Download Digestion - U of L Class Index

IV. Digestion, Absorption and Transport
• In order for food to be of any benefit to the body, and for the
body to get any nutrients from it, food must be eaten, digested an
the resultant nutrients absorbed from the intestinal tract and
transported to designated cells throughout the body
• This is done through the processes of ingestion, digestion,
absorption and transport
The Digestive System
• Provides two major functions; digestion and absorption
• Nutrients such as carbohydrates, fats and proteins are broken
down into their basic constituents and then absorbed as sugars,
fatty acids and amino acids and finally transported to their
designated destinations
• The gastrointestinal tract, a hollow tube from mouth to anus, is
the organ specifically involved in digestion and absorption
Digestion
• Involves anatomy, physiology and biochemistry
• It is the process of getting food ready for absorption
• Includes both the mechanical and chemical processes that break
food down into particles that can be absorbed
Gastrointestinal (GI) tract
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Also called the alimentary canal
Flexible, muscular tube that gets food ready for absorption
Mouth---------> anus - 26 feet
Principle organs - Stomach and Intestine
Steps Involved in Digestion
Before Digestion Begins……..
• Stimulation of the digestive processes begins even before food enters the mouth
• The sensory input, sight, sounds and smells, involved in preparing for and anticipating
the ingestion of food, play an important role in getting the body ready for taking in (or
ingesting) nutrients
• This cephalic response (nervous system response) makes the body feel hungry or have
the desire to ingest nutrients
• Before digestion begins, ingestion (the taking in of nutrients - generally through the
mouth) must occur
Digestion Begins in the Mouth
• Mechanical and chemical processes of chewing and swallowing
are under conscious control. The rest of the mechanical
processes involved in digestion are under unconscious or
involuntary control
• Food enters the mouth where it is reduced to a coarse mash
through a process called mastication
• Mastication - the breakdown of food by mandibular movement
(movement of the food around the mouth by the tongue, cheeks,
lips and lower jaw) and the subsequent breakdown of the food
into smaller, swallowable pieces
• The mouth contains two fluids that are involved in the initial
digestion of food; saliva and mucous
• Once the food has been chewed, broken down and partially
digested, it makes its way to the back of the mouth
• The movement of the tongue helps to position the food so that it
can be swallowed
• The chewed and partially digested food is called a BOLUS (ball
of chewed food mixed with saliva)
Chemical Processes
• Saliva - contains salivary amylase (initial breakdown of
carbohydrates) and lingual lipase (initial breakdown) of fats
• Mucous - slippery secretion secreted by the mucous membranes
lining the mouth; envelopes and lubricates the food, easing
passage from the mouth to the rest of the digestive tract
• Salivary amylase - begins starch breakdown by hydrolyzing the
glycosidic linkages (linkages connecting the individual glucose
molecules in starch) (yielding monosaccharides, disaccharides
and shorter polysaccharides)
• Lingual Lipase - secreted by the sublingual salivary gland under
the tongue
• - begins the breakdown of short chain fatty acids
Mouth to the Stomach
• As the bolus is swallowed, it enters into the pharynx.
• The pharynx is the part of the digestive tract responsible for swallowing and is
common to both the digestive and respiratory tracts
• Food passes through here on its way to the esophagus and air passes through here on
its way to and from the lungs
• A valve-like flap, called the epiglottis exists in the pharynx that serves to cover the
opening to the lungs when swallowing to prevent food from entering the lungs
• Sometimes the epiglottis gets stuck, or doesn’t completely cover
the opening and consequently food gets stuck or lodged in the
airway
• When this happens, the person can’t breathe and the food must
be dislodged in order to prevent suffocation
• If the food can’t be dislodged by coughing then a process called
the Heimlich can be used
Movement of Food
• Once the bolus is swallowed, it slides down the esophagus and is
conducted through the diaphragm to the stomach by peristalsis
• Peristalsis is the coordinated involuntary muscular contractions
and relaxations that aides the movement of the food down the
esophagus and also through the intestine
• Vomiting - reverse of peristalsis
Entrance to the Stomach
• Entrance to the stomach is gained through a muscular valve that
lies at the end of the esophagus and the beginning of the stomach
called the Cardiac Sphincter
Cardiac sphincter
• Muscle encircling the tube of the digestive tract that acts like a valve opening and
closing to allow food in
• Under unconscious control
• Relaxes to allow the bolus to enter the stomach and closes behind so that it can’t slip
back up
• Also prevents “reflux” of the stomach contents in to the esophagus thus preventing
heartburn
• NO CHEMICAL PROCESSES AIDING DIGESTION OCCURS IN THE ESOPHAGUS
Stomach
• The stomach is a pouch-like expanded portion of the G.I. Tract that serves as a
temporary storage place for food.
