BIO 226 Chap 22: Digestive System 2

• The esophagus includes the mucosa, submucosa, muscular layer, and adventitia.

Swallowing (Deglutition)

• Initiated voluntarily but proceeds automatically.
• Includes the buccal, pharyngeal, and esophageal phases.

Buccal Phase

• Strictly voluntary phase
• Begins with the compression of bolus against the hard palate.
• The tongue forces the bolus into the oropharynx.
• The soft palate also elevates, sealing off the nasopharynx.
• Entry into the oropharynx triggers a reflex response.

Pharyngeal Phase

• Begins with the stimulation of tactile receptors in uvula and palatine arches.
• Motor commands from the swallowing center (in the medulla oblongata) coordinate muscle contraction in pharyngeal muscles.
• Larynx is elevated, the epiglottis is folded, and the uvula and soft palate are elevated.
• The bolus is moved through the pharynx into the esophagus.

Esophageal Phase

• Begins as bolus is forced through the entrance to the esophagus.
• Bolus is pushed toward the stomach by peristalsis.
• The approach of the bolus triggers the opening of the lower esophageal sphincter and the bolus enters the stomach.
• Typical travel time is 9 seconds, but liquids may travel faster.
• A dry (poorly lubricated) bolus may require secondary peristaltic waves.

Peritoneal Cavity

• Encloses stomach and most of the intestinal tract.
• Lined by a serous membrane called the peritoneum.

Serous Membrane

• Divided into the serosa, or visceral peritoneum, and the parietal peritoneum.
• The visceral peritoneum covers organs enclosed by the peritoneal cavity.
• The parietal peritoneum lines the inner surface of the peritoneal cavity.
• Continuously secretes peritoneal fluid into the peritoneal cavity.
• About ~7 liters/day is secreted and reabsorbed.
• Volume at any one time is about 50 mL.
• A thin layer separates parietal and visceral surfaces.
• Allows sliding movements without friction or irritation.
• The rate of fluid moving into the cavity is accelerated by liver disease, kidney disease, and heart failure.
• Accumulation of peritoneal fluid can create abdominal swelling; ascites.

Mesenteries

• Dorsal and ventral mesenteries formed during embryonic development.
• These suspend the digestive tract and accessory organs and develop into other adult connections.

Dorsal Mesentery

• Becomes the greater omentum (omentum, fat), which is attached to the stomach and transverse colon.
• The greater omentum forms a large pouch extending inferiorly between anterior body wall and anterior surface of the small intestine.
• Contains adipose tissue that provides padding and protection.
• The dorsal mesentery also becomes the mesentery proper, which connects the small intestine to the posterior body wall.
• The mesocolon connects the large intestine to the posterior body wall.

Ventral Mesentery

• Becomes the lesser omentum, which connects the stomach to the liver and provides an access route for blood vessels.
• Also becomes the falciform ligament, which connects the liver to the anterior body wall.

Position of the Mesenteries

• Changing position of the mesenteries occurs with elongation of the digestive tract
• Some segments of the tract become fixed in position.
• Segments of the mesentery proper come into contact and fuse together.
• Much of the mesocolon fuses to the dorsal body wall.

Stomach

• Highly variable shape depending on contents.
• When empty, a muscular tube with a constricted lumen.
• Can expand to contain 1-1.5 liters of material (chyme) when full.
• Chyme is a viscous, highly acidic, soupy mixture formed from the combination of food, saliva, and gastric gland secretions.
• The lesser curvature forms the medial surface and is attached to the mesentery of the lesser omentum.
• The greater curvature forms the lateral and inferior surfaces and is attached to the mesentery of the greater omentum.

Stomach Regions

• The fundus, superior to the junction between the stomach and esophagus.
• The cardia, superior, medial portion within 3 cm of gastroesophageal junction.
• It secretes mucus to protect esophagus from stomach acid/enzymes.
• The body, the largest region that functions as a mixing bowl.
• The pylorus, a sharp curve of "J" of the stomach that frequently changes shape with mixing movements.

Muscle Layers and Rugae

• Layers of the muscularis externa include the oblique, circular, and longitudinal layers.
• The oblique layer strengthens the stomach wall and assists in mixing and churning to form chyme.
• Rugae (wrinkles) are prominent, temporary mucosal folds that allow gastric expansion by flattening out with stomach expansion.

Pyloric Part

• The pyloric antrum (antron, cavity) which is the portion connected to stomach body.
• The pyloric canal empties into the duodenum, the proximal segment of small intestine.
• The pyloric sphincter has a smooth muscle band regulating release of chyme into the duodenum.
• The pyloric orifice is the stomach outlet.

