DIGESTIVE SYSTEM

DAW SAN DAR TUN

ASSISTANT LECTURER

DEPARTMENT OF ZOOLOGY

UNIVERSITY OF MEDICINE,MAGWAY

Ingestion

• a process by which food is taken in through the mouth

• a reflex action which is involuntary

• it occurs when the food is put at the posterior position of the tongue

A process by which large food molecules

are broken down into smaller pieces

Digestion

Mechanical digestionbiting & chewing

Chemical digestionHydrolysis & enzymes(Simple soluble forms)

Ingestion and digestion of food

Oral cavity

Four kinds of teeth

Incisors – chisel-shaped teeth ( for biting )

canines – pointed teeth ( for tearing )

Premolars & molars – flattened , ridged teeth

( for grinding, pounding,

crushing )

Saliva (pH 6.8)

amylase(ptyalin)mucinwater

starch maltoselubricates

food

helps swallowing

dissolves

soluble food

Chemical Digestion

break down of starch molecules

into maltose molecules by salivary

amylase (from salivary glands)

Starch

maltoses

salivary glands

Peristalsis

Oesophagus

- the peristalsis passes the bolus to stomach

- no digestive enzyme is secreted

Stomach

Cardiac sphincter

(a valve –like ring of muscle )

- relaxes as the bolus

passes through,then

quickly closes

In the stomach,

- food is retained for 4-6 hours in man.

Layer of stomach muscle

contract &churn the bolus of food

with gastric juices to make it a soupy chyme.

Gastric juice

water mucusHCL enzymesalt

mucus cells mucus

oxyntic cells HCL (Hydrochloric acid)

chief cells pepsinogen (proenzyme)

90% 0.2-0.5%

In man

2-3 liters of gastric juice is secreted per day

The stomach and gastric glands

(Oxyntic cell)

Function of hydrochloric acid

(1) change the pH of food from 1 - 3.5

(2) activates

pepsinogen active pepsin

prorennin active rennin

(3) dissolves salts,bones etc. in food

(4) stop salivary enzyme action

(5) kill harmful bacteria

(6) splits nucleoproteins

(7) regulate pyloric sphincter

Hcl

nucleic acid + proteinHcl

Hcl

gastrin gastric juceactivate inhibit

under nervous reflex

Enterogasterone

Action of gastric juice

curd (cal. Paracaseinate)

peptones , proteoses

fatty acid , glycerol

rennin + milk protein(casein)

pepsin + milk curd

Lipase + fats

pepsin + protein proteoses, peptones, large polypeptides

split

split

split

split

To protect the stomach lining from the acid &enzyme

mucous cells secrete mucus

(that lines the stomach cavity)

An over abundance of acid

To protect the stomach lining from the acid &enzyme

mucous cells secrete mucus

(that lines the stomach cavity)

An over abundance of acid

To protect the stomach lining from the acid &enzyme

mucous cells secrete mucus

(that lines the stomach cavity)

An over abundance of acid

due to mucus failure

may lead to an ulcer

The semidigested acidic chyme

in form of small jets to duodenum

Emptying of stomach

- to the rate of processing of chyme into intestine.

Regurgitation of food

- from stomach to oesophagus

- & from duodenum to stomach by pyloric sphincter

is passed

due to rhythmic release of pylorus

is reciprocal

by cardiac sphincter

(which is controlled by hormone gastrin)

Intestinal hormone

Secretin liver and pancreatic cells

Cholecystokinin-pancreo-zymin

- cells of pancreatic acini

- contraction of gallbladder

- relaxation of sphincter Oddi

stimulate

stimulate

c.c.k.p.z

bile juice

• Sodium/potassium chloride

• -------------------- bicarbonates

• -------------------- glycocholates

• -------------------- taurocholates

water

• bilirubin• biliverdin

bile salts bile pigments

Cholesterol, lecithin, fatty acid

Functions of bile juice

(1) intensification of intestinal contraction (peristalsis)

(2) neutralization of acidity of HCL and inactivation of

pepsin

(3) emulsification of fats for easy action of pancreatic juice

(4) easy absorption of fatty acids, cholesterol and other

lipid by forming micelles

(5) absorption vitamin A,D.E & K

(6) excretion of the waste products, toxins etc.

Bile

- contain bile salts which emulsify lipids into smaller droplets without chemical change ( ∴ NOT digest fats)

- provide alkaline medium for enzymes to work

oil

oil droplets

• Fats are emulsified by bile salts.

• Micelles are tiny droplets consisting of fatty acids and monoglycerides complexed with bile salts.

• Micelles diffuse into epithelial cells.

– Resynthesized into triglycerides and pass into the lacteals of the lymphatic system.

pancreatic juice

• Amylase

• Lipase

• Endopeptidases

• Carboxypeptidase

• Nucleases

Water98%

Digestive enzyme

Actions of pancreatic juice

.

