Lecture 27 - Mouth to stomach

Oral cavity - secretion (4)

• Saliva moistens food and helps form a swallowable bolus

• Mucus lubricates the food and oral surfaces

• Bicarbonate buffer acids in the mouth

• Water and electrolytes provide the fluid environment

Oral cavity - digestion (3)

• Chewing breaks food into smaller pieces and mixes it with saliva: mechanical digestion

• Salivary amylase (ptyalin) begins starch digestion

• Lingual lipase is secreted in the mouth and acts mainly after food reaches the acidic stomach

Salivary control

Salivation is controlled by autonomic reflexes. Food in the mouth, sight, or smell can stimulate secretion

Oral cavity - absorption (3)

• The mouth is not a major site of nutrient absorption

• The thin mucosa and nearby blood vessels allow some drugs to be absorbed

• Motility: Swallowing

Oral phase of swallowing (2)

Chewing and saliva preparing the bolus

• The tongue pushes the bolus backwards into the pharynx

• This voluntary step initiates the swallowing reflex

Pharyngeal phase of swallowing

Protection → The larynx elevates, the vocal folds close, and the epiglottis helps cover the laryngeal entrance.

Breathing briefly pauses the bolus passes into the esophagus

Peristalsis

• Coordinated squeezing behind the bolus to move it forward

• Relaxation ahead of the bolus provides somewhere for it to go

• Circular and longitudinal muscles cooperate

Esophageal phase of swallowing

Peristalsis moves the bolus toward the stomach

Two forms of esophageal peristalsis:

Primary peristalsis → Follows a swallow

Secondary peristalsis → Triggered by distension from material remaining in the esophagus (another swallow is not required)

Esophagus (secretion and absorption, digestion, and motility)

Secretion and absorption → Mucus lubricate passage; nutrient absorption is negligible

Digestion → Salivary enzymes may continue acting within the bolus. The esophagus adds no digestive enzymes

Motility → Peristalsis moves the bolus; the lower esophageal sphincter (LES) relaxes to permit entry in to the stomach

Lower esophageal sphincter (LES)

Important to prevent passage of fluid from stomach back to esophagus

Gastroesophageal reflux disease

Esophagus mucosa not built for exposure to gastric secretions

Stomach - Gastric secretions (4)

Hydrochloric acid (HCL) → Creates the acidic environment

Pepsinogen → Inactivate precursor of the protein-digestion enzymes pepsin

Intrinsic factory → Needed for vitamin B12 absorption later, in the ileum

Mucus and bicarbonate → Help protect the stomach lining

Acid initates the pepsinogen activation; pepsin then activates more pepsinogen

Glands (4)

Parietal cells → HCL and intrinsic factor

Chief cells → Pepsinogen

Surface mucous cells → Protective mucus and bicarbonate

G cells → Gastrin, a hormone released into blood

Glad digestion and absorption

Digestion → Salivary amylase can act briefly within a bolus before acid inhibits it. Lipases begin fat digestion; pepsin begins protein digestion

Absorption → Limited overall. Some alcohol and certain drugs can be absorbed. The small intestine does most nutrient and water absorption

Gastric motility

• The stomach stores food, mixes it with secretion, and meters chyme into the duodenum

• Mixing break up and redistributes the contents

• Emptying delivers small amounts through the pylorus

• Emptying slows when the duodenum receives more acid of fat than it can readily handle