digestive system

Five Stages of Digestion

1. Ingestion - Intake of food

2. Digestion - Mechanical and Chemical Breakdown

3. Absorption - Uptake of nutrients

4. Compaction - Absorbing water, consolidating feces

5. Defecation - Elimination of feces

Define Mechanical Digestion?

The PHYSICAL break down of food into smaller particles (teeth)

Define Chemical Digestion?

A series of hydrolysis reactions that break down macromolecules into MONOMERS (into stuff we can use)

List the USABLE forms of these Nutrients:
- Polysaccharides
- Proteins
- Fats
- Nucleic Acids

Polysaccharides --\> MONOSACCHARIDES
Proteins --\> AMINO ACIDS
Fats --\> MONOGLYCERIDES AND FATTY ACIDS
Nucleic Acid --\> NUCLEOTIDES

What does the Digestive tract consist of?

Mouth, pharynx, esophagus, stomach, small and large intestine

What is the GI tract?

Gastrointestinal Tract (includes stomach and intestines)

Accessory Organs of the Digestive System?

Teeth, tongue, salivary glands, liver, gallbladder, pancreas

What is the "basic structural plan" most of the digestive tract follows?

- Mucosa (Epithelium)

- Submucosa

- Muscularis Externa (Inner circular, Outer longitudinal)

- Serosa (Mesothelium)

Nervous network in esophagus, stomach, and intestines that regulates digestive tract motility, secretion, and blood flow?

Enteric Nervous System

What are mesenteries? What is their function?

- Large folds of peritoneum that are double layered and fused

- Bind organs to one another and to the walls of abdominal cavity
- Provide highway for blood vessels, lymphatic vessels, and nerves to get to organs

Parietal Peritoneum is...?

A serous membrane that lines wall of abdominal cavity

The Parietal Peritoneum forms two mesenteries, what are they?

1. Dorsal Mesentery
- Two-layered membrane extending to digestive tract

2. Anterior (ventral) mesentery
- Two layers of the mesentery come together on far side of organ (doesn't say which), and form another sheet of tissue ^

What are the lesser and greater omentum? Where are they found on the stomach?

They are mesenteries

Lesser omentum is found on the LESSER curvature of the stomach, extends to the liver

Greater omentum found on the greater curvature of the stomach, covers small intestine

What is meant by Intraperitoneal and Retroperitoneal?

Intraperitoneal - When an organ is enclosed by mesenteries on BOTH sides

Retroperitoneal - When an organ is against the posterior body wall, covered by peritoneum on ANTERIOR side only

The mouth is also known as..?

The oral or buccal cavity

Function of teeth is to...?

masticate (chew) food to smaller pieces

Four types of teeth

incisors, canines, premolars, molars

Functions of Saliva

Cleanses mouth
Dissolves food chemicals for taste
Moistens food; compacts into bolus
Begins breakdown of starch with enzyme amylase

List the solutes found in saliva and functions:

Salivary Amylase - Begins STARCH digestion in the mouth

Lingual Lipase - ACTIVATED BY STOMACH ACID, begins digestion of fat after food is swallowed

Mucus - Binds and lubricates mass of food, aids in swallowing

Lysozyme - Enzyme that kills bacteria

Immunoglobulin A (IgA) - Inhibits bacterial growth

Electrolytes - Should know this

Intrinsic salivary glands

Small glands dispersed amid other oral tissues

Consist of:
Lingual glands - In the tongue
Labial glands - Inside of lips
Palatine glands - Roof of mouth
Buccal glands - Inside of cheek

Extrinsic Salivary glands consist of...?

Parotid - Just below the earlobe

Sublingual - Below the tongue

Submandibular - Below the bottom jaw

Each extrinsic salivary gland has....?

DUCTS that allow them to secrete saliva into the mouth

The pharynx has a superficial layer of circular skeletal muscles that form \____________.

