Digestive System

• Ingestion put food in
• Mastication Chewing
• Propulsion & Mixing peristalsis and segmentation
• Digestion enzymatic breakdown of food- secretion of enzymes, acids, buffers
• Absorption of nutrients, vitamins, and electrolytes across the gut wall
• Compaction of waste material into feces
• Elimination excretion of solid waste

1) corrosive effect of acids

2) Protects against abrasion

3) pathogens: pH of 1-3; Peyer's patches in ileum

1) Salivary glands

2) Liver

3) Gall bladder

4) Pancreas

1) Internal Mucosa (Innermost layer)

2) Submucosa

3) Muscularis Externa

4) Serosa

innermost layer

• has MUCOUS EPITHELIUM: Goblet cells
• has Lamina propria
• Muscularis mucosa

made of thick connective tissue

contains nerves, blood vessels, small glands

nerves of this layer form the submucosal plexus- A parasympathetic ganglionic plexus; some sympathetic input too

exocrine glands that secrete buffers and enzymes

2 layers of muscle; myenteric plexus lies between (has parasympathetic and symp response)

• (inner layer- circular smooth muscle)
• (outer layer - longitudinal muscle)

The intramural Plexus consists of the Submucosal and Myenteric PLexus.

Important in the control of movement and secretion!!!

Serosa= Visceral Peritoneum= ADVENTITIA

formed of serous membrane and loose connective tissue

Mesenteries is a double layered membrane firmed by the visceral and parietal peritoneum.

Mesenteries provide a route for blood vessels, nerves, and lymphatics.

7 liters a day of serous fluid is secreted by peritoneal lining to lubricate the peritoneum

Found throughout the GI tract

IMportant in movement of the food through peristalsis, and mechanical breakdown

shorter than skeletal muscle

single nucleus, centrally within the cell

actin and myosin are present, but the are not arranged in sarcomeres- therefore NO striations

muscle fibers twist like corkscrew

Ca++ triggers the contraction, as in skeletal and cardiac muscle

Most of the Ca++ enters through the extracellular fluid

Smooth muscle can function over a wide range of lengths- this is plasticity.

a "stretched" muscle adapts and can still contract

this stretch is difference from cardiac and skeletal- plasticity is important for digestive organs bc they undergo volume changes

Located within the muscularis mucosa and the muscularis externa

Pacemaker cells "pacesetter cells" depolarize spontaneously

Swallowing

moves the food from the oral cavity to the esophagus

Mixing contractions that occur in the small intestine

Secretions serve to lubricate, liquefy, and digest the food.

Secretions include:

• 1) Mucus- which is secreted through the food tube
• 2) WATER
• 3) enzymes

Propulsion- takes about 24-36 hrs to move the food the entire length of the food tube.

Peristalsis- method of which food is moved through the tube

Peristalsis is stimulated by chemical Ach or physical actions, stretch.

Local peristalsis is stimulated by sensory receptors in wall of GI tract (myenteric reflexes)

1) CIrcular Contractions: pushes the food forward

Stretch receptors trigger relaxation ahead to allow the food to move forward.

2) Longitudinal Muscles- contract and shorten the tube (behind)

STRETCH

Stretch before a sphincter---------> Relaxation up ahead

Stretch after a sphincter--------------> Contract the sphincter behind it

Occurs in the small intestine

Churning of the food assure a thorough mixing of the contents

NO NET DIRECTIONAL MOVEMENT IN SEGMENTATION

accomplished by circular muscles of the muscularis externa

Mucus is secreted and present throughout the entire tract

lubricates the food and helps protect the lining of the tube from abrasion and acidity

large amount of water is added to liquify the food

water moves into the GI tract by osmosis

Enzymes are secreted in the:

Mouth, stomach, and pancreas

• Breakdown into small, absorbable fragments
• -Mechanical: mastication
• -Enzymatic- uses enzymes; occurs mostly in small intestines

• carbohydrates----> monosaccharides
• proteins-------------> amino acids
• lipids------------------> glycerol and fatty acids

1) Simple DIffusion: Movement from [high} to [low}

2) Facilitated Diffusion: Involves a carrier protein; still passive

3) Active Transport: ATP used; moves from [low] to [high]

4) Cotransport: 2 molecules are transported together in the same direction

1) Circulatory system: amino acids and monosaccharides

2) Lacteals: fatty acids and lipids

Functions:

