PHYSIOLOGY

The Digestive System

Assist. Prof. Esra Nur Yiğit

(esra.yigit@medipol.edu.tr)

2023-2024 Spring Semester

Lectures

• Lecture 1 : Introduction to Physiology

• Lecture 2 : The Cell - I
• Lecture 3 : The Cell - II
• Lecture 4 : Musculoskeletal System
• Lecture 5 : Cardiovascular System I: Blood
• Lecture 6 : Cardiovascular System II: Heart
• Lecture 7 : Respiratory System
• Lecture 8 : Digestive (Gastrointenstinal) System
• Lecture 9: Urinary System
• Lecture 10: Nervous System - I
• Lecture 11: Nervous System - II
• Lecture 12: Endocrine System
• Lecture 13: Reproductive System
About this lecture

❑ Function of the digestive system

❑ Gastrointestinal tract

❑ Accessory digestive organs

❑ Movement of GI tract
❑ Peristalsis
❑ Segmentation
❑ Digestion and absorption of food
❑ Carbonhydrates
❑ Proteins
❑ Fats
Functions of digestive system

• Motility
• Secretion
• Digestion
• Absorption
• Storage and elimination
• Immune barrier
Motility

Movement of food through the digestive tract through the processes of:

• Ingestion: Taking food into the mouth.

• Mastication: Chewing the food and mixing it with saliva.
• Deglutition: Swallowing food.
• Peristalsis and segmentation: Rhythmic, wavelike contractions (peristalsis), and
mixing contractions in different segments (segmentation), move food through the
gastrointestinal tract.
Secretion

Includes both exocrine and endocrine secretions.

• Exocrine secretions: Water, hydrochloric acid, bicarbonate, and many digestive

enzymes are secreted into the lumen of the gastrointestinal tract.
• The stomach alone, for example, secretes 2 to 3 liters of gastric juice a day.

• Endocrine secretions: The stomach and small intestine secrete a number of

hormones that help to regulate the digestive system.
Digestion

Breakdown of food molecules into their smaller subunits, which can be absorbed.
Absorption

This refers to the passage of digested end products into the blood or lymph.

Lacteal: lymphatic capillary that absorbs dietary fats in the villi of the small intestine.
Storage and elimination

This refers to the temporary storage and subsequent elimination of indigestible food
molecules.
Immune barrier

The simple columnar epithelium that lines the intestine, with its tight junctions
between cells, provides a physical barrier to the penetration of pathological
organisms and their toxins. Also, cells of the immune system reside in the connective
tissue located just under the epithelium to promote immune responses.
The digestive system

Gastrointestinal (GI) tract

• Oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine

Accessory digestive organs

• Teeth
• Tongue
• Salivary glands
• Liver, gallbladder
• Pancreas
Layers of the Gastrointestinal Tract

The four tunics of the GI tract, from the inside out, are; Mucosa, Submucosa,
Muscularis, and Serosa
Layers of the Gastrointestinal Tract

Mucosa;
The mucosa, which lines the lumen of the GI
tract, is the absorptive and major secretory layer.
It consists of a simple columnar epithelium
supported by the lamina propria.

Lamina propria, a thin layer of areolar

connective tissue containing numerous lymph
nodules, which are important in protecting
against disease.

External to the lamina propria is a thin layer of

smooth muscle called the muscularis
mucosae. This is the muscle layer responsible
for the numerous small folds in certain portions
of the GI tract.
Layers of the Gastrointestinal Tract

Mucosa;
Specialized goblet cells in the mucosa secrete mucus throughout most of the GI tract.
Layers of the Gastrointestinal Tract

Submucosa;
Submucosa is a highly vascular layer of
connective tissue that serves the mucosa.

Absorbed molecules that pass through the

columnar epithelial cells of the mucosa
enter into blood and lymphatic vessels of the
submucosa. In addition to blood vessels, the
submucosa contains glands and nerve
plexuses.
Layers of the Gastrointestinal Tract

Muscularis;
Responsible for segmental
contractions and peristaltic
movement through the GI tract.

