All organs and tissues involved in ingesting and digesting food and removing waste from the body constitute the digestive system. To better understand the digestive process, it is necessary to know the digestive system.
Digestive system
The digestive system consists of the digestive tract, the route of food in the body and organs that help digestion. The schematic representation of the digestive system is given below;
Image: Blausen Medical – Wikimedia Commons, CC BY 4.0
Digestive tract organs;
1. Mouth
Both chemical and mechanical digestion take place in the mouth. The teeth break food into smaller pieces, while the tongue shapes it into a bolus. The oral cavity is lined with a mucous membrane.
Saliva from three pairs of salivary glands around the mouth—under the jaw, under the tongue and near the ears—is released into the oral cavity. It moistens food, while the amylase it contains begins the chemical digestion of starch in the mouth.
2. Pharynx
The pharynx is the passage between the mouth and the esophagus. No mechanical or chemical digestion takes place there. It helps prevent swallowed food from entering the trachea: during swallowing, the epiglottis closes over the airway, directing the bolus into the esophagus.
3. Esophagus
The esophagus is the passage through which food moves from the pharynx to the stomach. Smooth muscles in its wall contract and relax during digestion, producing peristaltic movements that move the bolus toward the stomach.
While the peristaltic movement is considered mechanical digestion in some sources, it is not accepted in some sources. We prefer to regard peristaltic motion as mechanical digestion.
4. Stomach
It can be said that it is the main organ of digestion. It can be thought of as a bag where digestion is carried out and the consumed food is stored for a while. At the stomach entrance, there is a valve called “cardia” that prevents the stomach contents (chyme or chymus) from passing into the esophagus.
In cases where the cardia cannot fully perform its function, the highly acidic chyme may pass into the esophagus and a burning sensation (reflux) may occur in the esophagus.
Both chemical and mechanical digestion take place in the stomach. When the bolus reaches and contacts the stomach wall, the stomach secretes gastric juice containing water, mucus, HCl and pepsinogen. Under the action of HCl, pepsinogen is converted to pepsin, which chemically digests proteins.
In addition to digestion in the stomach, absorption of water, some vitamins and minerals also occur.
5. Small intestine
The adult small intestine is approximately 6–7 meters long and has three parts. The first 25 cm form the duodenum, followed by the jejunum and then the ileum. The duodenum is particularly important for digestion, while absorption predominates in the jejunum and ileum.
The small intestine folds in the abdominal cavity. It consists of connective tissue, smooth muscle and epithelial tissue. In the inner surface of the small intestine, there are folds called “villi” and “microvilli” that increase the surface area.
Thanks to the villi and microvilli, the small intestine of an adult has a very large surface area of 120 m2. Considering that 120 m2 is the surface area of an average house, it is pretty interesting that a human small intestine has such a large surface area.
Dextrinase, sucrase, lactase, maltase, dipeptidase, tripeptidase, aminopeptidase and enterokinase enzymes are secreted from the small intestine. The digestion of carbohydrates, fats and proteins is completed in the small intestine and the absorption of the building blocks (monosaccharides, amino acids, fatty acids and glycerol) is carried out.
6. Large intestine
The canal from the small intestine to the anus is called the large intestine. It is about 2 meters long and is folded in the abdominal cavity. The cecum is located at the junction of the large intestine and the small intestine.
At the end of the caecum is a protrusion called the appendix. The part where it joins the anus is called the “rectum.” Protrusions such as villi and microvilli are not found in the large intestine.
No digestion takes place in the large intestine. However, the absorption of water, vitamins, and minerals takes place.
7. Anus
It is the part where wastes formed after digestion are removed from the body.
Organs that help digestion;
1. Salivary glands
There are three pairs of salivary glands around the mouth, under the chin, under the tongue, and under the ear. The mission of the salivary glands is to help digest food by secreting saliva. The saliva contains water, mucus, electrolytes, amylase and lysozyme enzymes.
