Digestive System

Journey through the Digestive System

How food becomes poo

...and part of you!

From start to finish...

All of the food that you consume is processed in a very long, hollow tube called the alimentary canal, or digestive tract. It begins at your mouth and ends at your anus. 

Digestion vs. Absorption

There are two processes that take place in your digestive tract. Food is broken down into smaller particles through digestion.

These particles can then be transported throughout your body via your blood by passing through the walls of the digestive tract (absorption).

Multiple Choice

Which process is chewing involved in?

• Digestion

• Absorption

How does food pass through the alimentary canal?

Skeletal muscle is involved in certain parts of this process, such as when you chew your food; however, most of the work is done by two layers of smooth muscle. These smooth muscles conduct peristalsis, rhythmically contracting and relaxing to push food from one segment of the digestive tract to the next.

It is similar to pushing toothpaste out of a tube. And you don’t even have to think about it!

Peristalsis is how food is pushed from one segment of the digestive tract to the next.

Multiple Choice

REVIEW! What part of the nervous system controls peristalsis (and digestion, in general)?

• Sympathetic Nervous System

• Parasympathetic Nervous System

Multiple Choice

What type of muscle is found in your digestive tract?

• Smooth

• Cardiac

• Skeletal

Chewing

• Chewing your food is called mastication. You use your teeth to break down large pieces of food into smaller fragments. 

• Salivary glands secrete saliva, a watery fluid containing mucus and a digestive enzyme salivary amylase which helps to break down sugars in your food (“amyl-” - sugar; “-ase” - enzyme).

• Your tongue does more than just tasting, too. It is continuously repositioning the food in your mouth until a ball-like bolus is formed. Then, it pushes the bolus toward your throat for swallowing.

Your tongue continuously repositions food in your mouth until a bolus is formed.

Multiple Choice

Which is a synonym for chewing?

• Digestion

• Mastication

• Absorption

Multiple Choice

What type of molecule is amylase responsible for digesting?

• Carbohydrates

• Lipids

• Proteins

Multiple Select

Which functions does the tongue help with? (Check ALL that apply.)

• Swallowing

• Tasting

• Absorbing

Swallowing

The tongue pushes the food from the mouth into the pharynx (which we studied in the respiratory system). By swallowing, food is supposed to enter your esophagus. But to make sure that you don’t accidentally choke on your food, a flap of tissue called the epiglottis covers the trachea.

During the swallowing motion, the epiglottis covers the top of the trachea. Peristalsis pushes food down the esophagus.

Multiple Choice

What body part prevents you from choking on your food?

• Pharynx

• Epiglottis

• Trachea

• Esophagus

Sphincters?

• Throughout your digestive tract are a series of sphincters, rings of muscle that close and guard various openings (such as the anus or the openings of the stomach).

• Swallowing pushes food past the pharyngoesophageal sphincter into the esophagus. Food is then able to pass into the stomach with the relaxing of the gastroesophageal sphincter.    

One sphincter separates the pharynx from the esophagus. Another sphincter separates the esophagus from the stomach.

Multiple Choice

Which sphincter separates the esophagus from the stomach?

• Pharyngoesophageal

• Gastroesophageal

Open Ended

Sometimes, reflux of stomach contents into the esophagus causes heartburn, or pyrosis. The burning sensation is a result of the high acidity of stomach contents.

What other words start with “pyro-”?

What do those words mean?

The Stomach

Your stomach has thicker muscle than the rest of the digestive tract. Three layers of muscle arranged in different orientations allow it to effectively churn and mix the food with gastric juice. This thick, pastelike mixture is called chyme. 

Multiple Choice

What is the term used to describe digested food in the stomach?

• Bolus

• Chyme

• Feces

These are some (but not all) of your stomach’s digestive functions:

• Secretes gastric juice, which includes hydrochloric acid and digestive enzymes (for breaking down lipids and proteins)

• Regulates how much chyme is delivered at a time to the small intestine

• Absorbs small quantities of water and dissolved substances.