• Approximate 1 litre capacity
• Comprised of three sets of muscles; circular, longitudinal and horizontal
• The coordinated movement of these muscles results in the “churning action” of the
stomach. This mixes the food with the gastric juices to form a semiliquid mass called
chyme
• Digestion occurs via mechanical processes and chemical processes
• Mechanical processes involve the churning of the food to mix it with the gastric juices
while at the same time breaking it down further
• The chemical processes of digestion in the stomach occur via hydrolysis of the food by
HCl and the digestive enzymes in the gastric juices
• Enteric Coating - mucopolysaccharide (mucous) secreted by the stomach lining that
prevents autodigestion of the stomach by the gastric juices
• Food is retained in the stomach for 2-3 hours (depending on its
composition)
• After it is thoroughly mixed and liquified, it moves to the bottom
of the stomach where it leaves to enter the small intestine by
another valve called the pyloric sphincter
Regulation of Gastric Secretion
• Secretions of the stomach are under both nervous and hormonal control
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Governed by signals from the brain,
stomach and small intestine
signals work like an on/off switch - stimulation or inhibition of gastric juice secretion
three phases of gastric secretion
– cephalic
– gastric
– intestinal
Cephalic phase
• Occurs before food enters the stomach, even before food enters
the mouth
• nervous signals are sent by the brain to get the stomach ready to
receive food
• in this stage the gastric secretions are stimulated by the
thought,sight, smell and taste of food
Gastric phase
• Once food enters the stomach it presence (bulk) stimulates the secretion of the gastric
juices
• The bulk stretches the local nerves at the top of the stomach
• This stretching signals the brain that food is present and a hormone, gastrin, is released
from the upper part of the stomach
• Gastrin stimulates the secretion of the gastric juices containing HCl and the digestive
enzymes such as pepsinogen, gastric lipase, etc.
Enzymes
• Proteins
• Enzymes are very important in biochemical reactions
• They speed up the rate of a specific biochemical reaction without
themselves being altered
• Each reaction has its own specific enzyme(s)
Components of Gastric Juice
• HCl
– deactivates salivary amylase (stops starch digestion) (enzyme does not
function in an acidic environment
– kills many bacteria consumed with the food
– provides an acidic environment (pH 1-2) for gastric enzyme function
– begins protein digestions by converting pepsinogen (precursor) to
pepsin (protein digesting enzyme (enzyme requires an acidic pH)
• Gastric Lipase
– starts fat digestion
– fats leave the stomach only partially processed
• Bicarbonate
– neutralizes the acidity of the chyme before it enters the small intestine
• Vitamin B12
– becomes attached to a protein carrier - this protects the vitamin against
the harsh environment of the stomach
• Carbohydrates pass though the stomach to the small intestine as
the stomach does not secrete enzymes for carbohydrate digestion
and it inactivates amylase from the mouth
• Therefore carbohydrates leave first, proteins and fats remain
behind for more extensive processing
• Fats generally leave last which is why a high fat meal has the
“stick to your ribs” quality
Intestinal phase
• Begins when the chyme passes into the small intestine
• Pyloric sphincter (pylorus)
– this is a valve between the stomach and the small intestine. It
prevents back-up of the intestinal contents into the stomach
and regulates the flow of the chyme from the stomach to the
small intestine
• The passage of chyme into the small intestine triggers a series of
events that regulates the release of chyme into the small intestine
stomach
• Motility is slowed, gastric secretions are shut off and the release
