Functional anatomy of Stomach .
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| Parts of stomach . |
[1]. Stomach is a hollow organ situated just below the diaphragm on the left side in the abdominal cavity.
[2]. Volume of empty stomach is 50 mL.
[3]. Under normal conditions, it can expand to accommodate 1 L to 1.5 L of solids and liquids. However, it is capable of expanding still further up to 4 L.
Parts of Stomach .
In humans, stomach has four parts:
1. Cardiac region .
2. Fundus .
3. Body or corpus .
4. Pyloric region.
1. Cardiac Region .
[1]. Cardiac region is the upper part of the stomach where esophagus opens.
[2]. The opening is guarded by a sphincter called cardiac sphincter, which opens only towards stomach. This portion is also known as cardiac end.
2. Fundus .
[1]. Fundus is a small dome shaped structure.
[2]. It is elevated above the level of esophageal opening.
3. Body or Corpus .
[1]. Body is the largest part of stomach forming about 75% to 80% of the whole stomach.
[2]. It extends from just below the fundus up to the pyloric region .
4. Pyloric Region .
[1]. Pyloric region has two parts, antrum and pyloric canal. The body of stomach ends in antrum.
[2]. Junction between body and antrum is marked by an angular notch called incisura angularis.
[3]. Antrum is continued as the narrow canal, which is called pyloric canal or pyloric end.
[4]. Pyloric canal opens into first part of small intestine called duodenum.
[5]. The opening of pyloric canal is guarded by a sphincter called pyloric sphincter. It opens towards duodenum.
[6]. Stomach has two curvatures. One on the right side is lesser curvature and the other on left side is greater curvature.
Structure of Stomach Wall .
Stomach wall is formed by four layers of structures:
[1] . Outer serous layer: It is formed by peritoneum
[2] . Muscular layer: It is made up of three layers of smooth muscle fibers, namely inner oblique, middle circular and outer longitudinal layers
[3] . Sub-mucus layer: It is formed by areolar tissue, blood vessels, lymph vessels and Meissner nerve plexus.
[4] . Inner mucus layer: It is lined by mucus secreting columnar epithelial cells.
[4.1]. The gastric glands are situated in this layer. Under resting conditions, the mucosa of the stomach is thrown into many folds. These folds are called rugae.
[4.2]. The rugae disappear when the stomach is distended after meals. Throughout the inner mucus layer, small depressions called gastric pits are present.
[4.3]. Glands of the stomach open into these pits. Inner surface of mucus layer is covered by 2 mm thick mucus.
Glands of Stomach – Gastric Glands .
[1]. Glands of the stomach or gastric glands are tubular structures made up of different types of cells .
[2]. These glands open into the stomach cavity via gastric pits.
Classification of Glands of the Stomach .
Gastric glands are classified into three types, on the basis of their location in the stomach:
1. Fundic glands or main gastric glands or oxyntic glands: It is situated in body and fundus of stomach
2. Pyloric glands: It is present in the pyloric part of the stomach
3. Cardiac glands: It is located in the cardiac region of the stomach.
Structure of Gastric Glands .
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| Gastric Glands . |
1. Fundic Glands .
[1]. Fundic glands are considered as the typical gastric glands .
[2]. These glands are long and tubular.
[3]. Each gland has three parts, viz. body, neck and isthmus.
Cells of fundic glands .
1. Chief cells or pepsinogen cells
2. Parietal cells or oxyntic cells
3. Mucus neck cells
4. Enterochromaffin (EC) cells or Kulchitsky cells
5. Enterochromaffin like (ECL) cells.
[4]. Parietal cells are different from other cells of the gland because of the presence of canaliculi (singular = canaliculus).
[5]. Parietal cells empty their secretions into the lumen of the gland through the canaliculi. But, other cells empty their secretions directly into lumen of the gland.
2. Pyloric Glands .
Pyloric glands are short and tortuous in nature. These glands are formed by G cells, mucus cells, EC cells and ECL cells.
