The document summarizes the key parts and functions of the digestive system, beginning with the mouth. It describes the salivary glands, tongue, and role of saliva in beginning digestion of starches. It then discusses swallowing and the esophagus before focusing on the stomach, including its parts, secretions like gastric juice that contain enzymes like pepsin, and roles in further digesting proteins and temporarily storing food.

1. Digestive system
UNIT III

3. Parts of Digestive System
- The main component of the digestive system is the
digestive tract/alimentary canal.
- There are the glands which help in the digestive process,
these are salivary gland, liver, gallbladder, pancreas and
intestinal glands.
- The main parts are the mouth, pharynx, esophagus,
stomach, small intestine, large intestine.
- The last part of large intestine is the rectum and anal canal
which ends at the anus.

4. MOUTH
- Mouth is the beginning of digestive tract and is guarded by
two lips with the cheeks at the sides.
- It contains the tongue and teeth
- It opens posteriorly into the pharynx.
- Roof of mouth is formed by palate.
- It is divided into anterior hard palate which is bony part, is
longer and separates mouth cavity from the nasal cavity
- The posterior part of the palate, the soft palate, is like a
curtain. It is muscular, shorter and hangs down at the back.

7. Tongue
The tongue is muscular organ within the mouth
cavity
Function of tongue
- It helps in swallowing of food. It cleans the
mouth
- It helps in speech
- It helps in taste

8. Salivary gland
Three pairs of salivary glands are parotid glands,
submandibular glands and sublingual glands
Parotid glands are situated one on each side of
face just below the external acoustic meatus. Each
gland has a parotid duct, opening into the vestibule
of mouth opposite the 2nd upper molar tooth.

9. Submandibular glands lie one on each side of
face under the angle of jaw. The duct of
submandibular gland opens on the side of the
frenulum(tongue attaches to the floor of mouth)
Sublingual glands lie under mucus membrane of
the floor of mouth in front of submandibular
glands and they open by 8-10 ducts in the floor of
the mouth.

11. FUNCTIONS ASSOCIATED WITH MOUTH
Digestion and absorption:
- Food consists for nutrients like carbohydrates,
proteins, fats and small quantities of vitamins and
minerals.
- Most of these have to be broken down into simpler
substances so that these can be absorbed and
transported to the tissues.

12. - This process of digestion begins from mouth. Saliva
lubricates and moistens the food and chewing
converts it into a soft mass of bolus.
- The secretory function of salivary glands is under the
stimulatory and inhibitory control of
parasympathetic and sympathetic nervous system
respectively.
- Sight, smell and thought of delicious food produce
saliva.

13.  Saliva
- Saliva is a clear liquid made by several glands in mouth.
- It is an important part of a healthy body
- Saliva contains the enzyme Amylase that breaks down
starches into maltose and dextrose.
- Saliva has 99.5% water.
- It Contains many important substances, including mucus,
electrolytes, antibacterial compounds and various enzymes
that help to digest food and keep the teeth strong.

15. Composition of saliva
- Saliva consists of 99.5% water, the remaining 0.5% is
dissolved substances including amylase enzymes that
help to digest food and keep the teeth strong.
Regulation of saliva secretion:
- Salivary fluid secretion largely depends upon cholinergic
signaling from parasympathetic nerves whereas the
protein content of saliva is additionally dependent upon
signaling by neuropeptides by sympathetic nerves and
the release of noradrenaline.

16. Stimulate the salivary centers present in brainstem
Presence of food in oral cavity
Mechanical stimulus by food
bolus
Chemical substances of food
Stimulation of taste receptors
Afferent impulses are carried by
7th,9th and 10th cranial nerve
Stimulation of mechanoreceptors
Afferent impulses are carried by 5th
cranial nerve
Efferent impulses along 7th and 9th cranial nerves to salivary glands
Stimulate the secretion of saliva from the glands
Stimulation of mechanoreceptors
Afferent impulses are carried by
5th cranial nerve

17. Functions of saliva:
- Saliva helps moisten the food during mastication
and helps in chewing
- Dissolves the food in forming the bolus and
facilitates swallowing.
- Helps cleanse the teeth.
- Chemical digestion: breaks down starch by the
function of “Salivary Amylase”.

18.  Solvent effect: Dissolves food and allows the
tongue to taste food
 Salivary amylase digests the cooked starch into
maltose.
 pH buffering effect: Prevents sudden changes in
pH
 Lubricating effect: Moisturizes the inside of the
mouth and creates smoother speech

19.  Cleaning effect: washes away food debris and
bacteria remaining in the mouth
 Antibacterial effect: lysozyme, peroxidase,
immunoglobulin A and lactoferrin fight against
pathogenic microorganisms.
 Supplies minerals, including calcium and
phosphorus to teeth.

20. Swallowing and Its Stages
- Mouth, pharynx and esophagus participate in the
process of swallowing
- Once the food is masticated and made into bolus by
mixing with saliva, then the process of swallowing is
initiated voluntarily.