of food into the small intestine slowed so that thorough digestion
can take place
Control of Stomach Emptying
• The amount of chyme leaving the stomach through the pyloric
sphincter is about 1-2 tsp every 30 seconds
• Consequently the stomach empties over a period of time rather
than as a bulk emptying
• Rate is determined by the size and composition of the meal and
control from the small intestine
• Chyme passing through the pyloric sphincter causes the small intestine to distend
which then closes the valve (as well as stimulation by the low pH) until the food
moves down the intestine after processing thus this will slow stomach emptying
• Large meals will take longer to leave the stomach than smaller
• The composition (amount of protein, fat and carbohydrate) of the meal will determine
the rate of stomach emptying due to the amount of processing required
• In addition, out moods can affect stomach emptying (sadness & fear -slow emptying;
excitement & aggression-increase emptying)
The Small Intestine
• 15 - 30 feet long
• approximately 95% of all digestion takes place in the small
intestine
• comprised of the
– duodenum (12 inches)
– jejunum(8 feet) (most digestion takes place here)
– ileum (11 feet)
• all three parts have different functions within the small intestine
but they are noncompartmentalized
• Digestion within the small intestine is aided by secretions from itself, the gallbladder
and the pancreas
• When the chyme enters the small intestine through the pyloric sphincter, the pH is
very acidic
• the nerve cells sense this and stimulate the pyloric sphincter to close and this stops
more chyme from entering
• at any given time only about 1-2 tsp enters the small intestine
• as it enters the small intestine, it bypasses the bile duct
• The bile duct secretes the fluids from the gall bladder and the pancreas
• Bicarbonate is secreted to neutralize the acidic chyme and to provide an environment
conducive for the digestive enzymes to work in
• Once the chyme is sufficiently neutralized then the nerve cells will stimulate the
pyloric sphincter to relax and more chyme will be allowed to enter until the pH causes
the nerve cells to contract the pyloric sphincter
Gall bladder
• Stores and secretes bile
• bile is synthesized in the liver and excess is concentrated and
stored in the gall bladder
• necessary for fat digestion and absorption
• secretes its contents through the bile duct in to the small intestine
when required
Bile
• Contains “surfactants” to help solubilize fats
• serves to emulsify fats
• makes them partially soluble in water thus allowing for aqueous
enzymes to break down fat in an aqueous environment
Pancreas
• Secretes pancreatic juices into the small intestine via special
ducts
• pancreatic juices contain enzymes (proteases, lipases and
amylase) and bicarbonate
• bicarbonate serves to neutralize the HCl from the stomach
allowing for the functioning of the enzymes for further digestion
Cystic Fibrosis
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Inherited disease of infants and children
pancreas is often affected by the thick mucous that blocks the ducts
as a result the cells die
pancreas can’t deliver the enzymes into the small intestine
carbohydrate, fat and protein digestion is affected
• no amylase, trypsin, lipase, etc.
• enzymes must be given orally otherwise the undigested nutrients go to the
large intestine
• this causes bacterial buildup, acids and gases are produced and the
individual is very uncomfortable and in considerable pain
• the consequent result is malabsorption and eventually severe malnutrition
As well, in the intestine, there are
• Various nutrient degrading enzymes are present on the surface of
the small intestine (ex. Maltase)
• In the neutral pH of the small intestine bacteria thrive
• Bacterial action leads to vitamin formation (ex. Vitamin K) and
as well, they secrete some enzymes that aid in digestion (ex.
