the above ppt delas with digestive system and digestion, absorption of carbohydrates, fats, proteins.

1. Human Physiology

2. Digestive system

3. Introduction to Digestive system
• Digestive system, also called gastrointestinal system, comprises of the organs
like mouth, esophagus, stomach, small intestine, large intestine and accessory
organs like liver and pancreas.
• The main function of the gastrointestinal system is ingestion, secretion,
digestion, absorption, motility and egestion.
• This is achieved by processing food into molecules that can be absorbed and
utilized by the cells of the body.
• Food is broken down, bit by bit, until the molecules are small enough to be
absorbed and the remaining unabsorbed waste eliminated

4. • Digestion and absorption occur
in the digestive tract. After the
nutrients are absorbed, they are
available to all cells in the body
and are utilized by the body cells
in metabolism.
• The digestive system prepares
nutrients for utilization by body
cells through six activities, or
functions namely, ingestion,
propulsion, mechanical
breakdown, chemical digestion,
absorption, and elimination.

5. Overview Of Digestive System

6. Parts of Digestive System
Gastrointestinal (GI) tract,
• Tube within a tube
• Direct link/path between organs
• Structures are,
• Mouth
• Pharynx
• Oesophagus
• Stomach
• Small intestine
• Large Intestine
• Rectum

7. ◾ Teeth mechanically break
down food into small pieces.
Tongue mixes food with
saliva (contains amylase,
which helps break down
starch).
◾ Epiglottis is a flap-like
structure at the back of the
throat that closes over the
trachea preventing food
from entering it.
Mouth

8. Esophagus
• Esophagus is a hollow tube, like structure which is 25cm in length. It is
guarded on either side by two sphincters: upper esophageal sphincter
(UES) and lower esophageal sphincter (LES).
• Upper one fourth of esophagus is skeletal in nature so UES opens
automatically for entry of food.
• Lower two third of esophagus is smooth muscle in nature and LES is
involuntary. Increase in pressure in the esophagus or distension of
esophagus results in opening of the LES allowing the entry of bolus into
the stomach.
• Food entering from the pharynx into the esophagus initiates the primary
peristaltic waves due to luminal stretch at the rate of 3-5cm/s. They push
the bolus rapidly into the stomach.
• The transit time is about 10s. It is faster in erect posture. If the bolus is not
cleared in a single go, secondary and tertiary peristalsis sets in.
• Upper esophageal sphincter (UES) refers to the high-pressure zone
located in between the pharynx and the cervical esophagus. The
physiological role of this sphincter is to protect against reflux of food into
the airways as well as prevent entry of air into the digestive tract
• Normal LES function allows food transit from the esophagus into the
stomach and prevents the reflux of gastric contents back into the
esophagus.

9. Stomach
Stomach is a hollow visceral organ in
continuation of esophagus present below
the diaphragm. Stomach contains the
following parts: cardia, fundus, body and
pylorus. Cross section of stomach wall
shows four layers: mucosa, submucosa,
muscle layer and serosa.
Muscle layer contains three layer of
muscle inner oblique, middle circular and
outer longitudinal.
Mucosa is thrown into folds which
increases the surface area for digestion.
Stomach on either end has sphincter: upper
end is called lower esophageal sphincter
and lower end is called pyloric sphincter.
• J-shaped muscular bag that stores the food you eat,
breaks it down into tiny pieces.
• Mixes food with digestive juices that contain enzymes to
break down proteins and lipids.
• Acid in the stomach kills bacteria.
• Food found in the stomach is called chyme.

10. Gastric glands of the fundus and body have a variety of secretory cells
• Cardiac glands – secrete soluble mucus.
• Main gastric glands (short duct, long alveoli) have mucous neck cells secrete
viscid mucus, chief/peptic cells secrete pepsinogen and parietal/oxyntic cells
secrete Hydrochloric acid and intrinsic factor.
• Pyloric glands (long duct, short alveoli) secrete mainly mucous.
• G cells secrete Gastrin which is a local hormone of stomach.
• Enterochromaffin cells secrete histamine.

