Mechanical and Chemical Digestion in the Oral cavity

The gastrointestinal tract starts in the oral cavity where your teeth grind and chew food, breaking it into small manageable pieces. This chewing process, known as mastication, is temporalis dependent upon powerful muscles muscle (masseter and temporalis), as well as smaller muscles that permit fine control; they move the mandible (lower jawbone) against the upper jaw and enable crushing of relatively hard food. muscle. Mastication causes exocrine glands under the tongue and in the back called saliva which performs two essential functions. It moistens and compacts the chewed food so your tongue can roll it into a ball (bolus) and push it masseter to the back of your mouth for swallowing and easy passage through the pharynx and In addition, saliva contains digestive enzymes (e.g. salivary amylase) which begin the breakdown of carbohydrates. Mastication and saliva secretion work in harmony:chewing increases the surface area of foods which helps to accelerate the

esophagus. breakdown of starch molecules into simple sugars by the digestive enzymes.

the mouth, except for the release of

Almost no protein or fat digestion occurs

lingual lipase an enzyme secreted by Ebner's glands on the dorsal surface of the

tongue. The actions of the teeth and tongue prepare food for swallowing. When you

are ready to swallow, the tongue pushes a piece of chewed food (a bolus) toward the

back of your throat and into the opening of the esophagus - the tube which leads to the

stomach. To prevent food in the throat from rising into the nasal cavity or moving

down the windpipe (trachea), the act of swallowing triggers two involuntary events.

The soft palate (the back of the roof of the mouth) closes off the nasal cavity while the

epiglottis, a flap of cartilage attached to the root of the tongue tilts downward to seal

the trachea. The swallowing procedure is regulated by nerves in the medulla

oblongata and pons. The reflex is instigated by receptors in the throat as a bolus of

food is pushed to the back of the mouth by the tongue.

Swallowing process

The three stages of swallowing are as follows:

Stage 1: The Oral Stage

 

Stage 2: The Pharyngeal Stage

The larynx lifts up to meet the epiglottis, which lowers, making a seal that prevents material from entering the windpipe. This is important as it stops food or liquid from being aspirated into the lungs.

Stage 3: The oesophageal Stage

The bolus is passed into the oesophagus by

automatic contractions of the pharynx. It then travels to the stomach by gravity and

reflex action. This stage of swallowing is entirely automatic and cannot be controlled.

Peristalsis

Once the food ball enters the esophagus, it is pushed towards the cardiac sphincter by smooth muscle contractions called peristalsis. Food travels from the

mouth to the stomach in about 4 to 8 seconds. Peristalsis occurs throughout the length of the digestive tract and is responsible for keeping things moving and the occasional strange sounds that arise. The digestive tract is surrounded by both circular and longitudinal smooth muscle that allows for rhythmic contractions or peristalsis,

Food in stomach

Food enters the stomach from the

esophagus, through the lower esophageal sphincter. The stomach is the part

where physical and chemical breakdown

of food really begins.It operates like a food

mixer, churning the food bolus to a pulp

called chyme, and releasing numerous

chemicals such as hich are digestive hormones, enzymes and gastric juices which help to break down food molecules in the chyme into small particles for absorption into the bloodstream. An capacity expands to about 1 liter of food, and may expand to hold as much as 4 liters. empty stomach has a volume of approximately 50 ml. But typically after a meal, its The chyme slowly exits the stomach via the pyloric sphincter or valve and passes into the duodenum - the first segment of the small intestine - where digestion continues.

 Structure of Stomach

Shaped like the letter "j", the stomach is a large stretchy bag situated in the middle of the chest behind the liver, between the esophagus and the first section of the
small intestine (the duodenum). It is subdivided into 4 regions each with different cells and gastric functions:

 (1) the Cardiac region, where the contents of
the esophagus empty into the stomach through the lower esophageal or cardiac
sphincter; 

(2) the Fundus, an expanded area curving up
above the esophageal opening;
(3) the Body, the central and
largest region;, 