Cellulases for the breakdown of cellulose
• It is difficult for infections to get established in the small
intestine due to the high density of “natural” bacteria (Natural
flora)
• Defending cells (phagocytes) are also present to confer
immunity against intestinal diseases
• Segmentation - periodic squeezing or partitioning of the intestine
by its circular muscles. Tends to force food backward a few
inches and allows for better mixing with the juices
• Pancreatic amylase continues the breakdown of carbohydrate
started by salivary amylase
• Protein digesting enzymes break protein down into smaller units
• The cells lining the small intestine contain enzymes that further
break down sugars into monosaccharides and small polypeptides
into constituent amino acids
• Fat is mixed with bile to emulsify it so that the pancreatic
enzymes containing lipases can break down the fat
Control of Intestinal Secretions
• Two hormones control the release of bile and pancreatic
enzymes into the small intestine
• Secreted by the mucosal lining of the duodenum
• Secretin and Cholecystokinin (CCK)
• Work together to control digestion in the small intestine
Secretin
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Hormone
Secreted by the mucosal lining of the duodenum
Stimulates the pancreas to secrete sodium bicarbonate
Stimulates the liver to secrete bile into the gallbladder
Cholecystokinin (CCK)
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Hormone
Secreted by the mucosal cells lining the duodenum
Stimulates the pancreas to release its digestive enzymes
Stimulates the gallbladder to contract thus releasing its contents
into the duodenum
Absorption in the Small Intestine
• The main function of absorption is the transport of nutrients
across cell membranes
• The small intestine is the primary site of absorption of most
nutrients from digestion (water, vitamins, minerals, sugars,
amino acids, and fatty acids)
• Coordinated with digestion
• Although the small intestine is 10 ft long, it has a surface area
for absorption equivalent to 1/4 of an American football field
• The intestinal wall is an extremely convoluted surface with
hundreds of folds
• Each fold has hundreds of nipple-like projections called villi
• The villi are further covered with fingerlike projections called
the villus (brush border)
• On the surface of the villi are the microvilli which are projections of the plasma
membrane of the underlying cells
• The microvilli are covered with hairs or cilia that serve to trap nutrients
• Some partially digested nutrients are trapped in the microvilli where they are digested
further and then absorbed
• Within the microvilli there are many enzymes and pumps which recognize and absorb
nutrients (ex. Maltase, glucose transporter, etc.).
• Once absorbed they make their way to the lymphatic system======> blood system
or are absorbed directly into the blood system
• Broken down energy nutrients are absorbed
• Water, minerals and vitamins are absorbed
• Segmentation helps to facilitate better absorption
Illeocecal Valve
• This valve lies at the opening of the large intestine (colon) on the
lower right hand side of the abdomen
• Once the chyme has traveled the length of the small intestine it
leaves via the illeocecal valve and enters the large intestine
How are Nutrients Absorbed???
Structure of Cell Membranes
• The cell membrane is comprised of lipids, proteins and a small amount of
carbohydrate
• The major component of the membrane is lipids (generally phospholipids) and these
give the cell membrane flexibility
• Flexibility within the cell membrane is determined by the types and amounts of fatty
acids present (saturated or unsaturated)
• Flexibility allows the cell to change shape
• Phospholipids are the specific lipids that comprise the “lipid bilayer” (a layer of
lipids on the inner and outer surface of the cell membrane)
• Protein molecules are generally embedded in the phospholipid
bilayer with some transversing the membrane
• Proteins serve to:
• give integrity (structural support) to the cell membrane
• transport molecules across the membrane
• serve as surface receptors for hormones and regulatory
substances
• Carbohydrates constitute about 10% of the cell membrane
• Play an important role in the regulatory mechanisms of the cell
membrane
Methods of Absorption
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Can occur by one of four methods
Simple diffusion
Facilitated diffusion
Active Transport
Pinocytosis
Simple Diffusion
• Most direct method of moving nutrients across the membrane
• Those nutrients involved in simple diffusion can move freely in and out of the cell
without any help
• Movement or diffusion of the nutrients is based on a concentration gradient on either
side of the membrane
• Diffusion across the cell membrane keeps the concentration of the nutrient(s) equal
• Includes H2O, O2, CO2 and other small substances
Facilitated Diffusion
• Nutrients are transported across a membrane by means of a
carrier protein
• Concentration of specific nutrients may not be the same on both
sides of the membrane
• The nutrient becomes attached to the protein on the outside of
the cell and is released on the inside of the cell
• Includes fructose, vitamins and minerals
Active Transport
• Uses both a carrier protein and energy (ATP)
• Highly regulated means of transport
• Used to tightly control the concentration of specific nutrients
within cells
