Chapter 23

1. takes in food

2. Breaks it down in to nutrient molecules

3. Absorbs molecules

4. Rids body of indigestible remains

Functions of the digestive system [4]

Everything body needs to survive enters via digestion (besides oxygen)

Why is the digestive system necessary for survival?

1. Alimentary canal (AKA GI tract)

2. Group of accessory digestive organs

Two main parts of organs in the digestive system”:

1. Tongue

2. Salivary glands

3. pancreas

4. Gallbladder

5. teeth

6. Liver

Accessory digestive organs: [4]

Alimentary canal (aka GI tract)

Muscular tube that runs from mouth to anus:

Break down food (digests). Canal absorbs fragments through lining into blood, blood carries throughout body cells

Goal of alimentary canal:

• Mouth

• Pharynyx

• Esophagus

• Stomach

• Small intestine

• Large intestine

• Rectum

• anus

Organs part of the alimentary canal:

Mouth and anus open to outside. Connect external environment to external environment

Why is the GI tract technically outside the body?

Salivary glands

Pancreas

Liver

Large digestive glands [3]:

Ingestion

First major process in the digestive system (eating)

1. Swallowing

2. Peristalsis

Processes involved in propulsion:

Peristalsis

Major means of propulsion in the body. Occurs due to alternating contraction and relaxation of smooth muscle (completely involuntary)

1. Chewing

2. Churning

3. Segmentation

Processes involved in mechanical breakdown:

Churning

In the stomach, food gets moved and mixed around. Part of mechanical breakdown:

Segmentation

Mechanical breakdown that is only happening in the small intestine. Rhythmic contraction of smooth muscle mixes food with digestive juices. Makes the process of digestion/absorption more efficient because it is moving food masses to different part of the intestinal wall where absorption takes place:

Mechanical breakdown

Breaking food into smaller bits. Taking larger particles of food and breaking it into smaller particles (is still food)

Digestion

Where smaller pieces of food now get changed into molecules. Enzymes in the GI tract breaks down food to chemical building blocks

Small intestine

Digestion takes place primarily in the:

Absorption

Taking chemical building blocks and moving them into the blood stream or lymph so they can travel in the body

Small intestine

Where does absorption primarily take place?

Defication

Where indigestible solid substances get removed from the body

Peritoneum

Membrane that lines the abdominopelvic cavity

Parietal peritoneum

Lines the body wall in abdominopelvic cavity

Visceral peritoneum

Covers the external surfaces of most digestive organs in abdominopelvic cavity:

Peritoneal cavity

Space in between the visceral and parietal peritoneum

Intraperitoneal

Orans withing the peritoneal cavity:

Retroperitoneal

organs behind the peritoneal

Pancreas, duodenum, some parts of intestine

retroperitoneal organs:

mesentery

Double layer of that peritoneum.

• Provide routes for blood vessels, lymphatics and nerves to reach digestive organs ( important communication mechanisms get to abdominal organs)
• Hold organs in place
• Stores fat (deeper)

functions of mesentery: [3]

mucosa

innermost layer of the GI tract. secretes mucous and digestive enzymes and hormones. it can absorb digested food into blood in small intestine, and has substances in it that protect against infectious disease

lumen

space in gi tract where food passes through

submucosa

layer of gi tract with really rich blood supply ad lymphatic vessels in it. Lots of elastic fibers (stretchy). Elastic fibers allow things like the stomach to be able to stretch to hold food. Nerves here are the ones that ring the nutrients and bring nerve supply that is needed for Gi tract to function

Muscularis externa

Most external layer of muscle in GI tract. Muscles that create peristalsis, segmentation, and sphincters

Serosa/serosal layer

Surrounds the vessels and organs and tissues. Outermost layer of the GI tract. this is the visceral peritoneum.

Splanchnic circulation (arterial) and hepatic portal circulation (veinous)

Where does GI tract blood supply come from? [2]

Hepatic portal circulation

Circulation that collects nutrient rich blood from digestive viscera and delivers it to liver.

Splanchnic circulation

Circulation that receives one quarter of the cardiac output (arterial)

Needs such a rich blood supply because all nutrients that body cells need come from the digestive system, and nutrients need to be transported out.

Why does the GI tract require a rich blood supply?

Enteric nervous system

Nerve supply of the GI tract. Unique because the GI tract is outside the body, and not exposed to internal environment.

1. in submucosa (submucosal nerve plexus)

2. Between layers of smooth muscle (myenteric nerve plexus)

Where are two nerve plexuses in enteric nervous system?

Controls the segmentation and peristalsis (autonomic and controlled reflexes. Results in inhibition or enhancement of digestive activity.

What does the enteric nervous system control in GI tract?

1. Stretch (mechanoreceptors)

2. Chemical composition (chemoreceptors)

Receptors in the digestive system sense: [2]

1. smooth muscle

2. Glands (salivary, etc.)

Effector organs in enteric nervous system [2]

1. Greater contraction of smooth muscle

2. More release of digestive juices

3. More release of hormones

Response from effectors may be: [3 examples]

mouth

Only site of ingestion

oral (buccal) cavity

“mouth”

Palate

roof of the mouth

soft palate

Palate that rises up to block nasopharynx

Tongue

bundles of skeletal muscle fibers that helps move and reposition food while eating to form a bolus.

Extrinsic muscles in tongue

Muscles that allow tongue to change position

intrinsic muscles in the tongue

Muscles that allow tongue to change shape

lingual frenulum

How tongue is secured to the bottom of the mouth

Tongue tie

Short frenulum. Babies may have difficulty breastfeeding.

