The Digestive System The Anatomy

Anatomy We have what we call an alimentary canal This means we have 2 holes One for import, one for export This is in contrast to organisms such as jellyfish who have a gastrovascular cavity You guessed it…..just one hole for both in AND out Moral of the story: be glad you have 2 holes!!

Anatomy Connecting our 2 holes we have a series of tubes PLUS a bunch of accessory organs that assist in the digestion of foods

The Tubes Our alimentary canal, aka our gastrointestinal (GI) tract, consists of a continuous, coiled, hollow, muscular tube Organs Mouth Pharynx Esophagus Stomach Small intestine Large intestine

Let’s Draw a Picture!

1) Mouth Lined with a mucous membrane Important components Question: What type of membrane is that? Epithelial membrane Means it is comprised of epithelial tissue and connective tissue Important components Tongue Teeth Salivary glands

Ms. Mueller’s Teeth

2) Pharynx Passageway for food, fluids, and air Muscular passageway to propel food Called peristalsis

CAREFUL!! Do NOT confuse this structure with the larynx The pharynx is for food and air The larynx is for voice (we will talk about this in the respiratory system) Food should NOT be found here!!

True or False Food passes through the larynx on the way to the stomach.

3) Esophagus Runs from the pharynx to the stomach 4 layers (innermost to outermost) called tunics Mucosa Moist membrane that lines the lumen (the hollow part of the tube) Epithelial tissue, connective tissue, smooth muscle layer Submucosa Connective tissue Blood vessels, nerve endings, lymph organs

3) Esophagus Muscularis externa Serosa Inner circular layer and outer longitudinal layer of smooth muscle Serosa Outermost layer consisting of flat serous fluid-producing cells Visceral peritoneum and parietal peritoneum

Question What are the 4 tunics, in order, from external to internal? Serosa, muscularis externa, submucosa, mucosa

Location Matters Notice the location of the esophagus to the trachea

Question What prevents our food from “going down the wrong tube”? Answer: the epiglottis!! Flap of cartilage that directs food to the stomach and air to the lungs

4) Stomach C-shaped organ Has the same 4 tunics as the esophagus What were those again? Mucosa, submucosa, muscularis externa, serosa

4) Stomach Has 2 sphincters Let’s add to our picture! These acts as gatekeepers letting things in and out of the stomach Cardioesophageal sphincter connects the cardiac region of the stomach (closest to the heart) with the esophagus the pyloric sphincter connects the stomach to the small intestine Let’s add to our picture!

4) Stomach Contains large folds of mucosa called rugae This enables it to be flexible in its diameter When full, it can hold up to ONE GALLON of food

Surgery: Gastric Bypass Treatment for obesity Involves decreasing the size of the available storage space of the stomach Intended to make you feel full faster so that way you can eat less Surgery Squad! http://www.surgerysquad.com/surgeries/rny-gastric-bypass-surgery/

4) Stomach: Microscopic Pitted appearance called gastric pits that lead to gastric glands There is also a variety of cells that assist in digestion (We will get to those when we get to the physiology of digestion )

5) Small Intestine Connects to the stomach via the pyloric sphincter Coiled tube Average length: 8-18 feet!! Same 4 tunics of the stomach and esophagus 3 subdivisons Duodenum (5% of length) Jejunum (40% of length) Ilium (55% of length)

5) Small Intestine Contains ducts that connect to accessory organs Both ducts merge into 1 duct Pancreatic duct Connects the pancreas to the small intestine Common bile duct Connects the liver/gall bladder to the small intestine The gall bladder, liver, and pancreas are accessory organs to the GI tract

5) Small Intestine: Microscopic The small intestine has folds in it, just like the stomach Called circular folds Difference: these folds are not for stretching like the rugae of the stomach These folds are for increased surface area

5) SI: Microscopic More microscopically, villi are fingerlike projections of mucosa found facing the lumen of the SI on the circular folds Again, these function to increase the surface area

5) SI: Microscopic Even more microscopically, there are a series of microvilli on each villus Projections of the plasma membrane of the enterocytes Often referred to as the “brush border”

Let’s Draw a Picture Circular folds Villi Microvilli

Question Which of the following has the 4 tunics? A) esophagus B) SI C) stomach D) LI E) all except D

6) Large Intestine Larger in diameter than the SI, but shorter in length Frames the SI on 3 sides Subdivisions Cecum attaches the SI to the LI The appendix hangs off of this structure Called vermiform appendix because it looks like a rat tail Colon Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anal canal

Interesting….. Your anus is a sphincter That is, it has a muscular gatekeeper It has both voluntary and involuntary muscle so technically you can’t always control your elimination Elimination=defecating=going #2

Let’s draw a complete picture putting it all together!

Quiz Yourself Put the following organs in order: pharynx, mouth, small intestine, large intestine, stomach, esophagus Mouth, pharynx, esophagus, stomach, SI, LI Where does food pass through first: the cardioesophageal sphincter or the pyloric sphincter? Cardioesophageal and then pyloric What are the three accessory organs and how are they connected to our GI tract? Liver, gall bladder, and pancreas; SI What allows our stomach to expand? Rugae folds What is the purpose of the circular folds, villi, and microvilli? What organ contains these structures? Increase surface area; SI