• Includes such nutrients as glucose, amino acids, certain vitamins
and minerals
Pinocytosis
• Endocytosis
• Least common form of transport
• Especially important during infancy when large immunoglobulins are required to be
absorbed from the intestine (mother’s milk)
• Substances that enter via pinocytosis are engulfed by the cell membrane
• The engulfed substances are released once inside the cell
• The lost portion of the cell membrane is replaced
The Large Intestine
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Approximately 5 feet in length
serves to collect waste products of digestion to be excreted
Begins with the Cecum and ends with the Sigmoid colon
Between is the Ascending, Transverse and Descending portions of the
colon
• Chyme enters the colon from the small intestine through the illeocecal
valve and it travels through the large intestine to the rectum where it is
eventually excreted through another valve called the Anus
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Undigested and unabsorbed enters the large intestine
Not much digestion takes place after the chyme leaves the small intestine
Very sluggish peristalsis
Food remains in the large intestine for 24-72 hours
This allows the intestinal bacteria to breakdown undigested and unabsorbed nutrients
further
• Also allows time for the production of some nutrients by the intestinal microflora
(such as vitamin B and K) to be absorbed by the body
• Byproduct of bacterial metabolism is gas production or flatulence
• Undigested materials such as fiber, body secretions, water, bile salts, etc. are excreted
in the feces
• Water, some minerals and vitamins are absorbed in the large intestine
• The Anus is the final sphincter muscle at the end of the digestive tract
• Requires mass accumulation for stimulation
• “safety device”
• Defecation is the process whereby the feces (containing the waste material) is
eliminated from the colon
Once nutrients are absorbed, how are they
transported throughout the body????
Transport of Nutrients
• Transport is the movement of the nutrients throughout the body
once they have been absorbed for use by the body
• It also involves the movement of those nutrients that are not used
by the body and waste products of metabolism for excretion
• Involves the Cardiovascular system and the Lymphatic system
Cardiovascular System
• Has a pump (heart) to move the fluid (blood) through the system
• Closed system-blood flows continuously through
– Pulmonary circuit - circulates between the right side of the heart and lungs
– Systemic circuit - circulates between the left side of the heart and the other body parts
• Capillaries - Small vessels branching from an artery (connect arteries to veins) just
large enough to allow on rbc to cross
• oxygen, nutrients, water materials are exchanged across capillary walls
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Arteries - vessels that carry the blood away from the heart
Veins - vessels that carry blood back to the heart
Blood leaves the right side of the heart via the pulmonary artery to the lungs
In the lungs blood loses CO2 and picks up O2. Bood leaves the lungs and returns to the
left side of the heart (oxygenated) via the pulmonary vein
• Oxygenated blood leaves the left side of the heart via the Aorta
• Aorta - the main artery of the body that supplies oxygenated blood to the various
organs and tissues
• Blood leaving the aorta may either go to the upper body and head or to the lower body
• In the lower body, the blood may go either to the digestive tract and the liver or the
pelvis, kidneys and legs
• Oxygen depleted (CO2-rich) blood returns to the right side of the heart via veins
• Blood carries all the water soluble nutrients
• In the GI tract, blood picks up nutrients through the capillaries, which then empties it
into the venules to the veins and then to the heart.
• Nutrients in the blood are transported by veins to the capillaries of the tissues, etc
where they are deposited through the capillary walls
Hepatic Portal and Lymphatic System
• Nutrients from digestion can enter the blood either via the hepatic portal circulation or
the lymphatic system
• The Lymphatic System - comprised of a system of loosely organized vessels and
ducts in which the lymph (fluid) circulates
• No Pump - the lymph “squishes” from one tissue to another as the muscles contract
and relax
• Extremely important in the immune system
• Lymph - similar to blood but no rbc’s or platelets
• circulates between the cells of the body outside the vascular system
In the Small Intestine
• Water soluble products of digestion
• mucosal cells of the villi
• capillaries
• capillaries merge
• venules at the base of the villi
• venules merge
• larger veins
• hepatic portal vein
• liver
• processing of absorbed nutrients
• general circulation
The Liver
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The major metabolic organ of the the body, acts as a gatekeeper between substances absorbed from the
intestine and delivered to the rest of the body
It has many functions:
Storage
– glycogen
– vitamins
Production
– bile
– non essential amino acids
Detoxification
– site where alcohol, drugs and poisons are sequestered and broken down
The Thoracic Duct
• The Thoracic Duct is the “Link” between the lymphatic and
vascular systems
• Nutrients are lost through the villi of the GI tract into the
lymphatic system (collected interstially.