• **Although the stomach is not well suited for absorption, it can absorb alcohol efficiently. This is why the consumption of alcoholic beverages on an empty stomach can quickly increase blood levels of alcohol.

Multiple Select

Which of the following are functions of your stomach? (Check ALL that apply.)

• Regulating how quickly food passes into your small intestine

• Breaking down large pieces of food into smaller particles

• Swallowing

• Secreting gastric juice

The stomach gets a lot of attention clinically. Let’s take a look at some conditions involving the stomach...

Ulcers

The lining of your stomach has a mucous membrane that prevents gastric juices from burning other stomach tissues. If the stomach lining breaks down, for some reason, a lesion called an ulcer will form. Sometimes ulcers can be caused by the bacterium Helicobacter pylori (“spiral bacteria of the stomach”) and this will lead to painful bleeding.

Multiple Select

How would a doctor treat an ulcer caused by H. pylori bacteria? (Check ALL that apply.)

• Antacids that neutralize stomach acid

• Vaccination

• Antibiotics

• Antifungals

Hiatal Hernias

The stomach is in the abdominal cavity which means that the esophagus must pass through an opening in the diaphragm. If that opening is weakened or enlarged, the stomach may protrude, or herniate, from the abdominal cavity into the thoracic cavity.

Multiple Select

What organs belong in the thoracic cavity? (Check ALL that apply.)

• Thorax

• Lungs

• Heart

• Stomach

Pyloric Stenosis

• The pyloric sphincter separates your stomach from the entrance to your small intestine. It relaxes periodically, allowing chyme to pass through the opening (the pylorus).

• Occasionally during infancy, the pylorus is too narrow, and food is unable to move out of the stomach. This condition is called pyloric stenosis (“stomach narrowing”), and is characterized by PROJECTILE VOMITING after feeding.

• Fortunately, a simple surgical procedure corrects the defect. 👍

If the opening in the pyloric sphincter is too narrow, food cannot pass through and projectile vomiting will occur.

The Small Intestine

Once chyme leaves the stomach, it enters the small intestine. It gets its name from its appearance. Although it is over 20 feet in length, it is carefully wrapped in your abdomen and appears smaller than the large intestine (which is only 5 feet in length). If it weren’t for this wrapping of the small intestine, you would be three stories tall!

There are three section to the small intestine:

• The Duodenum (10 inches) - the “meeting point” for digestion; receives chyme from the stomach and secretions from several accessory organs, including the liver, gallbladder, and pancreas

• The Jejunum (8 feet) - some digestion and absorption of food occurs here

• The Ileum (12 feet!) - the ileocecal valve at the end prevents reflux of contents from the cecum (part of the large intestine) back into the ileum

Top to bottom: Duodenum (10 in), Jejunum (8 ft), Ileum (12 ft)

Multiple Choice

Which segment of the small intestine is the longest?

• Duodenum

• Jejunum

• Ileum

Multiple Choice

Which segment of the small intestine receives secretions from several accessory organs?

• Duodenum

• Jejunum

• Ileum

Maximum Nutrition

• Surface area (S.A.) is the key to your body maximizing what can be absorbed from the chyme.

• More S.A. = More Absorption

• The inner wall of the small intestine forms circular folds with fingerlike projections called villi.

Maximum Nutrition

• The individual epithelial cells of each villus form extension called microvilli. In other words, even the fingers have fingers!

Multiple Choice

What property of the small intestine maximizes its ability to absorb nutrients?

• High surface area

• The presence of normal gut flora

• Movement of food via peristalsis

And you have help, too!

• A number of bacterial species live within your body. This community of microorganisms is called the normal flora.

• Some of them, such as E. coli, synthesize vitamins in your gut. Basically, they pay their rent.

• The gases produced by the gut flora give feces its characteristic odor. Phew!

Now on to the Large Intestine!

The Cecum

The cecum is the entryway to the large intestine from the ileum (brown in diagram).

The appendix is attached to it. Even in the twenty-first century, we still have no idea what it does, if anything.

The colon comes next...

