical regulation– Production of hormones

• Gastrin, secretin

– Production of paracrine chemicals

• Histamine

• Help local reflexes in ENS control digestive environments as pH levels

Oral Cavity

• Mouth or oral cavity– Vestibule: Space between

lips or cheeks and alveolar processes

– Oral cavity proper

• Lips (labia) and cheeks• Palate: Oral cavity roof

– Hard and soft

• Palatine tonsils• Tongue: Involved in

speech, taste, mastication, swallowing

Salivary Glands

• Produce saliva– Prevents bacterial

infection

– Lubrication

– Contains salivary amylase

• Breaks down starch

• Three pairs– Parotid: Largest

– Submandibular

– Sublingual: Smallest

a. Functions of the oral cavity

In addition to chewing, or mastication in the oral cavity its enzyme contributes the initial stage of digestion.

The salivary glands release salivary amylase, which digests starch into glucose and the other polysaccharides to disaccharides. (Fig. 24-7 and Fig. 24.9)

Note that humans do not digest cellulose, since they do not have cellulase.

In addition, saliva contains lysozyme and immunoglobulin A to fight against bacteria.

Mucous release from the other glands in the oral cavity contains mucin, a proteoglycan, that contributes lubrication of the mouth.

The salivary glands stimulation is primarily through the parasympathetic nerve.

Higher centers of the brain also affect the activity of the salivary glands.

Pharynx and Esophagus

• Pharynx– Nasopharynx

– Oropharynx: Transmits food normally

– Laryngopharynx: Transmits food normally

• Esophagus– Transports food from

pharynx to stomach– Passes through

esophageal hiatus (opening) of diaphragm and ends at stomach

• Hiatal hernia

– Sphincters• Upper• Lower

Deglutition (Swallowing)

• Three phases– Voluntary

• Bolus of food moved by tongue from oral cavity to pharynx

– PharyngealReflex: Upper esophageal sphincter relaxes, elevated

pharynx opens the esophagus, food pushed into esophagus

– Esophageal• Reflex: Epiglottis is tipped posteriorly, larynx

elevated to prevent food from passing into larynx

Fig. 24-11 The swallowing process

Stomach Anatomy

• Openings– Gastroesophageal: To

esophagus

– Pyloric: To duodenum

• Regions– Cardiac

– Fundus

– Body

– Pyloric

Stomach Histology

• Layers– Serosa or visceral

peritoneum: Outermost

– Muscularis: Three layers• Outer longitudinal

• Middle circular

• Inner oblique

– Submucosa

– Mucosa

Stomach Histology

• Rugae: Folds in stomach when empty

• Gastric pits: Openings for gastric glands– Contain cells

• Surface mucous: Mucus• Mucous neck: Mucus • Parietal: Hydrochloric

acid and intrinsic factor • Chief: Pepsinogen • Endocrine: Regulatory

hormones

i. Secretions in the stomach (Fig. 24-13)

The mixture in the stomach is called chyme.

Stomach secretes mucus, hydrochloric acid, gastrin, intrinsic factor, and pepsinogen, a precursor to protease pepsin.

Alkaline mucus secreted from the mucus cells protects the epithelial cells from the acidic chyme and pepsin.

Parietal cells in the gastric glands secrete intrinsic factor and concentrated HCl. Intrinsic factor is a vitamin B12 binding glycoprotein for better absorption of B12.

Chief cells within the gastric glands secrete pepsinogen, which will be activated to pepsin by HCl.

i. Low pH in the stomach

Relatively high concentration of HCl released in the stomach has the following functions

(a) The value of pH in the stomach is 1 - 3.

(b) Most of bacteria are killed at this pH. But not all! - Pylori

(c) Inactivate amylase, thus no further digestion of carbohydrates.

(d) Many proteins are denatured.

(e) Pepsin, now activated at this pH, can digest these proteins

Fig. 24-14 Secretion of HCl

i. Secretion of H+

Release of H+ from parietal cell into the stomach starts with consumption of CO2 from blood. (Fig. 24-14) Note the names of players in this process: CA, ATP requiring proton/potassium exchange pump, bicarbonate/chloride shift, movements of potassium, bicarbonate and chloride ions.

i. Regulation of stomach secretion

2 - 3 L of gastric secretion/day.

Up to 700 ml/meal depending on the types of meals.

Both neuronal and hormonal regulations are possible. For details study Fig. 24-15.

