Notes
Swallowing and Gastric Filling
Sections
oral, esophageal, and early gastric digestion
Summary
Mouth
Physical breakdown
- Food into smaller particles → mastication (chewing)
Chemical breakdown
- Carbohydrates → salivary secretions
Secretions
- Secreted by salivary glands (parotid gland, sublingual, sbmandibular)
- Secreted in anticipation of and during food consumption.
- Salivary secretions:
– Salivary amylase: breaks down polysaccharides → maltose
– Mucus: moistens food, forms bolus
– Lysozyme: lyses bacteria
0.5% of saliva is enzymes and electrolytes; the rest is water.
Pharynx and Esophagus
Motility (movement)
- Food from oral cavity to stomach
Swallowing Oral, Pharyngeal and Esophageal phases.
Tongue
- Pushes to the back of the pharynx to initiate swallowing.
Pharynx
- Common passageway for both food and air, continuous with trachea
Epiglottis
- Laryngeal flap that prevents the bolus from entering the trachea.
Esophagus
- Esophageal sphincter is open (relaxed) to let passage of food
– Sphincters: modified, one-way valves that comprise smooth muscle; they regulate food movement through the alimentary canal.
– Peristalsis: unidirectional wave-like smooth muscle contractions to push food down the esophagus and into stomach
Gastric Digestion
- Peristalsis deposits food in the stomach
- Bolus passes through the gastroesophageal sphincter
Clinical Correlation: heartburn occurs when acidic contents of the stomach backflow into the esophagus.
Stomach Key Functions
- Temporary storage
- Slows food transit to the small intestine.
- Maximizes nutrient absorption.
- Physical breakdown (like in the mouth)
- Chemical breakdown of proteins → amino acids
– Salivary amylase (from mouth) continues carbohydrate breakdown in the stomach.
Gastric Phases
- Filling: food enters the stomach (through the gastroesophageal sphincter).
- Mixing: peristaltic contractions churn the food.
– Gastric juices secretion to produce chyme (solution of partially digested macromolecules) - Emptying, in which peristaltic contractions propel chyme into the small intestine.
Gastric Filling Phase In Depth
- Gastroesophageal sphincter is a passage-way for food between esophagus and stomach
- Anatomical divisions of stomach: fundus, body, antrum
- Smooth muscle lining
– Receptive relaxation: Stomach muscles "relax" to "receive" food - Smooth muscle walls reduce tone to expand stomach volume (in response to food reception)
– Stomach volume = 0.5L empty → expands to 0.8L to 4.0L during receptive relaxation
– Increase in volume allows stomach to accommodate food with little rise in pressure
(Note: Intertextual variation exists regarding the stomach's full capacity during receptive relaxation.)
– Facilitates temporary storage – stomach secretes chyme slowly, gives the small intestine more time to absorb nutrients.
Full-Length Text
- Here, we will learn about digestion in the upper half of the GI tract.
- Here, we'll focus on oral, esophageal, and early gastric digestion.
- We learn the final stages of gastric digestion, elsewhere.
- First start a table.
- Begin with the Key Functions of the digestive tract.
- Denote the following 5 major functions:
- Motility
- Secretion
- Digestion
- Absorption
- Protection
- And the minor function: Elimination.
In this tutorial, we will address motility and secretion.
- First, denote the mouth's key functions, which include:
- Physical breakdown of food into smaller particles
- Chemical breakdown of carbohydrates.
- Denote physical and chemical breakdown occur via:
- Mastication (aka chewing).
- Salivary secretions, which secrete in anticipation of and during food consumption.
Let's draw the oral cavity to illustrate these processes.
- Draw the cross section of a face in profile.
- Leave out the teeth for clarity.
- Indicate that mastication facilitates physical breakdown: the breakdown of food into smaller particles.
- Now draw the tongue.
- The pharynx.
- The 3 salivary glands:
- parotid
- sublingual
- submandibular
- Our salivary glands secrete more than a liter of saliva a day.
- Failure to swallow results in sialorrhea, the over production of saliva, and is a common consequence of Parksinson's disease, which is a common movement disorder.
- Denote that the salivary secretions include:
- Salivary amylase, which breaks down polysaccharides into maltose. It passes into the stomach where it continues to digest carbohydrates.
- Mucus, which moistens food and helps food bolus formation.
- Lysozyme, which lyses bacteria swallowed with our food.
- 0.5% of saliva is enzymes and electrolytes; the rest is water.
Now, let's move on to the next step in digestion: swallowing.
- Denote that the key functions of the pharynx and the esophagus is to move food: from the oral cavity to the stomach.
Let's illustrate the mechanics of swallowing.
- Write that swallowing includes an oral, pharyngeal and esophageal phase.
- Draw a bolus of food in the mouth, which the tongue pushes to the back of the pharynx to initiate swallowing.
- Show that the trachea is continuous with the pharynx.
- The pharynx is the common passageway for both air and food.
- So now draw the epiglottis, which is a laryngeal flap that prevents the bolus from entering the trachea.
- Show the esophagus with layers of smooth muscle.
- Specifically, indicate that the esophageal sphincter is open (relaxed).
- It contracts to close.
- Recall that sphincters are modified, one-way valves that comprise smooth muscle; they regulate food movement through the alimentary canal.
- Show that the bolus moves through the relaxed esophageal sphincter to enter the esophagus.
- Indicate that via peristalsis, which is unidirectional wave-like smooth muscle contractions, the food bolus is pushed down the esophagus.
- Draw the cross section of a stomach that connects to a duodenum.
- Show that peristalsis deposits the bolus of food in the stomach.
- Indicate that the bolus must pass through the gastroesophageal sphincter.
- As a clinical correlation, indicate that "heartburn" occurs when the acidic contents of the stomach backflow into the esophagus. Thus, it would be more aptly named: "esophageal burn."
Now, let's learn the stomach's role in digestion.
First, let's summarize the stomach's key functions.
- Denote that they include:
- Temporary storage to slow food transit to the small intestine and maximize nutrient absorption.
- Physical Breakdown (like in the mouth)
- Chemical Breakdown of proteins into their amino acids at the same time that salivary amylase from the mouth continues to breakdown carbohydrates in the stomach.
The stomach accomplishes these functions in three phases:
- Denote that they are:
- Filling, in which food enters the stomach (through the gastroesophageal sphincter).
- Mixing, in which peristaltic contractions churn the food (this occurs while the stomach lining secretes gastric juices to produce chyme – a solution of partially digested macromolecules).
- Emptying, in which peristaltic contractions propel chyme into the small intestine.
In this tutorial, we will learn the filling phase; we cover mixing and emptying elsewhere.
To begin, let's illustrate the basic features of the stomach.
- Label the following anatomical features: fundus, body, antrum, pyloric sphincter and duodenum
- They comprise the relevant anatomy of digestion.
First, let's illustrate the filling phase.
- Show that smooth muscle lines the stomach walls.
- Write that via receptive relaxation, the stomach muscles "relax" to "receive" food, meaning their walls reduce tone to expand stomach volume (in response to food reception):
- Indicate that the stomach volume is about 0.5L when empty, but can reach anywhere from 0.8L to 4.0L during receptive relaxation, an increase in volume that allows the stomach to accommodate food with little rise in pressure.
- Intertextual variation exists regarding the stomach's full capacity during receptive relaxation.
- Denote that receptive relaxation facilitates temporary storage.
- It enables the stomach to secrete chyme slowly to give the small intestine more time to absorb nutrients.