Digestion physiology second year
The Digestive system
 The digestive system consists of:
The alimentary canal
 Extends from the mouth to the anus.
 Includes the mouth, pharynx, esophagus,
stomach, small intestine, and large intestine.
The major accessory organs
 Salivary glands, liver, and pancreas.
 These organs secrete digestive juices
into the alimentary canal.
 Structure of digestive system:
1- Alimentary canal: it consists of:
2- The mouth
3- The pharynx
4- The esophagus
5- The stomach
6- The small
intestine
7- The large
intestine or
colon
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Contains The teeth , The tongue and The salivary glands
Leads to both the trachea and the esophagus.
The epiglottis blocks the trachea
The uvula blocks off the nose, while food is being swallowed.
Muscular tube which extends from the pharynx to the stomach.
Food moved along esophagus by peristalsis, and mucus secretion.
Sphincter muscles at the top and bottom of the esophagus.
J-shaped sac, which lies on the left side of the upper abdomen.
The stomach has two sphincters:
A. The cardiac sphincter closes off the top end of it
B. The pyloric sphincter, which closes off the bottom.
Length about 6 m & the total surface area about 300 m2
The surface is wrinkled for a greater surface area for absorption.
Includes: the duodenum, the jejunum, and the ileum
Begins with a blind pouch called the cecum that terminates in the
appendix, a finger-like extension ( function in immune system ).
Rectum the terminal portion of LI ( for storage of the feces ) AND
leads to external opening called the anus.
B. Accessory glands: salivary glands:
Salivary glands:  3 pairs , the parotids, submandibular, and sublingual.
The liver:  The largest gland in the body
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The pancreas: 
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B.
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Attached to under surface of it a gall bladder contains bile.
The pancreas lies between the stomach and the duodenum.
It contains two types of cells,
One secretes hormones (e.g. Insulin) into the blood stream,
The other secretes a digestive juice via the pancreatic duct into gut.
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Functions of the gastrointestinal tract
1. Secretion: Includes both exocrine and endocrine secretions.
 Hcl, H20, HC03-, bile, lipase, pepsin, amylase, trypsin, elastase and histamine are
secreted into the lumen of the GI tract
Endocrine:  Stomach and small intestine secrete hormones to regulate the GIT as Gastrin,
Secretin, CCK & Somatostatin.
Exocrine:
2. Motility: Movement of food through the gastrointestinal tract; e.g.:Ingestion:
Mastication:
Deglutition:
Peristalsis:
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Taking food into the mouth.
Chewing the food & mixing it with saliva.
Swallowing the food.
Rhythmic wave-like contractions that move food through GIT.
3. Digestion: It is the dissolving and breakdown of large particles of food into small molecules.
4. Absorption: It is the movement of the small digested molecules from the lumen of the GIT
across a layer of epithelial cells to enter blood or lymph.
The wall of the alimentary canal is formed of 4 layers
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Digestion
 Biochemical process by which the complex compounds in foods like carbohydrates, proteins,
and lipids are changed into more simple soluble units (glucose, fructose, galactose ,amino acids
,fatty acids, and glycerol), which can be easily absorbed through the intestinal wall.
 An enzyme is a biochemical catalysts ( chemical substance ) (special proteins) accelerates a
chemical reaction without itself being changed or consumed.
 Factors affecting digestive enzymes activity:
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Each enzymes has an optimum temperature. Excessive heat destroys their activity.
Each enzyme has an optimum ph.
Each enzyme acts upon a specific type of food called substrate.
Each enzyme secreted as a pro-enzyme (zymogen) inactive form.
Each enzyme has a specific activator and specific inhibitor.
Most enzyme is names are written with the suffix -ase, e.g. Maltase - sucrase - lactase.
Digestion in buccal cavity
 Mechanical digestion in the mouth :
1- The mouth broken down food by both mechanical and chemical means.
2- The teeth physically break down food through the process of mastication or chewing .
3- The tongue helps move the food around the mouth, allowing the different sets of teeth
variously to cut, tear, or grind the food
4- By the presence of food in the mouth, salivary glands are stimulated and secrete an increased
amount of saliva (the sensations of sight, taste, and smell also increase saliva flow).
Salivary glands:
 There are 3 pairs of salivary glands:
A. The parotid
B. Submandibular
C. Sublingual.
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Saliva
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Salivary glands secrete watery substances called saliva in the mouth cavity
About 1-1.5 liter/day of the saliva in the adult.
The saliva is colorless, and has a PH 7.4.
Saliva composed of
 99.5% water
 0.2% inorganic materials [chloride, phosphate, bicarbonate HCO3, Na+, k+, and Ca++]
 0.3% organic solids [mucin,albumin,and enzymes
 Choose The most important anions of Saliva are.