11. Functions of stomach
• The main functions of the stomach are:
• Bulk storage of undigested food
• Secretion
• Disruption of chemical bonds with acids and enzymes- Digestion
• Secretion of intrinsic factor
• Movement

12. Gastric Juice Composition
• Gastric Juice Gastric juice is a
mixture of secretions from
different gastric glands.
• Properties Of Gastric Juice
• Volume : 1200 mL/day to 1500
mL/day.
• Reaction : Gastric juice is highly
acidic with a pH of 0.9 to 1.2.
Acidity of gastric juice is due to
the presence of hydrochloric acid.
• Specific gravity : 1.002 to 1.004

13. Anatomy Of Small Intestine
• Small Intestine (SI) → crucial component of digestive
system → breakdown & absorption of nutrients & permits
body to function at its peak performance → via complex
network of blood vessels, nerves & muscles
• Avg. length = 3-5 mts ; 1. Duodenum 2. Jejunum 3. Ileum
• Duodenum → shortest (0.20-0.25 m or 0.8feet )
Location: proximal end= antrum of stomach separated by
• pylorus → distal end= blends into beginning of jejunum →
C-shaped → receives bile & pancreatic duct at Ampulla of
Vater
• Jejunum (=Empty in Latin) → 0.9 m or 3 feet → no clear
demarcation between jejunum & ileum
• Ileum → longest 1.8 m or 6 feet → thicker, more vascular,
• more mucosal folds → joins cecum at ileocecal sphincter
• Blood → superior mesenteric artery = arterial supply
• Veins → runs parallel and drains into superior mesenteric
• vein → nutrient-rich blood → liver via hepatic portal vein
• Parasympathetic nerve fibers from the vagus nerve &
sympathetic nerve fibers from thoracic splanchnic nerve

14. Histology of small intestine
Layers: 1. Mucosa, 2. Sub-mucosa, 3. Muscularis externa, 4. Outermost peritoneal layer

15. Typical cross section of gut

16. • The main functions of the small intestine are secretion, absorption and
movement. The epithelial cells of the small intestine secrete enzymes which
digest chyme into the smallest particles, making them available for
absorption. Concurrently the duodenum functions to mix food with bile and
pancreatic enzymes to continue the digestion of carbohydrates, fats, and
proteins.
• Small intestinal secretion is called Succus entericus or intestinal juice. Daily
secretion is about 2 to 2.5 lt / day with a pH – 7.5 to 8.6. Contains the
Intestinal enzyme like Peptidases, Disaccharidases (lactase, sucrase, and
maltase), Intestinal lipases and Enterokinase or Entero- peptidase. These
enzymes act on the food resulting in end products which are absorbed from
the intestine.

17. • Large intestine (LI) → terminal part → finish absorption of
nutrients and water, synthesize certain vitamins, form feces, &
eliminate feces
• Structure: runs from appendix to anus → frames SI on 3
sides → one half of SI but more than twice diameter (3
inches)
• Sub-division: Cecum, Colon, Rectum & Anus
• Ileocecal valve controls the flow of chyme from SI to LI
• Cecum: sac-like, 6cm long, receives contents of ileum &
continues absorption of water & salts
• Appendix: winding tube of 7.6cm → lymphoid tissue →
• vestigial but immunologic function/ bacterial reservoir
• Colon: Ascending colon → right hepatic flexure →
• transverse colon → left splenic flexure → descending colon
• → S-shaped Sigmoid colon → rectum
• *Retroperitonium *tethered to posterior abdominal wall
• Rectum: 20.3 cm → third sacral vertebrae → rectal valves
• Anal canal: in perenium completely out of abdominopelvic
cavity → 3.8-5cm → opens exterior of the body at anus
Anatomy Of Large Intestine

18. Anatomy Of Large Intestine
• Few enzyme-secreting cells are found
in the wall
• No circular folds or villi
• Other than in anal canal, mucosa is
simple columnar epithelium made
mostly of enterocytes (absorptive
cells) & goblet cells
• Goblet cells → mucous → eases
movement of feces & protects from
acids & gases produced by enteric
bacteria
• Enterocytes absorb water and salts
as well as vitamins (biotin,
pantothenic acid & vitamin K)
produced by your intestinal bacteria

19. Functions of large intestine
• Secretory functions: Secretes alkaline juice rich in mucus & HCO3
-.
• Synthetic functions: Synthesizes folic acid, Vit B12 & Vit K from bacterial flora
• Absorptive functions:
 Absorption of water & electrolytes.
 Organic substances like glucose & alcohol.
 Drugs like anesthetic agents, sedatives & steroids.
• Excretory functions: helps in excretion of heavy metals like mercury, lead, bismuth & arsenic
through feces.
• Storage function: Storage of feces
• Motility functions:
- Propulsive movements
- Haustral contraction
- Peristalsis
- Mass peristalsis
- Gastrocolic reflex
- Defecation reflex