The muscularis has an inner circular

and an outer longitudinal layer of
smooth muscle. Contractions of these
layers move the food through the tract
and physically mix the food with
digestive enzymes.
Layers of the Gastrointestinal Tract

Serosa;
The outer serosa completes the wall of the
GI tract. It consists of areolar connective
tissue covered with a layer of simple
squamous epithelium, and is continuous with
the mesentery.
Mouth

As food enters the mouth, it is mixed with saliva and masticated (chewed). The
cheeks and closed lips hold the food between the teeth during chewing. The tongue
continuously mixes food with saliva during chewing and initiates swallowing. Thus,
the breakdown of food begins before the food has even left the mouth.
Bolus

Bolus is defined as a ball-like mixture of food and saliva that forms in the mouth
during the chewing process. It usually has a similar colour to that of the food being
eaten with an alkaline pH due to the saliva it is mixed with.
Saliva

The principal glands of salivation are the parotid, submandibular, and sublingual
glands; in addition, there are many tiny glands. Daily secretion of saliva normally
ranges between 800 and 1500 milliliters

Saliva contains two major types of

protein secretion:
(1) a serous secretion that contains
ptyalin (α-amylase), which is an
enzyme for digesting starches, and
(2) mucus secretion that contains
mucin for lubricating and for
surface protective purposes.
Pharynx

From the mouth, food

passes posteriorly into the
oropharynx and
laryngopharynx, both of
which are common
passageways for food, fluids,
and air.
Swallowing (Deglutition)

Swallowing can be divided into

(1) Oral stage: Initiates the swallowing process (voluntary)
Bolus moves from oral cavity to oropharynx
(1) Pharyngeal stage: Involuntary and constitutes passage of food through
the oropharynx into the esophagus
(2) Esophageal stage: Involuntary phase that transports food from
esophagus to the stomach.

Upper esophageal
sphincter (UES) : high-
pressure zone located in
between the pharynx and
the cervical esophagus
Sphincter: circular muscle
Esophagus

Connects the pharynx to the stomach.

It is a muscular tube approximately 25 cm long.

Swallowed food is pushed from the oral to the

stomach end of the esophagus (and, afterward, of
the intestine) by a wavelike muscular contraction
called peristalsis.
Esophagus

The upper esophageal sphincter (UES) is a

bundle of muscles at the top of the esophagus. It
is closed if there is no swallowing action. UES
opening is triggered by swallowing.

The lower esophageal sphincter (LES) is a

bundle of muscles at the low end of the
esophagus, where it meets the stomach.
When the LES is closed, it prevents acid and
stomach contents from traveling backwards fro m
the stomach. The LES muscles are involuntary.

It is also called as cardiac sphincter.

Gastroesophageal reflux disease (GERD)
Stomach

The functions of the stomach are to store food, to initiate the digestion of
proteins, to kill bacteria with the strong acidity of gastric juice, and to move the food
into the small intestine as a pasty material called chyme.

Food→Bolus:
Teeth and saliva
in mouth (alkaline)

Bolus→Chyme:
Enzymes
in stomach (acidic)
Stomach

The cardia is the first part of the stomach below the esophagus. It contains the
cardiac sphincter.

The fundus is the rounded area that

lies to the left of the cardia and below
the diaphragm.

The body is the largest and main part

of the stomach. This is where food is
mixed and starts to break down.
 Stomach

The antrum is the lower part of the stomach. The antrum holds the broken-down
food until it is ready to be released into the small intestine. It is sometimes called
the pyloric antrum.

