produces several hydrolytic enzymes and an alkaline solution rich in bicarbonate that buffers the acidity of the chyme from the stomach; 

a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help in the further breakdown of the carbohydrates, protein, and fat in the chyme.

increases surface area of villi;

Large circular folds in the lining bear fingerlike projections called villi, and each epithelial cell of a villus has many microscopic appendages called microvilli that are exposed to the intestinal lumen.

The microvilli are the basis of the term “brush border” for the intestinal epithelium.

digestive tubes extending between a mouth and anus called complete digestive tracts; the system of organs that takes in food, digests it to extract energy and nutrients, and expels the remaining waste 

Bile is stored in the gallbladder until needed; It contains bile salts that act as detergents that aid in the digestion and absorption of fats; a yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes

secreted in response to the presence of amino acids (proteins) or fatty acids (lipids), causes the gallbladder to contract and release bile into the small intestine and triggers the release of pancreatic enzymes;

targets gall bladder for release of bile (enzymes on bile work w/food)

accessory gland that delivers saliva through ducts to the oral cavity in the presence of food;

under the control of the autonomic nervous system, which controls both the volume and type of saliva secreted; release water and glycoproteins called mucins

accessory gland with the metabolic versatility to interconvert various organic molecules, has first access to amino acids and sugars absorbed after a meal is digested;

modifies and regulates this varied mix before releasing materials back into the bloodstream; produces bile; organ with a wide range of functions: detoxification, protein synthesis, and production of biochemicals necessary for digestion

At the opening from the stomach to the small intestine, which helps regulate the passage of chyme into the intestine- A squirt at a time, it takes about 2 to 6 hours after a meal for the stomach to empty

contains a slippery glycoprotein called mucin, which protects the soft lining of the mouth from abrasion and lubricates the food for easier swallowing; also contains buffers that help prevent tooth decay by neutralizing acid in the mouth; Antibacterial agents in saliva kill many bacteria that enter the mouth with food; composed mostly of water, but also includes electrolytes, mucus, antibacterial compounds, and various enzymes

act as detergents that aid in the digestion and absorption of fats; 

invaginations of the epithelium around the villi, lined largely with younger epithelial cells which are involved primarily in secretion; intestinal gland-act as place where intestinal juice comes from-mucous produced (goblet cells)

o site of cell division→mitosis

small globules of fats mixed with cholesterol and coated with special proteins; transported by exocytosis out of epithelial cells and into lacteals

Stomach:

-located in the upper abdominal cavity, just below the diaphragm

-has 4 layers

-stores food and performs preliminary digestion:

1. accordion-like folds and a very elastic wall-->can stretch to accommodate about 2 L of food and fluid

Mechanical Digestion:

-churning-grinding chyme in stomach-about every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles

-As a result of mixing and enzyme action: recently swallowed meal-->a nutrient-rich broth known as acid chyme

Chemical Digestion:

-gastrin-hormone stimulates formation of HCl, gastic acid, pepsinogen

-secretes a digestive fluid called gastric juice and mixes this secretion with the food by the churning action of the smooth muscles in the stomach wall

-Gastric juice (all enzymes-pepsin + HCl + water) secreted by the epithelium lining numerous deep pits in the stomach wall: contains a high concentration of hydrochloric acid

-->pH=2

-pepsin-enzyme begins hydrolysis of proteins:

1. works well in strongly acidic environments

2. breaks peptide bonds adjacent to specific amino acids

-->producing smaller polypeptides

3. secreted in inactive form called pepsinogen by specialized chief cells in gastric pits

4. positive-feedback system-activated pepsin can activate more pepsinogen molecules

Stomach protects against self-digestion:

1. Parietal cells, also in the pits, secrete hydrochloric acid- converts pepsinogen to the active pepsin only when both reach the lumen of the stomach (minimizing self-digestion)

2. stomach’s second line of defense against self-digestion is a coating of mucus, secreted by epithelial cells, that protects the stomach lining

