How does Breakdown occur in the 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
The Lining of the Digestive Tract plays a defensive role, protects against?
1) corrosive effect of acids
2) Protects against abrasion
3) pathogens: pH of 1-3; Peyer's patches in ileum
What is included in the Accessory Organs of the digestive tract?
1) Salivary glands
3) Gall bladder
Four Major layers of the GI Tract
1) Internal Mucosa (Innermost layer)
3) Muscularis Externa
WHat goes on within the Internal Mucosa?
- has MUCOUS EPITHELIUM: Goblet cells
- has Lamina propria
- Muscularis mucosa
What occurs within the submucosa?
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
Whats important of the Muscular Externa?
2 layers of muscle; myenteric plexus lies between (has parasympathetic and symp response)
- (inner layer- circular smooth muscle)
- (outer layer - longitudinal muscle)
Intramural Plexus? What consists of it? WHy is it important?
The intramural Plexus consists of the Submucosal and Myenteric PLexus.
Important in the control of movement and secretion!!!
Serosa? which of the 4 layers? IMportance??
Serosa= Visceral Peritoneum= ADVENTITIA
formed of serous membrane and loose connective tissue
Importance of Mesenteries in the stomach
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
Importance of Smooth Muscle in the Digestive Tract
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
what does plasticity of smooth muscle mean??
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
Where are the pacemaker cells located? What are they?
Located within the muscularis mucosa and the muscularis externa
Pacemaker cells "pacesetter cells" depolarize spontaneously
moves the food from the oral cavity to the esophagus
Mixing contractions that occur in the small intestine
Functions of Secretions?
Secretions serve to lubricate, liquefy, and digest the food.
- 1) Mucus- which is secreted through the food tube
- 2) WATER
- 3) enzymes
Propulsion vs. Perstalsis
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
Methods of Peristalsis
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)
What is common stimulus for GI tract?
Stretch before a sphincter---------> Relaxation up ahead
Stretch after a sphincter--------------> Contract the sphincter behind it
Where does Segmentation occur? WHat is Segmentation?
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
Function of Secretions within the GI Tract?
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
Digestion: What happens?
- 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
Types of Absorptions?
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
How are the nutrients transported?
1) Circulatory system: amino acids and monosaccharides
2) Lacteals: fatty acids and lipids
Oral Cavity: Functions and what it consists of...
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
ORAL CAVITY, LABIA, PALATE, VESTIBULE, UVULA, TONGUE, LINGUAL FRENULUM
NAME THE 3 PAIRS OF SALIVARY GLANDS, AND WHERE LOCATED?
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
3 Phases of Deglutition/Swallowing
: 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
- 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
What is the Swallowing Center and what cranial nerves are involved with swallowing?
Medulla is the swallowing center
Name the 2 muscles of the Pharynx, and explain the function of the Pharynx.
The common passageway that leads to the digestive tract, via the esophagus, and to the lungs, via the trachea
- Pharyngeal constrictors- bolus movement
: elevate the larynx
Purpose of the Esophagus?
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!
Functions of the Stomach?
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)
Gastric Glands (4 Cell Types)
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)
Function of Parietal Cells
Parietal cells- secretes HCL and intrinsic factors (B12 absorption)
facilitates absorption of B12 in the gut wall
B12 is necessary for DNA synthesis and RBC formation
Function of Chief Cells
Function of Chief cells is to secrete pepsinogen- an inactive form of pepsin, which acts on proteins
Function of Endocrine Cells
******* GASTRIN STIMULATES PARIETAL AND CHIEF CELLS
******* GASTRIN INCREASES SECRETIONS AND MOTILITY
******* GASTRIN ALWAYS PROMOTES DIGESTION
Function of Mucous cells
Function of HCL in the stomach
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
Phases of Gastric Secretion (3)
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
What is the Pyloric Pump?
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
What promotes motility and relaxes the pyloric sphincter?
STRETCH, CNS reflexes, Gastrin
Functions of the Small Intestine AND 3 subdivisions?
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
Specialization of Duodenum?
