Digestive

Endocrine System

Hormones

Nervous System

Autonomic

• Parasympathetic

  • Increase activity of GI tract

  • Mostly through the vagus nerve

• Sympathetic

  • decreases activity of GI tract

• Enteric

  • influenced by sympathetic and parasympathetic

  • controls motility and secretion

    • includes: myenteric and submucosal plexus

Enzymes are

• proteins

• specific for one reaction

• can be used over and over again

• catalyze reactions to help them proceed at a faster rate

• specific pH and temperature

Carbohydrates

• we take in a lot of starch

• when we are eating, we are not breaking bonds. this occurs through chemical digestion

  • through the process of hydrolysis, the bonds are broken down by adding H2O and catalyzed by enzymes

Hydrolysis

• adding water to break down bonds

• our body has plenty of water

  • saliva

  • gastric juice

  • pancreatic juice

  • liver bile

Carbohydrate Digestion

Polysaccharides → disaccharides → monosaccharides → blood stream

Begins in mouth → salivary glands → amylase → breaks down starch polysaccharide to disaccharide

Stomach

not too much carbohydrate break down here unless amylase is hidden with food bolus

• stomach: pH 2

• amylase: pH 6.5

Duodenum

pancreatic amylase enters

Jejunum

brush border → simple columnar epithelial lining → cells have surface extensions called microvilli

Enzymes exist here

• lactase

• surcrase

• maltase

(disaccharides)

Carbohydrate Absorption

1. Polysaccharides are broken into oligo and disaccharides in reactions catalyzed by pancreatic amylase

2. brush border enzymes catalyze the breakdown of disaccharides into monosaccharides

3. the Na/K pump creates a gradient for Na absorption from the fluid in the lumen

4. the gradient drives the secondary active transport of glucose and galactose via the Na/glucose cotransporter (glucose is absorbed by cotransport with sodium ions)

5. fructose is absorbed by faciliated diffusion

6. all 3 monosaccharides cross the basal side of the enterocyte membrane and then diffuse into the blood

Protein Digestion

• proteins are made up of 20 different amino acids

• proteins are broken down into amino acids by enzymes through hydrolysis

• polypeptides → amino acids

• no protein digestion in mouth

• protein digestion begins in stomach

• most occurs in duodenum where pancreatic enzymes are secreted

• lumen of alimentary canal is made of protein

Enzymes are initially secreted as what?

an inactive form called a zymogen (proenzyme) → prevents digestion of stomach (which is also made of protein)

pepsinogen

inactive form (made in stomach)

pepsin

active form

protein digestion in duodenum

pancreas secretes zymogens and they are activated in the duodenum

1. trypsinogen → trypsin

• activated by enterokinase in duodenum

• enterokinase is a brush border enzyme in duodenum

2. chymotrypsinogen → Chymotrypsin

• activated by trypsin

3. procarboxypeptidase → carboxypeptidase

• activated by trypsin

protein digestion in jejunum

double amino acids are broken by brush order enzymes

depeptidase

aminopeptidases

absorption

• broken down food molecules are absorbed from lumen (hollow opening) of small intestine, through epithelial lining (simple columnar), into blood stream

  • most occurs in jejunum of small intestine

amino acid digestion and absorption in the small intestine

1. oligopeptides are broken down into free amino acids in reactions catalyzed by pancreatic and brush border enzymes

2. the Na/K pump creates a Na gradient

3. this gradient drives the secondary active transport of certain amino acids into the enterocyte

   1. amino acids cross the basal enterocyte membrane by facilitated diffusion and enter the blood

Digestion of Fats

• Triglycerides: Glycerol + 3 fatty acids

• triglyceride → monoglyceride + 2 fatty acids

  • by hydrolysis

• digestion of fat begins in the duodenum

• pancreas secretes lipase which works on surface of fat globule

• need bile (liver) to effectively break down larger fat globs into smaller globs (emulsification)

• bile + pancreatic lipase = significant fat digestion

Bile

• bile is not enzyme

• it is an excretory solution made by the liver

• it contains bile salts to emulsify fat

• allows the lipase to have more fat surface area to break it down

• pancreatic lipase breaks the triglyceride into monoglyceride and fatty acids

Emulsification

• bile from liver

  • emulsify fat by bile salt

• lipase from pancreas

  • breaks monoglyceride and fatty chains

Chemical Digestion and Absorption of Lipids

Absorption of monoglycerides and fatty acids

• cluster with bile salts to form micelles

• fat is released by micelles to diffuse into epithelial cells

• then combine with proteins to form chylomicrons

• enter lacteals and are transported to systemic circulation

Steps for Absorption of Lipids

1. large fat globules are emulsified by bile salts in duodenum

2. digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. these then associate with bile salts to form micelles which “Ferry” them into the intestinal mucosa

3. fatty acids and monoglycerides leave micelles and diffuse into epitelial cells. there are recombined and packaged with other lipoid substances and proteins to form chylomicrons

4. chylomicrons are extruded from the epithelial cells by exocytosis. the chylomicrons enter lacteals. they are carried away from the intestine by lymph

stomach mucosa

1. goblet cells

2. parietal cells

3. chief cells

4. enteroendocrine (G cells)