3. Cardiac Glands .
Cardiac glands are also short and tortuous in structure, with many mucus cells. EC cells, ECL cells and chief cells are also present in the cardiac glands .
Enteroendocrine Cells .
Enteroendocrine cells are the hormone secreting cells present in the glands or mucosa of gastrointestinal tract, particularly stomach and intestine. The enteroendocrine cells present in gastric glands are G cells, EC cells and ECL cells .
Function of the gastric gland .
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| Secretory function of cells in gastric glands . |
Function of the gastric gland is to secrete gastric juice. Secretory activities of different cells of gastric glands and enteroendocrine cells are as table above .
Function of Stomach .
1.Mechanical Function .
i. Storage Function .
[1]. Food is stored in the stomach for a long period, i.e. for 3 to 4 hours and emptied into the intestine slowly. The maximum capacity of stomach is up to 1.5 L.
[2]. Slow emptying of stomach provides enough time for proper digestion and absorption of food substances in the small intestine.
ii. Formation of Chyme .
Peristaltic movements of stomach mix the bolus with gastric juice and convert it into the semisolid material known as Chyme.
2. Digestive Function .
3. Protective Function .
4. Hemopoietic Function .
5. Excretory Function .
Many substances like toxins, alkaloids and metals are excreted through gastric juice.
Properties and Composition of Gastric Juice .
Gastric juice is a mixture of secretions from different gastric glands.
Properties of Gastric Juice .
Volume : 1200 mL/day to 1500 mL/day.
Reaction : Gastric juice is highly acidic with a pH of 0.9 to 1.2. Acidity of gastric juice is due to the presence of hydrochloric acid.
Specific gravity : 1.002 to 1.004
Composition of Gastric Juice .
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| Composition of gastric juice . |
[1]. Gastric juice contains 99.5% of water and 0.5% solids.
[2]. Solids are organic and inorganic substances.
Function of Gastric Juice .
1. Digestive Function .
[1]. Gastric juice acts mainly on proteins.
[2]. Proteolytic enzymes of the gastric juice are pepsin and rennin .
[3]. Gastric juice also contains some other enzymes like gastric lipase, gelatinase, urase and gastric amylase.
Pepsin .
[1]. Pepsin is secreted as inactive pepsinogen.
[2]. Pepsinogen is converted into pepsin by hydrochloric acid.
[3]. Optimum pH for activation of pepsinogen is below 6.
Action of pepsin
[1]. Pepsin converts proteins into proteases, peptones and polypeptides.
[2]. Pepsin also causes curdling and digestion of milk (casein).
Gastric Lipase .
[1]. Gastric lipase is a weak lipolytic enzyme when compared to pancreatic lipase.
[2]. It is active only when the pH is between 4 and 5 and becomes inactive at a pH below 2.5.
[3]. Gastric lipase is a tributyrase and it hydrolyzes tributyrin (butter fat) into fatty acids and glycerol.
Actions of Other Enzymes of Gastric Juice .
i. Gelatinase: Degrades type I and type V gelatin and type IV and V collagen (which are proteoglycans in meat) into peptides
ii. Urase: Acts on urea and produces ammonia
iii. Gastric amylase: Degrades starch (but its action is insignificant)
iv. Rennin: Curdles milk (present in animals only).
2. Hemopoietic Function .
[1]. Intrinsic factor of Castle, secreted by parietal cells of gastric glands plays an important role in erythropoiesis.
[2]. It is necessary for the absorption of vitamin B12 (which is called extrinsic factor) from GI tract into the blood.
[3]. Vitamin B12 is an important maturation factor during erythropoiesis.
[4]. Absence of intrinsic factor in gastric juice causes deficiency of vitamin B12, leading to pernicious anemia .
3. Protective Function – Function of Mucus .
[1]. 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.
[2]. It protects the gastric wall by the following ways:
Mucus:
[1]. Protects the stomach wall from irritation or mechanical injury, by virtue of its high viscosity.
[2]. Prevents the digestive action of pepsin on the wall of the stomach, particularly gastric mucosa.
[3]. Protects the gastric mucosa from hydrochloric acid of gastric juice because of its alkaline nature and its acid combining power.
