THE stomach is J-shaped portion of the alimentary canal which is situated in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity. The stomach has a cardiac sphincter where it joins with the oesophagus and a pyloric sphincter where it meets duodenum. It has two curvatures, the lesser curvature on the posterior surface and the greater curvature on the anterior surface.
The stomach is divided in three regions- the fundus, the body, and the pylorus. At the distal end of the pylorus is the pyloric sphincter. When the stomach is inactive, the sphincter is relaxed and open and when the stomach contains food, the sphincter gets closed.
All the organs of alimentary canal has a similar structure with small modifications according to their functions. The stomach wall also has 4 layers- adventitia, muscle layer, submucosa and mucosa.

Muscle layer:- the muscle layer of stomach wall has 3 layers instead of two in other organs which are
1. The outer layers having longitudinal fibers
2. Middle layer having circular fibers
3. Inner layer having oblique fibers.
This type of arrangement is done in order to provide the churning motion for gastric activity and also supports peristaltic movement. The circular muscle is strongest between the pylorus and pyloric sphincter.

Mucosa:– when the stomach is empty, the mucous membrane lining turns into the folds and the stomach is full, the folds are converted into smooth appearance. No. of gastric glands situated open on to it. They in turn consist of specialized cells which secretes gastric juice.

Functions of stomach

Functions of stomach are as follows-

• it provides temporary storage which allows the digestive enzymes to perform their functions
• chemical digestion of proteins-pepsin breaks proteins into polypeptides
• limited absorption of water, lipids, alcohols and some drugs
• defense against microbes
• prepares iron for absorption
• produces and secrets the intrinsic factors needed  for absorption o vitamin B12
• regulation of the  passageway for the gastric contents towards the duodenum.
• secretion of hormone gastrin

Gastric Juice

The size of stomach varies with the amount of food present in the stomach. when a meal has been eaten, the food is present in the stomach in layers and the last part of the meal remains in the fundus for long time. Mixing with gastric juice takes place gradually. Parasympathetic stimulation increases the motility of the stomach and secretion of gastric juice

About 2 liters  of gastric juice is secreted daily by gastric glands which are present in the mucosa. it consist of-

1. water
2. mineral salts
3. mucous
4.  hydrochloric acid
5. intrinsic factor
6. inactive enzymes like pepsinogens.

Functions of gastric juice

• water present further liquifies the swallowed  food
• HCl:- acidifies the food and therefore stops the action of salivary amylase; kills the microbes; provides the acidic environment  for the action of pepsin
• pepsinogens are activated to pepsins.
• intrinsic factor is necessary for the absorption of vitamin B12 from ileum
• mucus prevents the mechanical injury to the stomach wall.

Secretion of Gastric Juice

There is always a small quantity of gastric juice present in the stomach even when the stomach is empty; this is known as fasting juice. Secretion of gastric juice is maximum after 1 hour of the meal. The level of juice decline to the fasting juice level after 4 hours.

There are 3 phases of secretion of gastric juice-

1.  Cephalic phase:- this phase occur before the food reaches the stomach. It is due to the reflex stimulation of the vagus nerves which provides parasympathetic stimulation which initiates due to smell or taste of food. Sympathetic stimulation inhibits the secretion
2.  Gastric phase:-  the enteroendocrine cells in the pylorus and the duodenum secretes the hormone gastrin which are stimulated due to the presence of food in the stomach. The gastrin passes into the blood; through blood the gastrin reaches the stomach which stimulates the gastrin glands to produce more gastric juice. in this way secretion of digestive juice is completed after the meal. Gastric secretion stops when the Ph in the pylorus becomes 1.5 .
3.  Intestinal phase:-  when the partially digested content flows in the duodenum, two hormones-secretin and cholecystokinin(CCK)  are produced by small intestine which slows down the gastric juice secretion. The chyme in the duodenum becomes more thoroughly mixes with the pancreatic juice and bile when the emptying rate of stomach falls. the rate at which the stomach empties depends upon the type of food present; like carbohydrate stays for 2-3 hours, proteins takes more time but fats takes the longest time.