2. Liver and gallbladder
The liver can be called the body’s chemistry laboratory. It weighs approximately 1.5 kg and consists of 2 lobes, right and left. It is the heaviest organ in the body after the skin. It synthesizes many enzymes and hormones necessary for the body and converts many molecules.
The liver’s role in digestion is to secrete bile, which contributes to the mechanical digestion of fats. Bile is produced in the liver, stored in the gallbladder and released into the duodenum during digestion. It contains no enzymes; it breaks fat into smaller droplets.
3. Pancreas
The pancreas is an organ where enzymes necessary for digestion are produced. The pancreatic juice is secreted during digestion and this fluid is sent to the duodenum. Pancreatic juice contains water, bicarbonate ions, amylase, lipase, trypsinogen, chymotrypsinogen, carboxypeptidase and nuclease enzymes.
The high acidity (pH 2.0) of chyme reaching the duodenum from the stomach is neutralized by bicarbonates in pancreatic juice. This prevents high acidity from damaging the intestines.
Digestion of Food
Digestion is the process of breaking down food physically and chemically and making it suitable for use by the body. A person gets the energy he needs for his life activities and the molecules he needs for cell construction, enzyme and hormone production from food. In order to benefit from nutrients, that food must be broken down into its building blocks.
Digestion has two aspects: mechanical and chemical. Mechanical digestion uses physical actions such as crushing, grinding and reducing particle size, without enzymes. It therefore does not change the chemical structures of the nutrients.
1. Chewing and grinding performed with the help of teeth in the mouth,
2. Peristaltic movements along the esophagus,
3. The reduction of fat globules by bile fluid is mechanical digestion.
Mechanical digestion aims to break down the food into smaller sizes and increase the surface area on which the enzymes can act. Increasing the surface area facilitates chemical digestion. An example of this is that a bite that is swallowed without chewing is more difficult to digest.
In chemical digestion, the enzyme effect breaks the bonds in large molecules and the building blocks become free.
It has been stated in previous issues that food consists of different nutrients such as water, carbohydrates, fat, protein, vitamins and minerals. There is no single standard digestive procedure in which every nutrient is digested. The digestion and absorption procedures of carbohydrates, fats and proteins are different from each other.
For example, while the chemical digestion of carbohydrates takes place in the mouth and small intestine; the chemical digestion of proteins takes place in the stomach and small intestine. Naturally, the enzymes used in the chemical digestion of these items are also different from each other.
Water, vitamins and minerals pass through the digestive system without being broken down and are absorbed to the extent possible. Carbohydrates, fats and proteins are broken down into their building blocks—monosaccharides, fatty acids, glycerol and amino acids—which are then absorbed and made available to the body.
When food is eaten, the digestive system immediately begins the procedure to break down that food and obtain its building blocks. Even seeing the food or even the thought of that food is often enough to start the digestive activity. For example, when we see a food that we think is delicious, the salivary glands begin to work and they secrete saliva, causing our mouths to water.
The digestive system focuses on what nutrients the food contains rather than what it is. Let’s take bread, for example. Bread is food containing water (37%), carbohydrates (50%), protein (8%), fat (1%), minerals and vitamins.
If the events that took place from the moment the bread is eaten are listed;
1. The salivary glands release saliva, which moistens the bread. This makes chewing and grinding easier and helps the bolus pass through the pharynx and esophagus.
Salivary amylase also begins to break down the starch in the bread chemically. Bread contains other carbohydrates as well, but starch is the only carbohydrate that undergoes chemical digestion in the mouth.
Other carbohydrates reach the stomach without undergoing chemical digestion in the mouth. In the mouth, the proteins and fats in the bread do not undergo chemical digestion; Therefore, their chemical structure does not change.
2. Teeth perform mechanical digestion by chewing and grinding. Thus, a large food surface is provided for chemical digestion. All food undergoes mechanical digestion in the mouth.