4. Functions of Hydrochloric Acid .
Hydrochloric acid is present in the gastric juice:
[1]. Activates pepsinogen into pepsin
[2]. Kills some of the bacteria entering the stomach along with food substances. This action is called bacteriolytic action
[3]. Provides acid medium, which is necessary for the action of hormones.
Secretion of Gastric Juice .
Secretion of Pepsinogen .
[1]. Pepsinogen is synthesized from amino acids in the ribosomes attached to endoplasmic reticulum in chief cells.
[2]. Pepsinogen molecules are packed into zymogen granules by Golgi apparatus.
[3]. When zymogen granule is secreted into stomach from chief cells, the granule is dissolved and pepsinogen is released into gastric juice.
[4]. Pepsinogen is activated into pepsin by hydrochloric acid.
Secretion of Hydrochloric acid .
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| Secretion of hydrochloric acid in the parietal cell of gastric gland . |
[1]. According to Davenport theory, hydrochloric acid secretion is an active process that takes place in the canaliculi of parietal cells in gastric glands.
[2]. The energy for this process is derived from oxidation of glucose. Carbon dioxide is derived from metabolic activities of parietal cell.
[3]. Some amount of carbon dioxide is obtained from blood also. It combines with water to form carbonic acid in the presence of carbonic anhydrase. This enzyme is present in high concentration in parietal cells.
[4]. Carbonic acid is the most unstable compound and immediately splits into hydrogen ion and bicarbonate ion.
[5]. The hydrogen ion is actively pumped into the canaliculus of parietal cell. Simultaneously, the chloride ion is also pumped into canaliculus actively.
[6]. The chloride is derived from sodium chloride in the blood. Now, the hydrogen ion combines with chloride ion to form hydrochloric acid.
[7]. To compensate the loss of chloride ion, the bicarbonate ion from parietal cell enters the blood and combines with sodium to form sodium bicarbonate. Thus, the entire process is summarized as :
CO2 + H2 O + NaCl → HCl + NaHCO3 .
Factors Stimulating the Secretion of Hydrochloric Acid
1. Gastrin
2. Histamine
3. Vagal stimulation.
Factors Inhibiting the Secretion of Hydrochloric Acid .
1. Secretin
2. Gastric inhibitory polypeptide
3. Peptide YY.
Regulation of Gastric Secretion .
Regulation of gastric secretion and intestinal secretion is studied by some experimental procedures. Method of Study .
1. Pavlov Pouch .
[1]. Pavlov pouch is a small part of the stomach that is incompletely separated from the main portion and made into a small baglike pouch.
[2]. Pavlov pouch was designed by the Russian scientist Pavlov, in a dog during his studies on conditioned reflexes.
Procedure .
[1]. To prepare a Pavlov pouch, stomach of an anesthetized dog is divided into a larger part and a smaller part by making an incomplete incision.
[2]. The mucus membrane is completely divided. A small part of muscular coat called isthmus is retained.
[3]. Isthmus connects the two parts. The cut edges of major portions are stitched. Smaller part is also stitched, leaving a small outlet.
[4]. This outlet is brought out through the abdominal wall and used to drain the pouch.
Nerve supply of Pavlov pouch .
Pavlov pouch receives parasympathetic (vagus) nerve fibers through isthmus and sympathetic fibers through blood vessels.
Use of Pavlov pouch .
Pavlov pouch is used to demonstrate the different phases of gastric secretion, particularly the cephalic phase and used to demonstrate the role of vagus in cephalic phase.
2. Heidenhain Pouch .
[1]. Heidenhain pouch is the modified Pavlov pouch.
[2]. It is completely separated from main portion of stomach by cutting the isthmus without damaging blood vessels. So, the blood vessels are intact.
[3]. Thus, Heidenhain pouch does not have parasympathetic supply, but the sympathetic fibers remain intact through the blood vessels.
Uses of Heidenhain pouch .
Heidenhain pouch is useful to demonstrate the role of sympathetic nerve and the hormonal regulation of gastric secretion after vagotomy (cutting the vagus nerve).