Small intestine

At the proximal end of the small intestine, it joins with the stomach at the pyloric sphincter. it is about 2.5 cm in diameter and is about 5 m long and at the distal end the small intestine meets the caecum of large intestine at the ileocaecal valve . it lies in the abdominal cavity. it is divided in 3 parts

• Duodenum:– it is about 25 cm long and is present around the pancreas. the pancreatic juice and bile merges at the hepatopancreatic ampulla and the mixture than enters the duodenum through duodenal papilla
• Jejunum:- it is the middle part which is about 2 m long
• Ileum:- it is about 3m long and ends at the ileocaecal valve where it joins the caecum of the large intestine and this valve also prevents the back flow of the materials back into the small intestine from large intestine.

The wall of small intestine is made of 4 layers-

1. peritoneum:- the double layer of peritoneum attaches the ileum and jejunum to the posterior abdominal wall. this allows the blood vessels, lymph vessels, nerves to lie on the posterior abdominal wall and their branches then supply the small intestine.
2. muscle layer- same as in other organs of alimentary tract.
3. submucosa:- same as in the other organs of alimentary canal.
4. Mucosa:-the surface area of mucosa in small intestine is increased by the permanent circular folds, villi and microvilli.

Small intestine performs various functions which are described as follows-

• movement of contents from small intestine ahead by peristalsis.
• secretion of intestinal juice
• chemical digestion of carbohydrates, fats, proteins
• protection from microbes
• secretion of hormones like cholecystokinin(CCK) and secretin
• absorption of nutrients

Chemical digestion in small intestine takes place when the acid chime passes into the small intestine, it is mixed with the pancreatic juice , bile and the intestinal juice. digestion of nutrients is completed when-

• The carbohydrates break into monosaccharides
•  Proteins break into amino acids
• Fats are broken into fatty acids and glycerol.

the pancreatic juice is secreted by the exocrine pancreas. it is basic in nature, its consist of water, mineral salts, amylase , lipase, trysinogen and chymotrypsinogen.

the bile is secreted by liver and is stored in the gall bladder and the pH is 8. It consist of water, mineral salts, mucus, bile salts, bile pigments and cholesterol. Functions of bile include emulsification of fats, makes cholesterol and fatty acid soluble and excretion of bilirubin

the intestinal juice is secreted by the small intestine. it is also basic in nature and consist of water, mucus and mineral salts.

Absorption of nutrients from small intestine takes place by several processes like diffusion, osmosis, facilitated diffusion, active transport etc. the surface area through which the absorption of nutrients takes place in small intestine is increased by the circular folds and the villi. We will study the complete digestion and adsorption of nutrients in next upcoming articles.

Large intestine

it is about 1.5m long which begins at the caecum and ends at anal canal which is within the pelvis. the large intestine has diameter of 6.5cm and is larger than that of the small intestine. The large intestine is divided in four regions-

• Caecum:- inferior end is closed while it joins the ascending colon superiorly. it is about 8-9 cm long and contains large no. of lymphoid tissue. Appendix has no digestive function but when it is inflamed, it can cause problems.
• Colon:- colon has 4 parts having same structure and function-

1. ascending colon
2. transverse colon
3. descending colon
4. sigmoid colon

• Rectum:- it is about 13cm long and starts from the sigmoid colon and ends at the anal canal.
• Anal canal:-it is about 3.8cm long which leads to the exterior of large intestine. it has two shpincter – the internal sphincter which is controlled  by ANS and the external sphincter which is under voluntary control.

The wall of large intestine has four layers as in other organs of alimentary tract with certain modifications which are described as follows-

1.  in caecum and colon, the longitudinal muscles are present in collected form in 3 bands instead of a continuous layer.
2. in rectum, the longitudinal muscles  forms the basic structure and therefore completely surrounds the rectum and anal canal.
3.  in the submucosal layer there are more lymphoid tissue which is for defense action
4. the mucous of colon and upper part of rectum has no. of goblet cells.

Functions of large intestine

• Absorption:- the contents that pass from the ileum to caecum are mostly fluids and absorption water takes place in large intestine along with absorption of mineral salts, vitamins, some drugs.
• microbial activity:- the large intestine consist of large no. of bacteria which synthesize vitamin K and folic acids
• mass movement:- since peristalsis does not takes place  in  large intestine but only after every 30mins, a wave of peristalsis facilitates  the contents to flow into the large intestine and this mechanism is known as peristalsis.
• Defaecation:- it is the final stage of digestion in which the waste material is excreted through the anus. defaecation involves involuntary contraction of the muscle of rectum and relaxation of internal anal sphincter. contration of abdominal muscles and lowering the diaphragm also increases the intra-abdominal pressure which also facilitates defaecation.