3. After being broken into smaller pieces, the bread passes through the pharynx and esophagus into the stomach. Peristaltic movements propel the bolus, while mucus from the mucosal lining facilitates its passage. The bolus reaches the stomach from the pharynx in about 3 seconds.
4. When the bite touches the stomach wall, the stomach begins to secrete gastric juice. Gastric juice contains water, hydrochloric acid (HCl), pepsinogen enzyme and mucus. In the stomach, no chemical digestion is applied to the carbohydrates and fats in the bread.
Only proteins undergo chemical digestion in the stomach, and they are not broken down all the way to their monomers, amino acids. Instead, they are broken down into polypeptides.
It is no longer possible to talk about a bite or bread in the stomach. It would be more accurate to say a mixture containing nutrients. This mixture is called chyme and is highly acidic (pH 2). The chyme goes to the small intestine after the stomach.
5. When chyme reaches the duodenum, hormones released there stimulate the pancreas, liver, gallbladder and stomach. These signals stimulate the pancreas and liver to secrete enzymes, the gallbladder to release bile, and the stomach to slow its activity.
Nutrients are broken down into their monomers in the small intestine. Carbohydrates yield building blocks such as glucose, fructose and galactose; proteins previously broken down into polypeptides in the stomach yield amino acids; and fats yield glycerol and fatty acids.
Bile juice is not secreted for chemical digestion but for mechanical digestion. The bile fluid only allows the fat globules to break down into smaller pieces. After mechanical digestion, the chemical digestion of fats is carried out with the enzyme lipase. As the name suggests, indigestible carbohydrates, known as dietary fiber, are not digested.
6. Digestion is now complete in the small intestine. The bread has become an aqueous mixture of glucose, amino acids, fatty acids, glycerol, dietary fiber, vitamins and minerals. At this stage, the components other than dietary fiber are absorbed.
Glucose, amino acids, vitamin C and B group vitamins are absorbed in the small intestine and delivered to the liver via capillaries. Fatty acids, glycerol, vitamins A, D, E and K are absorbed and transferred to the bloodstream through lymph capillaries.
Generally, 98% of glucose, fructose, galactose and other monosaccharides, 95% of glycerol and fatty acids and 92% of amino acids can be absorbed by the body.
After digestion and absorption in the small intestine, the mixture that remains contains water, dietary fiber, minerals, some vitamins and unabsorbed building blocks of carbohydrates, proteins and fats. It then moves toward the large intestine.
7. Digestion no longer takes place in the large intestine. Water, minerals and some vitamins remaining in the mixture are absorbed there to the extent possible. Bacteria in the large intestine ferment the unabsorbed building blocks of carbohydrates, fats and proteins.
This fermentation benefits the body. Probiotic bacteria perform important functions, including strengthening immunity, preventing weight gain and regulating digestion. What remains after absorption and bacterial fermentation is waste and enters the excretion process.
Bread is given as an example because it contains all the nutrients. No matter how many different foods the individual is fed at dinner, the digestion procedure is carried out through the nutrients contained in the food.
For example, the digestion of meat follows the same nutrient-based process as bread. Because meat contains no carbohydrate, carbohydrate digestion and absorption do not take place.
The rest of the digestive process is the same. One difference is that digestion takes longer because meat has a high protein and fat content.
The digestion procedure can be schematized as follows;
In addition to the above information regarding absorption;
1. In the mouth, nicotine, some poisons and some minerals can be absorbed,
2. In the stomach, alcohol and some poisons can be absorbed.
References
Raymond, J. L., & Morrow, K. (2023). Krause ve Mahan Besin ve Beslenme Bakım Süreci (16. baskı; G. Akbulut, Çev. Ed.). Nobel Tıp Kitabevleri.
Aksoy, M. (Ed.). (2022). Beslenme Biyokimyası (7. baskı). Ankara Nobel Tıp Kitabevleri.
