1. Block 2 Notes
Digestive System

2. Overview of the digestive system
There are three main functions of the digestive system:
1. it ingests (takes in) food
2. breaks the food down so nutrients can be absorbed
3. eliminates what cannot be digested

3. Overview cont. Types of digestion:
1. mechanical – involves chewing food to break it down into smaller pieces (also uses the smooth muscles of the stomach to help churn the food)
Chemical digestion – breaks down large molecules of food into smaller substances using enzymes (the smaller stuff can be absorbed into the body’s cells more easily)
Remember – enzymes are proteins (a macromolecule) that speed up reactions (amylase is an enzyme found in saliva that starts the process of chemical digestion by breaking starches down)

4. Organs that are part of the digestive system
Tongue, esophagus, several sphincters, stomach, small intestine, large intestine, pancreas, liver, gall bladder, colon

5. The path food takes during digestion
Starts in the mouth with saliva (enzymes begin breaking down the food)
The tongue pushes chewed food to the back of the mouth which stimulates swallowing reflexes
Food is forced into the upper part of the esophagus (which is a muscular tube that connects the pharynx – or upper throat – to the stomach)
The wall of the esophagus is lined with smooth muscles that contract to move the food through the digestive system (this is called peristalsis) – even if you were upside down, food would still move toward your stomach
When you swallow, your epiglottis covers the trachea to prevent choking

6. Path of food cont.
When food leaves the esophagus, it passes through a round muscle called a sphincter, and into the stomach.
The walls of the stomach are made of 3 overlapping layers of smooth muscle that help digestion by contracting, which breaks down the food and mixes it with secretions of gastric glands that line the inner wall of the stomach
The gastric glands secrete (release) an acidic solution (if the sphincter allows any of this through, it causes heartburn)
Cells in the lining of the stomach secrete mucus to help prevent damage from the acid
Not much is absorbed in the stomach
The muscular walls of the stomach contract and push food farther along the digestive tract (at this point your food is the consistency of tomato soup), through another sphincter to the small intestine

7. Path of food cont.
Your small intestine is about 7 meters long; the first part is called the duodenum
The smooth muscles in the walls of your small intestine continue to push food farther through the digestive tract
The pancreas, liver and gallbladder all help with chemical digestion here
The pancreas produces enzymes that digest carbohydrates, proteins, and fats. It also releases a fluid that raises the pH to a little above 7 (this helps the intestine enzymes survive)
The liver is the largest organ of the body and makes bile, which helps break down fats (you make about 1 L of bile each day).
Extra bile is stored in the gallbladder
Digestion is completed here and most of the nutrients are absorbed in the small intestine, into the bloodstream through fingerlike structures called villi
The remaining food, now in a semi-liquid form called chyme, moves to the large intestine – this is the stuff that couldn’t be digested or absorbed

8. Path of food cont
The large intestine is the end of the digestive tract
It’s about 1.5 meters long and includes the colon, the rectum, and the appendix
The colon absorbs water from the chyme
The indigestible material gets more solid and becomes your feces (poop)
Peristalsis continues to move feces toward the rectum, causing the walls of the rectum to stretch.
This initiates a reflex that causes the final sphincter muscle to relax, and eliminate the feces through the anus
This entire process usually takes about hours (most of the time is spent in the large intestine)

9. Assignment
Love letter: Students will write a five paragraph love letter to their digestive system. (20 min)
Introduction
3 reasons why they love their digestive system
Conclusion/summary
Diagram: Students will spend the remaining time coloring and labeling a digestive system diagram hat will go into their notebooks. (20 min)