Anatomy and Physiology of the Digestive System

Overview of the Digestive System

By Zaki
So, what in the world is the Digestive System? Well, the Digestive System is composed of two groups of organ, which are the the Gastrointestinal Tract (GI) and the Accessory Digestive organs. The Gastrointestinal Tract (GI) is a continuous tube that extends from the mouth to the anus through thoracic and abdominopelvic cavities.

From the back.

From the front.

What organs include in the Gastrointestinal Tract? The organs of the Gastrointestinal Tract include the mouth, most of the pharynx, esophagus, stomach, small intestine, and large intestine.

And What organs include in the Accessory Digestive? you might ask, it include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. The teeth aid in the physical breakdown of food, and the tongue assists in chewing and swallowing. The other accessory digestive organs, however, never come into direct contact with food. They mainly produce or store secretions that flow into the GI tract through ducts. Basically the secretions aid in the chemical breakdown of food.

What else should you know about the Digestive System? There's one thing that you should know and that is the length of the Gastrointestinal Tract which is about 5-7 meters. THAT LONG! Incase you don't have a rope, you know what to do right? Well yeah buy a rope! 

Functions of the Digestive System

Overall, the digestive system performs six basic processes:

1. Ingestion: Taking food into mouth.
2. Secretion: Release of water, acid, buffers, and enzymes into lumen of GI tract.
3. Mixing and Propulsion: Churning and Propulsion of food through GI tract.
4. Digestion: Mechanical and Chemical breakdown of food.
5. Absorption: Passage of digested products from GI tract into blood and lymph.
6. Defecation:  Elimination of feces from GI tract.

The flow of taking food until you poop!

Layers of the Gastrointestinal Tract 

Before we go deeply on the anatomy and physiology, let me educate you guys the layers of the GI tract. The wall of GI tract from the lower esophagus to the anal has the same basic, four-layered arrangement of tissues. The four layers of the tract, from deep to superficial, are the Mucosa, Submucosa, Muscularis, and Serosa or Adventitia (basically the same). So yeah, that's it! Only 4 layers guys! It's that easy. Let me show you guys the structure of the layers below.

The 4 layers of the Gastrointestinal Tract.

What is Peritoneum?

The peritoneum is the largest serous membrane of the body, it consists of a layer of simple squamous epithelium with an underlying supporting layer of areolar connective tissue. The peritoneum is divided into the Parietal Peritoneum, which lines the wall of the abdominopelvic cavity, and the Visceral Peritoneum, which covers some of the organs in the cavity and is their serosa.
The slim space containing lubricating serous fluid that is between the parietal and visceral portions of the peritoneum is called the Peritoneal Cavity. Unlike the pericardium and pleurae, which smoothly cover the heart and lungs, the peritoneum contains large folds that weave between the viscera. The folds bind the organs the one another and to the walls of the abdominal cavity. They also contain blood vessels, lymphatic vessels, and nerves that supply the abdominal organs. There are five major peritoneal folds which are the greater omentum, falciform ligament, lesser omentum, mesentry, and mesocolon.

Midsagittal section showing the peritoneal folds.

The Gastrointestinal Tract Organs

Mouth 

In human anatomy, the mouth is the first portion of the alimentary canal that receives food and saliva. The primary role of a mouth its obviously to eat, but other than eating, the mouth also plays a good role in gossiping (I mean communication). The mouth is normally moist, because it is lined with a mucous membrane, and contains the teeth to chew your food. The lips mark transition from mucous membrane to skin, which covers most of the body. 

The mouth

 

Pharynx

The pharynx is actually part of the throat that is behind the mouth and nasal cavity and above the oesphagus and the larynx, or any tubes going down to the stomach and the lungs. The pharynx is an organ which can be found in the vertebrates and invertebrates, though the structure is not universally the same across all of those species obviously. Pharynx is part of the digestive system and also of the conducting zone of the respiratory system. Our pharynx is actually divided into three sections which are the nasopharynx, the oropharynx and laryngopharynx. Well, if you really want to know what these three sections is all about, you can go for a book.

The pharynx with the three sections.