Faeces is the waste material which is not digested in the small intestine. it consist of semisolid brown mass, the brown color is due to the presence of stercobilin. water is absorbed in large and small intestine, still it is found in the faeces. It consist of-

1. fiber-indigestible cellular plant and animal material
2. dead and live microbes
3. fatty acids
4. epithelial cells that shed from the GIT
5. mucous secreted by the large intestine

Multiple choice questions(MCQs) Based on Stomach & Gastric Juice Secretion

1. where is the stomach situated?
A. epigastric region of abdominal cavity
B. right hypochondriac region of abdominal cavity
c. in front of pelvic cavity
D. behind the pelvic cavity

2. which layer of stomach wall contains the gastric glands?
A. adventitia                  B. muscle layer
C. mucosa                      D. sub-mucosa

3. what is the function of stomach?
A. temporary storage B. chemical digestion of proteins
C. defence against microbes D. all of the above

4. which of the following is not the constituent of gastric juice?
A. water B. mineral salts
C. mucous D. none of the above

5. which of the following statement is NOT true?
A. stomach is divided in 3 regions
B. the proximal end of pylorus has pyloric sphincter
C. gastric juice secretes intrinsic factor
D. HCl is secreted by parietal cells

6. which phase of digestion starts before the food enters the stomach?
A. cephalic phase B. gastric phase
C. intestinal phase D. both A and B

7. Match the following-
a)outer layer of muscle layer                   1. Contains circular fibers
b)middle layer of muscle layer                2. Contains longitudinal fibers
c)inner layer of muscle layer                    3. Contains gastric glands
d)mucosa                                                     4. Contains oblique fibers

8. which of the following is NOT the function of gastric juice?
A. acidifies the food B. liquefies the food
C. preparation of iron for digestion D. kills microbes

9. what initiates the cephalic phase of digestion?
A. sympathetic stimulation B. parasympathetic stimulation
C. sense of taste and smell D. both B and C

10. which nutrients remains in the stomach for the longest time?
A. lipids B. proteins
C. carbohydrates D. fats

ANSWERS:–

1. epigastric region of abdominal cavity
2. mucosa
3. all of the above
4. none of the above
5. the proximal end of pylorus has pyloric sphincter
6. cephalic phase
7. a) – 2 b) – 1 c) – 4 d) – 3
8. preparation of iron for absorption
9. both B and C
10. fats

Multiple choice questions(MCQs) Based on Small Intestine & Large Intestine

1. Where small intestine joins the stomach?
A. fundus B. lesser curvature
C. greater curvature D. pyloric sphincter

2. How long is the small intestine?
A. 5 meters B. 2meters
C. 10meters D. 3meters

3. Which of the following are the constituents of pancreatic juice?
A. water B. inactive enzymes
C. mineral salts D. all of the above

4. At what point the pancreatic juice and bile merges before entering the duodenum?
A. duodenal papilla B. hepatic duct
C. duodenal impression D. hepatopancreatic ampulla

5. Match the following-
a)Pancreatic juice 1. Secreted by small intestine
b)Bile 2. Secreted by pancreas
c)Intestinal juice 3. Excreted though anus
d)Faeces 4. Secreted by liver

6. What is the length of the large intestine?
A. 3 meters B. 1.5 meters
C. 5 meters D. 2 meters

7. Which of the following statement is NOT true?
A. ileocaecal valve prevents back flow of contents
B. pancreatic juice is acidic in nature
C. defecation involves contraction of rectum muscles
D. in mass movement, peristalsis takes place after every 30 mins

8. what is the reason for the brown color of feces?
A. presence of fiber

B. shed epithelial cells

C. stercobilin

D. both B and C

9. WHAT is the function of small intestine?
A. mass movement                               B. defecation
C. microbial activity                             D. none of the above

10. Which of the following is NOT the content of bile
A. mineral salts                                  B. bile pigment
c. amylase                                            D. water

ANSWERS:-

1. pyloric sphincter
2. 5meters
3. all of the above
4. hepatopancreatic ampulla
5. a) – 2 b) – 4 c) – 1 d) – 3
6. 1.5 meters
7. pancreatic juice is acidic in nature
8. stercobilin
9. none of the above
10. amylase

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 REFRENCE:-1. Ross and Wilson-Anatomy and physiology in health and illness; 12th edition; page no.-: 297-301.

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