-stomach is closed off at either end:

cardiac orifice/sphincter: opening from the esophagus to the stomach-dilates only when a bolus driven by peristalsis arrives

-backflow of acid chyme from the stomach into the lower esophagus causes heartburn

-pyloris sphincter: opening from the stomach to the small intestine-helps regulate the passage of chyme into the intestine

-A squirt at a time-takes about 2 to 6 hours after a meal for the stomach to empty

Small Intestine:

-major organ of digestion and absorption

Mechanical Digestion:

1.peristalsis

2. segmentation-kneading of bread

(increases SA to increase contact w/enzymes)

Chemical Digestion:

1. acididty of chyme-->secretin-->bicarbonate-->pH becomes more basic, stimulates bile to be created by liver

2. protein and chyme stimulate CCK; presence of fatty acid

--> CCK; CCK-rels.:

1. pancreatic enzymes

2. triggers gall bladder to rel. bile

3. carbs, polysac (starch), disac (sucrose, maltose, lactose), mono (fructose)-->pancreatic enzymes rel.

4. enterogastrone-hormone that blocks gastrin-->decrease amount of acid, slows stomach churning-->stay full

*absorbed by microvilli:

1. go to lymphatic system

2. circulatory system

3. peristalsis continues in the jejunum, ileum

Sections of SI:

Duodenum: first 25 cm

1. acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gall bladder, and gland cells of the intestinal wall

2. brush border of the epithelial lining of the duodenum produces several digestive enzymes:

a. secreted into the lumen

b. bound to the surface of the epithelial cells

3. Most digestion completed here

Jejunum: absorption of nutrients and water

1. To enter the body, nutrients in the lumen must pass the lining of the digestive tract

2. enormous SA adaptation that greatly increases the rate of nutrient absorption

3. Nutrients absorbed across the intestinal epithelium and then across the unicellular epithelium of capillaries or lacteals

-two single layers of epithelial cells separate nutrients in the lumen of the intestine from the bloodstream

4. transport of nutrients across the epithelial cells can be passive, as molecules move down their concentration gradients from the lumen of the intestine into the epithelial cells, and then into capillaries (i.e. Fructose, a simple sugar)

a. Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream

b. Glycerol and fatty acids absorbed by epithelial cells are recombined into fats

i. fats are mixed with cholesterol and coated with special proteins to form small globules called chylomicrons- transported by exocytosis out of epithelial cells and into lacteals-->converge into the larger vessels of the lymphatic system-->draining into large veins that return blood to the heart

*capillaries and veins that drain nutrients away from the villi converge into the hepatic portal vein, which leads directly to the liver

Large Intestine:

-Reclaiming water is a major function

Sections:

1. cecum- has a fingerlike extension, the appendix, which makes a minor contribution to body defense

2. colon- recover water that has entered the alimentary canal as the solvent to various digestive juices

3. rectum-where feces are stored until they can be eliminated

-connected to the small intestine at a T-shaped junction where a sphincter controls the movement of materials

-More than 90% of the water is reabsorbed, most in the small intestine, the rest in the colon.

-Digestive wastes, the feces, become more solid as they are moved along the colon by peristalsis.

-Movement in the colon is sluggish, requiring 12 to 24 hours for material to travel the length of the organ.

-If the lining of the colon is irritated by a bacterial infection, less water than usual is reabsorbed, resulting in diarrhea.

-If insufficient water is absorbed because peristalsis moves the feces too slowly, the result is constipation.

-Living in the large intestine is a rich flora of mostly harmless bacteria (E.coli)

-As a by-product of their metabolism, many colon bacteria generate gases, including methane and hydrogen sulfide.

-Some bacteria produce vitamins, including biotin, folic acid, vitamin K, and several B vitamins, which supplement our dietary intake of vitamins.

-Feces contain masses of bacteria and undigested materials including cellulose.

-Although cellulose fibers have no caloric value to humans, their presence in the diet helps move food along the digestive tract.

-Between the rectum and the anus are two sphincters, one involuntary and one voluntary.

