Digestive system

Looking beyond what is visible to the naked eye makes the world a much more interesting place.

Fetal development, roughly speaking, begins with three tubes.

• The first tube is the vascular system with a central knot, the heart.
• The second tube develops parallel to the first along the back—this is the nervous system.
• The third tube, the intestinal tube, runs through the fetus from end to end.

The complex intestinal tube grows many of the body’s organs. It buds further out left and right for the later development of lungs, then further down, the intestinal tube bulges to form the liver, gallbladder, and pancreas. It gets clever and constructs the mouth, esophagus, and a little pouch for food storage. Last but not least, the intestines form.

The heart and the brain are central to life; however, without the digestive system, the core mechanisms of each would not function.

The gastrointestinal (GI) tract is joined by a tube from the opening of the mouth to the anus.

Organs of the digestive system consist of two groups:

• The alimentary canal is considered outside of the body because it is open to the external environment at each end, the mouth and anus. The canal includes the mouth, pharynx, esophagus, stomach, and small and large intestines.
• Accessory digestive organs include the tongue, teeth, salivary glands, gallbladder, liver, and pancreas. The tongue and teeth lie within the mouth, while the digestive glands connect to the GI tract through pathways called ducts. The glands produce several types of secretions that aid in the break down of food.

The gateway to the gut

Mouth. The mouth has four small points called parotid papillae. These four papillae are channels for the saliva glands to secrete saliva whenever necessary—for example, when chewing.

These little nubs are in the same position in everybody’s mouth.

• Two under the tongue.
• Two on the inside middle of the cheeks, opposite of the upper molars.

Pharynx. Before food or liquid enters the esophagus, the immune system investigates the tiny particles by using immune tissue known as the Waldeyer’s tonsillar ring.

The ring consists of three main areas:

• Lingual tonsils — the bumpy landscape on the back of the tongue.
• Palatine tonsils — located on either side of the throat.
• Adenoid tonsils and tubal tonsils — the roof of the pharynx

How does digestion work?

Peristalsis: The hollow organs contain a muscle layer in their wall that contracts and relaxes. This is called peristalsis—it pushes liquid and food through the long GI tube with a wave-like motion.

Esophagus: Swallowed food pushes into the esophagus. The muscular esophagus leads from the throat, bypasses the top of the stomach, and then passes into the stomach on the right-hand side. During activities when the abdominal muscles are tense, such as talking and laughing, the abdomen presses up against the stomach from below; therefore, if the esophagus entered the stomach from the top, food could easily escape the outer sphincter, called the lower esophageal sphincter.

Stomach: The second break down of food is in the stomach. The stomach has a side that is longer than the other causing the whole organ to bend, which creates a large fold inside of it. This folded-shape has a purpose—swallowed liquid can flow down the shorter side of the stomach, the right-handed side, and end up at the entrance of the small intestine, the pyloric sphincter. Cunningly, food is separated from the liquid so the digestive juices can break it down.

Small intestine: The small intestine, where the final break down of food occurs, meanders and folds for about 10–20 feet until passing into the large intestine. The folds create a large surface area. Without the folds, the small intestine would need to be three times as long as it is to provide enough surface area for the absorption of nutrients into the bloodstream.

Large intestine: Muscles push waste products, such as undigested food and older, sloughed off cells from the wall, into the large intestine. The large intestine changes the waste from liquid to stool by absorbing any remaining nutrients or water. The rectum then stores the stool for a bowel movement.

Digestive juices

Enzymes, found in digestive juices, haste the chemical reactions in the body and break down food into different nutrients.

Salivary glands. Saliva saturates food, so it can glide easily through the esophagus toward the stomach. It also comprises an enzyme that starts the break down of starches in the food.

Gastric glands. Gastric glands produce acid and an enzyme that digests protein. These glands are located in different regions of the stomach.

Pancreas. The juice from the pancreas contains numerous enzymes to break down proteins, fats, and carbohydrates. The pancreas delivers the enzymes to the small intestine through ducts.

Liver. Bile, a digestive juice from the liver, is stored in the gallbladder between meals. As a person eats, the gallbladder will squeeze the bile through the bile ducts, which is the connection between the gallbladder and liver to the small intestine. The bile is acidic and dissolves fat into the watery contents of the intestine.

Small intestine. To complete the digestion of starches and proteins, the digestive juices from the small intestine combine with pancreatic juice and bile. The break down of carbohydrates produces glucose that then absorbs into the bloodstream.

Microbiota.The microbiome in the small intestine produce enzymes needed to break down sugars.