1) analysis of material before swallowing

2) mastication: carried out by teeth, tongue, and parietal surface

3) lubrication: mucus and salivary secretions

4) digestion: of carbs by amylase

CONSISTS OF:

ORAL CAVITY, LABIA, PALATE, VESTIBULE, UVULA, TONGUE, LINGUAL FRENULUM

LOCATED WITHIN THE ORAL CAVITY

1) Parotid- The largest, has Amylase; serous secretions

2) Submandibular: under the mandible; some amylase; serous and mucous secretions

3) Sublingual: Under the tongue; mostly mucus secretions; no enzymes

1) Voluntary: the bolus is pressed against the hard palate----> pharynx

• 2) Pharyngeal : Soft palate is elevated, closing off the nasopharynx (uvula)
• The epiglottis closes and the vocal folds shuts the glottis; the larynx is elevated.

THis phase is unconscious and is stimulated by tactile receptors in the oropharynx

3) Esophageal - journey through esophagus to the entry of the stomach

bolus must be lubricated to travel down the esophagus (peanut butter)

Intramural plexus (submucosal plexus and myenteric plexus) initiates peristalsis

Medulla is the swallowing center

Trigeminal (V)

Glossopharyngeal (IX)

Vagus (X)

Accessory (XII)

The common passageway that leads to the digestive tract, via the esophagus, and to the lungs, via the trachea

• Muscles:
• Pharyngeal constrictors- bolus movement

Laryngeal elevators: elevate the larynx

The food tube that leads from the mouth to the stomach

Passes through the esophageal hiatus (opening) of the diaphragm (hiatal hernia)

1 ft long, 5-8 sec. to pass through

contains smooth and skeletal muscle!

1) BULK STORAGE: Rugae (pleats) allow for distension

2) Mechanical breakdown: CHURNING- TURNS FOOD INTO CHYME- A SOUPY MIXTURE MIXED WITH STOMACH ACIDS AND ENZYMES

3) Limited Digestion: via acid secretions and enzymes (Parietal Cells- release HCL)

4) Produce Intrinsic Factor: binds vitamin B12 to allow for absorption

5) PRODUCES FOOD INTO CHYME- food is turned into a "soupy" mixture once it leaves the stomach

CONTAINS GASTRIC PITS; LINED WITH GOBLET CELLS

MUCUS PROTECTS THE LINING OF THE STOMACH FROM EFFECTS OF THE ACIDS AND ENZYMES

• CONTAINS FOUR TYPES OF GASTRIC GLANDS
• (PARIETAL, CHEIF, ENDOCRINE, MUCOUS)

1) parietal cells- secrete HCL and intrinsic factors

2) chief cells- secrete pepsinogen an inactive form of pepsin

3) endocrine cells- secrete GASTRIN

4) Mucous cells- secrete mucous

parietal cells- secrete HCL and intrinsic factors (Allows absorption for vitamin B12)

Parietal cells- secretes HCL and intrinsic factors (B12 absorption)

INTRINSIC FACTORS:

facilitates absorption of B12 in the gut wall

B12 is necessary for DNA synthesis and RBC formation

Function of Chief cells is to secrete pepsinogen- an inactive form of pepsin, which acts on proteins

SECRETES GASTRIN

******* GASTRIN STIMULATES PARIETAL AND CHIEF CELLS

******* GASTRIN INCREASES SECRETIONS AND MOTILITY

******* GASTRIN ALWAYS PROMOTES DIGESTION

Secretes Mucus!

HCL is released by parietal cells

Histamine is released by endocrine cells- stimulates teh HCL release

Also activates pepsinogen to denature the proteins

Stops the carbohydrate breakdown of amylase which begun in the mouth

1) Cephalic Phase- thoughts, smell, taste stimulates para symp.- leads to an increase in motility and secretions

2) Gastric Phase- Arrival of food in the stomach- causes muscular reflexes- Gastrin is secreted

• 3) Intestinal Phase- arrival of chyme in the duodenum
• **** Stimulate gastric response if pH is above 3
• ***** Chyme inhibits gastric secretions if pH is below 2

Stretch is an IMPORTaNT stimulus in the GI tract

*** The pyloric sphincter usually remains partially closed. Peristaltic contractions force chyme through the sphincter. This contraction is referred to as the pyloric pump

STRETCH, CNS reflexes, Gastrin

MOST OF THE DIGESTION AND ABSORPTION OF NUTRIENTS OCCURS IN THE SMALL INTESTINE.

