SECT ION 2 , CHAPTER 17

THE DIGESTIVE SYSTEM

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• The pharynx is a cavity that connects the oral cavity to the esophagus

The Pharynx

• Nasopharynx • Superior to the soft palate• Passage for air during breathing

• Oropharynx • Posterior to the mouth• Passage for food during swallowing

and air during breathing.

• Laryngopharynx • Begins at the epiglottis • Extends to the esophagus.

Muscles of the Pharynx

• Muscles of the pharyngeal wall form an inner circular and outer longitudinal groups.

3 Constrictor Muscles – pulls walls inward

• Superior constrictor muscle• Middle constrictor muscle

Longitudinal muscles - elevate the pharynx when swallowing

Pharyngeal muscles are skeletal muscles that contract during swallowing (deglutition).

• Inferior constrictor muscle • Inferior constrictor forms much of the Upper esophageal sphincter (UES)• The UES prevents air from entering stomach

Figure 17.13 Muscles of the pharyngeal wall, posterior view.

The Swallowing Mechanism

• Swallowing can be divided into three stages:• Voluntary stage - saliva is mixed with chewed food• Swallowing begins the swallowing reflex• Peristalsis transports food through the esophagus to the stomach

The voluntary stage

• Food is chewed and mixed with saliva• Mass of food and saliva is called a bolus• The tongue forces food into the oropharynx

The Swallowing MechanismSwallowing Reflex• The soft palate raises, preventing food from entering the nasopharynx

• Longitudinal muscles elevate the hyoid bone and larynx• The tongue presses back, and the epiglottis closes• Inferior constrictor muscles relax, opening the esophagus.

Swallowing Reflex continued

The Swallowing Mechanism

• Superior constrictor muscles contract• Initiates a peristaltic wave that forces food into the esophagus.

Peristalsis – peristaltic waves move food through the esophagus.

The EsophagusThe esophagus is a 25cm long tube joining the laryngopharynx to the stomach.

Mucosal layer – stratified squamous epithelium with mucous glands

Esophageal Hiatus – opening in the diaphragm, passage for esophagus.

• Hiatal hernia – part of the stomach protrudes through the hiatus.

Lower Esophageal (Cardiac) sphincter – prevents regurgitation from the stomach.

The stomach has two layers of smooth muscle:• An inner circular layer• An outer longitudinal layer• (There may be a third inner layer of oblique fibers.)

The Stomach

The stomach is a J-shaped, pouch-like organ, about 25-30 centimeters long

Location: It’s inferior to the diaphragm in the upper-left abdominal quadrant.

Figure 17.17. Some parts of the stomach have three layers of muscle fibers.

Parts of the Stomach

Cardia – entrance Fundus – superior portionPyloris – funnel-shaped portion near the distal endBody– main part of the stomach.

The stomach can be divided into 4 regions

Pyloric sphincter – controls the rate of gastric emptying.

Rugae – gastric folds line the stomach

Lining of the Stomach

The stomach is lined with simple columnar epithelium along its gastric folds.

Deep pits (gastric pits) form gastric glands in the stomach’s mucosa.

Figure 17.19 Lining of the stomach. Gastric glands include mucous cells, parietal cells, and chief cells.

Gastric Glands contain 3 types of secretory cells:

Mucous Cells – secrete mucus

Chief Cells– secrete pepsinogen

Parietal Cells– secrete HCl

Secretions from these cells form gastric juice.

Gastric SecretionsPepsin – important digestive enzyme that breaks down proteins.

Pepsin is formed when the pepsinogen secreted by chief cells mixes with HCl secreted by parietal cells.

pepsinogeninactive HCl

pepsinactive

Mucus secreted by mucous cells protects the stomach from pepsin and HCl.

Intrinsic factor – secreted from the parietal cells• Is required for vitamin B12 absorption

Regulation of Gastric Secretions

The rate of gastric juice secretion is controlled both neurally and hormonally.

Neural Stimulus:

Hormonal Inhibition: Endocrine cells in the stomach secrete somatostatin, which inhibits gastric juice secretions.

Parasympathetic impulses (from the vagus nerve) release acetylcholine

Somatostatin acts as a “brake”, preventing unnecessary gastric juice secretions.

Acetylcholine has three actions:• Suppresses the secretion of somatostatin • Stimulates the secretion of gastric juices.• Stimulates the secretion of gastrin

Gastrin is a hormone that further increases the secretory activity of gastric glands.

