SMALL INTESTINE

Small Intestine Majority of digestion and absorption Figure 24.18 Small Intestine Majority of digestion and absorption Extends from pyloric sphincter  ileocecal sphincter Divided into 3 regions Same 4 layers as most of GI

Figure 24.18b

Figure 24.19b Histology of the small intestine

Mucosa Figure 24.19 S cells CCK cells K cells

Figure 24.20 a

Figure 24.20

Figure 24.20d Brush border of Absorptive cells Brush border enzymes work inside epithelial cells (surface of microvilli) Some absorptive cells slough off to act in the lumen

Submucosa Duodenum Ileum Duodenal glands (Brunner’s) Figure 24.20c Duodenum Duodenal glands (Brunner’s) Alkaline mucous Ileum Lymphatic nodules (Peyer’s patches)

Mechanical Digestion Segmentation and Peristalsis (MMC)

Chemical Digestion Read through your notes. Refer to your textbook if needed. Pay attention to enzymes and where they act and what they break down. Take note of when they are activated / inactivated. Focus on tables 24.4 and 24.5

TABLE 24.4

Absorption End products of digestion moves into the blood or lymph. Occurs by diffusion, facilitated diffusion, osmosis and various forms of active transport. MICELLES CHYLOMICRON

Water Absorption via osmosis Fig 24 Water Absorption via osmosis Fig 24.22 Based on electrolyte and nutrient [ ]

LARGE INTESTINE

Large Intestine (colon) Terminal portion of GI tract. Completion of absorption, production of vitamins and formation and expulsion of feces. Extends from the ileocecal sphincter to the anus. Cecum, colon, rectum, and anal canal. The colon is divided into the ascending, transverse, descending, and sigmoid portions.

Figure 24.23a

Eiploic appendages or omental appendices

Rectum and Anus Rectum Anal canal Figure 24.23b Rectum Anal canal Anal columns  mucous membrane  longitudinal folds Anus Internal anal sphincter  smooth muscle External anal sphincter  skeletal muscle

Figure 24.24a

Figure 24.24b

Figure 24.24c and d

Histology of the L.I Summary Mucosa Simple columnar epithelium Absorptive and goblet cells in crypts Submucosa Areolar connective tissue Muscularis Internal smooth muscle External longitudinal muscle Teniae coli Haustra Serosa Visceral peritoneum Omental appendices

Mechanical Digestion Chemical Digestion Gastroileal reflex Haustral churning Peristalsis Mass peristalsis Bacterial fermentation Absorption Feces Formation Water Electrolytes Some vitamins Inorganic salts Sloughed-off epithelial cells Bacteria Indigestible and unabsorbed food

Defecation Reflex

PHASES OF DIGESTION

Three Phases of Digestion Cephalic Phase Gastric Phase Intestinal Phase

Cephalic Phase Cerebral cortex, hypothalamus, and brainstem activated by sight, smell, thought, and taste of food. Brainstem activates parasympathetic nuclei: CN VII – salivation CN IX – salivation CN X – gastric juice Prepares mouth and stomach for food that is about to be eaten

Gastric Phase Begins when food reaches the stomach Promotes gastric secretion and motility Regulated by neural and hormonal activities

Gastric Phase – Neural Regulation Food entering stomach increases gastric juice pH and distends stomach walls. Chemoreceptors and stretch receptors in stomach Detect pH increase Detect distention Send nerve impulses to submucosal plexus. Submucosal plexus stimulates: Parietal cells to secrete HCl Muscularis to contract more vigorously Causes mixing and initiates gastric emptying.

Figure 24.25 Neural Feedback Loop Food entering stomach Disrupts homeostasis by increasing CONTROLLED CONDITION pH of gastric juice Distention (stretching) of stomach walls RECEPTORS Chemoreceptors and stretch receptors in stomach detect pH increase and distension Input Nerve impulses CONTROL CENTER Return to homeostasis when response brings pH of gastric juice and distention of stomach walls back to normal (pre-eating status) Submucosal plexus Output Nerve impulses (parasympathetic) EFFECTORS Smooth muscle in stomach wall Parietal cells HCI Parietal cells secrete HCI Smooth muscle contracts more vigorously RESPONSE Increase in acidity of stomach chyme; mixing of stomach contents; emptying of stomach

Gastric Phase – Hormonal Regulation Food entering stomach increases gastric juice pH and distends stomach walls. ANS is triggered to release acetylcholine. ACh stimulates G cells in pyloric antrum and secrete gastrin hormone into blood.

Gastrin’s Actions Major Effects Promotes gastric juice secretion Increases gastric motility Promotes gastric mucosal growth Minor Effects Constricts lower esophageal sphincter Relaxes pyloric sphincter

Intestinal Phase Begins when food enters the small intestine. Inhibits gastric secretion and motility. Promotes intestinal and pancreatic juice secretion, bile ejection, and intestinal motility. Regulated by neural and hormonal activities.

Intestinal Phase – Neural Regulation Gastric emptying of chyme into small intestine distends duodenal walls. Stretch receptors detect distention  nerve impulses to brainstem. Brainstem ↓ parasympathetic and ↑ sympathetic nerves to stomach and pyloric sphincter. Gastric motility and emptying is inhibited.

Intestinal Phase – Hormonal Regulation Chyme containing digested proteins and fatty acids stimulates intestinal glands to secrete cholecystokinin into blood. Acidic chyme stimulates intestinal glands to secrete secretin into blood.

Secretin’s Actions Major Effects Stimulates secretion of pancreatic juice and bile rich in HCO3- Minor Effects Inhibits gastric juice secretion Promotes normal pancreatic growth and maintenance Enhances CCK’s effects

CCK’s Actions Stimulates secretion of enzyme-rich pancreatic juice Major Effects Stimulates secretion of enzyme-rich pancreatic juice Causes ejection of bile from gallbladder Opens sphincter of hepatopancreatic ampulla Minor Effects Inhibits gastric emptying Promotes normal pancreatic growth and maintenance Enhances secretin’s effects

Table 24.8 Major Hormones that Control Digestion There are at least 10 other hormones involved in the GI tract

Development and Aging of the Digestive System Read the last page in your notes Read page 932 in your textbook With a friend, add any important information to your notes on these two topics.