Digestion Part II and III

Absorption of water-soluble, non-lipid nutrients by small intestine

- Arteriole (blood vessel) delivering blood to the villus is low on nutrients and high on oxygen (red). - Epithelial cells absorb nutrients (monosaccharides, amino acids, and nucleotides) through active transport - Capillary in villus receives nutrients absorbed by epithelial cell through active transport. - Venule leaving the villus transports blood to the liver through the hepatic portal vein. It is high on nutrients and low on oxygen (blue). - Liver stores nutrients, converts them to other nutrients, and distributes them in proper proportions and kinds before release into the bloodstream to the rest of the body.

Digestion and absorption of lipids

- Bile emulsifies triglyceride lipids in the small intestine. - The resulting smaller drops expose more surface area to lipases that break triglycerides down into glycerol, fatty acids, and monoglycerides. - Once these components enter epithelial cells, they are recombined into triglycerides, converted to lipoprotein, and enter the lacteals.

Absorption of fatty acids, glycerol, and monoglycerides (1/3) by small intestine

- Lymph vessel entering villus is low on lipids (micelles and triglycerides). - Epithelial cells absorb fatty acids, glycerol, and monoglycerides through diffusion. - Reassembled into triglycerides and converted to lipoprotein by adding protein - Lacteal/lymph capillary in villus receives lipoprotein from epithelial cells through exocytosis. - Lymph vessel leaving villus delivers lymph fluid to subclavian veins in neck. It is high on lipids (lipoprotein) - specifically chylomicrons.

Absorption of micelles (2/3) by small intestine

- Lymph vessel entering villus is low on lipids (micelles and triglycerides). - Epithelial cells absorb micelles through endocytosis. - Micelles are converted to lipoprotein by losing their bile and picking up soluble protein. - Lacteal/lymph capillary in villus receives lipoprotein from epithelial cells through exocytosis. - Lymph vessel leaving villus delivers lymph fluid to subclavian veins in neck. It is high on lipids (lipoprotein) - specifically chylomicrons.

Villi, microvilli, and submucosa

- Submucosa contains blood vessels, lymph vessels, and nerves. - Lymph vessels carry lymph fluid which contain lipids. - Blood vessels carry water-soluble, non-lipid nutrients. - Villi are made of epithelial cells that have 500 projections called microvilli per cell. - In less detail, the villus does the absorption. - However, the epithelial cells do the ACTUAL absorption.

Digestive enzymes

1. Salivary amylase (hydrolyzes starch into maltose and dextrins) 2. Gastric pepsin and inactive pepsinogen (hydrolyzes protein to peptones/peptides/polypeptides) 3. Pancreatic amylase (hydrolyzes starch into maltose) 4. Pancreatic trypsin (hydrolyzes protein into peptones/peptides/polypeptides) 5. Pancreatic carboxypeptidase "COOH compatible" (hydrolyzes peptones/peptides/polypeptides into amino acids) 6. Pancreatic lipase (hydrolyzes triglycerides into fatty acids, glycerol, and monoglycerides) 7. Pancreatic nuclease (hydrolyzes nucleic acids into nucleotides) 8. Intestinal enterokinase (activates trypsinogen into trypsin), 9. Intestinal erepsin "NH2 compatible" (hydrolyzes peptones/peptides/polypeptides into amino acids) 10. Intestinal sucrase (hydrolyzes sucrose into glucose and fructose) 11. Intestinal maltase (hydrolyzes maltose into glucose and glucose) 12. Intestinal lactase (hydrolyzes lactose into glucose and galactose)

Digestion and absorption of carbohydrates

Begins in the mouth and continues in the small intestine as long chains (starch) are broken into disaccharides (maltose) by salivary and pancreatic amylase. - Disaccharides are converted to monosaccharides by enzymes in the small intestine. (carbohydrases - maltase, lactase, sucrase) - Monosaccharides can enter small intestine epithelial cells and move into the bloodstream.

Pancreatic duct

Carries pancreatic juice from the pancreas to the duodenum - Juice contains enzymes that break down carbohydrates, nucleic acids, polypeptides, and micelles. - Also contains bicarbonate buffer, which neutralizes the acidic stomach contents

Small intestine

Chyme (liquefied contents) enters the seven meters of small intestine from the stomach when the pyloric sphincter relaxes. - Addition of numerous enzymes completes the digestion of food, and nutrients are absorbed into the blood. - First 10 inches is the duodenum - then jejunum - then ileum - Smooth on the outside, folded on the inside

Rectum

Compacts and stores feces until they can be eliminated through the anus

Homeostatic control of digestion

Digestive tract is under the control of the autonomic nervous system, hormones, and webs of neurons

Hormones regulating digestion

Gastrin, secretin, enterogastrone (GIP), and cholecystokinin (CCK)