Once or more each day, strong contractions of the colon create an urge to defecate.

Produces: 1.several hydrolytic enzymes

a. Pancreatic enzymes: amylase, trypsin, lipase include protein-digesting enzymes (proteases) that are secreted into the duodenum in inactive form

i. pancreatic proteases activated in the extracellular space within the duodenum

-signaled by secretin hormone

2. alkaline solution rich in bicarbonate-buffers the acidity of the chyme from the stomach

1. production of bile

a. stored in the gallbladder until needed

b. bile salts act as detergents that aid in the digestion and absorption of fats

c. bile pigments-by-products of red blood cell destruction in the liver

i. eliminated from the body with the feces

2. metabolic versatility to interconvert various organic molecules

a. first access to amino acids and sugars absorbed after a meal is digested

b. modifies and regulates this varied mix before releasing materials back into the bloodstream

i. helps regulate glucose levels in blood, ensuring that blood exiting the liver usually has a glucose concentration very close to 0.1%, regardless of carbohydrate content of the meal

*From the liver, blood travels to the heart, which pumps the blood and nutrients to all parts of the body.

*detoxify, regulates, produces proteins, and emulsifies fats (bile)

Jejunum (Small intestine)-depending on protein, fat or carbs signals diff enzymes

fats absorbed by microvilli-->hepatic portal vein-->liver

-rhythmic waves of contraction by smooth muscles in the walls of the canal, pushes food along

-After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested

1. the larynx moves downward and the trachea is opened, and peristalsis moves the bolus down the esophagus to the stomach

a. muscles at the very top of the esophagus are striated and, therefore, under voluntary control

b. Involuntary waves of contraction by smooth muscles in the rest of the esophagus then take over

2. Enzymatic digestion is completed as peristalsis moves the mixture of chyme and digestive juices along the small intestine.

3. Digestive wastes, the feces, become more solid as they are moved along the colon by peristalsis.

*Esophagus peristalsis to move forward

*SI peristalsis to mix chyme

-salivation may occur in anticipation because of learned associations between eating and the time of day, cooking odors, or other stimuli

-presence of food in the oral cavity-->a nervous reflex causes the salivary glands to deliver saliva through ducts to the oral cavity

-Think/see food (ANS symphatic/parasymphatic)--> brain stimulation increases blood flow to salivary glands

Functions of Saliva:

1. contains a slippery glycoprotein called mucin:

a. protects the soft lining of the mouth from abrasion

b. lubricates the food for easier swallowing

c. binds food to create bolus

2. contains buffers-prevent tooth decay by neutralizing acid in the mouth

3. Antibacterial agents in saliva kill many bacteria that enter the mouth with food

4. Evaporative-cooling method

5. contains enzymes (salivary amylase and lipases) to breakdown food: starch and fat

-enzyme that hydrolyzes starch and glycogen into smaller  (alpha bonds) polysaccharides and the disaccharide maltose

-breaks down starch (functions at pH 8) to maltose (2 unit glucose)

-stomach environment not conducive to amylase-->
protect teeth from sticky starch rather than breaking down starch

Overview:

1. chewing and swallowing-->food to pass down the esophagus to the stomach-->spends 2 to 6 hours being partially digested

2. Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours

3. In 12 to 24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus

Detailed Steps:

1. tongue tastes food, manipulates it during chewing, and

helps shape the food into a ball called a bolus

2. during swallowing-tongue pushes a bolus back into the oral cavity and into the pharynx

3. pharynx (throat) is a junction that opens to both the esophagus and the trachea (windpipe).

a. When we swallow, the top of the windpipe moves up so that its opening, the glottis, is blocked by a cartilaginous flap, the epiglottis.

i. ensures that a bolus will be guided into the entrance of the esophagus and not directed down the windpipe

b. When a food bolus reaches the pharynx, the larynx moves upward and the epiglottis tips over the glottis, closing off the trachea.