Phases of Gastric Secretion

Fig. 24-15a

Fig. 24-15b

Fig. 24-15c

Movements in Stomach

Small Intestine• Site of greatest amount of

digestion and absorption• Divisions

– Duodenum– Jejunum– Ileum: Peyer’s patches or

lymph nodules

• Modifications– Circular folds or plicae

circulares, villi, lacteal, microvilli

• Cells of mucosa– Absorptive, goblet, granular,

endocrine

Small Intestine Secretions

• Mucus – Protects against digestive enzymes and stomach acids

• Digestive enzymes– Disaccharidases: Break down disaccharides to

monosaccharides– Peptidases: Hydrolyze peptide bonds– Nucleases: Break down nucleic acids

• Duodenal glands – Stimulated by vagus nerve, secretin, chemical or tactile

irritation of duodenal mucosa

The primary center for digestion and adsorption. In three parts: duodenum, jejunum and ileum making up to 6 meters (Fig. 24-16)

In fact, the digestive function in the small intestine follows the food stuff being digested by pancreatic juice to relatively smaller molecules.

Observe the anatomy and histology of the intestinal wall to find intestinal glands, capillary networks in the villi, lacteal, goblet cells, etc. (Fig. 24-17)

i. Secretions in the small intestine (Fig. 24-16, 17)

Duodenal glands and Goblet cells release mucus.

The final stage of break down to small molecules so that they may be absorbed through villi.

Absorptive cells release digestive enzymes, such as aminopeptidase, peptidase, enterokinase (trypsinogen activator), amylase, sucrase, maltase, isomaltase, lactase and lipase.

These enzymes are bound to the membranes of the absorptive cell Microville.

Many other digestive enzymes are supplied from the pancreas.

i. Movement in the small intestine

Segmental contraction for mixing and peristaltic contractions for propelling are observed.

The contractions move at rate of 1 cm/min, thus taking 3 - 5 hours for chyme to move from the pylorus to the ileocecal junction.

ii. Absorption from the small intestine

Out of about 9 L of water enters the digestive system, the small intestine absorbs about 8 - 8.5L by osmosis.

Duodenum and Pancreas

Duodenum Anatomy and Histology

Pancreas

• Anatomy– Endocrine

• Pancreatic islets produce insulin and glucagon

– Exocrine• Acini produce digestive

enzymes

– Regions: Head, body, tail

• Secretions– Pancreatic juice

(exocrine)• Trypsin• Chymotrypsin• Carboxypeptidase• Pancreatic amylase• Pancreatic lipases• Enzymes that reduce

DNA and ribonucleic acid

a. Functions of the pancreas

The gross anatomy and cytology of the pancreas is shown in (Fig. 24-18). The pancreas has both endocrine and exocrine cells.

Endocrine: Pancreatic islet contact to blood stream.

Alpha cells – glucagonsBeta cells – insulin

Exocrine: Acini cells open to ducts and secrete enzymes

Bicarbonate Ion Production

i. Pancreatic juice to adjust pH.

The columnar epithelial cells of the pancreas contains bicarbonate ions, which will be released into the intralobular duct of the pancreas. The bicarbonate ion neutralize the acidic chyme and stops pepsin activity, while make it possible for the pancreatic enzymes to remain active in the small intestine. (Fig. 24.22 of Seeley)

i. Pancreatic enzymes

The acinar cells of the pancreas produce pancreatic enzymes.

Many proteolytic enzymes are released in the form of precursors.

They include, trypsinogen , chymotrypsinogen, and procarboxypeptidase and are activated by other enzymes, such as enterokinase.

Amylase, lipases are also present.

i. Control of pancreatic secretion

By both hormonal and neuronal means. (Fig. 24-22 and Fig. 24.23 of Seeley)Also study Table 24-1.

Fig. 24-22 Hormone action

Pancreatic Secretion Control

Liver

• Lobes– Major: Left and right

– Minor: Caudate and quadrate

• Ducts– Common hepatic

– Cystic• From gallbladder

– Common bile• Joins pancreatic duct at

hepatopancreatic ampulla

Functions of the Liver• Bile production

– Salts emulsify fats, contain pigments as bilirubin

• Storage– Glycogen, fat, vitamins, copper and iron

• Nutrient interconversion• Detoxification

– Hepatocytes remove ammonia and convert to urea

• Phagocytosis– Kupffer cells phagocytize worn-out and dying red and white blood

cells, some bacteria

• Synthesis– Albumins, fibrinogen, globulins, heparin, clotting factors

a. Liver functions

Anatomy and histology: (Fig. 24.19)i. Bile production

600 - 1000 ml/day

No digestive enzymes

Dilute and neutralize stomach acid and emulsify fats.

The pH of chyme is raised so that pancreatic enzymes can function.

Contains bilirubin from broken down hemoglobin.

Cholesterol, fast, fat-soluble hormone and lecithin are found.