20. Accessory Organs
Liver

21. Anatomy of liver -Introduction
It is the largest gland of the body.
• Occupying much of the right upper part of the abdominal cavity.
• It consists of both exocrine and endocrine parts.
• The liver performs a wide range of metabolic activities necessary for homeostasis, nutrition, and immune response

22. Location Shape, Size, And Colour
The liver almost fully occupies the right hypochondrium, Upper part of the epigastrium.
part of the left hypochondrium up to the left lateral (midclavicular) line.
Shape : The liver is wedge shaped and resembles a four-sided pyramid laid on one side with its
base directed towards the right and apex directed towards the left.
Colour : It is red-brown in colour.
Weight : In males 1.4 to 1.8kg.
In females 1.2 to 1.4kg.
At birth 125- 150 g. ( 255g at the age of 1 year , 430g at 2 years , 530g at 5 years )

23. Functions of liver

24. The liver has the ability to regenerate itself.
Only 10% to 20% of functioning liver is required to sustain life, although removal of the liver
results in death, usually within 24 hours.
The liver is integral to most metabolic functions of the body and performs more than 500 tasks.
The main functions of the liver include
(1)Metabolism of carbohydrate, protein, and fat
(2)Storage And Activation Of Vitamins And Minerals;
(3)Formation and excretion of bile
(4)Conversion of ammonia to urea
(5)Metabolism of steroids
(6)Detoxification of substances such as drugs, alcohol, and organic compounds;
(7)Function as a filter and flood chamber

25. Extra
Functions:
• Metabolic functions: metabolic processes related to carbohydrate, lipids and proteins
like synthesis & storage of glycogen, glycogenolysis, and gluconeogenesis, β-
oxidation of fatty acids, synthesis of fatty acids, formation of triglycerides, VLDL,
LDL, HDL, Cholesterol & phospholipids and synthesis of plasma proteins, clotting
factors like VII, IX, X (vit K), V, VIII. XI, XII, enzymes, urea, lipo-proteins
respectively happens in liver.
• Secretion of bile juice which contains bile salts and bile pigments
• Defense functions: production of immunoglobulin
• Metabolism of hormones
• Detoxification: Removal of poisonous substances from body like alcohol
• Reservoir of blood: during emergency the liver will be squeezed of the blood to
provide for the necessary organs of the body
• Heat production
• Haemopoietic function: in embryogenic days the liver acts as a site of hemopoiesis.
• Storage of vitamins and minerals

26. Bile Juice
• It is formed in liver hepatocytes and about 500ml is secreted per day.
• The bile juice contains bile salts, bile pigments, phospholipids, cholesterol and
electrolytes. The bile juice is stored and concentrated in the gall bladder.
• Hepatic bile is more alkaline compared to gall bladder bile.
Functions of bile salts:
• Amphipathic in nature
• Emulsification of fat (Detergent action)
• Micelle formation
• Increases the action of pancreatic lipase
• Choleretic (increases bile juice secretion) and cholagogue (increase gall bladder
contraction)
• Increases absorption of fat-soluble vitamin
• Keep cholesterol in soluble form
• Increases intestinal motility (laxative)

27. PANCREAS
• The pancreas is an abdominal organ that is
located behind the stomach and is
surrounded by other organs, including the
spleen, liver and small intestine. The
pancreas is about 6 inches (15.24
centimeters) long, oblong and flat. The
head of the pancreas is on the right side of
the abdomen and is connected to the
duodenum (the first section of the small
intestine) through a small tube called the
pancreatic duct. The narrow end of the
pancreas, called the tail, extends to the left
side of the body. Pancreas functions as both
endocrine and exocrine gland.
• Endocrine function: secretes pancreatic
hormones e.g., insulin, glucagon etc. which
mainly plays a role in glucose homeostasis.
• Exocrine function: secretes pancreatic
juice which contains enzymes that help in
the digestion of food.

28. Structure of Pancreas

29. • The bulk of the pancreas is composed
of pancreatic exocrine cells and their
associated ducts. Embedded within this
exocrine tissue are roughly one million
small clusters of cells called the Islets of
Langerhans, which are the endocrine
cells of the pancreas.
• Pancreatic exocrine cells are arranged in
grape-like clusters called acini (a single
one is an acinus).
• The exocrine cells themselves are packed
with membrane-bound secretory granules
which contain digestive enzymes that are
exocytosis into the lumen of the acinus.
From there these secretions flow into
larger intralobular ducts, which
eventually joins into the main pancreatic
duct which drains directly into the
duodenum.

