3.2 Digestive System Overview

A grain of rice is probably one of the smallest pieces of food you ever put in your mouth.  But even that is about 250 times longer than one of your cells.  So, how can your cells possibly use that grain of rice?

This is the first problem that your digestive system must solve if it’s going to keep you supplied with essential nutrients.

The digestive system faces two major problems:

1. Accessing nutrients.  The digestive system must break large food particles down into individual molecules that can be used by your cells.  This process of breaking down food into its component parts is called digestion.  Both chemical and mechanical break down occur in the digestive tract.
2. Circulating nutrients to all the cells of the body.  It’s not enough to have nutrients in your digestive tract; they must get to all the other cells of your body, too! Absorption is the process of getting nutrients from the digestive tract into the blood. Once in the blood, nutrients are transported throughout the body, to all of your cells.

Activities of the Digestive System

Digestion and absorption are the two main functions of the digestive system, but there are a lot of other activities involved that all work towards getting nutrients out of the food we eat and into our bloodstream:

Activities of the Digestive System Ingestion:  Intake of food (mouth) Propulsion:  Movement of food through GI tract Mixing:  Mixing food with digestive juices Mechanical Digestion:  Mechanical breakdown of food Chemical Digestion:  Enzymatic breakdown of food Absorption:  Uptake of nutrients from GI tract to blood or lymph Defecation:  Elimination of solid, indigestible waste

Propulsion

Propulsion is the act of moving something forward; in this case, moving food through the digestive tract. There are two main processes involved in propulsion: swallowing and peristalsis.

We’re all familiar with swallowing.  It’s a voluntary movement that sends food out the back of our mouth and into the esophagus.  Technically, swallowing is a reflex that we trigger by moving food to the back of our mouth with our tongues.  Next time you’re eating, I dare you to try to swallow without using your tongue.

Peristalsis refers to the waves of involuntary muscle contractions that move food through the lower digestive tract. Once food gets down to the lower esophagus, the walls of your digestive tract contract in a rolling wave that propels food through the stomach, small intestine, and large intestine.  It’s a bit like squeezing a baseball through a tube sock.  When you squeeze the walls of the sock together, it pushes the baseball forward.  Peristalsis is an involuntary motion, you can’t consciously control it.

Mixing

It’s vital to mix the foods you eat with the digestive juices.  The digestive juices are the fluids secreted into your digestive tract that contain the digestive enzymes.   Your tongue mixes food with saliva in your mouth.  Your stomach, a very muscular organ, churns and pounds food intensively, turning the mushy ball of chewed food and saliva that you swallow into a creamy paste.  In the small intestine, contractions occur that squeeze the contents of the intestine, mixing this paste with pancreatic juices and bile in a process called segmentation.

Mechanical and Chemical Digestion

There are two types of digestion in the body; mechanical and chemical. Mechanical digestion involves physically breaking food down into smaller pieces, usually through muscle contractions. The best example of this is chewing.

Chemical digestion uses digestive enzymes to break large food molecules into smaller nutrient molecules. This generally involves the breaking of chemical bonds.

 

The gastrointestinal (GI or digestive) tract, the passageway through which our food travels, is a “tube within a tube.” The trunk of our body is the outer tube and the GI tract is the interior tube, as shown below. Thus, even though the GI tract is within the body, the actual interior of the tract is technically outside of the body. This is because the contents have to be absorbed into the body. If it’s not absorbed, it will be excreted and never enter the body itself. It’s as if you never consumed it.

The organs that form the gastrointestinal tract (e.g., mouth, esophagus, stomach, small intestine, large intestine (aka colon), rectum, and anus) come into direct contact with the food or digestive contents.

The journey through the gastrointestinal tract starts in the mouth and ends in the anus as shown below:

Mouth –> Esophagus –> Stomach –> Small Intestine –> Large Intestine –> Rectum –> Anus

Sphincters

Sphincters are muscular openings that separate one compartment of the digestive tract from the next.  These openings act as one-way valves.  As a mass of food moves along, the sphincter closes behind it, ensuring that the food keeps moving forward through the digestive tract.

There are five sphincters in the digestive tract:

Upper esophageal sphincter:  Between the pharynx and esophagus

Lower esophageal (or gastroesophageal) sphincter:  Between the esophagus and stomach

Pyloric sphincter:  Between the stomach and the small intestine

Ileocecal valve:  Between the small intestine and the large intestine

Anus:  Between your digestive tract and the outside world

Accessory Organs

In addition to the GI tract, there are a number of accessory organs (e.g. salivary glands, pancreas, gallbladder, and liver) that play an integral role in digestion. The accessory organs do not come directly in contact with food or digestive contents, but still play a crucial role in the digestive process.

Digestive Enzymes

In addition to the digestive and accessory organs, there are a number of enzymes that are involved in digestion. We will go through each one in detail later, but this table should help give an overview of which enzymes are active at each location of the GI tract.

Table 3.21 Digestive enzymes

Location	Enzymes
Mouth	Salivary amylase Lingual lipase
Stomach	Pepsin
Small Intestine	Pancreatic amylase, Brush border disaccharidases Pancreatic lipase, Phospholipase A2, Cholesterol esterase Proteases, Brush border peptidases

Sometimes the name of a digestive enzyme helps  you remember what type of nutrient it acts on.  For example, lipases break down lipids, proteases break down proteins, and disaccharidases break down disaccharides.