Hormonal Stimulus:

Figure 17.20 The secretion of gastric juice is regulated in part by parasympathetic impulses that stimulate the release of gastric juice and gastrin.

3 Phases of Gastric Secretions

Celiac Phase – begins before food enters the stomach

• The vagus nerve stimulates gastric secretions in response to the taste, smell, sight, or the thought of food.

Gastric Phase– Starts when food enters the stomach• Accounts for most of the secretory activity “go for it”• Food distends the stomach wall, triggering gastric juice secretions.• Gastrin secretion ceases as the stomach pH approaches 1.5

Intestinal Phase– Begins when food enters the small intestine.

• Prepares for digestion “get ready”

• Acidic chyme in the duodenum stimulates sympathetic impulses, which inhibit gastric juice secretion “slow down”.

• Fats stimulate the release of intestinal somatostatin – inhibits gastric juice release.• Fats and proteins promote the release of cholecystokinin – decreases motility.

• Gastric enzymes begin breaking down proteins, but the stomach is not well-adapted to absorb digestive products

Gastric Absorption

• The stomach does absorb:• Some water• Certain salts• Certain lipid-soluble drugs• Alcohol

• The stomach absorbs very little nutrients.

Mixing and Emptying of the Stomach

• The stomach mixes food with gastric juice, forming chyme.

• Rhythmic muscle contractions push chyme through the pyloric sphincter into the duodenum a little at a time.

• Chyme entering the duodenum stretches the intestinal wall, stimulating an enterogastric reflex.

Enterogastric reflex – • Begins in the intestine (entero) and ends in stomach (gastric)

• As chyme fills the small intestine, sensory impulses conducted to the CNS reduce parasympathetic activity of the stomach.

• As a result, peristalsis in the stomach is inhibited, and intestinal filling slows.

Figure 17.22 The enterogastric reflex partially regulates the rate at which chyme leaves the stomach.

• The pancreas functions as both an endocrine and exocrine gland

• Its exocrine function is to secrete pancreatic juice

The Pancreas

Anatomy of the Pancreas

The head of the pancreas rests in the curvature of the duodenum.

Acinar cells – produce pancreatic juice• Acini – clusters of acinar cells• Acini surround small tubules that merge into a pancreatic duct.• The pancreatic duct empties into the duodenum.

Figure 17.23 The pancreas is closely associated with the duodenum.

Anatomy of the Pancreas

The pancreatic duct joins with the bile duct forming the hepatopancreatic ampulla.

Hepatopancratic sphincter – band of smooth muscle that regulates secretions into the duodenum.

Pancreatic juice contains enzymes and bicarbonate ions:

Pancreatic Juice

• Pancreatic amylase – splits glycogen into disaccharides

• Pancreatic lipase – breaks down triglycerides (lipids)

• Trypsin, chymotrypsin, and carboxylpeptidase – digest proteins

• Nucleases – digest nucleic acids

• Bicarbonate ions – neutralizes acidic chyme in the duodenum.

Enzymes

Alkaline Juice

Activation of Pancreatic Enzymes

• The pancreas secretes enzymes in the inactive form and must be activated by other enzymes within the small intestine.

Trypsinogen (inactive)

Secreted by pancreas

+ EnterokinaseActivating enzyme

Trypsin (active)

secreted by small intestine

Trypsin has two functions:• Digests proteins• Activates chymotrypsin & carboxylpeptidase.

Splits proteins

Regulations of Pancreatic Juice Secretions

The nervous and endocrine systems regulate the release of pancreatic juice.

Endocrine RegulationSecretin

Nervous Regulation Parasympathetic impulses stimulate the secretions of digestive enzymes

• Stimulates the secretion of alkaline pancreatic juice with few enzymes.• Secretin is released when acidic chyme enters the duodenum.• Bicarbonate ions in the pancreatic juice neutralize the chyme.

Cholecystokinin (CCK)

• CCK is released when chyme containing fats and proteins enters the small intestine.

• CCK stimulates the secretion of pancreatic juice rich in digestive enzymes.• It also stimulates the release of bile from the gall bladder.

Figure 17.24 Acidic chyme entering the duodenum from the stomach stimulates the release of secretin, which, in turn, stimulates the release of pancreatic juice.

END OF SECTION 2, CHAPTER 17