3. Bickel Pouch .
In this, even the sympathetic nerve fibers are cut by removing the blood vessels. So, Bickel pouch is a totally denervated pouch.
Uses of Bickel pouch .
Bickel pouch is used to demonstrate the role of hormones in gastric secretion.
4. Farrel and Ivy Pouch .
[1]. Farrel and Ivy pouch is prepared by completely removing the Bickel pouch from the stomach and transplanting it in the subcutaneous tissue of abdominal wall or thoracic wall in the same animal.
[2]. New blood vessels develop after some days. It is used for experimental purpose, when the new blood vessels are developed.
Uses of Farrel and Ivy pouch .
This pouch is useful to study the role of hormones during gastric and intestinal phases of gastric secretion.
5. Sham Feeding .
Sham feeding means the false feeding. It is another experimental procedure devised by Pavlov to demonstrate the regulation of gastric secretion.
Procedure .
1. A hole is made in the neck of an anesthetized dog
2. Esophagus is transversely cut and the cut ends are drawn out through the hole in the neck
3. When the dog eats food, it comes out through the cut end of the esophagus
4. But the dog has the satisfaction of eating the food. Hence it is called sham feeding.
[1]. This experimental procedure is supported by the preparation of Pavlov pouch with a fistula from the stomach.
[2]. The fistula opens to exterior and it is used to observe the gastric secretion. The animal is used for experimental purpose after a week, when healing is completed.
Advantage of sham feeding .
[1]. Sham feeding is useful to demonstrate the secretion of gastric juice during cephalic phase.
[2]. In the same animal after vagotomy, sham feeding does not induce gastric secretion. It proves the role of vagus nerve during cephalic phase.
Phases of Gastric Secretion .
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| Regulation of gastric secretion . |
[1]. Secretion of gastric juice is a continuous process.
[2]. But the quantity varies, depending upon time and stimulus.
[3]. Accordingly, gastric secretion occurs in three different phases:
1. Cephalic phase
2. Gastric phase
3. Intestinal phase.
[4]. In human beings, a fourth phase called interdigestive phase exists.
[5]. Each phase is regulated by neural mechanism or hormonal mechanism or both.
Cephalic Phase .
[1]. Secretion of gastric juice by the stimuli arising from head region (cephalus) is called cephalic phase.
[2]. This phase of gastric secretion is regulated by nervous mechanism.
[3]. The gastric juice secreted during this phase is called appetite juice.
[4]. During this phase, gastric secretion occurs even without the presence of food in stomach.
[5]. The quantity of the juice is less but it is rich in enzymes and hydrochloric acid.
[6]. Nervous mechanism regulates cephalic phase through reflex action.
Two types of reflexes occur :-
1. Unconditioned reflex
2. Conditioned reflex.
1. Unconditioned Reflex
[1]. Unconditioned reflex is the inborn reflex.
[2]. When food is placed in the mouth, salivary secretion is induced . Simultaneously, gastric secretion also occurs.
Stages of reflex action:
[1]. Presence of food in the mouth stimulates the taste buds and other receptors in the mouth
[2]. Sensory (afferent) impulses from mouth pass via afferent nerve fibers of glossopharyngeal and facial nerves to amygdala and appetite center present in hypothalamus .
[3]. From here, the efferent impulses pass through dorsal nucleus of vagus and vagal efferent nerve fibers to the wall of the stomach
[4]. Vagal efferent nerve endings secrete acetylcholine, which stimulates gastric secretion.
2. Conditioned Reflex .
[1]. Conditioned reflex is the reflex response acquired by previous experience .
[2]. Presence of food in the mouth is not necessary to elicit this reflex.
[3]. The sight, smell, hearing or thought of food, which induce salivary secretion induce gastric secretion also.
Stages of reflex action:
[1]. Impulses from the special sensory organs (eye, ear and nose) pass through afferent fibers of neural circuits to the cerebral cortex. Thinking of food stimulates the cerebral cortex directly
[2]. From cerebral cortex, the impulses pass through dorsal nucleus of vagus and vagal efferents and reach the stomach wall
[3]. Vagal nerve endings secrete acetylcholine, which stimulates the gastric secretion.