4. esophageal sphincter relaxes and the bolus enters the esophagus

5. larynx moves downward and the trachea is opened, and peristalsis moves the bolus down the esophagus to the stomach

Functions:

-Initiates food processing:

1. conducts food from the pharynx down to the stomach by peristalsis

*muscles at the very top of the esophagus are striated

-->voluntary control (skeletal muscle)

*middle third is both skeletal and smooth muscle

2. Involuntary waves of contraction by smooth muscles in the rest of the esophagus then take over

Controlled by:

-Most of the time the stomach is closed off at either end

-cardiac sphincter/orifice-opening from the esophagus to the stomach-->dilates only when a bolus driven by peristalsis arrives

1. occasional backflow of acid chyme from the stomach into the lower esophagus causes heartburn

Cardiac sphincter:

-usually stomach closed off at either end-->cardiac orifice

dilates only when a bolus driven by peristalsis arrives

-prevents movement back up into esophagus from the stomach→acid-reflux

Pyloric Sphincter:

-opening from the stomach to the small intestine

1. helps regulate the passage of chyme into the intestine

a. A squirt at a time, it takes about 2 to 6 hours after a meal for the stomach to empty.

Four types of secretory cells:

1. Goblet cells:

a. produce mucus to line surfaces

b. keeps acid away from surface to protect cells (basic)

• Aspirin-hits chem. pathway that triggers production of mucus→produce less mucus-stomach sensitive to acid

2. Parietal cells-produce HCL

3. Chief cells→pepsinogen (keep inactive until right acidity-lumen of stomach)-active/activates itself at low pH

*Cell types 1-3-->outside body (lumen)

4. enteroendocrine cells:

a. entero=w/in another structure; endocrine=producing hormones (inside-directly to blood)

b. gastrin-stimulates gastric juice (stimulating itself→positive feedback mech.)-controlled by acidity-too acidic turns off

c. ghrelin-controls hunger-hormone stimulates feeding (gas pedal)

d. obestatin-stop eating (brake)

• both ghrelin and obestatin made by cutting one protein into two pieces-can’t produce one without producing the other but can use one w/o the other-released separately

Gastric Juice: The stomach also secretes a digestive fluid called gastric juice and mixes this secretion with the food by the churning action of the smooth muscles in the stomach wall. Gastric juice is secreted by the epithelium lining numerous deep pits in the stomach wall.

With a high concentration of hydrochloric acid, the pH of the gastric juice is about 2. This acid disrupts the extracellular matrix that binds cells together. It kills most bacteria that are swallowed with food. Also present in gastric juice is pepsin, an enzyme that begins the hydrolysis of proteins.

Pepsin, which works well in strongly acidic environments, breaks peptide bonds adjacent to specific amino acids, producing smaller polypeptides. Pepsin is secreted in an inactive form called pepsinogen by specialized chief cells in gastric pits.

Hydrochloric acid: Parietal cells, also in the pits, secrete hydrochloric acid that converts pepsinogen to the active pepsin only when both reach the lumen of the stomach, minimizing self-digestion. In a positive-feedback system, activated pepsin can activate more pepsinogen molecules.

Mucus: The stomach’s second line of defense against self-digestion is a coating of mucus, secreted by epithelial cells, that protects the stomach lining. Still, the epithelium is continually eroded, and the epithelium is completely replaced by mitosis every three days. Gastric ulcers, lesions in the stomach lining, are caused by the acid-tolerant bacterium Heliobacter pylori.

Functions:

1. secreted in an inactive form called pepsinogen by specialized chief cells in gastric pits

3. enzyme that begins the hydrolysis of proteins

4. works well in strongly acidic environments-->breaks peptide bonds adjacent to specific amino acids, producing smaller polypeptides

5. positive-feedback system-activated pepsin can activate more pepsinogen molecules

Inactivated enzymes: 

1. Parietal cells-->secrete HCl:pepsinogen-->pepsin (only when both reach the lumen of the stomach) 

2. In a positive-feedback system, activated pepsin

-->activate more pepsinogen mols.

