Human Digestive System

• Chapter 24 will cover the digestive system, part 3 in this lecture.
• The topics include continuing with the small intestine, the large intestine, the defecation reflex, regulation of digestion, and phases of digestion.

The Stomach

• Composed of the lower esophageal sphincter, cardia, body, lesser curvature, pylorus, esophagus, duodenum, pyloric sphincter, pyloric canal, pyloric antrum, fundus, serosa, muscularis; longitudinal layer, circular layer, oblique layer, and great curvature.

Cells of Gastric Glands

• There are four main cell types: parietal cells, which are exocrine, chief cells, which are exocrine, mucous neck cells, which are exocrine, and G cells, which are endocrine.

The Small Intestine

• Segments of the Small Intestine: Duodenum, Jejunum, Ileum.

• Regions: Right Hypochondriac Region, Epigastric Region, Left Hypochondriac Region, Right Lumbar Region, Umbilical Region, Left Lumbar Region, Right Inguinal Region, Hypogastric Region, Cecum, Rectum, Left Inguinal Region.

  • The small intestine also includes the transverse colon (cut), ascending colon, descending colon, and sigmoid colon.

Summary Of Enzymes

• The Mouth contains saliva (water, bicarbonate ions, mucus) to moisten and lubricates food.

• Saliva neutralizes bacterial acids, and flushes bacteria from the oral cavity, and digests starch.

• Lysozyme has a weak antibacterial action.

• The Stomach contains hydrochloric acid, which kills bacteria, converting pepsinogen to pepsin.

• Pepsin then digests protein.

• Mucus protects the stomach lining and intrinsic factor binds to vitamin B12, which aids in its absorption.

• The Small Intestine and associated glands contains Bile salts.

• Bile salts emulsify fats.

• Bicarbonate ions neutralize stomach acid.

• Trypsin, chymotrypsin, carboxypeptidase digests protein.

• Pancreatic amylase digests starch.

• Lipase digests lipid (triglycerides).

• The Nucleases digest nucleic acid (DNA or RNA).

• Mucus protects the duodenum from stomach acid and digestive enzymes.

• Peptidases digest polypeptide.

• Sucrase digests sucrose.

• Lactase digests lactose.

• Maltase digests maltose.

The Small Intestine

• Intestinal juice is secreted by glands of the small intestine and is 1-2 liters daily.
• Intestinal Juice Contains water and mucus and is slightly alkaline. It provides liquid medium aiding absorption.

Brush Border Enzymes

• It inserts into the plasma membrane of the absorptive cells.

• Some enzymatic digestion occurs at the apical surface rather than just in the lumen.

• Examples:

• Carbohydrates: a-dextrinase, maltase, sucrase, lactase

• Proteins: aminopeptidase, dipeptidase, phosphatases

• Nucleosides: nucleosidases, phosphatases

Digestion In The Small Intestine

• Mechanical Digestion using Segmentation:

• It is localized, mixing contractions

• It causes sloshing of the chyme back and forth

• Mixes the chyme with digestive juices and brings it in contact with the mucosa for absorption

• There is no set pattern of contractions, so no particular direction of

Chemical Digestion of Carbohydrates

• Must break down polysaccharides to monosaccharides before they can be absorbed

• Mouth: So far, carbs have been acted on by salivary amylase, but the acidic pH of the stomach has inactivated it

• Small Intestine:

• Comes from pancreas: Pancreatic amylase will act on any remaining starch

• Also comes with brush border enzymes on the apical surface of the small intestine

• Small Intestine (brush border enzymes):

• Brush border enzymes act to break down small starches (disaccharides) into monosaccharides

  • α dextrinase: clips off one glucose molecule at a time
  • Sucrase: Breaks sucrose into glucose and fructose
  • Lactase: breaks lactose into glucose and galactose

• Maltase: Breaks maltose and maltotriose into 2 or 3 molecules of glucose

• Monosaccharides can then be absorbed

Lactose intolerance

• Mucosal cells of the small intestine fail to produce lactase

• Undigested lactose in chyme causes fluid to be retained in the feces

• Bacterial fermentation of the undigested lactose results in the production of gases