Experimental evidences to prove cephalic phase .
[1]. Unconditioned reflex of gastric secretion is proved by sham feeding along with Pavlov pouch (see above). After vagotomy, sham feeding does not cause gastric secretion. It proves the importance of vagus nerve in this phase.
[2]. Conditioned reflex of gastric secretion is proved by Pavlov pouch and bell dog experiment .
Gastric Phase .
[1]. Secretion of gastric juice when food enters the stomach is called gastric phase. This phase is regulated by both nervous and hormonal control.
[2]. Gastric juice secreted during this phase is rich in pepsinogen and hydrochloric acid.
Mechanisms involved in gastric phase are:
1. Nervous mechanism through local myenteric reflex and vago-vagal reflex
2. Hormonal mechanism through gastrin
Stimuli, which initiate these two mechanisms are:
1. Distention of stomach .
2. Mechanical stimulation of gastric mucosa by bulk of food .
3. Chemical stimulation of gastric mucosa by the food contents.
1. Nervous Mechanism
Local myenteric reflex .
[1]. Local myenteric reflex is the reflex elicited by stimulation of myenteric nerve plexus in stomach wall.
[2]. After entering stomach, the food particles stimulate the local nerve plexus present in the wall of the stomach.
[3]. These nerve fibers release acetylcholine, which stimulates the gastric glands to secrete a large quantity of gastric juice. Simultaneously, acetylcholine stimulates G cells to secrete gastrin .
Vagovagal reflex .
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| Vagovagal Reflex . |
[1]. Vagovagal reflex is the reflex which involves both afferent and efferent vagal fibers.
[2]. Entrance of bolus into the stomach stimulates the sensory (afferent) nerve endings of vagus and generates sensory impulses.
[3]. These sensory impulses are transmitted by sensory fibers of vagus to dorsal nucleus of vagus, located in medulla of brainstem.
[4]. This nucleus in turn, sends efferent impulses through the motor (efferent) fibers of vagus, back to stomach and cause secretion of gastric juice.
[5]. Since, both afferent and efferent impulses pass through vagus, this reflex is called vagovagal reflex .
2. Hormonal Mechanism – Gastrin .
[1]. Gastrin is a gastrointestinal hormone secreted by the G cells which are present in the pyloric glands of stomach.
[2]. Small amount of gastrin is also secreted in mucosa of upper small intestine.
[3]. In fetus, it is also secreted by islets of Langerhans in pancreas.
[4]. Gastrin is a polypeptide containing G14, G17 or G34 amino acids.
[5]. Gastrin is released when food enters stomach.
[6]. Mechanism involved in the release of gastrin may be the local nervous reflex or vagovagal reflex. [7]. Nerve endings release the neurotransmitter called gastrin releasing peptide, which stimulates the G cells to secrete gastrin.
Actions of gastrin on gastric secretion .
Gastrin stimulates the secretion of pepsinogen and hydrochloric acid by the gastric glands.
Experimental evidences of gastric phase .
[1]. Nervous mechanism of gastric secretion during gastric phase is proved by Pavlov pouch.
[2]. Hormonal mechanism of gastric secretion is proved by Heidenhain pouch, Bickel pouch and Farrel and Ivy pouch.
Intestinal Phase .
[1]. Intestinal phase is the secretion of gastric juice when Chyme enters the intestine.
[2]. When chyme enters the intestine, initially, the gastric secretion increases but later it stops.
[3]. Intestinal phase of gastric secretion is regulated by nervous and hormonal control.
Initial Stage of Intestinal Phase .
Chyme that enters the intestine stimulates the duodenal mucosa to release gastrin, which is transported to stomach by blood. There it increases gastric secretion.
Later Stage of Intestinal Phase .