Mucus: secreted by epithelial cells-->coating of mucus--> protects the stomach lining

Regulation of Pepsin:

1. Pepsin secreted in inactive form (pepsinogen) by specialized chief cells in gastric pits

a. Parietal cells, also in the pits, secrete HCl: converts pepsinogen-->active pepsin (only when both reach the lumen of the stomach)

2. positive-feedback system-activated pepsin can activate more pepsinogen mols.

Regulation of Acid Secretion:

1. negative feedback

Roles of Horomones:

Gastrin:

1. stimulates gastric juice (stimulating itself→positive feedback system)

2. positive feedback system-controlled by acidity-->too acidic turns itself off

Enterogastrone:

1. class of hormones-slows stomach down

a. triggered by lipids (hard to digest)-->stops gastrin production-->less acid-->slows down stomach emptying

• peristalsis, churning, movement of chyme all slowed down

Causes:

1. occurs when lining of organs is corroded by the acidic digestive juices secreted by stomach cells

a. Gastric ulcers-lesions in the stomach lining- caused by the acid-tolerant bacterium Heliobacter pylori

i. Ulcers treated w/antibiotics

2. Heliobacter pylori-bacteria impacts goblet cells-->slows down mucus production-->ulcers

Found in duodenum because:

*Most digestion completed while chyme is still in the duodenum:

1. overstimulation of gastrin-->increase stomach acid (denatures protein)-->erosion of mucosal lining

a. more common in the duodenum bc less protected by protective lining

1. Enzymatic digestion completed as peristalsis (then involuntary movement) moves the mixture of chyme and digestive juices along the small intestine.

a. pyloric sphincter-At opening from stomach to small intestine-helps regulate passage of chyme into the intestine i. squirt at a time-takes about 2-6 hrs after a meal for the stomach to empty

b. most digestion completed while chyme still in the duodenum

1. in the duodenum, acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gall bladder, and gland cells of the intestinal wall

2. pancreas produces several hydrolytic enzymes and an alkaline solution rich in bicarbonate that buffers the acidity of the chyme from the stomach

a. Pancreatic enzymes include protein-digesting enzymes (proteases) that are secreted into the duodenum in inactive form:

i. pancreatic proteases activated once they are in the extracellular space within the duodenum

Sequence: 

Roles:

Secretin: stimulates release of bicarbonate

•signal: acidity (homeostatic system)→bicarbonate (pancreas)→acidity goes away, no more secretin (what signals event, fixes with result)

Bile: It contains bile salts that act as detergents that aid in the digestion and absorption of fats.

Bicarbonate: buffers the acidity of chyme from the stomach

Pancreatic enzymes: protein-digesting enzymes (proteases) secreted into the duodenum in inactive form-->activated once they are in the extracellular space within the duodenum

Cholecystokinin (CCK): targets pancreas for release of enzymes; targets gall bladder for release of bile

•signal: lipids and proteins (peptides-smaller proteins)→CCK

Entergogastrone: class of hormones-slows stomach down; triggered by lipids (hard to digest)

•peristalsis, churning, movement of chyme all slowed down

Gall bladder: stores bile made in liver

•zymogens-inactive enzymes

-pancreas-produces trypsinogen-->trypsin: cleaves chymotrypsinogen by attacking arginine and lysine

-->chymotrypsinogen turns procarboxypeptidase into carboxpeptidase (proaminopeptidase?)

-proteases: proteins-->amino acids by proteases

-turn on? sep. production and secretion with trigger event (enzyme on surface of cells of duodenum-can’t move: enterokinase-kinase is enzyme that transfers phosphates-should be called enteropeptidase

1. protease to cut cork out of trypsinogen→trypsin-activates itself and all other enzymes

2. attacks arginine or lysine (specific amino acids; positively charged side chains)→activate other zymogens

•keep enzymes inactive until at right location

-if trypsin activates early→can damage pancreatic duct

Carbohydrates:

1. Chemical digestion-->chemical energy

a. begins in oral cavity

i. Saliva contains salivary amylase-enzyme that hydrolyzes starch and glycogen into smaller polysaccharides and the disaccharide maltose