The pylorus is the part of the stomach

that connects to the small intestine.
This region includes the pyloric
sphincter, which is a thick ring of
muscle that acts as a valve to control
the emptying of stomach contents
(chyme) into the duodenum (first part
of the small intestine). The pyloric
sphincter also prevents the contents of
the duodenum from going back into
the stomach.
Gastric Pits

Gastric pits are formed by invaginations of the surface epithelium.

They connect to gastric glands and thus allow the glandular products to be delivered
into the stomach lumen.
The cells that line the folds secrete various products into the stomach; these cells
form the exocrine gastric glands
 Fundic gland Pyloric gland
Gastric Pits

Mucous cells: secrete mucus

Parietal cells: secrete hydrochloric acid (HCl)
Chief (or zymogenic) cells: secrete pepsinogen, an inactive form of the protein-
digesting enzyme pepsin
Enterochromaffin-like (ECL) cells: secrete histamine and serotonin
G cells: secrete gastrin into the blood;
D cells: secrete somatostatin.
Hydrochloric Acid Secretion

The parietal cells secrete H+ (protons), at a pH as low as 0.8, into the gastric lumen
by primary active transport (involving carriers that function as an ATPase). These
carriers, known as H+ /K+ ATPase pumps,
 Pepsin and Hydrochloric Acid Secretion

The high concentration of HCl from the parietal cells makes

gastric juice very acidic, with a pH of less than 2.
This strong acidity serves three functions:
1. Ingested proteins are denatured at low pH—that is, their
tertiary structure is altered so that they become more
digestible.
2. Under acidic conditions, weak pepsinogen enzymes
partially digest each other—this frees the fully active pepsin
enzyme as small inhibitory fragments are removed.
3. Pepsin is more active under acidic conditions:
optimum pH =~ 2.0
 Absorption in Stomach

Proteins are only partially digested in the stomach by the action of

pepsin, while carbohydrates and fats are not digested.

Some of the digestion products can be absorbed.

Glucose, simple sugars and amino acids can also be absorbed.
Drugs can be absorbed in stomach.
SMALL INTESTINE

It is approximately 3 m long in a living person.

The first 20 to 30 cm extending from the pyloric sphincter is the duodenum. The
next two-fifths of the small intestine is the jejunum, and the last three-fifths is the
ileum. The ileum empties into the large intestine.
SMALL INTESTINE

Absorption of carbohydrates, lipids, amino acids, calcium, and iron occurs

primarily in the duodenum and jejunum.

Bile salts, vitamin B12, water, and electrolytes are absorbed primarily in the
ileum.
Villi and Microvilli

Intestinal villi (singular: villus) are

small, finger-like projections that
extend into the lumen of the small
intestine.
Each villus is approximately 0.5–1.6
mm in length (in humans).

Villi increase the internal surface

area of the intestinal walls
making available a greater surface
area for absorption.
Villi and Microvilli

The villi are covered with columnar

epithelial cells, among which are
interspersed mucus-secreting goblet
cells.

The lamina propria, which forms the

connective tissue core of each villus,
contains numerous lymphocytes, blood
capillaries, and a lymphatic vessel
called the central lacteal.
Villi and Microvilli

Absorbed monosaccharides and

amino acids enter the blood
capillaries

Absorbed fat enters the central

lacteals.
 Villi and Microvilli

Microvilli are formed by foldings at the apical

surface of each epithelial cell membrane.

These minute projections can be seen clearly

only in an electron microscope.

Increase the surface area for diffusion and

minimize any increase in volume, and are
involved in a wide variety of functions,
including absorption, secretion, cellular
adhesion, and mechanotransduction.
Intestinal Enzymes

The plasma membranes of the microvilli contain digestive enzymes that

hydrolyze disaccharides, polypeptides, and other substrates
Large Intestine

The large intestine is subdivided

into four main regions:
• the cecum,
• the colon,
• the rectum
• the anus

The ileocecal valve, located at

the opening between the ileum
and the large intestine, controls
the flow of chyme from the small
intestine to the large intestine.
Large Intestine

The large intestine has little or no digestive function, but it does absorb water and
electrolytes from the remaining chyme, as well as several B complex vitamins and
vitamin K.