 (4) the Pylorus, the narrow end of the
stomach that joins the small intestine at the pyloric sphincter. Like the cardiac
  sphincter the pyloric spinctrr is a ring of muscle that regulates the movement of food
out of the stomach.
The wall of the stomach is lined with millions of gastric glands, which
together secrete 400-800 ml of gastric juice at each meal. Several kinds of cells are found in the gastric glands

1.parietal cells

2.chief cells

3.mucus-secreting cells

4.hormone-secreting(endocrine) cells

a. Parietal cells

Parietal cells secrete hydrochloric acid

intrinsic factor

i. Hydrochloric acid (HCI)

Parietal cells contain a Ht and ATPase. This transmembrane protein secretes ions (protons) by active transport using

the energy of ATP, The concentration of it in

the gastric juice can be as high as 0.15 M, giving gastric juice a pH somewhat less than

1.

ii. Intrinsic factor

Intrinsic factor is a protein that binds ingested vitamin B,, and enables it to be

absorbed by the intestine.

b. Chief Cells

The chief cells synthesize and secrete pepsinogen, the precursor to the proteolytic enzyme pepsin.

c. Mucus secreting cells

Special cells secrete a protective coating called mucus, on the stomach walls

to prevent damage from gastric acids. Originally it was thought that peptic ulcers

were caused by an erosion of this mucus lining by these acids. However recent

research indicates that these ulcers are caused largely by the spread of a type of

bacteria called Helicobacter pylori bacterium into the gastric walls

d. Hormone secreting cells

Secretion by the gastric glands is stimulated by the hormone gastrin. Gastrin is released by endocrine cells in the stomach in response to the arrival of food. 

 Absorption in the stomach

Very little absorption occurs in stomach. However, some water, certain ions,

and drugs like aspirin and ethanol are absorbed from the stomach into the blood

(accounting for the quick relief of a headache after swallowing aspirin).

Functions of the Stomach

.

.

Although it is a very complex organ which performs a wide variety of digestive actions, the stomach has 3 main functions:

It stores the food.

 This allows us to cat a large number of food calories in a relatively short time and then digest it over a longer period. Without the stomach's storage capacity, we would need to eat very small amounts of food continuously throughout the day, because the small intestine digests food very slowly.

 It breaks down large fat and protein molecules in food, 

so they can be absorbed in the small intestine. To do this, the stomach releases a number of powerful gastric juices containing hydrochloric acid and other digestive

enzymes. In addition to breaking down food, these acidic juices (pH 1-3) also kill bacteria in the food. For easier digestion, powerful muscles in the stomach wall churn the food into a paste of porridge-like consistency, called chyme. This churning action also ensures that the secreted gastric acids and enzymes are thoroughly mixed with the food.

 It empties the partially digested chyme into the duodenum

 (the first segment of the small intestine) at a manageable speed, through the the pyloric sphincter. While the intestine is full and still digesting food, the stomach acts as storage area for food.

The absorption of food and water by the stomach is negligible, but iron and

highly fat-soluble substances like alcohol are absorbed directly.

stomach

Mechanism of Secretion of Gastric Juice

An interesting question is raised here. What causes gastric juice to be secreted? There are two possible answers to this question - chemical control and Nervous control. Sometimes even the sight, smell, taste or hearing of delicious food, stimulate the nervous system which orders for the secretion of small amount of gastric juice like watering of mouth. This is proved by the experiment of Russian, Pavlov. He cut the esophagus of a dog and left the cutend open to the outside stomach resulted in the secretion of about one fourth the normal amount of gastric. When he fed this dog, the food, of course, never reached the stomach, yet the stomach resulted in the secretion of about one fourth the normal amount of gastric  juice. This showed that the gastric secretion was under the reflex control and cutting of the gastric nerves proved it. If there are more proteins in the food, a message is given to the brain which in result

would order the gastric glands to secrete more gastric juice. A hormone called gastrin, controls the secretion of gastric juice.

Protein molecules stimulate the endocrine cells of the stomach to secrete gastrin. The

liberating gastrin is soon absorbed by the blood which carries it to the gastric glands to

secrete large amount of gastric juice in the stomach. The contact of the food with the

lining of the stomach also causes the cells to secrete gastrin which stimulated the

gastric glands to secrete gastric juice.