A. Chloride, bicarbonate & phosphate.
B. Chloride, Sodium & phosphate.
C. Chloride, bicarbonate & magnesium.
Functions of saliva:
A. Digestive function of salivary enzymes:
1- Salivary amylase: it initiates the digestion of a part of soluble starch into maltose
2- Maltase: it digests maltose sugar into glucose,
3- Salivary lipase (lingual lipase):
 Salivary lipase is a more potent form of lipase & has a ph optimum 4.5 - 5.4
 The enzyme
a. Catalyzes the first reaction in the digestion of dietary lipid
b. Has the ability to catalyze reactions without bile salts, so the lipolytic activity of it
continues for some time in the stomach.
c. Plays a large role in fat digestion in newborn infants as their pancreatic lipase still needs
some time to develop.
B. Other functions of saliva:
1- The mucous acts as a lubricant which facilitates mastication and swallowing, besides
2- Saliva dissolves certain substances then able to stimulate the taste buds of the tongue and
give the sensation of taste.
3- It has anti-bacterial effects, due to the presence of lysozyme enzyme.
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Digestion in the stomach
 Composition of gastric juice:
1- Gastric juice is a strongly acidic (ph 1-2), and colorless liquid.
2- The total quantity of the juice is about 2-2.5 liter of acidic juice per day.
3- The average composition of human gastric juice is as follows:
 Water: 99.45%.
 Solids: 0.55%;which can divided into: Inorganic: 0.15% (H , Cl , Na, K , calcium, phosphate, bicarbonate.... Etc.)
 Organic: 0.4% [mucin, intrinsic factor (vitamin B12 carrier ), and enzymes such as
pepsinogen , renin ,gastric lipase, lysozyme, gelatinase ].
4- The gastric glands contain different types of secretory cells:
• The body and fundus glands contain:a.
b.
c.
d.
Parietal (oxyntic) cells
Chief (peptic) cells
Mucus cells in the gland’s neck.
G cells in the antrum the glands
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Secrete hcl.
Secrete pepsinogen.
Secrete mucous
Contain endocrine cells (G-cells) secrete gastrin
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Functions of gastric digestive enzymes:
A. Inactive pepsinogen (zymogen) is converted into active pepsin, by the action of HCL
B. Pepsin hydrolyzes complex proteins to proteoses and peptones.
C. Prorennin (in some young mammals) is converted into active rennin by the action of HCL .
D. Rennin acts on protein of milk caseinogen (soluble) and converts it into insoluble calcium
paracaseinate in the presence of calcium ions.
E. Lipase enzyme: like salivary lipase, gastric lipase functions on non-emulsified fats and converts
it into fatty acid and diglycerides.
Functions of HCL :
1. It gives the optimum ph of stomach (1-2}.
2. It activates pepsinogen enzyme into pepsin.
3. It converts the prorennin into active rennin.
4. It has antibacterial action.
5. It converts carbohydrates into mono-, di-, and polysaccharides, and proteins to meta-proteins.
Control of gastric juice secretion:
1- Reflex stimulation of the vagus nerve.
2- Hormonal stimulation:
A. Gastrin stimulates acid secretion.
B. Secretin stimulates pepsin secretion, and reduces acid secretion from the stomach by
inhibiting gastrin release from g cells.
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Digestion in the small intestine
 Most of the digestive actions that take place in the small intestine are carried out in the
duodenum and brought about by secretions from liver, pancreas, and intestine itself.
The pancreatic juice:
 Pancreas acts as an endocrine and an exocrine gland.
Pancreas as endocrine gland
Pancreas as exocrine gland
Produces hormones (such as insulin, and glucagon) which
released directly (without ducts) into the circulatory system.
Secretes digestive juice, which is transported via the pancreatic
duct to the duodenum.
 Composition of pancreatic juice:
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Alkaline (ph 7.5-8.0), due to the high concentration of bicarbonate ions.
Quantity: 1200-1500 ml/day
Contain Water and electrolytes (mainly bicarbonate ions).
Digestive enzymes for proteins (trypsinogen, chemotrypsinogen, procarboxy-peptidase), lipids
(lipase), carbohydrates (amylase), and nucleic acids (nucleases).
5- The pancreatic secretion flows through a long pancreatic duct that joins the common bile duct
before it empties into the duodenum.
6- The opening of the joined ducts is surrounded by sphincter of Oddi.
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Pancreatic enzymes:
Enzymes for protein digestion:
Enzymes for Nucleic acid digestion:
Enzymes for carbohydrates digestion:
1.
2.
1.
2.
Trypsin and chymotrypsin: Endopeptidases .
Carboxypeptidases: Exopeptidases.
Ribonuclease
Deoxyribonuclease:
These enzymes digest RNA and DNA.
1. Pancreatic amylase digests starch to maltose
The liver
 The liver is the largest gland in the body.
 The gall-bladder is attached to the under surface of the liver , contains a green colored fluid
called bile.
 When the chyme from the stomach enters the duodenum, the gall bladder contracts and
discharges the bile on the foods by stimulation of Cholecystokinin hormone
General functions of the liver:
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It produces the bile which used in fat digestion.
The liver stores excess of glucose in the form glycogen.
At starvation, liver breaks down the glycogen again into glucose which released into the blood.