Functions of the Stomach

• Includes the temporary storage of ingested food.
• Provides mechanical digestion of ingested food.
• Allows for the chemical digestion of food through the action of acid and enzymes.
• Involved in the production of intrinsic factor.

Layers of the Stomach Wall

• The mucosa is composed of simple columnar epithelium.
• It produces a layer of alkaline mucus, protecting epithelial cells against acid and enzymes in gastric lumen.
• The life span of a gastric epithelial cell is only 3–7 days.
• Submucosa
• Muscular layer includes oblique, circular, and longitudinal layers
• Serosa

Gastric Glands

• Glands in the fundus and body secrete most of the acid and enzymes enabling gastric digestion.
• Dominated by parietal cells and chief cells.
• Secrete ~1.5 L of gastric juice each day.
• Glands in the pylorus secrete mucus and hormones that coordinate and control digestive activity.

Gastric Pits

• Shallow depressions opening onto the gastric surface.
• Active stem cells at the base of each pit replace superficial cells shed into the chyme.
• Each pit communicates with several gastric glands.

Cells of the Gastric Glands

• Parietal cells secrete intrinsic factor/glycoprotein to aid in vitamin B12 absorption, and hydrochloric acid (HCl).
• HCI activates pepsinogen and keeps stomach at pH 1.5-2.
• G cells (enteroendocrine cells produce a variety of hormones.
• Chief cells secrete pepsinogen, which is activated by HCl to become pepsin.
• Newborns also produce rennin and gastric lipase.
• Rennin and gastric lipase are enzymes important for the digestion of milk.

HCI Production

• Parietal cells do not create HCI in their cytoplasm to avoid destroying themselves.
• H+ and Cl- are transported and secreted separately.
• H+ is generated as carbonic anhydrase converts CO2 and H2O to carbonic acid.
• Carbonic acid dissociates into bicarbonate ions and hydrogen ions.
• Bicarbonate is ejected into the interstitial fluid in exchange for a chloride ion.
• If gastric glands are very active, the amount of bicarbonate released is enough to increase the pH of the blood.
• Sudden influx of bicarbonate ions is called the alkaline tide.
• Chloride ions diffuse across the cell and exit into the lumen of the gastric gland.
• Hydrogen ions are actively transported into the gastric gland lumen.

Small Intestine

• Plays a key role in nutrient digestion and absorption.
• 90% percent involves absorption, most of the remaining 10% occur in the large intestine.
• Approximately 6 m (19.7 ft) in length but the diameter ranges from 4 cm (1.6 in) near stomach to 2.5 cm (1 in.) near large intestine.
• Includes the duodenum, jejunum, and ileum.

Specialized Structures of Small Intestine

• Intestinal structures increase surface area to increase absorption.
• Circular folds (plicae circulares) are series of transverse folds along the intestinal lining.
• They are permanent features as there are roughly 800 folds in the small intestine, mostly in the jejunum.
• Intestinal villi (singular, villus) are fingerlike projections of mucosa, covered by epithelial cells.
• Surfaces are covered with microvilli.

Intestinal Glands

• Located at the bases of villi.
• Near base stem cells divide and produce epithelial cells.
• Paneth cells at the base have a role in innate immunity by releasing defensins and lysozyme.
• The internal structure of a villus includes an extensive capillary network in the lamina propria.
• Capilaries carries absorbed nutrients to the hepatic portal circulation.
• The lymphatic capillary (lacteal) transports materials that cannot enter blood capillaries.
• An example includes absorbed fatty acids assembled into protein-lipid packages (chylomicrons) too large to diffuse into bloodstream.
• Therefore chylomicrons are transported by lymphatic system to venous circulation.
• Smooth muscle in muscularis mucosae is within villi.
• The smooth muscle move villi back and forth, exposing surfaces to intestinal contents.
• Also squeezes lacteal, assisting in lymph movement.

Brush Border

• Carpet of microvilli on the surface of the epithelial cells.
• Increases surface area for absorption.
• The absorption contains enzymes that digest materials as epithelial cells can then absorb breakdown products.

Segments of the Small Intestine

• Includes the duodenum, the jejunum, and the ileum.

Duodenum

• 25 cm. (10 in) in length
• Shortest segment
• Closest to the stomach.
• Is a mixing bowl as it receives chyme from stomach and digestive secretions from liver, gallbladder, and pancreas.
• Mostly retroperitoneal.
• Duodenal glands produce mucous secretions.
• Few circular folds and small villi.
• Main function is to neutralize acidic chyme.