Proteins: 

1. Proteins-->amino acids

a. pepsin in gastric juice-enzyme begins hydrolysis of proteins

i. works well in strongly acidic environments

ii. breaks peptide bonds adjacent to specific amino acids, producing smaller polypeptides

iii. secreted in inactive form called pepsinogen by specialized chief cells in gastric pits

•pepsinogen in pH 2 refolds, cuts off piece of itself→active pepsin (specific enzyme-only cuts at tyrosine in protein)

•only activates when food in environment

-Parietal cells, also in the pits, secrete HCl: pepsinogen

-->active pepsin (only when both reach the lumen of the stomach)

v. Pancreatic enzymes include protein-digesting enzymes (proteases) secreted into duodenum in inactive form

-->pancreatic proteases activated in the extracellular space within the duodenum

• protease-enzyme breaks down protein (cells, body made of protein)-produced in inactive form→zymogen 

(-ogen=inactive)

Lipids:primary structure: triglyceride-glycerol (3C) backbone w/three fatty acids (saturated or unsaturated)

•mix w/water by adding bile-->add lipases (enzymes)-if block, fat goes to colon where bacteria eats it-cut off two of fatty acids→monoglyceride and two fatty acids→absorbed (break them to get them in)-->reassembled to triglycerides-can’t move in water→packaged w/proteins-surrounds fat, water-soluble on outside: protein-lipid package=chylomicrons-so large can’t sneak into capillaries

-->end up in the lymph-in each villus in SI-lacteal-small lymph vessel-->moves into blood→fatty acids→cells;

*absorbed into structure-too large to get into capillaries-can’t go from SI to blood supply directly→lymph (drainage system-set of vessels takes excess liquid, dumps into subclavian veins-if blocked→swelling-elephantitis)

•lipids→lymph→blood→liver (circuitous route); lipids hard to digest bc don’t like to interact with water→slows movement of chyme into stomach if high fat content

• emulsify so water-soluble enzymes have access (immiscible): bile (pdt. of liver, stored by gal bladder; rel. mixed w/chyme in S.I.)-->increase lipid SA (mech. digestion)

Nucleic Acids:

-Nucleic acids cleaved into nucleotides

-exonucleases (5’ or 3’)-chew in from opp. ends of RNA and DNA

-endonucleases-break long polymers into smaller pieces so more ends to attack

Function:

1. contains bile salts that act as detergents that aid in the digestion and absorption of fats

2. pathway to rid body of bilirubin (green/yellow of bruise)- liver helps remove, puts into bile-if can’t process bilirubin→jaundice (liver failure)

3. acts as an emulsifier-break fat into smaller droplets (enzymes to breakdown fat are water-soluble)

-if gall bladder removed-restrict fat intake bc less bile

Produced: Liver

Stored: Gall bladder

Composition: made up of phospholipids, cholesterol, neutral fats; also contains pigments that are by-products of red blood cell destruction in the liver--> eliminated from the body with the feces

Outside the body: 

1. To enter the body, nutrients in the lumen must pass the lining of the digestive tract. 

2. Nutrients are absorbed across the intestinal epithelium and then across the unicellular epithelium of capillaries or lacteals. 

a. Only these two single layers of epithelial cells separate nutrients in the lumen of the intestine from the bloodstream.

1. A few nutrients absorbed in stomach and large intestine, but most absorption takes place in the small intestine: major organ of digestion and absorption

a. enormous SA of the small intestine is an adaptation that greatly increases the rate of nutrient absorption

2. The jejunum and ileum, function mainly in the absorption of nutrients and water.

a. jejunum-absorption (40% of length of S.I.)

