Chapter 24 - The Digestive System Flashcards
Why do we need digestive system?
To absorb nutrients from our environment into our body
Anabolism
Build up essential compounds from absorbed nutrients
Catabolism
Break essential compounds down to supply energy to our cells
Two main components:
-Digestive Tract (GI Tract, Alimentary Canal)
-Accessory Organs
Digestive system function: break things down into small enough pieces to be absorbed (cross epithelium of digestive tract)
Digestive Tract: Pathway that food travels from (also known as alimentary canal)
Some of these structures perform digestive functions.
Overall, this is a pathway/tube for food.
Accessory Organs
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.
Ingestion only happens at oral cavity.
Defecation only happens at anus.
These are the only processes that only happen at one place.
Everything in the middle happens at multiple places.
Mechanical vs Chemical Digestion
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
Propulsion (moving food)
Moving food via 2 ways:
Peristalsis: from A to B
Segmentation: back and forth
Along this path we have secretion and absorption
Secretion: putting enzymes, buffers, acids into the lumen of the digestive tract
Absorption: going the other way (from lumen into body tissues across epithelium)
(Epithelium)
Covers all internal and external surfaces of the body.
***Stratified squamous epithelium is found at all entrance/exit points of the body.
Muscular layer is what moves food through digestive tract.
Myenteric plexus gives the directions for this.
Visceral layer is pretty thin, just covers digestive tract
Sometimes, serosa can join together on different sides, forming a double layer “mesentery”.
Mesenteries help suspend and anchor our intestines.
Pacesetter cells depolarize (on their own) along the whole digestive tract to help in moving food.
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.
Segmentation helps with both mechanical (increasing surface area via mixing) and chemical digestion (if there are secretions added).
Not a forward progression, but rather a “back and forth” movement.
Help for increasing amount of time food is in contact with secreted enzymes.
pH is pretty neutral until the stomach.
In oral cavity, there is chemical digestion of only carbohydrates and lipids.
**No proteins until the stomach!!!
No nutrient absorption here!! Nutrient absorption doesn’t happen until the small intestine.
Tongue = skeletal muscle (voluntary)
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
Teeth
Dentin makes up bulk of tooth (expands to root).
Vessels in pulp cavity bring nutrients to tooth.
Salivary glands (accessory organ #3)
**All three are paired structures!
IgA antibodies are found in saliva!!
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)
**Whilst tongue starts chemical digestion of lipids (lingual lipase), the salivary glands start chemical digestion of carbohydrates (salivary amylase).
!!!!!!!!!
Within oral cavity, the accessory organs are the major contributors to digestive processes.
(LECTURE 2)
Mastication (Chewing)
**Stratified squamous epithelium is found in oral cavity, pharynx, and esophagus.
**Simple columnar epithelium lines the stomach and intestines!
Esophagus
-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)
Deglutition (Swallowing)
-Peristaltic wave triggers opening of lower esophageal sphincter
Stomach
-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
Gastrin: increases what the digestive processes in the stomach are
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.
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.
