Gastrointestinal system

• Overview• Digestion of nutrients• Absorption of

nutrients and water• Principles of GI

regulation• GI secretion and

regulation• GI motility and

regulation

GI organization

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

• Accessory organs are emptying secretions into the tract.

Liver

Liver

• Hepatocytes– Are liver cells– Adjust circulating levels of nutrients

• Through selective absorption and secretion

– In a liver lobule form a series of irregular plates arranged like wheel spokes

– Many Kupffer cells (stellate reticuloendothelial cells) are located in sinusoidal lining

– As blood flows through sinusoids• Hepatocytes absorb solutes from plasma• And secrete materials such as plasma proteins

Liver

Liver

• Roles of the liver:

- Secretes bile

- Processes nutrients

- Remove wastes from the body (including old RBCs)

- Detoxify

- Secretes hormones

Liver Function

The Physiology of the Liver1. Metabolic regulation

2. Hematological regulation

3. Bile production

Liver Function

• Metabolic Regulation– The liver regulates:

1. Composition of circulating blood

2. Nutrient metabolism (carbohydrate, lipid & amino acid)

3. Waste product removal

4. Vitamin Storage (A, D, E & K)

5. Nutrient storage (iron)

6. Drug inactivation

Liver Function

• Composition of Circulating Blood

– All blood leaving absorptive surfaces of digestive tract

• Enters hepatic portal system

• Flows into the liver

– Liver cells extract nutrients or toxins from blood

• Before they reach systemic circulation through hepatic veins

– Liver removes and stores excess nutrients

• Corrects nutrient deficiencies by mobilizing stored reserves

or performing synthetic activities

Liver Function

• Hematological Regulation– Largest blood reservoir in the body

• Receives 25% of cardiac output• Functions of Hematological Regulation

1. Phagocytosis and antigen presentation2. Synthesis of plasma proteins3. Removal of circulating hormones4. Removal of antibodies5. Removal or storage of toxins6. Synthesis and secretion of bile

Liver Function

• The Functions of Bile– Dietary lipids are not water soluble– Mechanical processing in stomach creates large

drops containing lipids – Pancreatic lipase is not lipid soluble

• Interacts only at surface of lipid droplet

– Bile salts break droplets apart (emulsification)• Increases surface area exposed to enzymatic attack • Creates tiny emulsion droplets coated with bile salts

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• Lies posterior to stomach

– From duodenum toward spleen

• Is bound to posterior wall of abdominal cavity

• Is wrapped in thin, connective tissue capsuleFunctions of the Pancreas1. Endocrine cells of the

pancreatic islets:• Secrete insulin and

glucagon into bloodstream2. Exocrine cells:

• Acinar cells and epithelial cells of duct system secrete pancreatic juice

Pancreas• Pancreatic Enzymes

– Pancreatic alpha-amylase• A carbohydrase• Breaks down starches• Similar to salivary amylase

– Pancreatic lipase• Breaks down complex lipids• Releases products (e.g., fatty

acids) that are easily absorbed

• Pancreatic Enzymes – Nucleases

• Break down nucleic acids

– Proteolytic enzymes• Break certain proteins apart

• Proteases break large protein complexes

• Peptidases break small peptides into amino acids

• 70% of all pancreatic enzyme production

• Secreted as inactive proenzymes

• Activated after reaching small intestine

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

Absorptive state

Post-absorptive state

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

Coordination of Secretion & Absorption

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

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