• Symptoms include diarrhea, gas, bloating, and abdominal cramps after consumption of milk and other dairy products

• Either avoid dairy or take lactase supplements with dairy (LACTAID)

Chemical Digestion Of Proteins

• Must break down proteins to amino acids before they can be absorbed

• Stomach:

• So Far, proteins have been broken down into smaller peptide molecules in the stomach (by pepsin)

• Small intestine:

• They originate from the pancreas. Trypsin, Chymotrypsin, Carboxypeptidase, and elastase continue to break down proteins into small peptides

• Aminopeptidase and dipeptidase in brush borders help complete the break down process by cleaving off single amino acids (which can then be absorbed)

• Amino acids can then be absorbed

Chemical Digestion of Lipids

• Mouth and Stomach:

• Lipid digestion has occurred by lingual lipase and gastric lipases

• Small Intestine:

  • Originates from pancreas: Pancreatic lipase is most important in triglyceride digestion
  • Originates from liver and gallbladder. Bile salts emulsify fats into small lipid globules so pancreatic lipase can act more efficiently

• Fatty acids, glycerol, and monoglycerides can then be absorbed

Chemical Digestion of Nucleic Acids

• Small Intestine:

• From pancreas: Ribonuclease and deoxyribonuclease in pancreatic juice

• Nucleosidases and phosphatases in brush border

Absorption In The Small Intestine

• Digestion to this point is to produce small molecules that can be absorbed through epithelial cells lining the lumen

• This includes monosaccharides (glucose, fructose, galactose), single amino acids, dipeptides, and tripeptides, fatty acids, glycerol, and monoglycerides

• Different molecules are absorbed differently (diffusion, facilitated diffusion, osmosis, and active transport)

• About 90% of absorption occurs in the small intestine and the remaining 10% occurs in the stomach and large intestine

Absorption in the small intestine - Monosaccharides

• Absorbed by facilitated diffusion or active transport into the absorptive cell.

• Fructose is via facilitated diffusion, and glucose and galactose is via active transport

• The absorption of glucose is coupled to the active transport of sodium.

• Monosaccharides will move out of the absorptive cells and into capillaries via facilitated diffusion

Absorption in the small intestine - Amino Acids

• Most are absorbed as amino acids via active transport into blood

• Half of absorbed amino acids come from proteins in digestive juice and dead mucosal cells, and the other half comes from food

• Amino acids will move out of the absorptive cells and into the capillaries via diffusion

Absorption in the small intestine - Lipids

• All dietary lipids absorbed by simple diffusion

• Short-chain fatty acids are small enough they can be absorbed via simple diffusion then pass into the capillaries via diffusion

• Long-chain fatty acids and monoglycerides must be transported to the apical surface via micelles because they're large and hydrophobic

• Bile salts form micelles to ferry long-chain fatty acids and monoglycerides up the absorptive cell surface where they are absorbed and the micelle remains in the lumen

• Reform into triglycerides and are coated with a protein, forming chylomicrons

• Leave cell by exocytosis

• Enter lacteals to eventually enter blood with protein coat of chylomicron, keeping them suspended and separate

Long-chain fatty acids + Monoglycerides

• An enzyme called lipoprotein lipase acts to break down the triglyceride in the chylomicron into fatty acids and glycerol.

These products will then diffuse into hepatocytes and adipose cells to reform triglycerides

Absorption in the Small Intestine

• Electrolytes: from Gl secretions or food/liquid. Absorbed by active transport.

• Vitamins: Fat-soluble vitamins A, D, E, and K are absorbed by simple diffusion and transported with lipids in micelles.

• Most water-soluble vitamins are also absorbed by simple diffusion, except for B12, which combines with intrinsic factor and is absorbed by active transport

• Water: 9.3L - comes from ingestion (2.3L) and Gl secretions (7.0L). most absorbed in small intestine, some in large intestine. Only 100ml is excreted in feces. All water absorption by osmosis.