Goblet cells

secrete mucus: protects against acidic pH of gastric juice p

parietal cells

appear pink

• HCl: kills bacteria, activates pepsinogen, denatures proteins

• intrinsic factor: for absorption of vitamin b12

chief cells

appear dark blue

secrete pepsinogen

Enteroendocrine (G- cells)

• located at gastric glands and is an Endocrine cell

• gastrin: hormon → blood. regulates activity of stomach. stimulates parietal cells to secrete more HCl

• enterochrommafin cells: secrete serotonin. important in chemical and mechanical stimuli in the gastric lumen. important in secretory and peristaltic reflexes and signals to vagus nerve to signal brain in generation of nausea

• enterochrommafin like cells: secrete histamine. secreted locally (paracrine secretion) and stimulates parietal cells to secrete HCl

Hormones of the GI tract

Secretin

Cholecystokinin (CCK)

secretin

secreted by duodenum. Reduces acid secretion and stimulates bicarbonate buffer production by pancreas

CCK

secreted by duodenum. Causes release of digestive enzymes from pancreas and gallbladder for breakdown of protein and fat

Nervous System GI tract

Parasympathetic (Vagus)

Enterogastric reflex

Parasympathetic (Vagus)

can aid in increased of acid production in stomach

Enterogastric reflex

stimulated by entrance of food into duodenum. Help to shut down acid production in the stomach (Sympathetic nerves, CCK, secretion)

Control of Stomach

1. cephalic phase

2. gastric phase

3. intestinal phase

Cephalic phase

1st phases where gastric secretion begins by the act of seeing, smelling, or thinking about food

hypothalamus → medulla → vagus nerve (CN X) → gastric activity

Gastric phase

• acid secretion due to presence of food in stomach which causes stomach to stretch. also due to presence of amino acids in stomach

• ph drop occurs due to presence of food

• vagus nerve and myenteric plexus also stimulated

• gastrin is released : stimulates parietal cells to secrete HCl

  • increase in secretion of gastric juice and motility

• Ach, histamine, and gastrin all work together to increase gastrin secretions

Intestinal phase

• acidic chyme enters duodenum, pH less than 2

• gastric secretions inhibited

• low pH carries info via vagus nerve to medulla → reduce gastric secretions

• secretin and CCK produced by duodenum decrease gastric secretions

What secrets HCl

• Parietal cells - has receptors for

  • Gastrin: From G cells (hormonal)

  • acetylcholine: from nerve endings

  • histamine: paracrine secretion from neighboring cells

Accessory Organs

• liver

• gall bladder

  • pancreas

Liver anatomy

• 4 lobes

• Hepatic Portal Vein

• Hepatic Portal Artery

• Inferior vena cava

4 Lobes of the Liver

• left, right, caudate, quadrate

• falciform ligament separates right and left lobes

Hepatic Portal Vein

• goes from intestine to liver

• blood rich in nutrients

• no oxygen

Hepatic Portal Artery

• oxygenated blood from heart

• no nutrients

Inferior Vena Cava

Deoxygenated blood back to heart

Hepatocytes

form a chain

sinusoids

space between chains of hepatocytes

lined by simple squamous

kupffer cells

macrophages

engulf debris and damaged RBC

Hepatic Triads

branch of hepatic portal vein

branch of hepatic portal artery

branch of bile duct

blood pathway

Hepativ portal vein and artery → sinusoids → central vein → hepatic veins → inferior vena cava

Bile Pathway

Hepatocytes → Bile canalculi → bile duct → Lt/Rt Hepatic Duct → Common bile duct → duodenum

Functions of liver

• Blood coming from intestines is filtered here

• Removal of glucose, amino acids, iron, vitamins and other nutrients for metabolism or storage.

• Removal and degradation of toxins, bile pigments, and drugs.
  Secretes albumin, lipoproteins (fat transportation), clotting factors

• Between meals → Stored glycogen → Glucose → Circulation for use by cells of the body.

• globin used to make amino acids

• bile stuff

Function and Control of Gallbladder

• store and concentrate bile

  • water and ions absorbed

  • bile salts become more concentrated

• bile: bile salts, cholesterol, bile pigments (bilirubin), ions, water

• CCK, released after a meal, causes gallbladder to contract which releases bile

Gallstones

excess cholesterol

crystallization

obstruction of bile duct

Pancreas

exocrine pancreas

endocrine pancreas

Exocrine Pancreas

• serous acinus

• digestive enzymes

• 99% exocrine

Endocrine pancreas

Islets of Langerhans

• insulin and glucagon

• secrete the hormone insulin into bloodstream

  • 1% endocrine

what does exocrine pancreases make?

Digestive enzymes

bicarbonate

Digestive enzymes (Pancreas)

CCK stimulates trypsinogen, chymotrypsinogen, procarboxypeptidases

amylase → carbs

lipase → fats

Ribonuclease and dexoyriribonuclease → DNA and RNA

Bicarbonate

• neutralize acids in duodenum

• stimulated by secretin

Jaundice

Liver disfunction

• excess bilirubin in blood

• portal hypertension

Pancreatic Dysfunction

• cancer

  • block common bile duct

  • bilirubin in blood

  • pale colored stool

  • oily stool