Gastrointestinal system Overview Digestion of nutrients Absorption of nutrients and water Principles of GI regulation GI secretion and regulation GI motility

Gastrointestinal system

• Overview• Digestion of nutrients• Absorption of

nutrients and water• Principles of GI

regulation• GI secretion and

regulation• GI motility and

regulation


• Overview

• Digestion of nutrients

• Absorption of nutrients and water

• Fate of nutrients in the liver

• Principles of GI regulation

• GI secretion and regulation

• GI motility and regulation

Overview

• Food for body:

• Carbohydrates, proteins and lipids are absorbed in a form that can not be taken up by the cells food needs to be broken a small pieces (mechanical digestion) and broken down chemically (chemical digestion)

Steps in food digestion

• Carbohydrates, proteins and lipids are absorbed in a form that can not be taken up by the cells food needs to be broken a small pieces (mechanical digestion) and broken down chemically (chemical digestion)

• Chemical digestion refers to the degradation of:

1 2• 1- Carbohydrates ---> disaccharides ---> monosaccharides

• 2- Proteins ---> peptides ---> amino acids

• 3- Lipids ---> diglycerides ---> monoglycerides and fatty acids

GI organization

• Composed of the GI tract, a “tube” running from the esophagus to the rectum

• Accessory organs are emptying secretions into the tract.

Structure of the GI tract

• 4 layers:- 1- mucosa: epithelial cells (enterocytes)

role vary with location - lamina propria = layer of connective

tissue under the epithelial, containing blood vessels, nerves and lymphatic vessels (Peyer’s patches)- the muscularis mucosae: thin layer of smooth muscles

- 2- submucosa: layer of connective tissue rich containing the submucosal plexus (Meissner’s plexus)

- 3- muscularis externa – radial and longitudinal smooth muscles + Auerbach plexus(in contact with Meissner’s)

- 4- serosa:connective tissue continuying through the mesenteries (a thin membranes rich in blood and lymphatic capillaries) and the peritoneum (a double layer membrane surrounding the abdominal organs)


• Overview



• Fate of nutrients in the liver




Functions of the GI system

• Digestion:

- Mechanical digestion: breaking food in small particles so they are easily broken down by enzymes mouth and stomach

- Chemical digestion: pancreas and duodenum

• Nutrient absorption: small intestine

• Water reabsorption: colon

Functional anatomy of the GI system

• Mouth: mostly mechanical digestion mastication food is broken down in small particles so food particles can be chemically digested bolus

• Enzymes:- lingual amylase

carb. (step 1)- lingual lipase

lipid (step 1)

• Pharynx, esophagus: passageway for food (from mouth to stomach)

Stomach

• Stomach: bolus is mixed with gastric juice (liquid, mucus and enzymes) chyme

• Enzymes:- pepsin: protein (step 1)- gastric lipase

lipid (step 1)

Pepsin is first activated by HCl (pepsinogen pepsin)

Gastric juice pH=2

[chemical digestion is minimal)]

Liver

• Roles of the liver:

- Secretes bile

- Processes nutrients

- Remove wastes from the body (including old RBCs)

- Detoxify

- Secretes hormones

Liver (bile)

• Bile is secreted by the liver hepatocytes

• Bile composition: water, mucus, bile salts (emulsify lipids), bile pigments (biliverdin and bilirubin), cholesterol,

• The bile salts are reabsorbed back to the liver for reuse by the enterohepatic circulation

Pancreas

- Trypsinogen trypsin (enterokinase from duodenum).

- Chymotrypsinogen and carboxypeptidase chymotrypsin and active carboxypeptidase).

Step 1 of protein digestion

- Pancreatic amylase Step 1 carbohydrate digestion

- Pancreatic lipase step 1 lipid digestion

- Bicarbonates ions from the pancreatic juice help neutralize the acidity of the chyme.

Figure 20.10

Figure 20.4

Duodenum

Receive juices from pancreas, liver and its own wall

• * Secretion from the duodenum: They finish off the last step of digestion.

• - Peptidases (or dipeptidases) break off the bond between dipeptides to free 2 amino acids

• - Disaccharidase (maltase, sucrase, lactase) break off disaccharides into 2 monosaccharides (mostly glucose)

• - Intestinal lipase breaks off diglycerides into monoglycerides and fatty acids.

• Nutrients are completely degraded into forms that can be absorbed by cell (step 2 of chemical digestion)

Jejunum-Ileum

• Nutrients will be reabsorbed along the jejunum-ileum

• Brush border contains villi which increase the surface of absorption

• The villi are structured for nutrient absorption

Colon

• Reabsorb water from food and digestive juices


• Overview






Steps in food digestion

1 2• 1- Carbohydrates ---> disaccharides ---> monosaccharides

• 2- Proteins ---> peptides ---> amino acids

• 3- Lipids ---> diglycerides ---> monoglycerides and fatty acids

Figure 20.7

Absorption of carbohydrates

• Monosaccharides (mostly glucose) are absorbed

• The monomers are carried by transporter molecules across the epithelial cells and into the blood capillary present in the villus portal vein liver

Absorption of proteins

• Proteins are degraded into amino acids (a.a.)

• A.a. are carried by transporter molecules across the cells and into the blood capillaries portal circulation liver

Lipid absorption

• Lipids (triglycerides) are degraded to monoglycerides and fatty-acids.