The Large Intestine

• Also known as the Large bowel, it averages 1.5 m (4.9 ft), and diameter is 7.5 cm (3 in.)
• Major functions:

1. Reabsorbing water (and some electrolytes) and compacting intestinal contents into feces
2. Produce and absorb some vitamins ( vitamin K, vitamin B7)
3. Storing fecal material prior to defecation

• Starts at the junction of the ileum and the cecum via the ileocecal valve
• Four segments:

1. Cecum
2. Colon
3. Rectum
4. Anal canal

The Large Intestine Segments - Cecum

• Expanded pouch distal to the ileum. Cecum and Ileum collect and stores material, begins the process of compaction.

• Opening between the cecum and ileum is the ileocecal valve

• Attached to the appendix (also called the vermiform appendix)

• Contains numerous lymphoid nodules

• Appendicitis is inflammation of the appendix

Colon

• Subdivided into four regions: Ascending colon Transverse colon Descending colon Sigmoid colon
• Ascending and descending colon are retroperitoneal and attached to the abdominal wall
• Transverse and sigmoid colon are suspended by mesocolon Ascending colon
• From cecum to the right margin of peritoneal cavity to the inferior surface of the liver
• Bends sharply to the left at the right colic (hepatic) flexture Transverse colon - crosses abdomen from right-to-left
• supported by the transverse mesocolon
• Makes 90-degree turn at the left colic (splenic) flexure

Descending colon

• Moves inferiorly along the body's left side to the iliac fossa
• Ends at the sigmoid flexure Sigmoid colon (Sigmeidos, Greek letter) - S shaped segment: 15 cm long - it empties into the rectum Rectum - Forms last 15 cm of digestive tract
• Expandable for temporary feces storage - Movement of fecal material into the rectum triggers the urge to defecate

Histology Of The Large Intestine

• Mucosa: Lacks villi and circular folds, contains distinctive intestinal glands dominated by mucin-secreting goblet cells

• Mucosa Lubricates feces as it dries and is compact

• Mucosa does not produce enzymes

• Muscularis: Has an Outer longitudinal layer and an inner circular layer. portions of longitudinal muscles have thickened, and form tenia coli. Haustra: pouches which give the large intestine a puckered appearance.

• Serosa and Andventitia: Ascending and Descending colon are retroperitoneal = Adventitia. The transverse and sigmoid colon = serosa/

Omental (fatty) appendices: Teardrop-shaped sacs of fat in the serosa of the colon.

• Internal Anal Sphincter - Inner circular smooth muscle layer - not under voluntary control.

• External Anal Sphincter- outer skeletal muscle layer - and is under voluntary control.

• Anus (exit of The anal canal) - Epidermis here becomes keratinized.

Digestion In The Large Intestine

• There is very little digestion occurring (mechanical or chemical)

• Mucus. Glands in the LI secrete mucus, however, no enzymes are secreted

• Final stage of digestion through bacterial action

• Bacteria Acts to Ferment any remaining carbohydrates, decompose Bilirubin (Gives feces brown color), produces some Vitamins (B & K).

• Water and ions are absorbed and some vitamins

Absorption In The Large Intestine

• Accounts for less than 10 % of all nutrient absorption in the digestive tract
• Vitamin absorption includes Biotin, Vit K & B5 - produced by normal bacteria in the colon
• Water reabsorption prevents dehydration - includes absorption of over 1 L through osmosis. ~1500 mL of material enters colon however excretion equals-200 ml of feces

###Movement Through the GIT:

• Peristalsis: discussed earlier, this is rhythmic contractions that keep chyme moving through the digestive tract all the way to the cecum. The movement of chyme from the ileum into the cecum is regulated by the ileocecal sphincter

• Haustral Churning: Chyme accumulated in the ascending colon causes haustral churning. This is where the haustra become distended as they fill up, walls contract and squeeze the contents into the next haustrum

• Mass Peristalsis: Strong peristaltic wave which begins in the middle of the transverse colon. It usually occurs 3-4x per day, once food has entered the stomach.