The blood and bile flow: (Fig. 24.20)

Stimulates bile secretion - secretin from duodenum, by parasympathetic vegas nerve, increased blood flow in the liver, etc. (Fig. 24.21)

Blood and Bile Flow

ii. Storage

Liver cells extract sugar from the blood and store as glycogen. Fats, vitamins, copper and iron are also stored for a short while

Thus hepatocytes can control blood sugar level...

iii. Nutrient interconversion

Proteins to carbohydrates to fats etc., and back.

Further transformation of the nutrients. Hydroxylation of vitamin D.

i. Detoxification

Conversion of toxic substances to acceptable substance. For example ammonia is converted to urea for elimination through the kidney as urine.

ii. Phagocytosis

Hepatic phagocytic cells (Kupffer cellsKupffer cells) phagocyte old RBC, WBC, bacteria, etc.

iii. Synthesis

Many blood proteins are synthesized in the liver. Albumin, fibrinogen, globulin, heparin, clotting factors, etc.

Duct System

Gallbladder

• Bile is stored and concentrated

• Stimulated by cholecystokinin and vegal stimulation

• Dumps into small intestine

• Production of gallstones possible– Drastic dieting with rapid weight loss

a. Functions of the gallbladder

The gallbladder stores and concentrates bile produced by the liver.

After a meal, it releases choleostokinin from the duodenum and stimulates releases of bile into the small intestine. (Fig. 24-21)

Fig. 24-21 The Gallbladder

Secretin and cholecystokinin:

Inhibit gastric secretions in the stomach,

Stimulate secretion and release of bile of thegallbladder

Stimulate release of bicarbonate ion and digestive enzymes from the pancreas

Large Intestine

• Extends from ileocecal junction to anus

• Consists of cecum, colon, rectum, anal canal

• Movements sluggish (18-24 hours)

Fig. 24-24 Histology of the large intestine

Large Intestine

• Cecum– Blind sac, vermiform appendix attached

• Colon– Ascending, transverse, descending, sigmoid

• Rectum– Straight muscular tube

• Anal canal– Internal anal sphincter (smooth muscle) – External anal sphincter (skeletal muscle)

– Hemorrhoids: Vein enlargement or inflammation

a. Functions of the large intestine (Fig. 24.25)

Slow sluggish movement in the colon taking 18 - 24 hours. Water and salts are absorbed and microorganisms help in the formation of feces. 1500 ml of chyme entering the colon end up only 80 - 150 ml of feces.

Secretions of Large Intestine

• Mucus provides protection– Parasympathetic stimulation increases rate of

goblet cell secretion

• Pumps– Exchange of bicarbonate ions for chloride ions– Exchange of sodium ions for hydrogen ions

• Bacterial actions produce gases called flatus

Histology of Large Intestine

Movement in Large Intestine

• Mass movements– Common after meals

• Local reflexes in enteric plexus– Gastrocolic: Initiated by stomach

– Duodenocolic: Initiated by duodenum

• Defecation reflex– Distension of the rectal wall by feces

• Defecation– Usually accompanied by voluntary movements to expel feces

through abdominal cavity pressure caused by inspiration

Fig. 24-25 The defecation reflex

Digestion, Absorption, Transport

• Digestion – Breakdown of food molecules for absorption into

circulation • Mechanical: Breaks large food particles to small

• Chemical: Breaking of covalent bonds by digestive enzymes

• Absorption and transport– Molecules are moved out of digestive tract and

into circulation for distribution throughout body

Carbohydrates

• Consist of starches, glycogen, sucrose, lactose, glucose, fructose• Polysaccharides broken down to monosaccharides• Monosaccharides taken up by active transport or facilitated diffusion and

carried to liver • Glucose is transported to cells requiring energy

– Insulin influences rate of transport

Lipids

• Include triglycerides, phospholipids, steroids, fat-soluble vitamins

• Emulsification breaks down large lipid droplets to small

Lipoproteins

• Types– Chylomicrons

• Enter lymph

– VLDL

– LDL• Transports cholesterol

to cells

– HDL• Transports cholesterol

from cells to liver

Cholesterol over 200 mg/100 ml blood risks coronary heart disease (CHD).

Other risk factors are: hypertension, diabetes mellitus, cigarette smoking, low plasma high density lipids (HDL),

Obesity – low HDL

Exercise – high HDL and decreased LDL

Ingestion of fatty acids – increased plasma cholesterol, higher LDH, lower HDL

Proteins

• Pepsin breaks proteins into smaller polypeptide chains

• Proteolytic enzymes produce small peptide chains

– Dipeptides, tripeptides, amino acids

Water and Ions

• Water– Can move in either

direction across wall of small intestine depending on osmotic gradients

• Ions– Sodium, potassium,

calcium, magnesium, phosphate are actively transported

Effects of Aging

• Decrease in mucus layer, connective tissue, muscles and secretions

• Increased susceptibility to infections and toxic agents– Ulcerations and cancers