Digestion, Absorption, Motility, and Secretion

• Digestion is part of these four main processes:
  • Digestion
  • Absorption
  • Motility
  • Secretion

Digestion

• Splitting molecules, is both chemical and mechanical.
  • Chemical uses hydrolysis and enzymes.
  • Mechanical is the physical breaking apart, such as chewing.

Absorption

• Food molecules are moved in smallest form into the blood.
• This mostly occurs in the small intestine.
• Small pieces are moved across a boundary (tube, epithelium, muscle layer).

Motility

• This is muscular movement of food.
• Most compartments have special motility to move food.
• This is mostly smooth muscle, and involuntary.
• Exceptions are the front and back end, which is skeletal muscle that is under control.
• Motility squishes materials back and forth and moves them through the system.

Secretion

• This involves moving many various forms into the digestive compartment.
• Every compartment makes mucus to soften and lubricate.
• Some make acid (HCl) and enzymes.
• This softens, helps break down, and move food around.
• It makes food into smaller molecules.
• Hormones go into the blood and work through the digestive system.
• The digestive tract is layered with epithelium.
• Secretions either come from this layer or go into the lumen.
• The digestive tract is one long tube from mouth to anus, and anatomy evolves with physiology.

Overview of Organs

• Mouth softens food, lubricates, begins mechanical breakdown with biting, and chemical breakdown with saliva.
• Esophagus moves food from mouth to stomach and secretes mucus.
• Not every chemical can be broken down in the mouth, but initial breakdown starts there.
• Almost every compartment has mucus.
• The Gastrointestinal tract (GI) is the digestive tract.
• The Stomach's roles includes; storage, slow breakdown, involvement in chemical digestion, and absorption of alcohol.
• The stomach does not absorb food.
• The Large intestine makes feces and absorbs water, turning liquid into solid waste.
• Other compartments have enzyme specialization while most have motility specialization and restricted absorption.
• Mucus is a secretion.
• BOLUS: solid-ish food that comes in, in mouth and esophagus.
• CHYME: more liquid form, occurs first in stomach then in small intestine
• Accessory digestive organs secrete and aid digestion then share a common duct that enters the digestive tract in between stomach and small intestine.
• The Pancreas has digestive enzymes.
• The Gallbladder stores bile made in the Liver for breaking down fat.

Multistep Breakdown

• Food is eaten in complicated form but must be broken down for use
• Three categories for macros are: carbohydrates, proteins, and fats.

Carbs

• Polysaccharides are long chains of glucose molecules.
  • From plants it is starch and cellulose.
  • From animals it is glycogen.
• Molecules are broken down into disaccharides (2 sugars).
• Then broken down to a monosaccharide or single sugar.
• Salivary Amylase starts the breakdown of carbs in the mouth.

Protein

• Proteins are broken down into smaller peptides, then chemically broken down.
• Peptides are broken down to single unit amino acids.
• Absorbed in 5-10 peptide chains large.
• The body builds proteins mostly from single amino acids.
• Pepsin and trypsin are two common enzymes that break down protein that begins in the stomach.

Fats

• Fats are really big molecules, not water-soluble, and harder to cross or be absorbed in blood.
• Become a triglyceride when enter body: glycerol + 3 fatty acid chains - require digestion.
• Broken from that form to monoglyceride for easier absorption.
• Fatty acids separate for absorption.
• Most fats breakdown happens in small intestine and some in stomach

Complex Molecules

• Complicated molecules first undergo step/hydrolysis and brings to disaccharides (maltose).
• It then can break down single-sugar to glucose.
• Carbohydrates breakdown can start as early as the mouth from saliva.
• Most nutrients can't be chemically broken down until later, besides some now.
• The Large intestine takes up water.
• The Small intestine mostly chemically absorbs and digests.
• The Stomach does some mechanical digestion.

Phases of Digestion

• There are 3 or 4 main phases of digestion, named by location.

1 Cephalic phase:

• Happens before entering the stomach.
• It includes sensory systems that are connected to the digestive tract through the central nervous system and vagus nerve, which connects the brain to the digestive tract and sends signals.
• Long reflex/signals, where the process is relatively longer because it goes outside of the digestive tract.
• Brain processes related to food include memory, the smell of cooking beginning the stomach grumbling and salivating
• A feed forward process prepares the stomach with producing secretion and acid and secretion of HCl and pepsinogen.
• Sensory systems transmitted to digestive track and structures in the head help with digestion

Controls

• Volves contol what comes in/out of the stomach
• esophageal sphincter prevents backflow
• pyloric sphincter at the bottom which passes lots of digestion and absorption in the stomach.
• Small intestine works best with a little bit of chyme.
• The stomach is acidic.
• Small intestine enzymes work best in basic environment.

2. Gastric phase:

• It is a short reflex that is all done within digestive track and in the stomach.
• Stimulates gastrin release.
• Starts when food enters the stomach.
• Stimuli increases gastric secretion.

3. Intestinal phase:

• This is the small intestine influencing gastric secretion.
• Other phases are excitatory, but this phase is inhibitory.
• It shuts off flow of gastric juices when chyme begins to be emptied into the small intestine
• Lump everything from cephalic phase until are in the stomach than gastric phase
• The Duodenum is the very beginning of the small intestine, which connects to the stomach.

Anatomy of Digestive Track

• This is a tube lined by 4 major layers which has food in the middle.