The large intestine’s mucosa produces bicarbonates which neutralize acidity caused
by the synthesis of fatty acids. Furthermore, the large intestines’ mucosal layer acts
as a barrier, protecting against microbial infections.
Intestinal Microbiota

Microorganisms, primarily bacteria, are present in relatively small numbers in the

stomach and proximal portion of the small intestine. Their numbers increase in the
distal ileum and are greatest in the colon, where an estimated 1014 reside.

These microorganisms are known collectively as the intestinal microbiota or

microflora.

- They are involved in metabolic activities for increasing nutrient uptake.

- They also protect host against invasion by foreign microorganisms.
Feces

Normally, feces are made up of 75 percent water and 25 percent solid matter.

About 30 percent of the solid matter consists of dead bacteria;

About 30 percent consists of indigestible food matter such as cellulose;
10 to 20 percent is cholesterol and other fats;
10 to 20 percent is inorganic substances such as calcium phosphate and iron
phosphate;
2 to 3 percent is protein
Defecation

The external sphincter is

controlled by nerve fibers (part of
the somatic nervous system) and
therefore is under voluntary,
conscious, or subconscious
control.
Subconsciously, the external
sphincter is usually kept
continuously constricted unless
conscious signals inhibit the
constriction.
Accessory digestive
organs
Liver
Bile production and secretion

The liver produces and secretes 250 to 1500 ml of bile per day.

The major constituents of bile are bile pigment (bilirubin), bile salts, phospholipids
(mainly lecithin), cholesterol, and inorganic ions

Bile pigment, or bilirubin, is produced in the spleen, liver, and bone marrow as a
derivative of the heme groups (minus the iron) from hemoglobin
Bile production and secretion

Bile is secreted continually by the

liver cells, but most of it is normally
stored in the gallbladder until it is
needed in the duodenum.

The maximum volume that the

gallbladder can hold is only 30 to 60
milliliters
Bile production and secretion

The liver cells synthesize about 6 grams of bile salts daily.

Function of bile salts in fat digestion and
absorption

The bile salts have two important actions in the intestinal tract:
Detergent action on the fat particles in the food. This action, which decreases the
surface tension of the particles and allows agitation in the intestinal tract to break the
fat globules into small sizes, is called the emulsifying or detergent function of bile
salts.
Function of bile salts in fat digestion and
absorption

The bile salts have two important actions in the intestinal tract:
Detergent action on the fat particles in the food. This action, which decreases the
surface tension of the particles and allows agitation in the intestinal tract to break the
fat globules into small sizes, is called the emulsifying or detergent function of bile
salts.

Help in the absorption of (1) fatty acids, (2) monoglycerides, (3) cholesterol, and (4)
other lipids from the intestinal tract. They help in this absorption by forming small
physical complexes with these lipids; the complexes are called micelles, and they
are semisoluble in the chyme because of the electrical charges of the bile salts.
 Liver cells

The liver’s cells have pathogen recognition receptors that recognize PAMPs
(pathogen-associated molecular patterns), enabling them to scavenge blood-borne
bacteria.
 Liver cells

The liver can also remove hormones, drugs, and other biologically active
molecules from the blood by

(1) excretion of these compounds in the bile.

(2) phagocytosis by the Kupffer cells that line the sinusoids
(3) chemical alteration of these molecules within the hepatocytes.
Production of Plasma Proteins

Plasma albumin and some of the plasma globulins (with the exception of
immunoglobulins, or antibodies) and clotting factors (such as fibrinogen and
prothrombin) are produced by the liver.
Gallbladder
 Biliary Tract

Stores and concentrates bile

Liver → Bile ducts → Hepatic ducts
→ Cystic duct → Gallbladder
Bile is a yellowish green fluid
containing bile salts, bilirubin,
cholesterol, and other compounds.
Bile is continuously produced by the
liver

Contraction of the muscularis layer of

the gallbladder ejects bile through the
cystic duct into the common bile duct,
which conveys bile into the duodenum
Gallstones

Gallstones are hard mineral deposits that

form in the gallbladder and generally have
cholesterol as their major component.

