The closest most people come to learning about the stomach and what it does is probably a hazy memory of school biology class and the idea that it breaks down food. Whilst that’s certainly true, there’s a whole lot more to it than that. Contrary to popular belief the stomach is not the be-all and end-all of digestion. As we have already seen, digestion begins in the mouth and later we’ll see how digestion is ‘completed’ in the small intestine. So, what exactly goes on in the stomach?
In the first instance, it acts as a temporary storage container or a kind of buffering chamber that holds food until it can be passed on for further processing, meaning that a large meal can be consumed fairly quickly and then held until it is ready to be digested. The food doesn’t just sit there, though, it undergoes both physical processing, as the stomach churns up the food through muscle contraction, and enzymatic breakdown, primarily denaturing and cleaving proteins.
Anatomy of the Stomach
The stomach is divided into 4 distinct regions : the cardia, fundus, body, and pylorus.
The stomach is made up of four distinct regions: the cardia, fundus, body and pylorus. The cardia is where the stomach joins with the esophagus, below and to the side of this is the fundus which sits directly above the main part of the stomach simply known as the body. The funnel-shaped pylorus leads to the duodenum (the first part of the small intestine) and is made up of the pyloric antrum, pyloric canal and finally the pyloric sphincter that opens directly into the duodenum.
When the stomach is empty it deflates, much the same way a balloon does, with its inner surface forming large folded structures called ruga. Again, contrary to popular belief, the stomach size of an individual tends to be fixed and there’s no evidence to support the idea that eating less will cause the stomach to shrink.
The main function of the upper stomach (the fundus and upper body) is to hold food and expands accordingly when food enters. Here low-frequency contractions are responsible for creating a pressure gradient from the stomach to the small intestine, pushing the contents downwards.
The lower part of the stomach (the lower body and antrum) undergoes strong peristaltic contraction creating a kind of grinding effect on the contents reducing it down to a thick liquid called chyme.
The Stomach Wall
The histology (the microscopic structure of tissues) of the stomach differs between the four different areas with each section secreting different chemicals.
Looking at figure 2 you’ll see that the surface of the stomach, known as the epithelium, is evenly dotted with tiny holes leading to pits. The surface epithelium throughout the stomach release a protective bicarbonate rich mucus that serves to protect the stomach wall from the acid, digestive enzymes, pathogens and the ingested matter itself. These mucus producing cells extend down into the mouth of the pits. Cells within the pits differ depending on where the pit is in the stomach. The gastric gland shown in the figure is typical of one found in the fundus and upper body of the stomach.
At the top of the gastric pit sit neck cells that secrete the mucus that protects the stomach wall from the acid and digestive enzymes.
Parietal cells cells produce hydrochloric acid (HCl) and intrinsic factor, a glycoprotein required for the absorption of vitamin B12 later on in the small intestine. The HCl serves to create an acidic environment of between 1.5 and 3.5 (when food is present) which helps to denature proteins so that enzymes known as proteases can work more efficiently. This highly acidic environment also serves to kill any bacteria that entered the stomach along with the food bolus.
Chief cells secrete pepsinogen, an inactive form of the protease, pepsin. Upon release from the cell the HCl converts pepsinogen into pepsin. Chief cells are also responsible for the secretion of gastric lipase which, together with lingual lipase, breaks down fats.
Enteroendocrine cells, more commonly known as G cells, release a hormone called gastrin. Gastrin is secreted in response to stretching of the stomach or the presence of protein, it promotes the production of acid, its opposite number being somatostatin, which reduces acid secretion.
Throughout the denaturation and enzymatic breakdown of the food, the stomach, through muscle action, helps to render the contents into a liquid called chyme. Chyme is then released into the small intestine where it’s fully digested and absorbed.
Absorption in the Stomach
The stomach actually absorbs very few substances. Some drugs like aspirin and non-steroidal anti-inflammatories can be absorbed through the stomach, as can alcohol (ethanol). All of these are known to cause irritation when used for prolonged periods.
Despite the fact that few substances are absorbed by the stomach itself the chemical and physical processes that occur within it are vital to the later absorption of many nutrients.
The mix of acid and enzymes collectively known as gastric juice helps to ensure the proper digestion of food by breaking down proteins and helps the absorption of vitamins and minerals.
Gastric acid is required for calcium absorption and studies suggest that there may be a link between reduced gastric acid secretion and an increased risk of sustaining bone fractures(1). Studies also show that medications that reduce or prevent acid production lead to a reduced ability to absorb heme iron(2,3), zinc (4) folic acid (5) and vitamin B12 (6,7). Evidence also suggests that oxide minerals e.g. magnesium might not be absorbed well when there is insufficient stomach acid (8,9).
Stomach acid also promotes the production of secretin, a hormone that performs many functions within the digestive tract. Insufficient stomach acid can, in turn, lead to failure to produce secretin (10).
The Stomach and Bacteria
Strong hydrochloric acid, with a pH of 1-3, when food is present, works to denature proteins, activate pepsinogen into its active form pepsin, and generally make the environment as inhospitable as possible to bacteria and viruses. (The pH of gastric acid varies throughout the day, being lower when food is present and higher, around 4-5, when the stomach is empty.) And whilst we often think of the stomach in terms of digestion it has another key role; eradicating potentially harmful germs.
While acid and gastric enzymes make the place inhospitable, the mucus pulls double-duty; protecting the stomach lining from the hydrochloric acid and digestive enzymes, but also acts as a protective barrier against bacteria viruses.
The stomach is a harsh place, but there are some bacteria capable of living in the highly acidic environment. For a long time it was thought that the stomachs of healthy people didn’t contain any bacteria as the environment was simply too harsh. However, Staphylococcus, Streptococcus, Lactobacillus, Peptostreptococcus and some types of yeast are now known to live in the stomach of even healthy individuals(11).
One particular bacteria, Helicobacter Pylori, has been implicated in a number of diseases including cancer. In the next installment I’ll be taking a look at this bacteria in more detail along with some common myths associated with the stomach, possible dangers of antacid medication and why stomach acid is so important.
11. Willey, Joanne; Sherwood, Linda; Woolverton, Christopher (2011).Prescott’s Microbiology (8th ed.). New York:
Images courtesy of OpenStax College. The Stomach [Connexions Web site]. Available at: http://cnx.org/content/m46517/1.3/