Cephalic phase:

• Includes anything from head to beginning of stomach, or feedforward.
• A big secretion is saliva, which makes food less solid and begins chemical deconstruction with amylase.
• Mechanical processes include physically breaking food when chewing and with grinding teeth.
• The stomach is prepared for food with secretion or motility to get ready to process food and sensory input.

What happens in the mouth?

• Chewing
• Salivation
• There is a little bit of digestion.
• A mechanical break down is with Chewing - chewing
• Water + mucus and Amylase enzyme lubricate and soften
• Salivation: very initial carbohydrate breakdown,
• Polysaccharide becomes disaccharide
• Saliva is a solvent
• Solivent mixes and solution and doesn't complete until get to stomach

Pharynx

• Back of mouth and where food and air pass.
• Two tubes go into the esophagus and the trachea.
• If food goes down trachea it stops breathing and no intake of nutrient
• The Epiglottis is a little flap that
  • opens when not eating for breathing.
• closes when swallowing to close off the trachea and prevent breathing.
• Persistalsis pushes bolus in one forceful direction.

Epiglottis

• Closes airway when swallow food
• BOLUS is what is eaten and as it goes in, becomes less solid with lubrication and secretion.
• CHYME enters the stomach in more liquid than solid and secretion is mixed with food.
• There is right amount into small intestine, which means
  • with too much that cant be processed = dumping syndrome and absorbs a water, causing diarrhea.
• If a small material remains you are prepared for absorption

GI Tract layers:

• hole = food moving thru
• Interact with lumen is the epithelium (folds = epithelial layers, more pronounced in some compartments than others - surface area = indentations)
• Epithelial space for interaction with lumen
• Each tube has 4 layers.
• Mucosal layer faces the tube inside and interacts.
• Submucosa consists of connective tissues, nerves, and blood flow to support the digestive tract and hold epithelium.
• Muscularis externa 2 layers of smooth muscle for movement, in perpendicular directions and 2 diff arrangements
• Serosa is outermost layer, lots of connective tissue, wrapping holds tube, connects digestive tract to body.

Parts are more specialized for surface area

• Esophagus = not a lot of absorbing/secretion
• Plexus consists of bunch interlinked nerves.
• Can work without outisde influencce but outside influence exists.

Four Layers of GT Tract

• Lumen = opening, for food, closest layer is the epithelial
• built as mucosal layer that interacts with food
• Secretes for surface area
• smooth musles and nerves Serosa wrapping all around the tube and attached to body wall

Mucosa Lines

• Luminal surface of digestive tract, cavities exposed to external environment and internal organ
• Mucous membrane: inner epithelial layer, protects and allows secretion and absorption:
• exocrine glands secrete digestives juices,
• endocrine secrete of blood-borne,
• epithelial for absorption,
• muscularis, layer of SM,outermost, adjacent mucosal surface, folded for increasing absorption,small intestine, transit tube

Submucosa

• a thick layer of connective tissue, supports digestive tract and elasticity and its vessels/nerves

Muscularis Externa

Major SM Layer, Composition/Functio

• Inner Circular Layer: encircle, contract, Lumen decreases
• Outer Longitude Layers: Lengthwise, Shorters the tube

Myenteric Plexus

• between layers
• Muscle and Intestine

Activity Regulated

• submucosal/myenteric plexuses with hormones

Serosa - Outer Connecting

• Serous fluid for less friction
• suspended from inner wall, movement freedom

Stomach Roles

• The function is to store food for intestinal use
• Some Digestion can occur in stomach, but not main role
• Esophageal Sphincter control
• If contracts move down lower - up has weak musculature; lower parts more empty - more musculature

Function

• store food - if you eat to much for small intestine, very folded walls so it can expand + stretch (don't swamp small intestine)
• mechanical digestion = churning of stomach to go back and forth
• Mixing: Vigorous contractions churn food and mix it with gastric juices to form chyme.

Details

• Central - body = big occurrence
• Closer to small intestine – antrum = most muscular and to go into
• Break food in small molecules, Grinding
• Motility = peristalsis, send things into net antrum
• Stomach: Gastric Acid, protect and breakdown

Controls and Action of Pyloric Sphincter

• Pyloric sphincter determines to control moderate what small intestine gets
• Upper isn't strength lower parts of the stomach delivers
• Pyloric sphincter move backwards
• The process happens again of contractions for mixing of chyme
• Chyme cannot if to much that occurs from contraction and fully closing.
• how full + if over come back again

Mixing

• The contents in both play role and also, neural/hormonal repsonses

Esophogeal Sphincter

• Upper: air
• Lower: contents down allow

Functions

• gastric, gateway, prevent, work down, ensure
• Influence by of what's going on and or hormonal repsosns

Esophageal Sphincter

• Controls gastric emptying to control regulated amount to duodenum.
• Lets stomach contents pass into the small intestine
• Prevnets reflux of duodenal
• Works with antral contractions to break down food particles
• Retains good gut

Can Detect Stretching

• Stomach stretches from high amount and senses can
• Hormone- hormones enter can some got get work

Signal Feedback

• Stomach talks to stomach
  • gives feedback from working that
  • and releasing or in small intestine gives her
• Signals with track help it

Signaling

• Hormone from S1 is bocarbonic
• Work with stomach to duodenum =

Controls

• In the gastric juice the stomach the cells secrete for balance to juice and hormones

Cell Action for Digestion

• parietal cells secrete help
• Chief cells secrete help for breakdown,
• help that and fat, cell does that
• D cells cells make acid. they cells
• Control,balance for it it all down down is initial
• Final in intestine