Papillae

Peglike projections on tongue

1. House taste buds

2. Roughen surface to grip food

Functions of tongue papillae: [2]

1. Cleanses the mouth

2. Dissolves food so it can be tasted

3. Moistens food to create a compact bolus

4. contains an enzyme called amylase

Why is saliva important?

1. most outside oral cavity, in mouth region

2. Sublingual (under tongue)

3. Transported to mouth via ducts

location of salivary glands:

Mucin

protein in saliva that forms thick mucous that lubricates oral cavity and foodstuffs

1. digestive enzymes

2. mucin

3. IgA antibodies, lysozyme and defensins that protect against microorganisms

Important solutes in saliva [3]

Parasympathetic nervous system

How is salivation controlled?

Mastication

Chewing process

Teeth chew food (mastication)

Where does mechanical breakdown process start?

Incisors are for cutting off pieces of food
• Canines tear and pierce food
• Molars are for grinding and crushing

Different kinds of teeth and function [3]

1. Ingestion

2. Mechanical breakdown (chewing)

3. Propulsion (swallowing)

4. Digestion (enzymes in saliva digest starches)

Digestive processes that happen in the mouth [4]

Esophagus

Food passes through the pharynx into the:

esophagus pierces the diaphragm to enter the abdominopelvic cavity. It joins the stomach at the cardiac (gastroesophageal) sphincter

Where does esophagus join the stomach?

cardiac (gastroesophageal) sphincter

Sphincter that closes off so food does not go back up. Not a true sphincter. As long as food isn’t being swallowed, it remains closed.

Buccal phase

Phase of swallow where food is in mouth, bolus has been formed. Pushes the food back onto hard palate so at start of oropharynx. Tongue then comes up and blocks the mouth and food starts movement down through the pharynx

stomach

Temporary holding tank, food enters from the esophagus. Mechanical breakdown continues and chemical breakdown happens here.

Chyme

End result of food in the stomach. Thick, slurry, watery substance.

LUQ, hidden partially by liver. Can stretch all the way to the pelvis.

Where is the stomach located?

Pyloric sphincter

Sphincter at the end of the stomach. Allows food to pass from stomach to the intestine.

Connected by an omentum. Blends with other parts of the mesentery.

How is the stomach connected to the abdominal wall?

Muscular layers are different. There is an additional layer that gives it the ability to do churning, allowing it to be effective in mechanical breakdown

Difference in GI layers in stomach from rest of the GI tract:

Gastric pit

“Holes” in the stomach with cells that secrete gastric juices.

Parietal cells

Cells in gastric pits that secrete HCL and intrinsic factor

 Important because it is required in order for body to absorb vitamin B12.

why is intrinsic factor important?

Key to be able to digest proteins, is what makes stomach contents so acidic.

Importance of HCL in the stomach

Chief cells

Cells in gastric pits that secrete pepsinogen and lipases (important for digesting protein and fat)

Pepsin

Important protein secreted by gastric pits for digestion of protein:

Lipase

Fat digesting enzyme that digests 15% of lipids

Gastric juices

Collectively, the stuff that is secreted from gastric pits:

1. Cephalic phase

2. Gastric phase

3. Intestinal phase

What controls regulation secretion of gastric juices? [3 phases]

Cephalic phase

Phase happens before food enters the stomach. It is getting ready for food (phase of gastric juice secretion)

Gastric phase

Phase happens once food is in the stomach. Mechanoreceptors sense stretch, sending messages to cells in gastric pits and releases HCL (2/3 of juices) [phase of gastric juice secretion]

Intestinal phase

Phase of gastric juice secretion as stomach begins to empty

1. Thick layer of mucous protects it

2. The cells that create lining have really tight junctions. Cells joined together tightly and nothing can pass between epithelial cells.

3. If anything gets damaged, they are very quickly replaced. Surfaces are replaced every 3-5 days because they have to withstand a lot

Three ways the mucosal barrier protects the stomachL

Mucosal barrier

Harsh digestive conditions requires stomach to be protected. It is protected by the:

Peptic ulcer

When the mucosal barrier does not work, it can cause erosion in the stomach wall.

1. Propulsion (peristalsis)

2. Mechanical breakdown (churning)

3. Digestion (proteins and a small amount of fat)

4. Absorption (alcohol and aspirin)

Digestive processes in the stomach:

The larger the meal and more liquid, the faster it empties.

Effect of stomach distension and liquid on emptying rate:

Upward of 6 hours

How long can food stay in the stomach?

Production of bile

Main role of liver in digestive system:

Bile leaves each lobe of the liver via the right and left hepatic duct. It drains into the duodenum (small intestine) and excess is stored in the gallbladder

Pathway of bile through the liver:

it breaks fat down into tiny particles so they can be more easily digested (emulsifier).

Why is bile important for digestion?

1. bile salts

2. bilirubin

3. Cholesterol, triglycerides, phospholipids, and electrolytes.

Contents of bile: [3]

Bile salts

Most important substance in bile. Cholesterol derivatives that function in fat emulsification and absorption. Can be recycled in the body and used again

Bilirubin

Waste product of RBC breakdown that is secreted in bile.

Enterohepatic circulation

Whole circulation that recycles bile salts. Bile salts stay in circulation, keeps getting reused over and over again. Minimizes the amount of new bile salts that must be produced.