The digestive system breaks down food into simpler compounds that can be absorbed and used by the body. It includes the oral cavity, esophagus, stomach, small intestine, large intestine, and digestive glands. In the mouth, saliva containing the enzyme amylase begins breaking down carbohydrates. The stomach contains acids and enzymes that further break down food into a semi-solid mass called chyme. The small intestine completes digestion with enzymes from the pancreas and liver that break down proteins, carbohydrates, and lipids. The large intestine absorbs water from undigested food matter which is then excreted as feces.

1. Digestive System

2.  Digestion :
 Breakdown of food into simpler compounds , ready to be used by body ,
for energy purposes
 Digestive system : many organs : food into energy
 Disassembly process

3. Organs
1. Oral cavity/ Mouth
2. Esophagus
3. Stomach
4. Small Intestine ( Duodenum , Jejunum , ileum
5. Large Intestine ( Ascending colon , Transverse colon Descending
colon, rectum , anus)
6. Glands: Pancreas , liver, salivary glands

5. Various systems involved in digestion
Endocrine : Glands : Secrete hormones for digestion
Nervous System : Contraction and relaxation of muscles in the
stomach ,
Lymphatic system/ Circulatory system : Absorption of food
Bile acids from Liver, Pancreatic juice , stomach acid, enzymes

6. Digestion in Oral Cavity
 Selection of food: taste / smelled : aid of organs of sight and smell
 Tongue : Sensory and Muscularly organ : Taste buds
 Grinding / mastication : grinded by teeth : form bolus : easy passage through esophagus
 Lubrication : saliva : Salivary Glands: (below tongue)
 Saliva has mucus or water , an enzyme named AMYLASE
 Digestion : Amylase starts digestion of carbohydrates

7. Amylase
 Digestion of complex carbohydrates into simple ones
 Starch to simple sugar molecules

8. Saliva : Bolus
 Food moist and Lubricated
 Chemicals ( sodium bicarbonate) antiseptic for killing germs
 Bolus: Softened, partly digested, slimy food mass , rolled into oval lump
 Pushed down through pharynx into esophagus :
 Muscle contract to prevent backward flow

9. Swallowing
 Beginning of swallowing : voluntary
 Once reaches back of the mouth become involuntary
 Peristalsis : Characteristic movement of digestive tract : involuntary( not
under our control)
 Alternative contraction and relaxation of muscles

10. Continue from back of mouth , esophagus, stomach and along all
digestive tract
Antiperistalsis: Vomiting
Hunger pang: Peristaltic movement / Hunger contractions : initiated
due to low blood glucose levels
Movement of food aided by gravity
But peristaltic movement strong enough
Headstand/ astronauts

12. Sphincters:
Muscle rings separating one organ from the other
Smooth muscle contraction : move food in one direction , no
flow back
At the junction of esophagus and stomach : Cardiac Sphincter

13. Digestion in Stomach
 Cardiac sphincter opens on receiving peristalsis wave from esophagus
 Stomach : Elastic muscular bag , stores food temporarily , digests food
 Stomach : Muscular layer + Mucosal layer
 Muscle layer : contraction and digestion : Mechanical grinding or
digestion of food
 Three layers of smooth muscles
 Different shapes of muscles
 Longitudinal , circular , oblique

16. Stomach : Gastric Juice
 Mucosa / inner wall of stomach : Gastric Glands : secret many hormones
nad chemical
 Mucus :
 Hydrochloric Acid
 Enzyme : PEPSIN :
 All these make up the gastric juice
 Released on sight/ smell of food and quality

17. Mucus : Thick secretion : protects the inner wall from being
digested by the enzymes and from the HCl
HCl : maintains 2-3 pH of stomach
Proper action of PEPSIN
Softens food
Pepsin : Breakdown of Protein molecules

18. End of stomach
Food now called as chyme : Acid mixed food , semi
solid mass
Reaches Pyloric Sphincter
Opening and chyme pushed into Small intestine

19. Heart Burn/ GERD
 Gastro Esophageal Reflex Disease
 Burning sensation
 Back flush of acidic chime into esophagus
 Overeating / fatty food/ lying down immediately after food
 Caffeine, smoking, alcohol consumption

20. Digestion in Small Intestine
Remaining sugars , proteins , fats digested in Small intestine
Digestion that started from oral cavity ending in SI.
Smooth muscle : contractions : Digest
Enzymes and bile juice ( form liver) Pancreatic juice ( pancreas)
pour into SI

21. Enzymes of Small Intestine
 Firstly pH of the acidic chime changed by Alkaline secretions ( sodium
bicarbonate)
 Pancreas: Pancreatic duct : Pancreatic juice : has all enzymes to digest all
components of food carbohydrates, proteins, lipids/ fats

22. Enzymes OF Small Intestine
 AMYLASE : Carbohydrates
 LIPASE : fats
 TRYPSIN : proteins
 MALTASE Maltose sugar ( grains/ vegetables)
 LACTASE Milk sugar
 SUCRASE (sucrose)

23. Bile Juice
 Produced in live and stored for some time in Gall Bladder
 Through bile duct into SI
 Green watery fluid
 fats : Breakdown : Large globules to small globules ( Emulsification )
 Small globules easily digested by LIPASE

25. Types of digestion
Mechanical and Chemical
Mechanical : Contractions of Smooth muscles layers
along the digestive tract
Chemical : Through enzymes , chemical
Both digestion in all major organs
Figure 2.4

26. Large Intestine / colon
 Material passed into large intestine from the Small intestine contains undigested food , large
amounts of water, dissolved salts.
 Water and salts are absorbed into blood while undigested material rejected as feces.
 Undigested food: Plant fibers, dead bacteria, undigested fats/ proteins
 Water reabsorption is very important , if not absorbed due to any reason : Diarrhoea
 Water absorbed excessively : Constipation

27. Rectum:
 Last Part of intestine
 Feces stored temporarily
 Rejected through anus
 Anus is controlled by sphincters
 Sphincters give defecation reflex: Controlled by adults

28. Bacteria in Large Intestine
 Make a few vitamins and Vitamin K
 Very important for clotting