The liver absorbs lactic acid from the blood and converts it to glucose.
The liver regulates the levels of triglycerides, fatty acids, and cholesterol in the blood
The liver removes excess amino acids, hormones, and antibodies from the blood; the liver also
removes various toxins from the blood.
 The liver stores a variety of vitamins and minerals.
 The liver produces the major plasma proteins, such as albumin.
 The liver contains kupffer cells which are macrophages, and they are responsible for engulfing
pathogens, cellular debris, and damaged blood cells.
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The bile juice:
1- Composition of the bile juice:
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Liver cells produce 0.5-1.0 liter of bile each day, which is stored in the gall-bladder.
The bile is composed of
a.
b.
c.
d.
e.
98% water.
Mucin (acts as a lubricant).
Bile salts (sodium salts).
Bile pigments as bilirubin and biliverdin.
Other constituents such as cholesterol, neutral fats, phospholipids and electrolytes.
2- Role of bile juice in digestion:
A. Sodium salts cause:
1- Alkalinity of digestive juice.
2- Break down the fats into minute droplets (emulsification of fat)
B. Mucous acts as a lubricant.
The intestinal juice:
 Composition of the intestinal juice:
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Colorless or yellowish and alkaline fluid which is secreted by the intestinal wall cells.
In human, about 1-3 liters of intestinal juice are secreted per day.
Average ph = 8.3. (Ranges 7.3 -9)
Composed of water mucus , and organic and inorganic substances.
It synthesizes several enzymes called brush border enzymes such as amylase, maltase, lactase,
aminopeptidases, lipase, nucleotidase and enterokinase.
Brush border enzymes are:
 Carbohydrate digestive enzymes: maltase, sucrase and lactase.
 Protein digestive enzymes: Peptidases (aminopeptidases, carboxypeptidases and
Dipeptidases)
 Nucleotide digestive enzymes: Nucleosidases and phosphatases
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Digestion
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The process of making food absorbable by mechanically and enzymatically breaking down
into simpler chemical compounds.
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Functions of bile salts
1- Emulsification of fats:
 Bile salts reduce surface tension and emulsify fats into small droplets preparatory to their
digestion by pancreatic lipase.
2- Absorption of fats:
 Bile salts tend to form cylindrical discs called micelles, with the hydrophilic surface facing
out and a hydrophobic center containing fats.
 Thus, micelles keep fats in solution and transport them to the brush border of small
intestine where they absorbed.
Regulation of intestinal juice secretion:
 Intestinal secretion is regulated either by nervous and hormonal mechanisms.
 Both Parasympathetic effect and entrokinin hormone stimulates secretion of intestinal juice,
 While sympathetic action inhibits it.
Absorption
 Absorption means passage of the digested molecules from the intestinal lumen into the blood
stream through the intestinal lining.
 Pathways of absorption:
1- The veins of the portal system where amino acids, monosaccharides, short chain fatty
acids, water, and electrolytes pass into the blood stream.
2- The lymphatic (lacteal) vessels where the long chain fatty acids and •glycerol are
transported into the lymphatic system and then to the blood stream at the thoracic region.
 Sites of food absorption:
Stomach:
Small intestine
Colon:
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 It absorbs 20% of the alcohol ingested, as well as some short-chain fatty
acids
 In different parts of the small intestine, absorption of all kinds of nutrient
{glucose, fructose, galactose, amino acids, fatty acids, and glycerol,
vitamins, water and electrolytes) occurs.
 Absorption of sodium, potassium, water, acids, gases, some vitamins
which synthesized by micro-organisms takes place in colon.
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 Mechanisms of absorption:
Fluids and nutrients are absorbed by different mechanisms (methods).
Osmosis
Diffusion:
Active
transport:
 Means the movement of water through a partially permeable membrane
down a concentration gradient from a dilute solution to a concentrated
solution .
 Means the movement of particles from an area of high concentration to an
area of low concentration down a concentration gradient.
 Means the movement of substances against a concentration and electrical
gradient, with consumption of energy.
Notes
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Diffusion and osmosis are both types of passive transport - i.e., no energy is required for the
molecules to move into or out of the cell.
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Diffusion may be simple or facilitated
1- In facilitated diffusion, large molecules such as glucose and amino acids cross the plasma
membrane by the help of carrier proteins.
2- In simple diffusion, small molecules {e.g., c02 & 02) are passed through the membrane
without the need of carrier proteins.
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In active transport, a special carrier protein in the cell membrane is used to transport the
particle from one side to the other side of the membrane.
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Lipids absorption
 Micelles Formation.
 Chylomicrons Formation in epithelial cells .
 Chylomicrons are then transported by Exocytosis to lymph vessels.
Vitamin absorption
1- Fat soluble vitamins (A, D, E & K): are absorbed like other lipids.
2- Most water soluble vitamins are absorbed by Na+-dependent co-transport mechanism.
3- Vitamin B12: is absorbed from terminal ileum and needs intrinsic factor.
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