Jejunum

• Between the duodenum and ileum.
• About ~2.5 m/8.2 Ft long
• Has peritoneal cavity
• Marked by a sharp bend at it's beginning
• Large majority if digestion and absorptions takes place here

Ileum

• Ends at the ileocecal valve, which controls flow from the ileum into the cecum.
• Approximately 3.5 m (11.5 ft) in length and known to be the last segment
• Villi are relatively stumpy and its submucosa contains aggregated lymphoid nodules
• Few circular folds, unlike the distal portion of the small intestine

Small Intestine Hormones

• Many major hormones that regulate digestion include gastric activities
• Most are produced by the duodenum
• Duodenum coordinates gastric activity and digestive secretion according to characteristics of the arriving chyme

Gastrin

• Is a peptide hormone that is produced by the G cells
• G cells are in the pyloric antrum and certain parts of the duodenum's enteroendocrine walls

Secretin

• Secretin is a hormone that is released when chyme arrives in the duodenum.
• Secretin's primary effect is an increase in the secretion of buffers by the pancreas, which increase the pH of the chyme
• It also stimulates secretion of bile and reduces gastric mobility

Gastric Inhibitory Peptide "GIP"

• GIP helps with reduction gastric mobility as fats and carbohydrates-especially glucose-enter the small intestine
• Also accompanies the stimulus from insulin by the pancreatic islets. Overall GIP also stimulates lipid synthesis in adipose tissue and glucose in skeletal muscles

Cholecystokinin "CCK"

• CCK is hormone that is secreted when chyme arrives in the duodenum specifically lipids and partially digested proteins
• Also gives relaxation to the hepatopancreatic sphincter and contraction of the gallbladder- which give bile and pancreatic juice to the duodenum

Vasoactive Intestinal Peptide "VIP"

• VIP will help with stimulating intestines, capillaries, and suppress the production of acidity in the stomach

Enterocrinin

• Is triggered by enteroendorine cells of the duodenum in response to mobility from the vagus nerves before chyme, plus it also gives stimuli from an alkaline in the submucosal glands from acidic chyme

Phases of Gastric Secretion

• The gastric secretion relies on cephalic and local mechanism which includes the cephalic, gastric, and intestine phase.

Cephalic

• This phase starts when you think taste- or smell- food, all of which directs the CNS (Central Nervous System) to start preparing the stomach to receive its contents
• Also will help with any parasympathetic impulses and submucosal plexus all due to vagus stimulation and certain gastric cells and gastrin
• Gastric juice will increase due to any production during 500mL/h.
• Overall Cephalic lasts only a few minutes

Gastric Phase

• This phase will start after any stimuli from food arrives, the stimuli then increase the gastric content and start gastro activity
• The gastric secretions will mix contents causing an intense rise in parietal cells along with chief and other stomach structure
• Depending in severity this phase could last for 3-4 Hours

Intestinal Phase

• Will Start because of chyme from contents already in duodenum, this occurs after hrs of mixing
• Duodenum stimuli will help initialize (entero) to help in mucus production during stomach digestion
• In-Turn all this activity reduces gastric production, secretion, and even more contraction since chyme is now decreasing

Gastric Central Reflexes

• All happens stimuli because of gastro reaction when stomach walls are fully distended to their maximum.
• Gastro triggers will be accelerated along the small intestine
• Plus chime contents start moving into the small intestine once its distended.

The Large Intestine

• The large intestine is also known as a large bowel
• Average diameter length will be 7.5cm( or 3 in)
• Average long is 1.5m( or 4.9ft and 3 is expansion

1. The reabsorption of material is an increase in fecal action
2. Plus gives vitamins
3. The materials that are collected all help with the defecation process

• Three main sections the cecum, colon, and rectum,

Large Intestine Segments

• The first section involves the cecum
• The Distal all help with the Ileum function and also collecting material- to compaction- to feces
• Cecum also triggers- Appendix - by - Appendicitis, since there is a high amount of inflammation

Colon

• Larger than other small structures and includes four distinct regions

1. (ascending) or 2 ( transversal) or 3 (descending) the colon- 4. being the (sigmoid) all are very distinctive with each other

• Each location helps and connects wall structure along regions for the other two functions

Ascending

• This structure can be seen near the cecum near peritoneum and near your liver

Transversal

• Is always abdomen and travels from one end to another

Descending

• This structure mainly moves along near your left parts and the iliac fossa

Sigmoid

Greek (aka sigmeidos,) helps