Pharyngeal Constrictors (Superior, middle, inferior)

- The inferior pharyngeal constrictor (also known as the upper esophageal constrictor) is contracted when not swallowing, preventing air from entering digestive tract

Explain the importance of the lower esophageal sphincter:

- Food pauses here before it enters stomach

IT ALSO PREVENTS STOMACH CONTENTS FROM REGURGITATING

The stomach performs both \______ and \______ digestion.

Mechanical and Chemical Digestion

Four regions of the Stomach:

1. Cardiac Region
- Small region right where food enters

2. Fundic Region
- Dome-shaped portion superior to where food enters

3. Body Region
- Greatest portion of stomach

4. Pyloric Region
- Narrow pouch at inferior end of stomach

Sections of Pyloric region include?

Antrum (Funnel-like structure)

Pyloric canal

Pylorus - Narrow passage leading to duodenum

Pyloric sphincter - regulates passage of chyme into duodenum

Where does the stomach receive parasympathetic fibers? Sympathetic?

Parasympathetic fibers from Vagus
Sympathetic fibers from Celiac ganglia

Blood in the stomach comes from the \________?

Where does it go?

Blood in the stomach comes from the celiac trunk

Goes to liver through the Hepatic Portal Circulation

What are the wrinkles in an empty stomach called?

Gastric Rugae

What is the difference in the MUSCULARIS EXTERNA in the stomach when compared to other sections of the digestive tract?

THREE layers rather than TWO
- Outer longitudinal
- Middle Circular
- Inner oblique layers

What are gastric pits?

Depression in gastric mucosa
- Glands open at the bottom at each of these pits

Cells in the Gastric Pits of the Stomach:

Mucous Cells - Secrete Mucous

Regenerative (Stem) Cells - Divide rapidly as cells live ~ 3-6 days

PARIETAL CELLS - SECRETE HYDROCHLORIC ACID, INTRINSIC FACTOR, and ghrelin (KNOW THIS)

CHIEF CELLS - SECRETE GASTRIC LIPASE and PEPSINOGEN (KNOW THIS)

ENTEROENDOCRINE CELLS - Secrete hormones and paracrine messengers

Importance of Hydrochloric Acid (HCl)!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

- Hydrochloric Acid ACTIVATES PEPSIN AND LINGUAL LIPASE

- Converts Fe3+ to Fe2+

- NONSPECIFIC RESISTANCE

What are zymogens?

A group of enzymes that are secreted in INACTIVE form

Pepsinogen is a zymogen, how is pepsinogen converted to pepsin (activated)?

HYDROCHLORIC ACID removes amino acids, creating pepsin

Gastric lipase and lingual lipase have a minor role in digesting \____________.

Dietary fats (~15%)

What is the function of Intrinsic Factor?

Essential for the absorption of vitamin B12

Secretion of Intrinsic factor IS THE ONLY INDISPENSIBLE FUNCTION of the stomach

Why is vitamin B12 SO important?

Vitamin B12 is required for the synthesis of HEMOGLOBIN
- Deficiency causes anemia

Gastric Motility

Stomach exhibits a rhythm of peristaltic contractions about every 20 seconds that begin in the fundus & become stronger toward the pyloric region

- 3 mL of chyme enters duodenum at a time

Why is it so important 3 mL of chyme enters duodenum at a single time?

The pH in the duodenum is 7, the pH in the stomach is 2.

The chyme must be BUFFERED to prevent erosion of the duodenum

Three ways the stomach is protected?

Mucous coat - Thick mucus coat

Tight Junctions - Gastric juice CANNOT seep between cells

Epithelial cell replacement - Cells live 3-6 days, constant replication and replacement

Small intestine receives chyme from stomach and secretions from \_______ and \________.

Liver and Pancreas

Function of Liver

Secretes and creates BILE

Anatomy of liver (4 lobes, ligaments)

Four Lobes
- Right, Left, Caudate, Quadrate

Falciform Ligament
- Separates left and right Lobes

What is the porta hepatis?