30. Composition of pancreatic juice
Pancreatic juice
• Daily secretion: 1200-1500ml with a specific gravity: 1010-1018. It is alkaline in nature with a pH: 7.8 – 8.4.
• Main components of the juice are digestive enzymes with bicarbonate ions which is four times more than the plasma. Like other
juices 99.5% is water and 0.5% is solids with organic and inorganic compounds.
• Enzymes present in juice are:
• Proteolytic enzymes: all the proteolytic enzymes are produced in inactive form. The main reason for being inactive is to protect
the pancreatic linings. When the juice is secreted into the intestine presence of an enzyme called “Enterokinase” present in the
brush border of small intestine activates trypsinogen enzyme which later activates itself and other proteolytic enzymes.
Following are the proteolytic enzymes:
Enterokinase
Trypsinogen Trypsin
Chemotrypsin
Procarboxypeptidases
Pronucleases
• Lipases: Pancreatic co-lipases, pancreatic lipase, phospholipase A, Cholesterol hydrolases acts on the lipid component of the food. The pancreatic
lipase and co lipase to act on lipids first must be emulsified by the bile. So, these enzymes act on emulsified fat. They convert the fat into fatty acids and
triglycerides
• Pancreatic α amylase: Pancreatic α amylase acts on both cooked and uncooked carbohydrate component of the food, converting it into
disaccharides.
Self-activation
All are activated by
Trypsin

31. Digestion and Absorption of Carbohydrates

32. Digestion of Carbohydrates
• Carbohydrate is available in the diet in three major forms. Monosaccharides like glucose, sucrose, a
disaccharide known commonly as sugar, lactose, also a disaccharide found in milk and starches, large
polysaccharides present in plant foods like grains and potatoes. Carbohydrate also contains large
amount of cellulose, which is not digested in human GIT but an essential component of food that helps
in easy defecation.
• In mouth and stomach: when food is taken through mouth and chewed it mixes with the saliva present
in the mouth. This salivary juice contains an enzyme called salivary α amylase (ptyalin) which is mainly
secreted by the parotid glands. This enzyme hydrolyzes the cooked starch into disaccharides maltose
and other small polymers of glucose. As food in the mouth stays for less than 2-5minutes, not more than
5% of starch is hydrolyzed. Upon swallowing the food now called bolus reaches stomach and
sometimes the digestion continues in the fundus and body of the stomach if the pH of the stomach is
more than 4.0. When the gastric acid concentration is increased salivary amylase activity is diminished
and approx. 30-40% of the starch is further hydrolyzed to maltose.
• In small intestine: bolus reaching the small intestine is called chyme, is acted upon by the pancreatic
secretion. This pancreatic secretion contains the enzyme α amylase which is more powerful than
salivary amylase. It acts on both cooked and uncooked starch. So as the chyme enters the small
intestine, by the action of the pancreatic α amylase all the carbohydrate is digested within 15-30
minutes. So, carbohydrates are hydrolyzed into maltose and other small glucose polymer before leaving
duodenum or upper jejunum.
• Further intestinal enzymes like lactase, sucrase, maltase and α-dextrinase splits the disaccharides like
lactose, sucrose, maltose and dextrins into their constituent monosaccharides.

33. Enzyme Substrate Product
Ptyalin-salivary amylase Cooked starch
1:4 linkage
Maltose, maltotriose,
α-dextrin
Pancreatic amylase Cooked &
uncooked starch
Same as above
Succus entericus
Maltase, trehalase
α-dextrinase
Maltose, maltotriose, α
dextrin, trehalose
glucose
Lactase lactose glucose & galactose
Sucrase sucrose glucose and fructose
• Lactase enzyme breaks down the disaccharide lactose into galactose and glucose. Sucrose by the action of sucrase, splits
into a molecule of fructose and glucose likewise maltose by the action of maltase enzyme, splits into two molecules of
glucose.
• So ultimately monosaccharides form the end products of the carbohydrate digestion. The glucose forms the major bulk of
final product of digestion with minute quantity of galactose and fructose which are all water soluble and absorbed into the
portal blood.