1. Amylase + carbohydrates maltosesplits

2. Lipase + fatssplits Fatty acids & glycerol

some into diglycerides

trypsin , chymotrypsin

Peptone, proteosesmall peptides

3. Endopeptidases + protein splits

4. Carboxypeptidase + polypeptides amino acids

5. Nucleases + nucleotidesD.N.A , R.N.A splits

splits

nucleosides

lipase glycerol + 3 fatty acids

lipase

glycerol

fatty acids

lipid

lipid

pancreatic proteases

proteinDipeptides/ polypeptides

protein

dipeptide

protease

disaccharides(eg. maltose)

monosaccharides(eg. glucose)

carbohydrase

disaccharide

intestinal carbohydrase

monosaccharide

intestinal proteases

dipeptides amino acids

proteaseamino acid

Succus entericus(Intestinal juice)

-is released from Crypts of Lieberkuhn

-Is an alkaline fluid – pH 7.5 – 8.0

- (about 2 liters secreted daily in man )

Crypts of Lieberkuhn

succus entericus (intestinal juice)

1. Erepsin

exopeptidases- (aminopeptidases ,carbooxypeptidases)

2 .Carbohydrases maltasesucraselactase

3. Enterokinase -

1.Peptidase + small peptide , dipeptide Amino acids

Maltase + maltose

Sucrase +sucrose

Lactase +lactose

Glucose

Glucose & fructose

Glucose & galactose

split

split

split

3.Enterokinase activates trypsinogenchymotrypsinogen

trypsinchymotrypsin

convert

Action of succus entericus

2.Carbohydrases

Absorption and assimilation

AbsorptionEnd products of digestion

- monosaccharide sugars ( glucose, fructose )

- amino acids

-fatty acids

- glycerol

- nitrogenous bases

- pentose sugars

simple soluble substances

can easily diffuse through cell membranes

About 9 liters of contents

(1 1/2 liter of digested nutrients &

rest water)

are absorbed daily

from gastrointestinal tract in man.

In stomach of man ,absorption

- water,

- glucose,

- alcohol

- some drugs

The intestine alone is well adapted

- whose wall is produced into villi

increasing the area

for absorption about 600 times in man.

lacteal

Epithelial

cell

arteriole

venule

blood

capaillaries

lymph

vessel

villi

Structure of Villi

Villi havedense network capillarieslacteala single lymph vessel

SugarSaltWaterVitaminsNitrogen basesamino acid

digestive juice

- Sugars

-salt,water

-vitamins

-nitrogen bases

-amino acids

-digestive juice etc

are absorbed into blood capillaries

absorbed by mucosal cells forming triglycerides in their smooth endoplasmic reticulum

Fatty acids

Monoglycerides

-the fat molecules are synthesized into

cholesterolWithin the reticulum,

phospholipids

These fat globules encased in a protein coat

chylomicrons

make their way out of absorptive cells

as chyle & enter lacteals.

Lacteals conduct chylomicrons

into larger lymphatics

--- carry them to thoracic lymphatic ducts

--- from they reach venous blood

jugular or subclavian vein

In the large intestine only water is absorbed.

via

Assimilation

The amino acids

monosaccharides

& then go to the heart for circulation in the different parts

of the body where tissues pick up their requirements.

The amino acids are building materials &

constantly needed for formation of

protoplasm.

The excess of amino acids are subjected to the process of

deamination in liver forming ammonia

----which is soon convered into urea

----excreted by the kidneys.

Assimilation

The amino acids

monosaccharides

& then go to the heart for circulation in the different parts

of the body where tissues pick up their requirements.

The amino acids are building materials &

constantly needed

for formation of protoplasm.

First coming into liver through portal circulation after process of absorption

The excess of amino acids

the process of deamination

in liver forming ammonia

----which is soon converted

into urea

----excreted by the kidneys.

The excess of monosaccharides stored

in the form of glycogen-------- in the liver cells.

Whenever glucose level falls down in the blood then

---glycogen is soon converted into--------glucose

( glycogenolysis)

----to maintain its normal level in the blood.

Liver is also concerned with fat metabolism &

---the fatty globules are found filled -------liver cells.

Energr is also released from the fats.

The fats maino acids

carbohydrates

The excess of monosaccharides stored

glycogen--- in the liver cells.

glucose level falls down in the blood then

-glycogen is converted into glucose ( glycogenolysis)

-to maintain its

normal level

in the blood.

Liver is also concerned with fat metabolism &

---the fatty globules are found filled -------in liver cells.

Energy is also released from the fats.

The fats amino acids

carbohydratesby the liver

Egestion ( Defaecation)

After absorption of the digested food,

--remaining contents ,bile pigments ,others

----whatever left ----------- is called faeces.

Vegetable foods form large amount of faeces

Milk forms relatively large amount of faeces

(due to large amount of unabsorbed salt in it.)

Animal proteins are absorbed 98%- 99%

Vegetable proteins to lesser extent.

large intestine

pass into

where water is absorbed from them

Egestion ( Defaecation)

After absorption of the digested food,

--remaining contents ,bile pigments ,others

----whatever left ----------- is called faeces.

Vegetable foods form large amount of faeces

Milk forms relatively large amount of faeces

(due to large amount of unabsorbed salt in it.)

Animal proteins are absorbed 98%- 99%

Vegetable proteins to lesser extent.

animal foodsthan

large intestine

pass into

where water is absorbed from them

The faeces are collected

in distal end of large intestine

the rectum

their removal through anus

defaecation or egestion

Table 1: Enzymes in Digestion

Area Secretion/Enzyme Substrate Product

Mouth Saliva - amylase Starch Maltose

StomachGastric Juice -

pepsin, lipaseProtein, Lipids

Peptides,

fattyacids/gycerol

Duodenum

Pancreatic juice -

amylase, trypsin,

chymotrypsin,

carboxypeptidase,

lipase

Starch, protein,

protein, peptides,

lipids

Maltose, peptides,

peptides, amino

acids,

fattyacids/glycerol

Ileum

None secreted,

remain on villi cells,

Maltase, sucrase,

lactase, peptidase

maltose, sucrose,

lactose, peptides

glucose,

glucose/fructose,

glucose/galactose,

amino acids.