SWALLOWING & GASTRIC FILLING
Physical breakdown
Chemical breakdown
- Carbohydrates - salivary secretions
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
- Temporary storage
- Slows food transit to the small intestine.
- Maximizes nutrient absorption.
- Physical breakdown (like in the mouth)
- Chemical breakdown of proteins to amino acids
– Salivary amylase (from mouth) continues carbohydrate breakdown in the stomach.
Gastric Phases
1. Filling: food enters the stomach (through the gastroesophageal sphincter).
2. Mixing: peristaltic contractions churn the food.
– Gastric juices secretion to produce chyme (solution of partially digested macromolecules)
3. Emptying, in which peristaltic contractions propel chyme into the small intestine.
Gastric Filling Phase
- 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.