1) Duodenum- 12in long

2) Jejunum- middle region; lots of villi

3) Ileum- last part; leads to the colon

1st part of Small Intestine

receives the acidic chyme from the stomach

has abundant muscle glands

has short villi

MOST ABSORPTION OCCURS HERE

Plicae and Villi are prominent over the first half and decrease as you approach the colon

PEYER's PATCHES- clusters of lymph nodes found here

Most abundant as you approach the colon

PLicae is located in the inner lining of the small intestine.

Plicae have villi- villi are columnar epithelium and are covered with many microvilli for absorption

Each villus is supplied with blood vessels, nerve endings, and terminal lymphatics called LACTEALS

Fatty acids which do not cross capillary walls are absorbed into the lacteals

In the INTESTINAL CRYPTS/INTESTINAL GLANDS ( ALONG THE GASTRIC PITS)

UPON EXFOLIATION, ENTEROKINASE IS RELEASED

ENTEROKINASE - Is an intracellular enzyme released during exfoliation of mitosis

ENTEROKINASE activates proenzymes of the pancreas- these then dump into the duodenum

1.8L a day

Most of the fluid moves by osmosis out of the mucosa and into the lumen

*** Activity of intestinal glands occurs in response to stretch

Regulated by hormones and ANS

Parasymp increases (vagus nerve X); Symp decreases

1) Gastrin- ALWAYS PROMOTES MOVEMENT AND DIGESTION. Increases enzymatic activity and motility. Release is triggered by Stretch

2) Secretin- Causes RELEASE of HCO from the pancreas; release is triggered by arrival of acidic chyme from stomach into the duodenum

3) CKG- CHOLECYTOKININ - Causes gallbladder to contract and release bile AND release of pancreatic enzymes; inhibitory to stomach

4) GIP- Gastric Inhibitory Peptide; Inhibit Gastric Secretions

1) METABOLIC REGULATION

2) HEMATOLOGICAL REGULATION

3) BILE PRODUCTION

1) Storage of glucose as glycogen

2) Maintain normal amino acid and fatty acid concentrations

3) Interconversion of nutrients

4) Makes and releases Cholesterol

5) Inactivation of TOxins: AMonia is converted to urea

6) Storage of IRON reserves

7) Storage of fat solubles (Vit A, D, E)

1) Synthesis of plasma proteins (albumins)

2) Synthesis of clotting factors

3) Synthesis of angiotensinogen

4) Synthesis of compliment proteins

5) Synthesis of Heparin

6) Phagocytosis of inefficient RBC's by Kupffer cells

7) BLOOD RESERVOIR (1L)

8) Absorption and breakdown of circulating hormones

9) Absorption and inactivation of lipid soluble drugs

1) Bile emulsifies fat

2) Bile contains H20, Ions, Bilirubin, Lipids

3) Bile helps buffer chyme entering the duodenum

4) Bile salts are derived from cholesterol... recycled 90%

RECEIVES BLOOD:

1) Hepatic Artery (1/3)

• 2) Hepatic Portal Vein (2/3)
• All Blood LEAVES Hepatic Portal Vein, then joins inferior Vena Cava

Blood flows slowly through the liver aloowing time for absorption by the hepatocytes.

At the corners of 6 lobules.

consists of:

• 1) Hepatic Portal Vein
• 2) Bile Duct
• 3) Artery

Bile flows to the hepatic duct----> Cystic Duct--------> Gall bladder to be stored

Bile flows in the opposite direction from the flow of blood

Not an Enzyme!!!!!!!!

Stores and modifies Bile

a muscular organ

CKG stimulates release of Bile (relaxes the sphincters and gall bladder contracts)

Always release bile when food enter the duodenum; more is ALWAYS act on Fatty Foods

Emulsifies fat into droplets to increase the surface area for lipases to work

Function is to produce pancreatic juice

• Has Exocrine and Endocrine Functions
• 2 components:

1) Aqueous Component- Mostly bicarbonate ions (H20)

2) Enzymatic Component- Produced by Acinar Cells---> THE DIGESTIVE ENZYMES

Acini cells (digestive enzymes)

Contain Islet cells:

1) Alpha cells- Secrete Glucagon

2) Beta Cells- Secrete Insulin

1) Lipases: act on lipids

2) Carbohydrases: Act on Carbs

3) Proteolytic: Act on proteins

• a) Protinases- breakdown proteins to peptide fragments
• b) Peptidases: Act on peptides--> amino acide, dipeptides, tripeptides