The Colon has four functions:

• Absorbing water and certain electrolytes

• Synthesis of certain vitamins by gut bacteria

• Temporary waste storage (feces)

• Elimination of waste from the body (defecation)

Multiple Choice

If a tumor were to grow in the large intestine, which process would most likely be impeded?

• Digestion

• Absorption

• Defecation

• Mastication

The Mesentery

Your intestines are carefully arranged so that they fit inside your abdomen. A double fold of membranous tissue called the mesentery attaches the intestines to the posterior abdominal wall. It also aids in fat storage and allows nerves, blood and lymph vessels to supply the intestines.

In 2012, it was discovered that the mesentery was actually a single organ!

The mesentery (yellow) attaches the intestines to the posterior abdominal wall.

Multiple Choice

Which statement best supports the assertion that the mesentery is an organ rather than a tissue?

• It is composed of similar cells that carry out a specific function.

• It is a collection of tissues that perform a specific function.

• It attached the intestines to the posterior abdominal wall.

The end of the line...

The rectum is an 8-inch section at the end of the large intestine. It terminates at the anus where feces passes through. Solid stool collects here (remember that the colon absorbs water) and the rectum holds it until it is time for evacuation.

Any organ that aids in digestion and is NOT part of the alimentary canal is called an accessory organ.

Let’s take a look at a few!

The Liver

• The liver is the largest gland in your body.

• It is roughly about the size of a football!

Here are several of the liver's roles:

• Secretion of bile - dissolves lipids in chyme

• Detoxification - changes toxic substances into substances more easily eliminated from the body

• Metabolism of carbohydrates - removes extra glucose from blood and converts it into glycogen; converts glycogen back into glucose when blood levels are low

Multiple Choice

Which of the liver’s functions would most likely be involved in the consumption of alcoholic beverages?

• Bile secretion

• Detoxification

• Metabolism of carbohydrates

What is Bile?

• Secreted by the liver

• Stored in the gallbladder; saved for future use in the duodenum

• Biliary Tree - ducts (delivery tubes) that connect the liver, gallbladder, and duodenum

Bile

Bile is greenish-yellow in color.

Its pigments bilirubin (“bile-red”) and biliverdin (“bile green”) are formed from the hemoglobin of worn-out red blood cells.

Jaundice

If the bile ducts become blocked, the flow of bile stops, leading to a backup in the liver and accumulation of these pigments in the blood. Bilirubin especially becomes deposited in the skin and turns it yellow.

The person experiencing this symptom is called jaundiced.

Multiple Choice

When someone develops jaundice, what problem are they most likely experiencing?

• Blockage of blood vessels

• Blockage of the pyloric sphincter

• Blockage of the duodenum

• Blockage of bile ducts

The Pancreas

The liver is not the only accessory organ that aids in digestion.

Enzymes secreted by the pancreas are the most important of all digestive enzymes!

Your pancreas also secretes bicarbonate (a basic substance) which neutralizes highly acidic chyme coming from your stomach into the duodenum.

Multiple Choice

Which accessory organ secretes bicarbonate to neutralize stomach acid?

• Liver

• Pancreas

• Cecum

• Appendix

Multiple Choice

Which accessory organ secretes chemicals that dissolve fats?

• Liver

• Pancreas

• Cecum

• Appendix

Multiple Choice

Which accessory organ would cause virtually no health impacts if removed from your body?

• Liver

• Pancreas

• Appendix

• A portion of the Jejunum

Let's review!

The last four slides of this lesson contain 3 multiple choice questions and 1 open-ended question.

Multiple Choice

The rings of circular smooth muscle that separate segments of the digestive tract are called…

• Valves

• Sphincters

• Gates

• Checkpoints

Multiple Choice

The rhythmic contractions that push food through the digestive tract are called…

• Mastication

• Absorption

• Digestion

• Peristalsis

Multiple Choice

Which term means the passage of nutrients from the digestive tract into the bloodstream?

• Mastication

• Absorption

• Digestion

• Peristalsis

Open Ended

What questions do you have about the digestive system?

What would you like to learn more about?