When food enters your stomach, it is mechanically broken down by a process called churning. In this lesson you will learn about this process and the chemical break down of proteins within the stomach by the enzyme pepsin.
We previously learned that the stomach is a C-shaped organ that lies on the left side of your abdominal cavity.
We also learned that the mucosa, or moist inner layer, of the stomach is dotted with millions of gastric pits, which lead to gastric glands. It is these gastric glands that secrete digestive juices needed to break down complex foods into simple nutrients. In this lesson, you will learn about the different juices that are secreted into the stomach and the role they play in digestion of your food.
By the time food reaches the stomach, it’s already undergone some digestion. The food has been physically broken down into smaller pieces by chewing. The starches in the food have also been acted upon chemically thanks to the enzyme salivary amylase, found in your saliva from your salivary glands.
To this point, digestion has been moving along quite briskly. After all, it does not take long to chew and swallow your food. However, when food reaches the stomach, it takes its time passing through. In fact, depending on the types of foods you eat and the size of your meal, food will typically stay in the stomach for two to six hours before moving on to the small intestine.
You can think of your stomach as both a temporary storage sack for food as well as a site for digestion. The walls of your stomach contain layers of smooth muscle. As food enters your stomach, the walls begin to stretch, prompting the smooth muscle to contract. This muscle activity leads to the physical mixing and breaking down of food in the stomach, known as churning.
In addition to the mechanical digestion caused by churning, there is also chemical digestion happening within the stomach. Specifically, we see the chemical digestion of proteins by digestive fluids. These acidic digestive fluids that are secreted by the gastric glands in the mucous membrane of the stomach are collectively referred to as gastric juices.
We previously learned that secretions of gastric juices from the gastric glands are regulated by the parasympathetic nervous system. This involuntary nervous system increases secretions when activated by the sight, smell or physical presence of food in the digestive tract. In addition, the presence of food and the change in the pH of the stomach stimulate the release of a hormone called gastrin. This is a hormone secreted by glands in the mucous membrane of the stomach that stimulates the production of gastric juices.
So, you can see that there are a couple of triggers for the production of gastric juices, and this leads to a surprisingly large volume of gastric juice produced during the day. Under normal circumstances, approximately two or three liters of gastric juices are produced in your stomach in a 24-hour period.
That is enough to fill at least one 2-liter bottle of soda, and this is all happening inside of you without your conscious awareness.
Chief Cells and Pepsin
When we look closely at the gastric glands, we see that they are lined by epithelial cells. There are several different types of cells within the glands, and they secrete different products. One of the important products of gastric juice that is secreted by these cells is pepsin.
Pepsin is the chief digestive enzyme in the stomach that breaks down proteins. You can think of pepsin as the ‘chief’ digestive enzyme in the stomach because it is produced by the chief cells. This can help you remember the term, but I should clarify that pepsin is the active protein-digesting enzyme. The chief cells actually produce pepsinogen (an inactive form of pepsin).It is an interesting fact that many enzymes in the digestive system are initially produced in their inactive form. We previously learned that an inactive form of an enzyme is called a zymogen.
Zymogens, like pepsinogen, must be activated before they can actually perform their duty. So, you might be wondering why your digestive system would go through the extra step of producing an inactive form of an enzyme? Well, the answer becomes clear if we think about a chief cell that produces the inactive pepsinogen. The chief cell itself contains many proteins. So, if the chief cell directly produced the protein-digesting enzyme, pepsin, it would actually eat itself.
Parietal Cells and HCl
Once the inactive pepsinogen is secreted by the chief cells, it’s inside the stomach.
And it needs to be converted to its active form, pepsin, before it can work to break down the proteins in your stomach. To do this, we need hydrochloric acid (HCl), which is a very strong acid. The presence of hydrochloric acid creates an acidic environment in your stomach that is needed to convert pepsinogen to pepsin. Hydrochloric acid is produced right in your stomach, and the gastric glands contain specialized epithelial cells called parietal cells that produce hydrochloric acid.
So, you might be thinking it could be dangerous to have hydrochloric acid in your stomach. And you’d be right if it wasn’t for the fact that your stomach’s epithelial cells are constantly producing a sticky alkaline mucus, which clings to the stomach lining and protects the stomach from the corrosive acid. This system works perfectly in a normal healthy body, but if the protective lining stops working correctly and the lining breaks down, the result can be a peptic ulcer. A peptic ulcer is a lesion of the mucosa. When this protective lining stops working, the digestive system cannot defend itself against the digestive action of pepsin, and you end up with a ‘peptic’ ulcer.
The most common locations for peptic ulcers are in the last part of the stomach or the first section of the small intestine.
After food has been churned in the stomach and mixed with the gastric juices, it becomes chyme. Chyme is a semifluid mass of partially digested food about the consistency of a heavy cream.
It might help you to think of the mechanical and chemical digestion that takes place in your stomach as the process that takes place when you’re preparing a cake mix. When you have a cake mix, you take the dry floury ingredients, and you continually mix them with liquid until they become uniformly consistent. When this consistency is reached, it’s ready to enter the structure that controls the passage of chyme into the small intestine, which we previously learned is named the pyloric sphincter.
Let’s review. In the stomach, we see both mechanical and chemical digestion taking place.
Mechanical digestion is due to a process called churning, which is the physical mixing and breaking down of food in the stomach. The chemical digestion of proteins happens due to acidic digestive fluids that are secreted by the gastric glands, called gastric juices. Secretions of gastric juices are regulated by both neural and hormonal factors. The sight, smell and physical presence of food stimulates the parasympathetic nervous system to increase secretions. In addition, gastrin is a hormone secreted by glands in the mucous membrane of the stomach that stimulates the production of gastric juices.One important component of gastric juice is pepsin. Pepsin is the chief digestive enzyme in the stomach that breaks down proteins.
We see that chief cells produce pepsinogen (an inactive form of pepsin). Pepsinogen is converted to pepsin when the parietal cells found within the gastric glands secrete hydrochloric acid. The presence of hydrochloric acid creates an acidic environment within the stomach that is needed to convert pepsinogen to pepsin.Mucus that is produced by the epithelial cells of the stomach help protect the lining of the stomach from the corrosive hydrochloric acid and pepsin. If this protective lining fails, it can result in a peptic ulcer, which is a lesion of the mucosa.
After food has been churned in the stomach and mixed with the gastric juices, it becomes a semifluid mass of partially digested food known as chyme. Chyme is now ready to exit the stomach through the pyloric sphincter on its way to the small intestine.
- Describe the mechanical and chemical digestive process of the stomach
- Identify the glands and secretions necessary for digestion
- Recognize the result if the stomach lining breaks down