Being the first compartment of the digestive tract with any significant volume, the stomach is where a meal is stored before it is passed on to the remainder of the digestive tract. However the stomach is not just a simple repository for food and performs two major functions:
A. MIXING
Sophisticated dining etiquette usually requires that individual courses are separated in time and preferable washed down with a glass of fine wine to complement each dish. This is quite amusing because the first thing that happens to the meal when it arrives in the stomach is that it is mixed up into one big gooey soup. Few people would contemplate mixing up a huge blob of smoked-mackerel pate and a big serve of pavlova and eating it; yet this is exactly what happens when a meal hits the stomach.
The term gastric is used to describe anything pertaining to the stomach, so the mixing of food by the cement mixer-like actions of the stomach is referred to as gastric motility.
1. Gastric Motility
Gastric motility has a number of important functions:
(a) The mixing of food, liquid and the exocrine secretions of the stomach produces a more homogenous substrate and enables more effective chemical digestion.
(b) The churning of the food helps breakdown lumps of food and therefore contributes to the process of mechanical digestion.
(c) The propulsive action of the stomach is responsible for the passing of food onto the small intestine.
Gastric motility is produced by the coordinated contraction of the smooth muscle layers in the wall of the stomach. In addition to the circular and longitudinal muscle layers the muscularis externa of the stomach contains a third layer of smooth muscle cells on the inside of the circular muscle layer. The orientation of the long axis of these smooth muscles cells is intermediate between the other two layers and for this reason this layer is referred to as the oblique muscle layer.
Gastric motility involves a wave of contraction that begins in the fundus
and travels down through the body and into the pyloric antrum. This involves the contraction of all three layers of smooth muscle and results in a wave of gastric contents being propelled towards the pyloric sphincter
This cycle of activity is repeated again and again
2. Control of Gastric Motility
The basic rhythm of the stomach is set around 3 contractions per minute and is established by pacemaker smooth muscle cells. The cells are electrically-coupled to adjacent smooth muscle cells and this enables the spread of the electrical signals that initiate the contractions.
When the stomach is empty, these waves of contraction are very weak (although can sometimes be detected as ‘hunger pangs’ during prolonged fasting) but increase in magnitude (but not frequency) as the stomach fills.
Variation in the size of these contractions is brought about by a number of neural and hormonal mechanisms that are subdivided into three distinct phases:
(a) Cephalic Phase
The thought, sight, smell and taste of food causes an increase in parasympathetic tone to the smooth muscle that increases the force of contraction and prepare the stomach for the arrival of food.(b) Gastric Phase
The physical presence of food in the stomach is detected by mechanosensitive neurones in the enteric nervous system and the G cells of the gastric glands that secrete the hormone gastrin.The chemical presence of food (particularly the products of protein digestion) is also detected by G cells that secrete the hormone gastrin.
Activity in the mechanosensitive neurones of the enteric nervous sustem and increasing circulating levels of gastrin result in depolarisation of the smooth muscle of the stomach and therefore an increase in the force of contraction. Interestingly, both caffeine and alcohol are also very potent stimulants of gastric motility. Perhaps this is why some people like to finish a big meal with a glass of port and a cup of coffee?
(c) Intestinal Phase
Gastric motility is switched off by a lowering of the pH of the duodenum as the acidic contents of the stomach (see below) pass through the pyloric sphincter. This inhibitory effect is mediated by the acidic pH causing the release of secretin and cholecystokinin from enteroendocrine cells in the duodenum.The combined action of the three phases ensures that gastric motility is increased in preparation for and during a meal but is reduced if gastric emptying is too large for the capacity of the small intestine. This ensures that food only moves into the small intestine at a carefully controlled rate that does not exceed the ability of the small intestine to process it.
B. CHEMICAL DIGESTION
The second major function of the stomach is to continue the process of chemical digestion that was initiated by the actions of salivary amylase in the mouth. Unlike the mouth however, the substances that are involved in chemical digestion are all derived from glands in the walls of the stomach known as gastric glands.
1. Gastric Glands
The inner wall of the stomach is covered in microscopic gastric glands that penetrate deep into the mucosa layer. The epithelium that lines the stomach extends down into these gastric glands but contains a number of specialised cells:
(a) Mucous Neck Cells
As their name suggests these cells are located near the entrance of the gastric glands and secrete a thick, sticky mucus that helps form a mucosal barrier that protects the walls of the stomach.(b) Stem Cells
The gastric glands contain a significant number of undifferentiated stem cells. Division of these cells gives rise to new epithelial cells and has the capacity renew the entire gastric epithelium every 5 days.(c) Parietal Cells
Parietal cells are found in the middle-depths of the gastric glands and secrete two very important substances:(i) Hydrochloric Acid (HCl) that is responsible for the very acidic nature of the stomach contents. This HCl is directly responsible for the breakdown of various types of connective tissue found in animal and plant foods, acts as an important line of defense by killing most bacteria that we might ingest and is also required for the activation of some very important enzymes (see below).
