Chapter 24 - The Digestive System

Question 1

Why do we need digestive system?

Answer 1

To absorb nutrients from our environment into our body

Question 2

Anabolism

Answer 2

Build up essential compounds from absorbed nutrients

Question 3

Catabolism

Answer 3

Break essential compounds down to supply energy to our cells

Question 4

Two main components:
-Digestive Tract (GI Tract, Alimentary Canal)
-Accessory Organs

Question 5

Digestive system function: break things down into small enough pieces to be absorbed (cross epithelium of digestive tract)

Question 6

Digestive Tract: Pathway that food travels from (also known as alimentary canal)

Answer 6

Some of these structures perform digestive functions.

Overall, this is a pathway/tube for food.

Question 7

Accessory Organs

Answer 7

Along the way of the digestive tract, we see many accessory organs.

Accessory organs contribute to digestive processes, and may or may not come into contact with food.

Question 8

Ingestion only happens at oral cavity.
Defecation only happens at anus.

Answer 8

These are the only processes that only happen at one place.

Everything in the middle happens at multiple places.

Question 9

Mechanical vs Chemical Digestion

Answer 9

Mechanical digestion: physically breaking something apart to make smaller pieces
- This increases surface area for enzymes to work on (ex: teeth).

Chemical digestion: Enzymes breaking chemical bonds down into small monomers, so that nutrients can be more easily absorbed across epithelium

Question 10

Propulsion (moving food)

Answer 10

Moving food via 2 ways:

Peristalsis: from A to B
Segmentation: back and forth

Question 11

Along this path we have secretion and absorption

Answer 11

Secretion: putting enzymes, buffers, acids into the lumen of the digestive tract

Absorption: going the other way (from lumen into body tissues across epithelium)

Question 12

(Epithelium)

Answer 12

Covers all internal and external surfaces of the body.

***Stratified squamous epithelium is found at all entrance/exit points of the body.

Question 13

Muscular layer is what moves food through digestive tract.

Myenteric plexus gives the directions for this.

Question 14

Visceral layer is pretty thin, just covers digestive tract

Answer 14

Sometimes, serosa can join together on different sides, forming a double layer “mesentery”.

Mesenteries help suspend and anchor our intestines.

Question 15

Pacesetter cells depolarize (on their own) along the whole digestive tract to help in moving food.

Answer 15

Contraction of inner circular layer narrows diameter, happens behind food to push it forward.

Contraction of outer longitudinal layer shortens pathway, allowing for less distance for food to move forward.

Question 16

Segmentation helps with both mechanical (increasing surface area via mixing) and chemical digestion (if there are secretions added).

Answer 16

Not a forward progression, but rather a “back and forth” movement.

Help for increasing amount of time food is in contact with secreted enzymes.

Question 17

pH is pretty neutral until the stomach.

Question 18

In oral cavity, there is chemical digestion of only carbohydrates and lipids.

**No proteins until the stomach!!!

Answer 18

No nutrient absorption here!! Nutrient absorption doesn’t happen until the small intestine.

Question 19

Tongue = skeletal muscle (voluntary)

Answer 19

As you push tongue up against hard palate, food can be broken down via compression and abrasion -> increases surface area.

Tongue can also manipulate where food goes/moves it around.

Mucins = mucus that lubricates food

Lingual lipase = MADE in tongue, breaks down/digests LIPIDS

(first word = where it’s made,
second = what it digests)

**Chemical digestion of lipids begins here with the tongue!!

Speech: consonants can be formed

Question 20

Teeth

Answer 20

Dentin makes up bulk of tooth (expands to root).

Vessels in pulp cavity bring nutrients to tooth.

Question 21

Salivary glands (accessory organ #3)

**All three are paired structures!

IgA antibodies are found in saliva!!

Answer 21

The only accessory organ that doesn’t come into direct contact with food, and so direct channels for secretions to food are needed.

Parotid glands make salivary amylase. *lots of it

Sublingual glands make mucus of which contains both buffers and lubricants.
As we take in foods that are not of neutral pH, we’ll need buffers in order to maintain a neutral pH.

Submandibular glands produce a mixture of everything. (produces over 70% of all mucin fluid in mouth)

Question 22

**Whilst tongue starts chemical digestion of lipids (lingual lipase), the salivary glands start chemical digestion of carbohydrates (salivary amylase).

!!!!!!!!!

Question 23

Within oral cavity, the accessory organs are the major contributors to digestive processes.

Question 24

(LECTURE 2)

Question 25

Mastication (Chewing)

Question 26

**Stratified squamous epithelium is found in oral cavity, pharynx, and esophagus.

Answer 26

**Simple columnar epithelium lines the stomach and intestines!

Question 27

Esophagus

Answer 27

-Starts at level of cricoid cartilage

-At esophageal hiatus, esophagus pierces through diaphragm and joins with the stomach

-**In submucosa, we have esophageal glands that secrete mucus to help lubricate the pathway as food is being moved down

-Top third of esophagus is voluntary skeletal muscle (allows you to swallow on will), which then transitions into involuntary smooth muscle

-Outermost layer (visceral peritoneum) is termed “adventitia” instead of serosa
-Has collagen fibers that better serve to support and anchor the esophagus to surrounding structures

-Without food, the esophagus collapses (same with stomach)

Question 28

Deglutition (Swallowing)

Answer 28

-Peristaltic wave triggers opening of lower esophageal sphincter

Question 29

Stomach

Answer 29

-Place where we can temporarily store food as well as do digestive processes

-There is more muscle in the stomach than anywhere else

-pH is much lower than anywhere else due to acids - mainly HCl (aids in chemical digestion via breaking apart food and/or denaturing molecules)
-HCl can break apart connective tissue, muscle, and denatures proteins (unfolding them, so that enzymes can work on them)

-HCl itself does NOT perform chemical digestion

-Pepsin, an ACTIVATED enzyme in the stomach, digests protein (first place in digestive tract where protein is chemically digested)

-Simple columnar epithelium (good for secretions but not quite absorptions yet)

-Anywhere you have muscle in the stomach, you have more than normal of it

Question 30

Gastrin: increases what the digestive processes in the stomach are

Question 31

Cephalic Phase:
- Sensory stimuli triggers stomach to prepare for incoming food.
- Sensory stimuli triggers secretions via the CNS (vagus nerve). Messages sent from vagus nerve to submucosal plexus (governs mucosal and submucosal layers). In turn, movements are started and secretions of HCl by parietal cells and pepsinogen by chief cells increase. Mucus cells and G cells are also stimulated.

Answer 31

When food actually gets to stomach, that’s what triggers the gastric phase - digestive processes such as contractions/mechanical digestion are increased.

When food leaves, you don’t want to increase what the stomach is doing, and so all processes are slowing down (intestinal phase).

**All of these happen through neural/hormonal reflexes.