What is gastric emptying?
Gastric emptying is the process by which the stomach empties its contents into the duodenum of the small intestine for further digestion of food and absorption of nutrients. While this may seem like a simple process, it is carefully coordinated so as not to overwhelm the duodenum with large amounts of partially digested food mixed with the acidic gastric secretions, which is collectively known as chyme.
How does gastric emptying work?
The stomach is a muscular, hollow organ. When food enters the stomach, it is churned by the stomach contractions (peristalsis) with gastric secretions (refer to Gastric Acid Secretion) and this allows for both mechanical and chemical digestion. Most of this churning occurs within the body of the stomach where the muscle contractions are weak.
The contractions lower down the stomach, near the pylorus, are more intense. This pushes the more fluid chyme through the pylorus while undigested food particles are forced higher up into the stomach for further breakdown. These stronger peristaltic waves that occur near the pylorus propel the fluid chyme through the pylorus into the duodenum in a pump-like action. This is referred to as the ‘pyloric pump‘.
The distal part of pylorus has a thick muscular wall arranged in a circular manner which remains contracted in a normal state. This is known as the pyloric sphincter. Even though it is contracted, the sphincter is not totally closed and there is gap which allows fluids like water or chyme to move through into the duodenum but prevents the movement of larger food particles.
What promotes or inhibits gastric emptying?
The vagus nerve is mainly responsible for parasympathetic stimulation to the stomach. This increases peristalsis and opens the pyloric sphincter. Sympathetic stimulation via the celiac plexus inhibits peristalsis and the opening of the sphincter. This is influenced by brain stem as well as stimuli from the sensory nerve endings in the gastric epithelium. Refer to Stomach Nerves for more information on the stomach nerve supply.
Certain local factors within the stomach and duodenum promote the pumping of chyme through the pyloric sphincter and decreases the tone of the sphincter muscles, thereby widening the opening. The local factors within the stomach that promote gastric emptying are relatively weak compared to the control exerted by the duodenum. Stretching of the stomach wall and the hormone gastrin increases the activity of the pyloric pump. Since gastrin secretion is increased by the presence of certain foods in the stomach, this can also be seen as a factor for increasing the activity of the pyloric pump.
A more potent mechanism of controlling gastric emptying is from the inhibitory effect of the enteric reflexes originating from the duodenum. These inhibitory reflexes slow down gastric emptying and are dependent on :
- Acidity of intestinal chyme
- Distension of the duodenal wall
- Irritation of the mucosal lining of the duodenum
- Concentration of the intestinal chyme
- Certain foods, like the breakdown products of protein and fat
Apart from the nervous reflexes that inhibit gastric emptying, certain hormones also play a part in gastric emptying. The hormone that plays the biggest role is cholecystokinin (CCK) and to a lesser degree, gastric inhibitory polypeptide (GIP) is involved. These hormones are secreted by the duodenum and jejunum in response to certain foods in the lumen of the small intestine. Refer to Digestive Hormones for more information on triggers and actions of these hormones. CCK and GIP inhibit gastric emptying.
The greater the stimulation by the inhibitory reflexes and the greater the CCK and GIP secretion, the slower the rate of gastric emptying.