The Digestive System In A Nutshell- Part One

You eat. Food is broken down. Nutrients are absorbed. The rest is waste. Right?

While most of us have the basic understanding of our digestive system, the mechanisms and structures that work together to help us survive are a bit more complex. In part one (this one!) of our series we’re going to delve into the five main GI structures and what they do to keep you functioning.

The Mouth

mouthThis is the part we know, it’s where food enters. What we may not know is that the mouth is lined with a mucous membrane to protect it. This membrane contains cells and glands that occasionally secrete mucous.

Your salivary glands produce roughly 1.5L of saliva each day (seems like a lot of drool right??).

This saliva contains fun enzymes like amylase– responsible for starting off the digestion of starch – and lipase– begins the (long) process of fat breakdown. Once the food is chewed and formed into a ball it is passed along.

The more you chew your food, the more time the enzymes have to break it down resulting in better digestion.

The Esophagus

esophagusIn simple terms, this is a transporting tube to the stomach. It is roughly 10 inches in length and again, lined with a mucous membrane (we seem to have a few of those!). It ends with the lower esophageal sphincter (LES) which regulates both food entering the stomach and prevents acidic juices coming up.

Heartburn is caused by the sphincter opening up more than it should or at the wrong times. Diet alone doesn’t cause this but foods like; chocolate, peppermint, alcohol, caffeine, citrus & fried foods can exacerbate symptoms. Food moves through the esophagus with wave like contractions and the help of gravity.



Heartburn symptoms can be reduced by staying upright after eating- this allows gravity to work its magic and reduces the risk of the LES allowing acids back into the esophagus

The Stomach

tummyThe first major stage of digestion. When empty, your stomach is fairly small. However, and depending on the meal, your stomach can enlarge to a volume of nearly 4L (50times what it is empty!!).

There are three parts of the stomach (fundus, body, antrum) each playing a slightly different role. On top of those parts, there are four layers. The initial layer (gastric mucosa) is where gastric juices and thus digestive enzymes are secreted. Pepsin, hydrochloric acid and intrinsic factor all aid in the breakdown of the chewed food.

Intrinsic Factor (IF) is necessary for B12 absorption. Since levels can vary depending on lifestyle factors, sometimes a B12 injection is needed to bypass the need for IF

Minimal absorption actually takes place in the stomach. Only some drugs, water, few vitamins, alcohol and specific short chain fatty acids are able to cross the stomach lining. While the stomach is therefore mainly digestive, it is also protective.

During digestion the acidity of our stomach increases. This acidity aids in the breakdown of food, but it also helps destroy most pathogenic bacteria swallowed as well. In total it takes between 1-4 hours for the stomach to empty depending on the meal.

Small Intestine

intestineThis is where absorption actually begins. There are three parts to the small intestine. The second being where most absorption occurs, but it is in the third section where B12, bile salts and any other products of digestion are taken in.

Absorption in the small intestine is slow. About 1 cm per minute. So it takes anywhere from 4-8hours for the digested products to pass through.

In the first part of the small intestine, juices secreted from the pancreas help neutralize the acidity and further breakdown any food stuff. This is also when bile is released from the gallbladder to aid in the emulsification of fat (helps it become more soluble and ready for absorption).

Consume soluble fiber!!! Bile salts are partially re-cycled in the small intestine. Without this re-absorption new bile salt production would be unable to keep up with the digestive process.

How does this occur? Soluble dietary fiber can bind to a portion of the bile salts in the small intestine and promote their elimination from the body. Since cholesterol is required for the formation of new bile salts, this causes more cholesterol to be used.

Therefore, dietary fiber can lower blood cholesterol

Large Intestine

large intstineThe large intestine is (you guessed it!) larger. It is also a slow moving structure. It takes another 12-25 hours for food to be fully passed through the body.

Through the large intestine, sodium, potassium, acids, gases and remaining water are absorbed. What is leftover, the semi solid mix, is waste.

The large intestine is a complex place. It contains a unique blend of bacteria which makes it the most metabolically active organ in the body. Intestinal bacteria are responsible for the fermentation of fiber, oligosaccharides and sugars that our bodies failed to digest and absorb. This fermentation turns the above listed carbohydrates into short chain fatty acids…and produces strong scented gas in the process.

Keeping a healthy balance of gut bacteria is essential as it helps:


  • Prevent bad bacteria and yeast from colonizing in the gut
  • Removes carcinogens (dead cells, bacterial cells, collagen & elastin from food)
  • Improves immune system
  • Prevents allergy
  • Prevents IBS


So there you have it. The main structures in your digestive system. Other organs involved (liver/gallbladder /pancreas) will be looked at in depth in the second part of this series.

I hope this quick summary helped you begin to understand how much work is involved in everyday functions. Your digestive system works hard to keep your body running optimally, so reward it with real, nutrient dense food! And as always, if you need help contact me to set up a free consultation.

Happy Eating ๐Ÿ™‚

References: “The Essentials of Sport & Exercise Nutrition”. Berardi, John & Andrews, Ryan. Precision Nutrition Inc, 2012.