DEPARTMENT OF ZOOLOGY

UNIVERSITY OF MEDICINE,MAGWAY

Digestive System 9

• Gastric juices

contain

• Pepsin

• Rennin

• lipase

enzymes

The action of gastric juice

1.Pepsin (secreted by chief cells in the form of pepsinogen

& activated by Hcl)

Pepsin+protein proteoses,peptones

&large polypeptides

2.Rennin (secreted by chief cells in young ruminants)

Rennin +soluble milk protein casein

insoluble milk curd

calcium paracaseinate

- which is then digested by pepsin.

split

changes

Rennin is not secreted in adult man & infants so milk

is digested by pepsin.

3. There is also a trace of lipase in gastric juice

but fat digestion due to this is negligible

protease

peptone

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 jet 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 cardia

(which is controlled by hormone gastrin)

Intestine Presence of Hcl &

semidigested food to release two hormones

in duodenum

1.Secretin

2, cholecystokinin-pancreo-zymin(CCK-PZ).

Secretin,

- on reaching the pancreas & liver with blood circulation,

stimulates its wall

stimulates their cells

CCK-PZ , on reaching pancreas with blood

-stimulates cells of pancreatic acini

-to secrete & release

It also stimulates the contraction of gallbladder &

relaxation of sphincter of Oddi

Simultaneously,

both bile &

pancreatic juice

pancreatic juice

are released into duodenum

Bilecontains:1.water

2.bile salts(sodium,potassium chlorides,glycocholates,taurocholates)

3.bile pigments(bilirubin,biliverdin formed by breakdown of haemoglobin in liver)

4.cholesterol

5.lecithin

6.Fatty acids

In man,

600-1000ml of bile is secreted per day.

-yellowish brown alkaline fluid( pH 7.7),

-no digestive enzyme

Functions in many other ways:

1.Intensification of intestinal contractions.

2.Neutralization of acidity of Hcl& inactivation of

pepsin.

3.Emulsification of fats for easy action of

pancreatic juice.

4.Easy absorption of fatty acids,cholesterol & other

lipids by forming micelles.

5.Absorption of vitamin A,D,E &K.

6.Excretion of the waste products, toxins etc.

3.Emulsification of fats for easy action of pancreatic juice.

Absorption of Nutrients

• Fats are emulsified by bile salts.

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

• Micelles diffuse into epithelial cells.

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

Pancreatic juice

has 98% water & digestive enzymes

- is highly alkaline (pH 7.5-8.3 )

- digestive juice (500-800ml per day) &

- has following actions:

1.Amylase + carbohydrates maltose

2.Lipase +fats Fatty acids & glycerol

some into diglycerides

split

split

3.Endopeptidases---two pepsin like enzymes

trypsin chymotrypsin

inactive forms trypsinogen chymotrypsinogen

activated by

enterokinase of succus entericus

Endopeptidases +protein

trypsin ,chymotrypsin

Peptone,proteose

Smaller peptides

split

4. Carboxypeptidase---zinc containing exopeptidase

that acts upon carboxyl terminals of

polypeptidase forming amino acids.

Carboxypeptidase + polypeptides amino acids

5.Nucleases component nucleotides nucleosideshydrolyse

Protein Digestion

Succus entericus(Intestinal juice)

• is released from Crypts of Lieberkuhn

- It is an alkaline fluid – pH 7.5 – 8.0 (about 2 liters

• secreated daily in man )

• Following actions

• 1.Erepsin-exopeptidase including(aminopeptidases &

carboxypeptidases)

• hydrolyzing small peptides &dipeptides amino acids

2.Carbohydrases- include maltase

sucrase

lactose

Maltase+ maltose 2 molecules of glucose

Sucrase + sucrose glucose + fructose

Lactose + lactose glucose + galactose

splitting

Carbohydrate Digestion - Small Intestine

3.Enterokinase

trypsinogen

chymotrypsinogen

-Mucus secreted by Brunner’sglands

spreads upon the mucous membrane

preventing action of digestive juice on gut wall.

pancreatic juiceactivates

Absorption monosaccharide sugars

amino acids

fatty acids

glycerol

nitrogenous bases

pentose sugar

-which are 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.

(Glucose,fructose etc)

End products

of digestion

- water,

- glucose,

- alcohol

- some drugs

The intestine alone is well adapted

- whose wall is produced into villi

- increasing the area for absorption about 600times in

man.

of blood capillaries

a sigle lymph vessel

lacteal

Absorption occur in stomach of man.

for absorption of digested food

The villi have dense network

Microscopic Anatomy of the Small Intestine

- Sugars

-salt,water

-vitamins

-nitrogen bases

-amino acids

-digestive juice etc

Fatty acids

monoglycerides

are absorbed into blood capillaries

absorbed by mucosal cells forming triglycerides in their smooth endoplasmic reticulum

Within the reticulum,

-the fat molecules are synthesized into

cholesterol

phospholipids

These fat globules encased in a protein coat

chylomicrons

Chylomicrons make their way out of absorptive cells

as chyle & enter lacteals.

Lacteals conduct chylomicrons into larger lymphatics

---which carry them to thoracic lymphatic ducts

---where from they reach venous blood

jugular or subclavian vein

In the large intestine only water is absorbed.

are called

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.

First coming into liver through portal circulation after process of absorption

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

also converted into

by 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

animal foodsthan

The faeces are collected

in distal end of large intestine

the rectum

their removal through anus

defaecation or egestion

called

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

pancreatic proteases

proteinDipeptides/ polypeptides

protein

dipeptide

protease

intestinal proteases

dipeptides amino acids

proteaseamino acid

pancreatic lipasesglycerol + 3 fatty acids

lipase

glycerol

fatty acids

lipid

lipid

disaccharides(eg. maltose)

monosaccharides(eg. glucose)

carbohydrase

disaccharide

intestinal carbohydrase

monosaccharide

• The liver is responsible for the synthesis of many proteins - some of which are vital for normal function, and some of which can be used as markers of liver disease

• The main proteins synthesised by the liver are summarised here :

Final Digestion Products• Final digestion products absorbed by cells lining

small intestine.– Carbohydrates:

• Monosaccharides– Proteins:

• Amino acids• Chains of 2 or 3 amino acids

– Fats: • Fatty acids• Glycerol• Monoglycerides

– Vitamins, minerals, water and some larger fat-like compounds such as cholesterol are not broken down before they are absorbed.

Sources Energy

content

Use

Carbohydrates

Proteins

Fats

Cereals, grains, breads, fruits and vegetables

4.1 calories per gram

For energy and raw materials

Dairy products, poultry, fish, meat and grains

4.1 calories per gram

For energy and raw materials

Oils, butter,Margarine, fried foods and chips

9.3 calories per gram

For energy storage and raw materials