AOHS Foundations of Anatomy and Physiology II Lesson

AOHS Foundations of Anatomy and Physiology II Lesson 18 Physiology of the Digestive System Copyright © 2014‒ 2016 NAF. All rights reserved.

By the end of this presentation, you will know: • The roles of the digestive system • The functions of each organ in the digestive system • Fluids produced by digestive organs that aid in digestion • Which organs are involved in the breakdown and absorption of food

Your food becomes part of you: the stuff you’re made of is built from things you’ve eaten The cells on your skin’s surface, for example, are made of molecules your body built from the food you ate. What types of food molecules would your body use to make a new skin cell?

Digestion occurs in four steps: 1. Ingestion: taking food in 2. Digestion: breaking food down into usable molecules 3. Absorption: getting usable molecules from the digestive tract into the bloodstream 4. Defecation: getting rid of wastes

There are two categories of digestive system organs 1. The gastrointestinal (GI) tract, or alimentary canal, is a long muscular tube that winds its way through your body. 2. Accessory organs provide fluids that play different roles in breaking down food.

Ingestion and digestion both begin when you put food in your mouth In your mouth, your teeth crush food and your tongue mixes it with saliva to start digestion. The saliva comes from salivary glands and begins the chemical process of digestion.

Ingestion continues as food moves through the pharynx to the esophagus The pharynx and esophagus, both muscular organs, propel food toward the stomach using a mechanism called peristalsis. How does your body respond when you get something “down the wrong pipe”?

Peristalsis is a wave of coordinated muscle contraction that moves along the length of a tube Many of the organs in your digestive tract use peristalsis to keep food moving through them.

In the stomach, food is mashed into much smaller pieces The stomach works on food both physically and chemically. Physically, the stomach churns food, mashing and grinding it into smaller pieces. What do you think is happening when your stomach growls?

Cells in the stomach secrete enzymes and acid that start the chemical digestion of proteins Some cells in the stomach produce hydrochloric acid (HCl), which activates the protein-digesting enzymes. Food mixed with digestive enzymes is called chyme. HCl also makes the stomach very acidic.

From the stomach, chyme is pushed into the small intestine The small intestine averages 15‒ 20 feet long and has three sections: Section Average length Duodenum (do-AH-de-num) 2 feet Jejunum (je-JOO-num) 6 feet Ileum (IL-ee-um) 10 feet

Bile from the liver breaks down fat molecules Bile, made in the liver, is stored in the gallbladder before it enters the small intestine. Bile contains salts that help break down large fat globules into smaller ones. Some people have their gallbladder removed. How do you think this affects their digestion?

The liver has many other functions that impact all body systems The liver processes wastes and toxins, stores glucose and other nutrients, and produces many enzymes and hormones. The liver is one of the largest organs in the body, as well as one of the few that is able to regenerate itself.

The pancreas produces a juice of enzymes that help digest every category of food molecule Pancreatic juice is alkaline and neutralizes the acidic chyme arriving from the stomach.

The human body makes 22 digestive enzymes Digestive enzymes are categorized by the kind of molecules they work on. They break each type of molecule down into specific parts. Type of molecule Products of breakdown Carbohydrates Simple sugars Proteins Amino acids Fatty acids and glycerol DNA and RNA Nucleic acids The products of this breakdown process are recycled into other molecules the body can use. How do you end up eating DNA?

Inside your small intestine, the epithelial surface has many projections that increase surface area Tiny projections called villi on the inner layer of the small intestine contain dense capillary beds. The cells that make up the villi, in turn, have tiny projections, called microvilli, on their plasma membranes. Why is it important that there are lots of capillaries and lots of surface area in the small intestine?

When the chyme reaches the end of the small intestine, most of the nutrients have been absorbed Material moves from the small into the large intestine, where it becomes feces. The large intestine prepares feces to be removed from the body. Where is the appendix located?

Feces is made of indigestible food parts, bodily waste, and billions of bacteria In the large intestine, much of the water is absorbed from feces back into the body. Bacteria in the large intestine digest some food residue that we don’t. Some of these bacteria produce sulfur dioxide, a gas with characteristic odor. A colored microscope image of human feces. The pink objects are bacteria. About half the mass of human feces is bacteria from the gut.

Long, slow peristaltic contractions of the large intestine move feces to the rectum Fiber—indigestible plant material— makes these contractions stronger and helps feces pass through the large intestine more easily.

The defecation reflex causes the involuntary anal sphincter to relax The brain consciously decides when to open the voluntary anal sphincter.

Babies drink milk because they lack the enzymes to digest solid food It takes about six months for a baby’s digestive system to begin producing enzymes needed to break down solid food. Our digestive systems continue to change throughout our lives.

Researchers are just beginning to appreciate the role of bacteria in the digestive system There are 10 times more bacterial cells in your digestive tract than there are cells that make up your body. Scientists are only beginning to understand the role these microbes play. A colored microscope image of naturally occurring (harmless) E. coli bacteria (red) among the microvilli of cells in the small intestine. What have you heard about the microbiome, or gut flora?