• They are absorbed into the cell by diffusion. The cell resynthesizes triglycerides. Because TG are not soluble in H2O, the TG are surrounded with proteins and packaged into chylomicrons

• The chylomicrons are emptied into lymphatic capillaries, the lacteal lymph circulation blood cells and liver

Absorption of minerals

• Sodium: active absorption in jejunum-ileum. Chloride follow by electromagnetic attraction.

• Potassium: passive secretion or absorption, depending on lumenal concentration if diarrhea, hypokalemia due to loss of K+

• HCO3-: secreted by pancreas, neutralizes H+ from stomach. Used as a buffer

• Calcium: need an active transport to cross the intestinal epithelium. Absorption promoted by a derivative of Vit D

• Iron: actively reabsorbed. Stored as ferritin

• Water: Two liters of fluids are taken as food or drink per day. In addition, 7 liters are used to secrete digestive jiuces need to reabsorb most of H2O.

• H2O reabsorbed throughout the small and large intestines. Colon is especially designed to reabsorb H2O.


• Overview• Functional anatomy of the GI system• Digestion and absorption of nutrients and

water• Fate of nutrients in the liver• Principles of GI regulation• GI secretion and regulation• GI motility and regulation

Fate of nutrients

• Glucose:- used as needed by liver cell- blood stocked on glucose- glycogen syntesized- TG synthesized if needed and sent to adipose tissue

Amino acids:- used to restock the blood- used by the liver to synthesize its own proteins- used to synthesize blood proteins- if excess: a.a. are deaminated NH2 used to make urea and the rest used for energy or stored as TG

Lipids (next slide)

Lipidshttp://www.lipidlibrary.co.uk/Lipids/lipoprot/index.htm

http://www.lipidlibrary.co.uk/Lipids/lipoprot/index.htm

http://www.lipidlibrary.co.uk/Lipids/lipoprot/index.htm

Absorptive state

Post-absorptive state


• Overview






GI secretion and regulation

• Need to regulate GI function to changing states (after or before meals..)

• Sensors: 3 different receptors:- mechanoreceptors: monitor state of distention- chemoreceptors monitor concentrations of substances in the lumen- osmoreceptors monitor osmolarity of lumen contents

• Integrating centers: CNS and enteric NS

• Effectors: endocrine, exocrine cells, smooth muscles

Control pathways• Both hormonal and neural• Short pathways: involves automatic regulation within the enteric

system itself• Long pathways: involves the CNS (somatic and autonomic)• Three phases: cephalic, gastric and intestinal phases

Cephalic phase: salivary and gastric secretions

• Salivary secretion stimulated by parasympathetic NS by odors, sight, taste saliva fluid and rich in enzymes

• Stimulated by sympathetic NS thick secretion, rich in proteins

• Gastric secretion: increase acid and enzymes secretion in response to sight, smell and taste of food

Gastric phase

• Stimuli: presence of food in the stomach (both distention and nutrients)

• Stimulation of the parasympathetic NS and secretion of gastrin (hormone)

• Response: increased motility and juice secretion

Intestinal phase

• Arrival of nutrients in duodenum decreased gastric secretion and motility

• Promotes secretion of cholecystokinin (CCK) and secretin- CCK promotes:

- increased pancreatic enzyme secretion- gallbladder contraction and sphincter of Oddi

relaxation- secretin promotes:

- bicarbonate ion secretion (pancreas)- bile secretion


• Overview






Motility and its regulation

• The wall has 2 layers of smooth muscles with radial and longitudinal fibers. The fibers communicate through gap junctions

• The parasympathetic NS stimulates smooth muscle contraction

• Peristalsis: waves of contraction of longitudinal muscle fibers moving down the GI tract

• Segmentation: in small intestine for mixing chyme

• Chewing and swallowing:

Gastric motility

• Gastric motility increases with the presence of gastrin and decreased under the influence of CCK, secretin and gastric inhibitory peptide (GIP)

• Vomiting: – - emotional stress, severe

pain, illnesses, toxins stimulate the vomiting center in the medulla oblongata

– sensation of nausea, increased HR, skin paleness is followed by food coming back up

Motility in the small intestine

• Segmentation and peristalsis increased by distention of the wall

• Intestino-intestinal reflex: severe distention or injury inhibits motility in the region.

• Ileo-gastric reflex: distension of ileum inhibits gastric motility

• Gastro-ileal reflex: presence of chyme in stomach increases motility in ileum

Motility in the colon

• Haustration: like segmentation, for mixing

• Colono-colonic reflex: distension in 1 part of the colon induces relaxation in other parts

• Gastro-colic reflex: a meal in the stomach increases colonic motility

• Defecation: - triggered by distention of the rectal wall- signal sent to sacral parasympathetic and cortex- smooth muscle anal sphincter open- if the person decides to go to the bathroom open voluntary muscle sphincter

•

Clinical applications

• Diarrhea: can be due to bacterial, parasite infections, toxins, bowel irritation

• Laxatives:– - bulk-producing agents– Stool softeners– Hydratants (salines and

osmotic)– Stimulant or irritant

• Intestinal obstruction

• Hirshprung disease

Documents

Gastrointestinal system Overview Digestion of nutrients Absorption of nutrients and water Principles of GI regulation GI secretion and regulation GI motility