34. Absorption of the
Carbohydrates
• Carbohydrates are mainly absorbed in monosaccharides forms and very small
portion is absorbed in disaccharides form. As glucose forms the major bulk of the
carbohydrate digestion, we will discuss the absorption of the glucose.
• Glucose is mainly absorbed by secondary active transport* where it is
transported with another ion, sodium. Always glucose gets absorbed along with
sodium via sodium glucose co-transporter mechanism.
• As it is a secondary active transport, it happens in two stages. First there is
activation of the sodium potassium ATP pump present in the basolateral side of
the intestinal epithelial cells. With its activation, three sodium ions are pushed
into the ECF in exchange with the potassium ion into the ICF of the cells.
• This creates a depletion of sodium ion concentration in the cell which in turn
activates the SGLT 1(sodium glucose linked transporter 1) present in the brush
border of the intestinal epithelial cell. This pump transports sodium along with
glucose into the interior of the epithelial cell. Once glucose enters the epithelial
cells it diffuses through the basolateral membrane into the paracellular space*
and ultimately into the blood by the transport protein called GLUT 2.
• Absorption of the galactose is like glucose, where they also utilize the
transporters SGLT1 and GLUT2 to transport galactose across basolateral and
luminal side of the epithelial cells.
• Fructose is absorbed by facilitated diffusion* and not coupled with sodium
transport. Fructose enters the epithelial cells by GLUT5 and then into the blood
by the GLUT2.
• As fructose is not co transported with sodium, its absorption is very less
compared to glucose and galactose. Part of fructose that enters the intestinal cell
by GLUT5 gets phosphorylated and converts into glucose and then absorbed into
the blood.
Fig: Absorption of Glucose, galactose and
fructose into the blood through intestinal
epithelial

35. Continue….
* Secondary active transport is a kind of active transport that uses
electrochemical energy. It takes place across a biological membrane where
a transporter protein couples the movement of an electrochemical ion
(typically Na+ or H+) down its electrochemical gradient to the upward
movement of another molecule or an ion against a concentration or
electrochemical gradient.
• An electrochemical gradient is a difference of electrical charges across
a differentially permeable membrane.
• Electrochemical Gradient
• Electrochemical gradient exists whenever there is a net difference in
charges. The positive and negative charges of a cell are separated by a
membrane, where the inside of the cell has extra negative charges than
outside. The membrane potential of a cell is -40 to -80 millivolts.
• The cell has higher potassium concentration inside the cell but lower
sodium concentration than the extracellular fluid. The sodium ions will
move inside the cell based on the concentration gradient and voltage
across the membrane. The voltage across the membrane facilitates the
movement of potassium into the cell, but its concentration gradient
drives it out of the cell. The combination of voltage across the
membrane and the concentration gradient that facilitates the movement
of ions is called the electrochemical gradient.
* Passing through the intercellular spaces in between epithelial cells.
* Facilitated diffusion is a type of passive transport that uses specialized
proteins, such as channel proteins and carrier proteins for transport across
the biological membrane.

36. Digestion of Proteins
• Proteins in the diet are present in long chains due to the peptide linkages.
Digestion of protein starts in the stomach as salivary juice doesn’t contain any
proteolytic enzymes.
• In stomach: Pepsin secreted in the gastric juice is most active at pH 2 to 3 and
becomes inactive when pH is above 5.
• There are two important types of cells in the gastric glands that secrete the
enzyme and acid.
• The parietal cells secrete hydrochloric acid which makes the gastric secretion
acidic in nature and provides a favorable environment for the activation of the
proteolytic enzyme pepsinogen.
• Pepsinogen is secreted by the peptic cells of the gastric glands which is present
in inactive form i.e., zymogen form. In the presence of hydrochloric acid, it gets
activated to pepsin which then acts on the proteins of the food. Pepsin initiates
the protein digestion by acting on the protein and hydrolyses it into proteoses,
peptones and polypeptides.

37. • Pepsin also acts on the collagen component of the meat and helps in its digestion.
Nearly 20-30% of the protein digestion is initiated in the stomach and later continued in
the small intestine.
• In small intestine: Proteolytic enzymes secreted from the pancreas causes the protein
digestion in the duodenum and jejunum. The chyme containing partially digested
protein enters the intestine and is acted upon by trypsin, chymotrypsin,
carboxypeptidase and elastase. These enzymes are present in the inactive form and its
activation happens when trypsinogen comes in contact with enterokinase present in the
brush border of the intestine converting it into trypsin. This trypsin once formed can
activate other proteolytic enzymes and further activation of trypsinogen.
• Trypsin and chymotrypsin acts on proteins breaking it into small polypeptides.
Carboxypeptidases then act on the carboxyl end of the polypeptides dividing it into
tripeptides and dipeptides.
• Elastase digests elastin, the protein of animal origin. These dipeptides and tripeptides
formed by the action of the pancreatic enzyme undergo further digestion by the
intestinal juice enzymes like amino polypeptidase and dipeptidases. Nearly 99% of the
protein is digested into individual amino acids and these are easily transported across
the microvilli to the inner aspect of the cell.