Irregular opening between quadrate and caudate lobes
- Point of entry for Hepatic portal vein and hepatic artery

What is included in the hepatic triad?

Bile Ductule, Hepatic Artery, Hepatic Portal vein

Functions of Hepatocytes?

After a meal, they absorb:
- Glucose, amino acids, iron, vitamins

Break down stored glycogen and release glucose into blood

Remove and degrade:
- Hormones, toxins, bile pigments

Secrete into blood:
- Albumin, lipoproteins, clotting factors, angiotensinogen

What are bile canaliculi?

small ducts between hepatocytes that collect bile produced by the hepatocytes

Follow the path of bile beginning at the hepatocytes:

1. Hepatocytes
2. Bile Canaliculi
3. Ductule of the Hepatic triad
4. Right or Left Hepatic Duct
5. Common Hepatic Duct (Convergence of two previous)
6. Bile Duct (Convergence of Common Hepatic duct and Cystic Duct)

What is the expanded chamber in the duodenum where the contents of the Bile Duct and Pancreatic Duct secrete?

Hepatopancreatic ampulla
- Contains Hepatopancreatic sphincter, regulating passage of bile and pancreatic juice

What is the gallbladder? Function?

Sac located on the underside of liver

FUNCTION IS TO STORE AND CONCENTRATE BILE, NOT MAKE IT!!

What is bilirubin?

Principal pigment derived from the decomposition of hemoglobin (yellow-ish)

What do the bacteria in the large intestine convert bilirubin to?

Urobilinogen (responsible for brown color of feces)

What are bile acids (bile salts)? Function?

Steroids synthesized from CHOLESTEROL

These are what 'emulsify' the fatty acids and break them down.

What is it called when 80% of bile is reabsorbed?

Enterohepatic circulation
- Cholesterol and bile acids are used over and over, 2-3x during digestion

Two Glands of the Pancreas

Endocrine - Secrete insulin and glucagon INTO THE BLOODSTREAM

Exocrine - Secretes pancreatic juice INTO ANOTHER DUCT (small intestine)

Pancreatic Ducts secrete bicarbonate, what is the function?

Buffers HCl that comes from stomach (raises pH)

What are the 3 pancreatic zymogens?

Trypsinogen
Chymotrypsinogen
Procarboxypeptidase

What converts each of the pancreatic zymogens? What are they converted to?

Trypsinogen converted to TRYPSIN by ENTEROKINASE

Chymotrypsinogen converted to CHYMOTRYPSIN by TRYPSIN

Procarboxypeptidase converted to CARBOXYPEPTIDASE by TRYPSIN

Functions of pancreatic amylase and lipase?

Pancreatic amylase - Digests starch

Pancreatic lipase - Digest fat

Where does all absorption occur??

Small intestine

3 regions of the small intestine?

Duodenum
Jejunum
Ileum
(NOTE, ileocecal valve - a sphincter - marks the junction with the large intestine)

Duodenum (actions and anatomy)

- Ends at a sharp bend called duodenojejunal flexure
- (mostly) Retroperitoneal
- Bile acids physically break up fats
- PEPSIN IS INACTIVATED

Jejunum (actions and anatomy)

MOST DIGESTION AND ABSOPRTION OCCURS HERE

Ileum (actions and anatomy)

Peyer patches - lymphatic nodules (visible to naked eye)

Ileocecal junction - end of small intestine

Ileocecal valve - Sphincter formed, protrudes into cecum

Surface Area (Small intestine)

Circular folds - Increase SA 2-3x
Villi - Increase SA by 10x
Microvilli - Increase SA 20x

Villi contain what two types of cells?

Absorptive cells and Goblet cells (mucus)

What three vessels run through each villi?

Arteriole, Venule, and LACTEAL

Microvilli form a \_________ and contain \__________.