Sencer, E., & Orhan, Y. (2005). Beslenme. İstanbul Medikal Yayıncılık.
Gökalp, H. Y., Nas, S., & Certel, M. (2002). Biyokimya-I: Temel Yapılar ve Kavramlar. Pamukkale Üniversitesi Mühendislik Fakültesi.
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). (2017). Your Digestive System & How it Works. National Institutes of Health.
Here’s an article that might interest you; Energy Metabolism – I: Energy Production in the Body
Halil İbrahim Akgül
Halil Ibrahim Akgul, Ph.D., is an Assistant Professor of Food Engineering at Bayburt University, where he works in the Division of Food Sciences within the Faculty of Engineering. He received his B.Sc. in Food Engineering from Akdeniz University in 2010, followed by an M.Sc. and a Ph.D. in Food Engineering from Atatürk University in 2015 and 2020, respectively. His formally recognised academic fields are Food Sciences and Engineering, Food Technologies, Dairy and Dairy Products Technology, and Food Microbiology.
Dr. Akgul has more than fifteen years of professional, research, and academic experience in food science. Between 2011 and 2013, he worked as the food engineer responsible for operations at the Pilot Dairy and Bread Factory of Atatürk University’s Directorate of Agricultural Enterprises. This industrial experience, combined with his subsequent academic research, gave him extensive practical and scientific knowledge of milk processing, dairy production, food quality, hygiene, and product development.
His research is primarily focused on milk and dairy products, dairy microbiology, starter cultures, lactic acid bacteria, exopolysaccharides, butter technology, and the physicochemical, microbiological, sensory, and nutritional properties of foods. His studies have examined the quality and fatty-acid composition of traditional butter, aroma formation in butter produced with different starter cultures, raw-milk quality, the characterisation and antioxidant activity of bacterial exopolysaccharides, traditional dairy products such as çökelek and karın kaymağı, colostrum powder, and the use of fruit-derived ingredients in ice cream production.
His academic record includes peer-reviewed research articles published in national and international journals, including Mljekarstvo, Acta Scientiarum Technology, the Journal of Food Safety and Food Quality, the International Journal of Food Engineering, the Atatürk University Journal of Agricultural Faculty, and ATA Food Journal. Four of the research articles currently listed in his university profile were published in SCI-Expanded-indexed journals. He has also contributed to twelve national and international conference papers and presentations covering dairy products, food microbiology, butter quality, fatty-acid composition, starter cultures, exopolysaccharide-producing bacteria, colostrum, traditional foods, and functional ice-cream formulations.
Dr. Akgul has participated as a researcher or adviser in eleven completed scientific and applied research projects. These projects have addressed the modernisation of pilot dairy-processing facilities; the production and quality characteristics of colostrum powder; the physicochemical and microbiological quality of butter and raw milk; the identification of exopolysaccharide-producing lactic acid bacteria; the development and production optimisation of starter cultures for mould-ripened cheese; the standardisation of traditional dairy products; and the use of sloe berry and persimmon fibre in ice-cream production.
His teaching experience spans undergraduate, associate-degree, and graduate education. The courses he has taught include Food Chemistry, Analytical Chemistry, General Chemistry, Organic Chemistry, Fermented Dairy Products Technology, Butter Production and Processing Technology, Healthy Living and Nutrition, Organic Agriculture and Food, Popular Science, Digital Literacy, and Information Technologies. Through these courses, he combines fundamental food chemistry and engineering principles with dairy technology, nutrition, scientific literacy, and the practical communication of scientific knowledge.
He is the founder and Editor-in-Chief of Gıda ve Sağlık, a multilingual digital publication devoted to food, nutrition, health, food chemistry, food microbiology, and culinary science. Through this platform, he draws upon his academic background, teaching experience, research activity, and practical knowledge of food production to present scientifically grounded information in a detailed and accessible form for an international readership.