Endocrine

Glands that secrete hormones into the bloodstream which regulate digestion

Exocrine

Glands that secrete things through ducts outside of the bloodstream

Surface area of small intestine

Greatly increased by folding of lining (mucosa) - Each large fold and furrow is covered with millions (6) of microscopic finger-like projections called villi, increasing the absorptive surface area of the small intestine 600-fold. - Lining produces enzymes that complete the digestion of peptides and sugars. - Surface cells of the villi have many small projections called microvilli. - These greatly increase the surface area for absorption. - Each villus is hollow (closed on one end and open on the other) and contains blood capillaries surrounding a lymph capillary (LACTEAL). Small intestine increased surface area from four adaptations: 1. Length 2. Internal folding 3. Villi on the folds 4. Microvilli on the villi

Physical non-enzyme digestion agents

Increase the surface area (reduce the size) of food molecules and/or activate inactive enzymes. 13. Liver, gallbladder bile - emulsification of fats and oils (triglyceride lipids) 14. Gastric hydrochloric acid (HCl) - coagulation of protein and activation of pepsinogen 15. Gastric rennin - coagulation of milk protein 16. Pancreatic and intestinal bicarbonate (HCO3-) - buffers acidic pH to alkaline pH in the small intestine. Chewing of food is a physical (mechanical) non-enzyme digestion action that exposes a greater surface area of food to enzyme activity.

Chylomicrons

Lipoprotein in lymph fluid carried by lymph vessels that carries lipids: triglycerides and cholesterol

Secretin (buffer pH)

Location of gland tissue producing the hormone: Endocrine cells in the lining (wall) of the duodenum Stimulation of hormone secretion: Acidic chyme and peristalsis in the duodenum increases the conversion of inactive prosecretin to active secretin. Hormone target organ: 1. Exocrine cells in the pancreas 2. Exocrine cells in the liver Activity regulated by the hormone: Stimulates the secretion of HCO3- (bicarbonate) and bile into the duodenum to change (buffer) the pH from 2-3 to 8-9

Cholecystokinin (CCK)

Location of gland tissue producing the hormone: Endocrine cells in the lining (wall) of the duodenum Stimulation of hormone secretion: Fatty chyme (food) and peristalsis in the duodenum and small intestine Hormone target organ: 1. Gall bladder 2. Pancreas exocrine tissue 3. Pyloric sphincter muscle Activity regulated by the hormone: 1. Contractions cause release of bile into duodenum. 2. Secretion of enzymes into duodenum 3. Contracts and closes

Enterogastrine (GIP)

Location of gland tissue producing the hormone: Endocrine cells in the lining (wall) of the duodenum Stimulation of hormone secretion: Fatty chyme (food) and peristalsis in the duodenum and small intestine Hormone target organ: Muscles in the wall of the stomach and small intestine Activity regulated by the hormone: Peristalsis contractions slow down giving more time to digest and absorb the food.

Digestion and absorption of nucleic acids

Nucleic acids are hydrolyzed into nucleotides by pancreatic nuclease in the small intestine. They then are able to cross into the epithelial cells of the small intestine, and move into the bloodstream.

Bile production

One of many functions of liver is the production of bile. - Bile contains cholesterol, pigment from the breakdown of red blood cells, and salts. - Bile is stored in the gallbladder and is released into the small intestine through the common bile duct. - The bile salts help break down triglyceride lipid droplets into micelles so they can be chemically digested or directly absorbed. (emulsification)

Digestion and absorption of proteins

Proteins are hydrolyzed into polypeptides/peptones/peptides beginning in the stomach by pepsin and in the small intestine by trypsin. - Peptidases in small intestine (erepsin and carboxypeptidase) hydrolyze them into amino acids. - Active site of erepsin/aminopeptidase (small intestine) binds to amino end of polypeptide and hydrolyzes it into amino acids. - Active site of carboxypeptidase (small intestine) binds to carboxyl end of polypeptide and hydrolyzes it into amino acids. - Single amino acids are able to cross into epithelial cells of the small intestine and, like monosaccharides, move into the bloodstream. Protein --> polypeptides/peptides/peptones --> amino acids

Large intestine/colon

Receives indigestible material and reabsorbs large amounts of water from the small intestine after nutrients have been absorbed - regulates water and ion reabsorption into the bloodstream, absorbs vitamin and minerals, and houses beneficial bacteria (e.coli) that produce vitamins such as vitamin K

Gastrin (75%) (25% by nerves)

Stomach hormone Location of gland tissue producing the hormone: Endocrine (G) cells in the gastric pits/glands of the stomach lining Stimulation of hormone secretion: Peristalsis and food in the stomach Hormone target organ: Exocrine cells (chief cells and parietal cells) in the gastric pits of the stomach lining Activity regulated by the hormone: Stimulates the secretion of pepsin enzyme and HCl for protein digestion and coagulation

Pyloric sphincter

The stomach contents pass through the pyloric sphincter to the small intestine. - Common bile duct from the gallbladder adds bile, which breaks down triglyceride lipid droplets (emulsification) - Common bile duct is joined by pancreatic duct from the pancreas, which adds bicarbonate to neutralize the acid and adds enzymes which, along with those from the intestine's lining, complete digestion. - Once digestive enzymes have broken large food molecules into small ones such as monosaccharides, amino acids, fatty acids, glycerol, monoglycerides micelles, and nucleotides, absorption of these nutrients can occur. * Before the small intestine can absorb nutrients, they must be broken down to monomers by enzyme hydrolysis.