21. - There are the three stages of swallowing
First stage:
- Bolus is pushed into the pharynx with the help of tongue and cheeks.
- Once the bolus reaches the pharynx, the palatoglossal arches close the way
back to the mouth
Second stage:
- At the same time, soft palate closes off the nasopharynx to prevent the
bolus entering into the nose.
- Epiglottis moves backwards to close the opening of larynx like a lid.
- This stage controlled by swallowing center in medulla oblongata and lower
pons of brain.
- Muscles of pharynx push the bolus down through the upper esophageal
sphincter.

22. Third stage:
- Bolus itself stimulates peristalsis of esophageal wall with
pushes the bolus into stomach by relaxation of cardiac
sphincter.
- 4-8 sec required

24. ESOPHAGUS
- It is a 25cm long muscular tube.
- It starts at the level of 6th cervical vertebra, runs the
most of its part through the thoracic cavity.
- At the level of 10th thoracic vertebra, its enters the
diaphragm to reach abdominal cavity, where again it
has a small course to continue with the stomach
- It lies behind the trachea in upper part of its course,
and then it passes anterior to thoracic aorta.

27. Functions of Esophagus
- It transports material from the mouth to the stomach
- Prevent gastroesophageal reflux (GER) by providing
an important barrier, (lower esophageal sphincter
LES)
- Prevent Retrograde flow of gastric contents into the
esophagus.
- It has peristaltic action and moves swallowed food
down to the stomach

29. STOMACH
- It acts as a mechanical mixer, chemical digester and
temporary storehouse of food.
- Most dilated part of digestive tube.
- Capacity in adults is 1-1.5L

30. Parts
- Junction of esophagus and stomach is the cardio-esophageal
junction.
Stomach consists of:
- Two ends, the proximal cardiac and distal pyloric end
- Two borders, the lesser curvature and the greater
curvature.
- Two surfaces, anterosuperior and posteroinferior surface

31. - Part of the stomach above the gastroesophageal
junction is called the fundus of the stomach
- The main part is the body and the distal part is the
pyloric part
- This part possesses a sphincter, called pyloric
sphincter.

33. Functions
Its size is determined by the amount of food
ingested. The stomach has several functions:
- It serves as a temporary store for food and delivers
its contents slowly into the intestines
- It initiates the digestion of proteins
- Gastric secretions are highly acidic and kill harmful
microorganisms carried with the food

34. - It secretes gastric juice which contains intrinsic
factor for the absorption of vitamin B12
- Substances like alcohol, some drugs and glucose are
absorbed in the stomach
- Mechanical breakdown of food contents occurs by
the peristaltic movements of stomach. The addition
of gastric juices liquefies the contents to chyme.

35. Gastric juice

36. Function of Gastric juice
Digestive function
- The juice secreted are pepsin, gastric lipase, and
other enzymes.
- Acts mainly on Proteins.
- Proteolytic enzyme of the gastric juice is pepsin
- Also contain other enzyme like gastric lipase,
gelatinase, urease and gastric amylase

37.  Pepsin:
- It secreted as inactive pepsinogen
- Pepsinogen is converted into pepsin by HCI
- pH for activation of pepsinogen is below 6
- Action of pepsin: pepsin converts proteins into
proteases, peptones and polypeptides.
- Pepsin also causes curdling and digestion of milk.

38.  Gastric lipase:
- It is a weak lipolytic enzyme when compared to
pancreatic lipase.
- Active only when the pH is between 4 and 5 and
becomes inactive at a pH below 2.5
- Gastric lipase is a tributyrates and it hydrolyzes
tributyrin into fatty acids and glycerol.

39.  Actions of other enzymes of gastric juice:
- Gelatinase degrades type I and type V gelatin and
type IV and V collagen into peptides.
- Urease: Acts on urea and produces ammonia
- Gastric amylase degrades starch
- Rennin curdles milk (present in animals only)

40.  Functions of hydrochloric acid: it is present in
gastric juice
- It activates pepsinogen into pepsin
- Kills some of the bacteria entering the stomach
along with food substances. This action is called
bacteriolytic action
- Provides acid medium, which is necessary for the
action of hormones

41. - HCl kills the bacteria present in the food and
provides acidic medium to activate pepsinogen into
pepsin
- Pepsin begins the digestion of proteins and breaks
proteins to polypeptides
- Gastric lipase breaks down triglycerides in fat
molecules

42. Hemopoietic Function
- Intrinsic factor of Castle, secreted by parietal cells of
gastric glands plays an important role in erythropoiesis.
- It is necessary for the absorption of vitamin B12 form
the GI tract into the blood
- Vitamin B12 is an important maturation factor during
erythropoiesis.
- Absence of intrinsic factor in gastric juice causes
deficiency of vitamin b12, leading to pernicious anemia.

43. Protective Function
- Mucus is a mucoprotein, secreted by mucus neck cells of
the gastric glands and surface mucus cells in fundus,
body and other parts of stomach.
- Protects the stomach wall from irritation or mechanical
injury, by virtue of its high viscosity
- Prevents the digestive action of pepsin on the wall of the
stomach and gastric mucosa
- Protects the gastric mucosa from HCL of gastric juice
because of its alkaline nature and its acid combining
power.