Gallstones can block the hepatic, cystic, or

common bile ducts and evoke the pain and
nausea of biliary colic.
Gallstones are usually detected by
abdominal ultrasound, and the symptoms
they produce are usually ended with surgical
removal of the gallbladder.
 Pancreas

The pancreas is a soft, glandular organ that has both exocrine and endocrine functions.
 Pancreas

The endocrine function is performed by clusters of cells called the pancreatic islets, or
islets of Langerhans that secrete the hormones insulin and glucagon into the blood.

As an exocrine gland, the pancreas secretes pancreatic juice from acinar cells through the
pancreatic duct into the duodenum.
Pancreatic Juice

Pancreatic juice contains bicarbonate and about 20 different digestive enzymes.

These enzymes include:

Amylase, which digests starch;

Trypsin, which digests protein
Lipase, which digests triglycerides.
DIGESTION AND
ABSORPTION OF
FOOD
 Digestion of Carbohydrates

Saliva, which contains the digestive

enzyme ptyalin (an α-amylase)
secreted mainly by the parotid
glands.

Starch digestion sometimes

continues in the body and fundus of
the stomach for as long as 1 hour
before the food becomes mixed with
the stomach secretions.
 Digestion of Carbohydrates

Digestion by Pancreatic Amylase.

Pancreatic secretion, contains a large quantity
of α-amylase that is almost identical in its function
to the α-amylase of saliva but is several times as
powerful. Therefore, within 15 to 30 minutes after
the chyme empties from the stomach into the
duodenum and mixes with pancreatic juice, virtually
all the carbohydrates will have become digested.

In general, the carbohydrates are almost totally

converted into maltose and/or other small glucose
polymers before passing beyond the duodenum or
upper jejunum.
DIGESTION OF PROTEINS

Dietary proteins are chemically long chains of amino acids bound together by
peptide linkages.
 Digestion of Proteins in the Stomach

Pepsin, an important peptic enzyme of the stomach, is most active at a pH of 2.0

to 3.0 and is inactive at a pH above about 5.0.

Pepsin only initiates the process

of protein digestion, usually
providing only 10 to 20 percent of
the total protein digestion to
convert the protein to proteoses,
peptones, and a few
polypeptides.
 Most Protein Digestion Results From Actions of
Pancreatic Proteolytic Enzymes.

Most protein digestion occurs in the upper small intestine, in the duodenum and
jejunum, under the influence of proteolytic enzymes from pancreatic secretion.

Major proteolytic pancreatic enzymes;

• Trypsin,
• Chymotrypsin,
• Carboxypolypeptidase,
• Elastase
Most Protein Digestion Results From Actions of
Pancreatic Proteolytic Enzymes.

Both trypsin and chymotrypsin split protein molecules into small polypeptides.

Carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends
of the polypeptides.

Elastase digests elastin fibers

DIGESTION OF FATS

By far the most abundant fats of the diet are the neutral fats, also known as
triglycerides, each molecule of which is composed of a glycerol and three fatty acid
side chains.
DIGESTION OF FATS

In mouth: Lipases are not present in saliva hence, fats are not digested here.

In stomach: Gastric juice contains small amount of gastric lipase which converts
some fats into monoglycerides and fatty acids.

In small intestine: Bile salts of the bile break down fat droplets into many small
ones by reducing the surface tension of fat droplets.
This process is called emulsification. This increases lipase action on fat. Lipase is
present in the pancreatic juice and intestinal juice.

Pancreatic lipase is the principle enzyme for digestion of fat.