1st part of Small Intestine
receives the acidic chyme from the stomach
has abundant muscle glands
has short villi
Specialization of Jejunum?
MOST ABSORPTION OCCURS HERE
Plicae and Villi are prominent over the first half and decrease as you approach the colon
What is the specialization of the Ileum?
PEYER's PATCHES- clusters of lymph nodes found here
Most abundant as you approach the colon
WHat are Plicae and where are they located?
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
Function of Lacteals?
Fatty acids which do not cross capillary walls are absorbed into the lacteals
WHere does mitosis of the villi occur ?
In the INTESTINAL CRYPTS/INTESTINAL GLANDS ( ALONG THE GASTRIC PITS)
UPON EXFOLIATION, ENTEROKINASE IS RELEASED
WHAT IS ENTEROKINASE?
ENTEROKINASE - Is an intracellular enzyme released during exfoliation of mitosis
ENTEROKINASE activates proenzymes of the pancreas- these then dump into the duodenum
Intestinal Sections... What Regulates??
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
Important Hormones to Know; Intestinal Hormones released by Small Intestine
(4 Hormones of the Intestines: G.S.C.G)
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
THE LIVER AND ITS FUNCTIONS
(3 MAIN FUNCTIONS)
1) METABOLIC REGULATION
2) HEMATOLOGICAL REGULATION
3) BILE PRODUCTION
METABOLIC REGULATION IN THE LIVER
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)
HEMATOLOGICAL REGULATION OF LIVER
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
BILE PRODUCTION WITHIN THE LIVER
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%
Circulation through the Liver
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.
WHat is a TRIAD?
At the corners of 6 lobules.
- 1) Hepatic Portal Vein
- 2) Bile Duct
- 3) Artery
HOW DOES THE FLOW OF BILE OCCUR?
Bile flows to the hepatic duct----> Cystic Duct--------> Gall bladder to be stored
Bile flows in the opposite direction from the flow of blood
Function of the Gall Bladder?
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
Functions of Pancreas... What Are the 2 Components?
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
Exocrine Functions of Pancreas?
Acini cells (digestive enzymes)
Endocrine Functions of Pancreas?
Contain Islet cells:
1) Alpha cells- Secrete Glucagon
2) Beta Cells- Secrete Insulin
Enzymes of The Pancreas
: act on lipids
: Act on Carbs
: Act on proteins
- a) Protinases- breakdown proteins to peptide fragments
- b) Peptidases: Act on peptides--> amino acide, dipeptides, tripeptides
: Act on nucleic acids (DNA, RNA)
How do the Pancreatic Secretions Occur???
Pancreatic Secretions occur in response to hormones (CKG and Secretin) released from the duodenum
How is Secretin Released and what causes it?
Secretin is released from the arrival of acidic chyme into the duodenum and releases HCO3- and buffers from the pancreas
What does CKG release? WHat Causes CKG?
CKG causes gallbladder to release bile and release pancreatic enzymes, inhibitory to stomach
1) Amylase is enzyme released in mouth
2) Pancreatic Lipase
- 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
FUNCTIONS OF THE COLON
1) COMPACT FECES
2) ABSORPTION OF VITAMINS
3) STORES FECES PRIOR TO DEFECATION
FEATURES OF THE COLON?
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
Both the stomach and colon are lined with what??
HOWEVER---> The COLON ALSO HAS CRYPTS...
These crypts have MANY goblet cells (Mucus)
Crypts with goblet cells are known as INtestinal Glands
THE MASS MOVEMENT!!!
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!!!!
THE RECTUM; 2 Sphincters
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)
Digestion of Carbohydrates
Begins in mouth with salivary amylase.
Digestion is completed in the small intestine
Glucose uptake is facilitated by insulin
Digestion of Proteins
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
WHAT ARE THE TWO BEST KNOWN LIPID TRANSPORTERS IN THE BLOOD?
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
UP-Regulation vs DOWN-Regulation with cholesterol
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
Vitamins in Digestion
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
IONS in Digestion
Na, K, Mg, and PO are ACTIVELY TRANSPORTED into the small intestine
CL can move passively or if alone may be actively transported