Mechanism	Digestive aid	Molecules broken down
Mouth — chewing	Saliva	Starches
Esophagus — swallowing	N/A	N/A
Stomach — upper muscles relax and lower muscle contract to mix food with acid	Stomach acid	Protein
Small intestine — peristalsis	Digestive juice	Protein, starches, and carbohydrates
Pancreas	Pancreatic juice	Protein, starches, and fats
Liver	Bile acids	Fats

How is the digestive process regulated?

Hormones and nerve mechanisms regulate the digestion process. The microbiome also plays a crucial role in the teamwork between hormones, nerves, and organs.

Gut feeling. People often mention the term “gut feeling” when discussing a situation or someone they met for the first time. The gut-brain axis connection is not just metaphorical—signals from the gut can reach different parts of the brain. The gut commands a fleet of signaling substances and nerve-insulation materials. This axis is often associated with the enteric nervous system.

Nerves. Two types of nerve regulators aid the action of the digestive system:

• Extrinsic: or outside, nerves connect the digestive organs to the spinal cord and brain. These nerves cause the muscle layer to either contract or relax.
• Intrinsic: or inside, nerves are triggered to release different substances that delay or speed up the movement of food and the production of digestive juices when food stretches the walls of the GI tract.

Hormones: The functions of the digestive system rely on the stomach and small intestine to manufacture and release hormones. These hormones regulate appetite and stimulate the production of digestive juices.

Why is digestion necessary?

Digestion breaks down nutrients from food into smaller molecules so the blood and the lymphatic system can carry them to the cells throughout the body for growth, energy, and cell repair.

Carbohydrates: Carbohydrates are the starches, sugars, and fiber found in food. Depending on their chemical structure, carbohydrates are either simple or complex.

Protein: Foods such as eggs, beans, and meat digest into tiny molecules, called amino acids. The amino acids are absorbed into the blood via the small intestine.

Fats: Fat molecules help the body absorb vitamins and are a rich source of energy. Fat molecules are broken down into fatty acids and glycerol during digestion.

Vitamins: Vitamins are classified by how they dissolve. The liver and fatty tissues store fat-soluble vitamins, whereas the body flushes out extra water-soluble vitamins in the urine.

• Fat-soluble vitamins include vitamins A, D, E, and K.
• Water-soluble vitamins include vitamin C and all the B vitamins.

Facts to remember

• Digestion begins with chewing and ends in the small intestine.
• Digestion works by peristalsis, moving food through the GI tract.
• Food mixes with digestive juices as it moves through the GI tract, causing food to break down into smaller molecules.
• Digestive juices break food down into different nutrients through enzymes.
• Nutrients are absorbed through the walls of the small intestine.
• Waste products pass out of the body through the large intestine as solid matter called stool.
• Nerve and hormone regulators control the digestive process.

Interesting facts

• Saliva is filtered blood. Calcium, hormones, and products of our immune system enter the saliva from the blood.
• Saliva contains an analgesic, or painkiller, that is stronger than morphine. It is called opiorphin; discovered in 2006.
• Lying on the left side can help a bloated stomach.
• The surface area of the GI tract is one hundred times greater than the area of the skin.
• Recent research on the gut-brain axis suggests that the microbes in your gut could strongly influence your food choices.

Resources

Your digestive system and how it works: https://www.niddk.nih.gov/health-information/health-topics/Anatomy/your-digestive-system/Pages/anatomy.aspx

Journal of gastrointestinal and digestive system: https://www.omicsonline.org/gastrointestinal-digestive-system.php

Kids health — your digestive system: https://kidshealth.org/en/kids/digestive-system.html

Overview of the digestive system: https://www.hopkinsmedicine.org/healthlibrary/conditions/digestive_disorders/digestive_system_an_overview_85,P00380/

Microbial endocrinology in the microbiome-gut-brain axis: how bacterial production and utilization of neurochemicals influence behavior: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3828163/

Is eating behavior manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms: https://www.ncbi.nlm.nih.gov/pubmed/25103109

Modulation of gut microbiota-brain axis by probiotics, prebiotics, and diet: https://www.ncbi.nlm.nih.gov/pubmed/26306709

Akkermansia muciniphila and improved metabolic health during a dietary intervention in obesity: relationship with gut microbiome richness and ecology. https://www.ncbi.nlm.nih.gov/pubmed/26100928

Human opiorphin is a naturally occurring antidepressant acting selectively on enkephalin-dependent delta-opioid pathways: https://www.ncbi.nlm.nih.gov/pubmed/20610867