1. enormous SA of SI-->greatly increases the rate of nutrient absorption

a. Large circular folds in the lining bear fingerlike projections called villi, and each epithelial cell of a villus has many microscopic appendages called microvilli that are exposed to the intestinal lumen.

i. microvilli are the basis of the term “brush border” for the intestinal epithelium (increase SA of mucosal layer)

2. if SI flat-->no specialization of cells-SA would be much less-need a lot of SA contact

1. passive transport-molecules move down their concentration gradients from the lumen of the intestine into the epithelial cells, and then into capillaries

a. Fructose, a simple sugar, moves by diffusion alone down its concentration gradient

2. Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream.

3. Glycerol and fatty acids absorbed by epithelial cells are recombined into fats.

a. fats are mixed with cholesterol and coated with special proteins to form small globules called chylomicrons

i. Chylomicrons are transported by exocytosis out of epithelial cells and into lacteals.

ii. lacteals converge into the larger vessels of the lymphatic system, eventually draining into large veins that return blood to the heart

*The capillaries and veins that drain nutrients away from the villi converge into the hepatic portal vein, which leads directly to the liver.

iii. fats+water+bile-->add lipases (enzymes)-cut off two of the fatty acids→monoglyceride and two fatty acids→absorbed into cells (break them to get them in)

-->reassembled into triglycerides-can’t move in water→packaged w/proteins-surrounds fat, water-soluble on outside: protein-lipid package=chylomicrons-->so large can’t sneak into capillaries, end up in the lymph-in each villus in SI-lacteal-small lymph vessel-->moves into blood→fatty acids→cells

1. Glycerol and fatty acids absorbed by epithelial cells are recombined into fats. 

2. The fats are mixed with cholesterol and coated with special proteins to form small globules called chylomicrons. 

3. Chylomicrons transported by exocytosis out of epithelial cells and into lacteals.

4. lacteals converge into the larger vessels of the lymphatic system, eventually draining into large veins that return blood to the heart. 

*The capillaries and veins that drain nutrients away from the villi converge into the hepatic portal vein, which leads directly to the liver.

Lipoprotein:

-a biochemical assembly that contains both proteins and lipids: high density (HDL) and low density (LDL) lipoproteins which enable fats to be carried in the blood stream

1. The function of lipoprotein particles is to transport water, insoluble lipids (fats) and cholesterol around the body in the blood. 

a. have hydrophilic groups of phospholipids, cholesterol and apoproteins directed outward-->soluble in the salt water-based blood pool

b. Triglyceride-fats and cholesterol esters are carried internally, shielded from the water by the phospholipid monolayer and the apoproteins.

2. lipoprotein metabolism- handling of lipoproteins in the body:

a. two pathways-exogenous and endogenous-depending in large part on whether the lipoproteins in question are composed chiefly of dietary (exogenous) lipids or whether they originated in the liver (endogenous)

i. Exogenous pathway:

-Epithelial cells lining the small intestine readily absorb lipids from the diet-including triglycerides, phospholipids, and cholesterol-->assembled into chylomicrons-->secreted from the intestinal epithelial cells into the lymphatic circulation-->bypass the liver circulation and are drained via the thoracic duct into the bloodstream-->HDL particles donate to the nascent chylomicron-->Mature chylomicrons activate lipoprotein lipase (LPL), an enzyme on endothelial cells lining the blood vessels-->LPL catalyzes the hydrolysis of triacylglycerol (i.e. glycerol covalently joined to three fatty acids) that ultimately releases glycerol and fatty acids from the chylomicrons-->Glycerol and fatty acids can then be absorbed in peripheral tissues, especially adipose and muscle, for energy and storage-->The hydrolyzed chylomicrons are now considered chylomicron remnants

-->The chylomicron remnants continue circulating until they interact with chylomicron remnant receptors, found chiefly in the liver-->endocytosis of the chylomicron remnants

-->hydrolyzed within lysosome-->Lysosomal hydrolysis releases glycerol and fatty acids into the cell, which can be used for energy or stored for later use

• HDL, LDL-protein that carries cholesterol around

-HDL-high density-more protein than lipids-good form of cholesterol-->brings to liver→cleans

-LDL-low density-more lipids than protein-bad cholesterol

• problems delivering fatty acids, cholesterol to inside of blood vessels-->deposits