(ii) Intrinsic Factor which is required for the absorption of vitamin B12 by the small intestine.
(d) Chief Cells
These cells are located deeper in the gastric glands and secrete a family of proteins known as pepsinogens. Pepsinogens are inactive but are quickly converted to enzymes known collectively as pepsins by HCl (and subsequently by the actions of pepsins themselves). Pepsins play an important role in protein digestion because they target dietary proteins and cleave these to form polypeptides and amino acids.(e) Enteroendocrine Cells
At the base of the gastric glands are a population of cells that don’t contribute to gastric juice but secrete hormones into the bloodstream and are therefore enteroendocrine cells These cells are known as G Cells and secrete the peptide hormone gastrin into the bloodstream. Gastrin plays a number of very important roles in the control of difestive system function.The exocrine secretions of the mucous neck cells, chief cells and parietal cells flow out of the gastric glands into the lumen of the stomach and form what we refer to as gastric juice. Although the gastric glands themselves are very small, the large number of these result in 2-3 litres of gastric juice being secreted in adults each day.
2. Control of Gastric Juice Secretion
During fasting there is a small but significant volume of gastric juice secretion which referred to as basal secretion. The changes in the volume of secretion of gastric juice that accompany a meal involve three phases that are similar in many respects to those responsible for gastric motility:
(a) Cephalic Phase
Gastric juice is increased by a variety of stimuli that involve the head. The thought, sight, smell and presence of food in the mouth all increase the volume of gastric juice by the gastric glands by a number of specific mechanisms:(i) Parasympathetic postganglionic neurones release acetylcholine that binds to muscarinic receptors on parietal cells and chief cells. This causes a DIRECT increase in the volume of HCl and pepsinogens secretion
(ii) Parasympathetic postganglionic neurones also stimulate the release of gastrin from G cells and histamine from mast cells located in the underlying lamina propria. These two chemicals increase acid secretion in an INDIRECT fashion :
- Gastrin (acting as a hormone) targets the parietal cells and chief cells to increase HCl and pepsinogen secretion
- Histamine (acting in a paracrine fashion) binds to receptors on the parietal cells and increases HCl secretion
Through these direct and indirect mechanisms the cephalic phase results in an elevation of gastric juice secretion of up to 30% of maximal secretion.
(b) Gastric Phase
The arrival of food into the stomach causes further stimulation of gastric juice secretion through a number of mechanisms:(i) Distension of the stomach wall by food results in the activation of enteric nervous system neurones and parasympathetic postganglionic neurones
(ii) The presence of polypeptides and amino acids (the products of protein digestion) stimulate G cells to further increase the levels of gastrin secretion
Consistent with social conventions both caffeine and alcohol also increases gastric juice secretion by acting on G cells and elevating circulating gastrin levels.
Stimuli affecting the stomach during the gastric phase produce a further elevation of gastric juice secretion of up to 60% of maximal secretion levels.
(c) Intestinal Phase
Finally, the arrival of food in the small intestine has two different effects on gastric juice secretion:(i) The presence of food (in particular polypeptides and amino acids) in the small intestine results in the release of gastrin from G cells located in the wall of the duodenum. This enhances gastric juice secretion by up to 10% of total.
(ii) However as the pH of the duodenum drops (as a consequence of more food arriving from the stomach) secretin and cholecystokinin (from enteroendocrine cells in the duodenum) are released into the circulation and inhibit the activity of chief and parietal cells directly (as well as by reducing the secretion of gastrin by G cells). Consequently the volume of gastric juice secretion decreases.
In summary, the three phases responsible for the regulation of gastric juice secretion involve neural, paracrine and hormonal control systems. Through the cooperative actions of these systems gastric juice is elevated in preparation for a meal.
The end result of all the activity in the stomach is a complex mixture of food, saliva and gastric juice known as chyme. The amount of time that a meal spends in the stomach is highly variable and depends on the consistency of the meal (liquid or solid), its size and dietary composition (relative proportion of carbohydrate, protein and fats).
A light, fairly liquid lunch might be processed by your stomach in around 30 minutes. At the other end of the scale a family-sized pizza might spend up to four hours before being passed on to the small intestine (although this may be accelerated by the parallel consumption of a few stubbies of beer).
Remarkably, despite the time that food spends in the stomach, there is virtually no absorption of nutrients. As a result of their high lipid solubility however, both alcohol and certain drugs (such as aspirin) are able to diffuse through the gastric mucosa and reach the underlying blood vessels. The abosrption of alcohol and aspirin by the stomach helps explain the rapid rate of action and pressumably their popularity as 'party drugs' and 'morning after party drugs' respectively.