4) Nucleases: Act on nucleic acids (DNA, RNA)

Pancreatic Secretions occur in response to hormones (CKG and Secretin) released from the duodenum

Secretin is released from the arrival of acidic chyme into the duodenum and releases HCO3- and buffers from the pancreas

CKG causes gallbladder to release bile and release pancreatic enzymes, inhibitory to stomach

1) Amylase is enzyme released in mouth

2) Pancreatic Lipase

3) Nucleases

• 4) Protinases: 70% Released as proenzymes and are converted to active form after secretion.
• a) Trypsin ( ACTIVATES ALL THE OTHER ENZYMES)
• b) Chymotrypsin
• c) Carboxypeptidase
• d) Elastase

1) COMPACT FECES

2) ABSORPTION OF VITAMINS

3) STORES FECES PRIOR TO DEFECATION

1) HAUSTRAE: POUCHES SEEN EXTERNALLY

2) EPIPLOIC APPENDAGES: FAT FILLED POUCHES

3) CECUM- BLIND ended pouch at the very beginning of the colon "cul de sac"

4) Taenia coli- 3 longitudinal bands of muscle seen on external structure

5) Appendix- Hangs of the cecum at the juncture of the small intestine and the colon; HAS LYMPHATIC TISSUE

6) Ileocecal valve: At the entry of the colon

Gastric Pits!

HOWEVER---> The COLON ALSO HAS CRYPTS...

These crypts have MANY goblet cells (Mucus)

Crypts with goblet cells are known as INtestinal Glands

Powerful peristaltic movement that occur a few times/day.

THE STIMULUS IS DISTENSION OF THE STOMACH AND TEH DUODENUM.

INTESTINAL NERVE PLEXUSES RELAY THE SIGNAL.

THIS MASS MOVEMENT FORCES CHYME INTO THE RECTUM WHICH PRODUCES THE URGE TO DEFECATE!!!!

Last 5-6 inches

External Sphincter- Skeletal Muscle and under voluntary control

Internal Sphincter- is smooth muscle and under involuntary control

(When feces passes through the internal sphincter, the external sphincter automatically shuts and voluntary action is required to open the external sphincter to pooh)

Begins in mouth with salivary amylase.

Digestion is completed in the small intestine

Glucose uptake is facilitated by insulin

Begins in stomach with pepsin

Most of the digestion of proteins occurs in the small intestine- many proteases and peptidases

Using Cotransport mechanisms, amino acids, dipeptides, and tripeptides are the final end product that are absorbed across the gut.

once inside the gut wall, tripeptides and dipeptides are broken down into amino acids

Lipids are emulsified by bile salts in the small intestine to form micelles.

Lipases breakdown the lipids to free fatty acids and glycerol.

When the micelles contact cell membrane, fatty acids and glycerol, triglcerides diffuse into the epithelial cells

once inside, triglycerides gain a protein coat and are now called chylomicrons

then the chylomicrons leave the epithelial cells and enter the lacteals in the villi (too large to enter the capillaries in the villi)

LIPIDS ARE NOT WATER SOLUBLE- THUS ARE TRANSPORTED IN THE BLOOD WITH A PROTEIN COAT

LDL "bad" low density lipoproteins; transports cholesterol to the cells. If the cells are full. the cholesterol stays in circulation and can produce plaques

HDL "good" high density lipoproteins; takes cholesterol out of blood

GENETICALLY PREDETERMINED

UP-Regulation: when cholesterol is in short supply, the cell up regulates its cholesterol receptors ( takes out of the blood)

DOWN-Regulation- when a cell has an abundance of cholesterol, the cell DOWN-Regulates its cholesterol receptors

DIET PLAYS AN IMPORTANT ROLE IN REGULATION OF THE NUMBER OF CHOLESTEROL RECEPTORS A CELL EXPRESSES

All water soluble vitamins (EXCEPT B12) Are easily absorbed

B12 REQUIRES INTRINSIC FACTOR -FROM parietal cells

Fat soluble vitamins are absorbed in micelles

Colonic Bacteria are an important source of some vitamins, especially vitamin K, which is important with blood clotting

Na, K, Mg, and PO are ACTIVELY TRANSPORTED into the small intestine

CL can move passively or if alone may be actively transported