After the initial increase, there is a decrease or complete stoppage of gastric secretion. Gastric secretion is inhibited by two factors:
1. Enterogastric reflex
2. Gastrointestinal (GI) hormones.
1. Enterogastric reflex .
[1]. Enterogastric reflex inhibits the gastric secretion and motility.
[2]. It is due to the distention of intestinal mucosa by chyme or chemical or osmotic irritation of intestinal mucosa by chemical substances in the chyme.
[3]. It is mediated by myenteric nerve (Auerbach) plexus and vagus.
2. Gastrointestinal hormones .
[1]. Presence of chyme in the intestine stimulates the secretion of many GI hormones from intestinal mucosa and other structures.
[2]. All these hormones inhibit the gastric secretion. Some of these hormones inhibit the gastric motility also.
GI hormones which inhibit gastric secretion:
[1]. Secretin: Secreted by the presence of acid chyme in the intestine
[2]. Cholecystokinin: Secreted by the presence of chyme containing fats and amino acids in intestine
[3]. Gastric inhibitory peptide (GIP): Secreted by the presence of chyme containing glucose and fats in the intestine
[4]. Vasoactive intestinal polypeptide (VIP): Secreted by the presence of acidic chyme in intestine
[5]. Peptide YY: Secreted by the presence of fatty chyme in intestine.
In addition to these hormones, pancreas also secretes a hormone called somatostatin during intestinal phase. It also inhibits gastric secretion.
Thus, enterogastric reflex and intestinal hormones collectively apply a strong brake on the secretion and motility of stomach during intestinal phase.
Experimental evidences for intestinal phase .
Intestinal phase of gastric secretion is demonstrated by Bickel pouch and Farrel and Ivy pouch.
Interdigestive Phase .
[1]. Secretion of small amount of gastric juice in between meals (or during period of fasting) is called interdigestive phase.
[2]. Gastric secretion during this phase is mainly due to the hormones like gastrin. This phase of gastric secretion is demonstrated by Farrel and Ivy pouch.
Factors Influencing Gastric Method .
Gastric secretion is also influenced by some factors which increase the gastric secretion by stimulating gastric mucosa such as:
1. Alcohol .
2. Caffeine.
Collection of Gastric Juice .
[1]. In human beings, the gastric juice is collected by using Ryle tube. The tube is made out of rubber or plastic.
[2]. It is passed through nostril or mouth and through esophagus into the stomach. A line is marked in the tube.
[3]. The entrance of the tip of the tube into stomach is indicated when this line comes near the mouth. Then, the contents of stomach are collected by means of aspiration.
Gastric Analysis .
[1]. For analysis, the gastric juice is collected from patient only in the morning.
[2]. Analysis of the gastric juice is done for the diagnosis of ulcer and other disorders of stomach.
[3]. Gastric juice is analyzed for the following:
1. Measurement of peptic activity .
2. Measurement of gastric acidity:
Total acid, free acid (hydrochloric acid) and combined acid.
Method of Gastric Analysis .
1. Fractional Test Meal (FTM) .
[1]. After overnight fasting, the gastric juice is collected. Then, the patient takes a small test meal called fractional test meal (FTM).
[2]. Typical test meals are:
i. A piece of bread and a cup of tea .
ii. Wheat biscuit and 400 mL of water iii. 300 mL of oatmeal gruel.
Fractional gastric analysis
[1]. After the ingestion of a test meal, gastric juice is collected at every 15th minute for a period of two and a half hours.
[2]. All these samples are analyzed for peptic activity and acidity.
2. Nocturnal Gastric Analysis .
[1]. Patient is given a clear liquid diet at noon and at 5 pm. At 7.30 pm, the tube is introduced into the patient’s stomach.
[2]. Then from 8 pm to 8 am, hourly samples of gastric juice are collected and analyzed.
3. Histamine Test .
[1]. After overnight fasting, the stomach is emptied in the morning by aspiration. Then histamine is injected subcutaneously (0.01 mg/kg).
[2]. Histamine stimulates secretion of hydrochloric acid in the stomach.
[3]. After 30 minutes, 4 samples of gastric juice are collected over a period of 1 hour at 15 minutes interval and analyzed.
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