• Migrating Motility Complexes (MMC): Type of peristalsis occurring between meals - Happens every 90-240 minutes Begins in lower portion of stomach/migrates down the small intestine/reaches the ileum; another MMC begins, which Keeps undigested food and bacteria moving into the large intestine

###Defecation Reflex:

• Begins with distension of the rectal wall after the arrival of feces

• Triggered by stretch receptors in the rectal wall

• Involves two positive feedback loops: long and short reflex

Long Reflex- Coordinated by the sacral parasympathetic system

Short Reflex- Stimulates the myenteric plexus in the sigmoid colon and rectum

Voluntary Relaxation: Allows defecation to occur at a convenient time

Steps for defecation reflex

• Mass Peristalsis: pushes material into the rectum; distention initiating a release

• Sensory impulses are sent to the sacral spinal cord; as well, motor impulses travel with parasympathetic nerves back to the colon/rectum/anus-longitudinal mussels which shortens rectum while increasing pressure

• This shortens the Rectum- however, a voluntary contraction aids to increase pressure enabling the opening with the internal anal sphincter

###Regulation of Digestion is Local

• Local factors interact with the neural and hormonal mechanisms to regulate digestive activities

• Factors: Primary stimulus for digestion activities

• pH changes of the contents in the lumen/physical distortion of digestive track/presence of chemicals (nutrients or chemical messengers that can be released by the mucosa)

Neural Control:

• Short/Myenteric reflexes- triggered by Chemo/Stretch receptors in the digestive track walls- while controlling neurons located in the Myenteric Plexus

Long Reflexes:

• Hgher control which involves intemeurons and motor neurons of the CNS
• Controls large scale peristalsis (moving from one region to another)
• May include parasympathetic motor fibers innervating myenteric plexus

###Hormonal Control Mechanisms

• At least 18 hormones that affect digestive function

Functions Affect other systems Peptides produced by enteroendocrine cells

Key Hormones: Gastrin (G cells in Stomach)- Target (Paritel cells)/no effect CCK-(Endocrine in the intestinal)-Target(Gall bladder, etc.) Secretin (Endocrine of small intestine)- Target (pancreas, stomach) GIP- Endocrine small intestine- Target (Beta cells in endocrine pancreas)/no effect

Most digestive hormones produced by the Duo Enum (coordinates gastric secretion and digestive activity according to the arriving chyme

• CCK targets partially digested food in enzymes (stimuli is rich in digestive enzymes along with the gallbladder and relaxation of the hepatopatreatic sphinguer. Additionally, it slows the gastic emptying via hypththalmus

• Secretin: cells in the small intestine: stimulates the flow in pancreatic juice that is rich in bicarbonate ions that buffer acidic chime in duodenum

Phases Of Digestion

• Gastric secretion occurs in three phases

• Cephalic phase

• Gastric phase

• Intestinal phase; During each phase secretion of juice in small intestine is stimulated or inhibited.

Cephalic phase

• Begins when you see feel taste/think of food

• Directed by CNS to prepare mouth and stomach for food (salivation)

• Parasympathetic stimulation increased

• Vagus response- activates plexus to prepare the mouth for glands and or juices (-500ml/h)

Phase usually lasts only minutes Gastric phase Stimuli begins as food arrives and or increases for the gastric and the material

• Hormonal controls- Vagus nerve and hormonal/increase the strength of action potentials for muscular control/increased mucus secretion by parietal

Phase can last approx 3-4 hours- Distension of the duodenum then inhibits the Duomeum and stimulates the Enterogastic reflex

Enterogastic: slows the excitement of Kim through the stomach

Interstital: has increased mucus- parasympathetic response is to inhibit hormones and also decrease motility- Hormone action is stimulated by Secritan and CCK- is the result

Test your knowledge about the human digestive system, including cell types in gastric glands, the primary functions of the large and small intestines, the defecation reflex, and vitamin absorption. Understand the phases of digestion, the importance of digestive secretions, and fluid balance.