38. Absorption Of
Proteins
• The amino acids that are formed as the end
products of the protein digestion are absorbed in
the same mechanism as that of the glucose.
• It is transported with sodium as a co transporter.
The amino acids bind with specific transport
protein along with sodium and then enter the
cells by the electrochemical gradient created by
the sodium potassium pump located in the
basolateral side of the enterocytes.
• Some of the amino acids get transported by the
special membrane proteins like fructose by
facilitated diffusion. Because of the diverse
nature of binding of the amino acids and
peptides, there are about 10 different types of
transport proteins in the intestinal epithelial cells.
• For transporting into the enterocytes there are 7
different types in which five are dependent on
sodium and 2 are independent of sodium. To
transport out of the enterocytes there are 5
different transport system in which two are
dependent on sodium and 3 are independent of
sodium.
Fig: Absorption of sodium, glucose, amino acids,
chloride through the enterocytes.

39. Digestion Of Fats

40. Lipid Digestion In The Mouth
• A few things happen in the mouth that start the process of lipid digestion. Chewing
mechanically breaks food into smaller particles and mixes them with saliva.
• An enzyme called is produced by cells on the tongue (“lingual” means relating to the tongue)
and begins some enzymatic digestion of triglycerides, cleaving individual fatty acids from the
glycerol backbone.
Lipid Digestion In The Stomach
• In the stomach, mixing and churning helps to disperse food particles and fat molecules. Cells in
the stomach produce another lipase, called (“gastric” means relating to the stomach) that also
contributes to enzymatic digestion of triglycerides.
• Lingual lipase swallowed with food and saliva also remains active in the stomach. But
together, these two lipases play only a minor role in fat digestion (except in the case of infants,
as explained below), and most enzymatic digestion happens in the small intestine.

41. Overview of lipid digestion in the human
gastrointestinal tract

42. Lipid Digestion In The Small Intestine
• As the stomach contents enter the small intestine, most of the dietary lipids are undigested and clustered in large
droplets. , which is made in the liver and stored in the gallbladder, is released into the duodenum, the first section
of the small intestine. Bile salts have both a hydrophobic and a hydrophilic side, so they are attracted to both fats
and water.
• This makes them effective emulsifiers, meaning that they break large fat globules into smaller droplets.
Emulsification makes lipids more accessible to digestive enzymes by increasing the surface area for them to act
(see Fig next slide).
• The pancreas secretes into the small intestine to enzymatically digest triglycerides. Triglycerides are broken down
to fatty acids, monoglycerides (glycerol backbone with one fatty acid still attached), and some free glycerol.
Cholesterol and fat-soluble vitamins do not need to be enzymatically digested
Lipid Absorption From The Small Intestine
• Next, those products of fat digestion (fatty acids, monoglycerides, glycerol, cholesterol, and fat-soluble vitamins)
need to enter into the circulation so that they can be used by cells around the body. Again, bile helps with this
process.
• Bile salts cluster around the products of fat digestion to form structures called micells , which help the fats get
close enough to the microvilli of intestinal cells so that they can be absorbed. The products of fat digestion
diffuse across the membrane of the intestinal cells, and bile salts are recycled back to do more work emulsifying
fat and forming micelles.

43. Lipid digestion and absorption in the small intestine.

44. • Once inside the intestinal cell, short- and medium-chain fatty acids and glycerol can be directly absorbed into the
bloodstream, but larger lipids such as long-chain fatty acids, monoglycerides, fat-soluble vitamins, and
cholesterol need help with absorption and transport to the bloodstream.
• Long-chain fatty acids and monoglycerides reassemble into triglycerides within the intestinal cell, and along
with cholesterol and fat-soluble vitamins, are then incorporated into transport vehicles called chylomicrons. are
large structures with a core of triglycerides and cholesterol and an outer membrane made up of phospholipids,
interspersed with proteins (called apolipoproteins) and cholesterol.
• This outer membrane makes them water-soluble so that they can travel in the aqueous environment of the body.
Chylomicrons from the small intestine travel first into lymph vessels, which then deliver them to the
bloodstream.
Structure of a chylomicron. Cholesterol is not shown in this figure, but chylomicrons contain
cholesterol in both the lipid core and embedded on the surface of the structure.