Brush border

Contains brush border enzymes

Define Contact Digestion:

Chyme must come in contact with the brush border for digestion to occur

Three functions of contractions in the small intestine:

1. Mix chyme with intestinal juice, bile, and pancreatic juice
2. Churn chyme and bring it into contact
3. Move contents to large intestine

Segmentation is...?

Alternating contraction and relaxation of adjacent regions of the small intestine (more or less random contractions)

Peristalsis....?

Involuntary waves of muscle contraction that keep food moving along in one direction through the digestive system.

3 monosaccharides are

1. Glucose
2. Fructose
3. Galactose

3 disaccharides (What two monosaccharides make each?)

Maltose \= Glucose + Glucose
Sucrose \= Glucose + Fructose
Lactose \= Glucose + Galactose

What enzymes break down each disaccharide?

Maltose broken down by MALTASE
Sucrose broken down by SUCRASE
Lactose broken down by LACTASE

Polysaccharides are broken down (mostly) by...?
What are ALL polysaccharides made from?

All made from GLUCOSE chains

Broken down (mostly) by amylase

What form must all carbohydrates be in for reabsorption?

MONOSACCHARIDES

Group of enzymes that digest proteins...?

Proteases or peptidases (Peptide bonds are present)

Proteins MUST be broken down into SINGLE amino acids

What form must lipids be in for absorption? What digests them?

MONOGLYCERIDES

Lipase is the enzyme that digests them

Starch (Path of digestion)

- First digested to oligosaccharides
- Oligosaccharides --\> Maltose (Disaccharide)
- Maltose --\> Glucose (2 of them)

What does salivary amylase break down starches to in the mouth?

Starch \-------\> OLIGOSACCHARIDES

Pancreatic amylase converts \_________ to \__________ when reaching the small intestine?

Oligosaccharides to MALTOSE

What three sources do amino acids come from?

Dietary proteins, Digestive enzymes digested by each other, Sloughed epithelial cells digested by enzymes

Endogenous Amino acids come from....

Digestive enzymes digested by each other AND sloughed epithelial cells digested by enzymes

- 30g a day

Exogenous amino acids come from....

From our diet

- 44-60g a day

Protein digestion pathway

- Begins in stomach
- Pepsin hydrolyzes bonds (tyrosine and phenylalenine)

- Continues into small intestine
- Pepsin BECOMES INACTIVATED
- TRYPSIN AND CHYMOTRYPSIN take over
- Polypeptides --\> smaller peptides

Function of carboxypeptidase...?

Removes amino acids from the CARBOXYL group end of the protein chain

Function of aminopeptidase...?

Removes amino acids from the NH2 (AMINO) group
- THIS IS A BRUSH BORDER ENZYME

Function of dipeptidase..?

Splits dipeptides down the middle, releases two amino acids
- THIS IS A BRUSH BORDER ENZYME

Where do the sodium-dependent amino acid cotransporters move amino acids? (Step before entering the blood stream)

INTO THE EPITHELIAL CELLS

Where does facilitated diffusion move the amino acids?

Out of the cell, INTO THE BLOOD

Lingual lipase and gastric lipase break down...?

Fats

Emulsification droplets are broken down further by...

Bile, lecithin, and agitation
- exposes more fat to enzymes

Product of lipase action is ?

2 FREE FATTY ACIDS
1 MONOGLYCERIDE (Absorptive)

Within the intestinal cells, where do free fatty acids and monoglycerides move?

Into the smooth endoplasmic reticulum, where they are resynthesized into triglycerides

Triglycerides are coated with \_______ and \______, and form \_______?

Coated with PHOSPHOLIPIDS and PROTEINS

FORMS CHYLOMICRONS

nucleic acid digestion and absorption (not super important for exam)

-Nucleic acids are broken down into nucleotides
-Brush border enzymes digest nucleotides into sugars, phosphates, and nitrogenous bases
•Absorbed by active transport

Which vitamins are fat soluble?

Vitamins A, D, E, K (ADEK \= a deck)