Esophagus

The esophagus is mainly called the "tube of food". I just created that. It's an organ in vertebrates through which food passes from the pharynx to the stomach. The esophagus is a fibromuscular tube, about 25 damn centimetres in adult. It travels behind the trachea and heart, passes through the diaphragm and empties into the uppermost region of stomach. If you just imagine the tube is something like the water slides.

Imagine these fellas are the food you ate just now and they are traveling down to your stomach.

The esophagus.

The esophagus secretes mucus and transport food into the stomach. It does not produce digestive enzymes, and it does not carry on absorption.

Stomach

The stomach is probably the happiest part you tap after you eat something. Everyone does that! It is involved in the second phase of digestion, following chewing. The stomach is located between the esophagus and the small intestine. It secretes digestive enzymes and gastric acid to aid in food digestion. The pyloric sphincter controls the passage of partially digested food from the stomach into the duodenum where peristalsis takes over to move this through the rest of the intestines. You can see the pyloric sphincter in the diagram below.

The stomach in you.

The functions of the stomach are:

1. Mixes saliva, food, and gastric juice to form chyme.
2. Serves as reservoir for food before release into small intestine.
3. Secretes gastric juice, which contains HCl (kills bacteria and denatures protein), pepsin (begins the digestion of proteins), intrinsic factors (aids absorption of vitamin B12), and gastric lipase (aids digestion of triglycerides).
4. Secretes gastrin into blood.

Small Intestine

The small intestine is between the stomach and the large intestine, and is where most of the end absorption of food takes place. The small intestine actually has three regions and there are the duodenum, jejunum, and ileum. The primary function of small intestine is to absorp nutrients and minerals from food. Easy work there right?

The small intestine with three different regions.

The functions of the small intestine:

1. Segmentations mix chyme with digestive juices and bring food into contact with mucosa for absorption, peritalsis propels chyme through small intestine.
2. Completes digestion of carbohydrates, proteins, and lipids, begins and completes digestion of nucleic acids.
3. Absorbs about 90% of nutrients and water that pass through digestive system.

Large Intestine

The large intestine is the last part of GIT system. Water is absorbed here and the remaining waste material is stored as poop before being removed by pooping. Large intestine is the combination of the cecum, colon, rectum, and also anal canal.

The large intestine with its combination.

The functions of the large intestine:

1. Haustral churning, peritalsis, and mass peristalsis drive contents of colon into rectum.
2. Bacteria in large intestine convert proteins to amino acids, break down amino acids, and produce some B vitamins and vitamin K.
3. Absorbing some water, ions, and vitamins,
4. Forming feces.
5. Defecating (emptying rectum).

The Accessory Digestive Organs

Teeth

The teeth is functioned to mechanically break down food by cutting and crushing them in preparation before you swallow and for further digestion. Without teeth, you'll be having soups for your entire life.

That's you having soup with no teeth.

 Just for your information, there are four types of teeth and there are incisors, canines, premolars and molars each with specific function.

 

Thats my teeth right there!

Tongue

The tongue is a muscular organ in the mouth and it manipulates food for chewing, and is used in the act of swallowing. Without tongue, you can't eat and also talk and also to taste. It is senstive and kept moist by the saliva. Just for your information, the tongue serves as a natural means of cleaning the teeth.

The tongue

 

Liver and Gallbladder

The liver is a vital organ. In human, it is located in the upper right quadrant of the abdomen, below the diaphragm. The liver has a wide range of functions, including detoxification of various metabolites, protein synthesis, and the production of biochemicals necessary for digestion. It produces bile, an alkaline compound which aids in digestion via the emulsification of lipids. The gallbladder, a small pouch that sits just under the liver, stores bile produced by the liver.

Shows the liver in the human body.

The liver.

 The gallbladder is a small organ where bile is stored and concentrated before it is released into the small intestine. Just so you know, humans can live without a gallbladder. It sits just beneath the right lobe of the liver.

The gallbladder.

Pancreas


The pancreas is a glandular organ in the digestive system and also endocrine system. It is located in the abdominal cavity behind the stomach. It secretes pancreatic juice containing digestive enzymes that assist digestion and absorption of nutrients in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.

The pancreas.

 

Well that's all folks. Thank god I helped you with some educational info right? You're welcome.

Reference

1. Principles of Anatomy and Physiology, Volume 2, 13th Edition by Gerard J. Tortora and Bryan Derrickson.