• The lymph is colleced via the thoracic duct located behind the
heart where it is emptied into the subclavian vein which makes
its way to the heart via the right atrium
Which nutrients are absorbed where?
• Water soluble nutrients are generally absorbed into and transported by the
blood
• Fat soluble nutrients and fat are absorbed and transported by the lymph
• Lymph
– fat soluble vitamins
– most fat
– most fatty acids
• Stomach
– glucose
– alcohol
– some water
• Small Intestine
– amino acids, monoglycerides, water soluble vitamins minerals, glycerol, most water
• Large Intestine
– water
Regulation and Control
• Coordination of digestion and absorption involves both the
nervous system and the exocrine and endocrine glands
• Regulation is a feedback mechanism
• A feedback mechanism is one where
The Nervous System
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Comprised of the Central Nervous System and the Peripheral Nervous System
Central Nervous System - brain and spinal cord
Peripheral Nervous System - vast complex of wiring extending from the CNS
Further divided into the Somatic Nervous System (which gives rise to voluntary
muscle control) and the Autonomic Nervous System (which gives rise to the
involuntary functions)
• The Autonomic Nervous System is further divided into the Parasympathetic Branch
which regulates the body between stress and the Sympathetic Branch which helps the
body respond to stress
• Exocrine glands - secrete their products via duct into a cavity
• Endocrine glands - secrete their products directly into the
bloodstream to be transported to the target organ or area
• Feedback inhibition - means of regulation whereby the products
(once in sufficient amount) or the condition (once attained) shut
off the system until required again
Maintenance of Stomach pH
• Food enters the stomach
• nerve receptors in the stomach are stimulated by the presence of the food
• Gastrin (hormone) is secreted
• Secretion of Gastrin stimulates other glands to produce HCl
• When a pH of 1.5 is reached, Gastrin is no longer produced and the other glands
secreting HCl are shut off
• the process is repeated when more food or fluid enters the stomach or when the pH of
the stomach increases and digestion is not complete
Opening of the Pyloric Sphincter
• Relaxation of the pyloric sphincter causes leakage of the acidic chyme into the small
intestine
• The pyloric muscle cells on the intestinal side sense the stomach acid and this causes
the pyloric sphincter to close tightly and not allow more acidic chyme through
• Pancreatic bicarbonate neutralizes the medium around the pyloric sphincter
• When the medium is neutralized, the pyloric sphincter relaxes and the whole process
starts ovwer
• In this way a small amount of chyme is allowed into the small intestine at a time
Neutralization of the Chyme
• The presence of the acidic chyme in the duodenum stimulates the cells in the walls of
the intestine to secrete the hormone secretin into the blood
• The secretin circulates through the pancreas thus stimulating the release of the
pancreatic juices (containing bicarbonate)
• This neutralizes the acidic chyme
• The pH of the intestine becomes alkaline thus shutting off the stimulation of the
duodenal cells to produce secretin
• The pancreas then no longer secretes pancreatic juices
When digestive process go awry
• Problems with any step along the way of digestion, absorption and
transport can inhibit the ability of the body to obtain nutrients from food
• Can lead to many types of problems, both acute and chronic
Ulcers
• Lining of the stomach or duodenum of small intestine gets an ulcer or a
sore as a result of the acid eating through it
• Bacterial infections, stress, poor diet are some causes
• Typical symptoms - pain approx. 2 hours after eating, heartburn
• Calcium is a thought to be a culprit as it stimulates gastrin which then
stimulates HCl
• Antibiotics have been found to cure about 90% of all ulcers (pyloric
bacterial)
Heartburn
• Cardiac sphincter muscle doesn’t function as well as it should - weak, relaxes
• Stomach contents move back up to the esophagus (reflux)
• Stomach has a mucopolysaccharide coating but the esophagus doesn’t - leads to a
burning sensation
• Eating too large of meals, lying down after eating, spicy foods, certain hormones
(pregnancy) all augment the problem
• Eat smaller more frequent meals, don’t lie down after eating, drugs (Tagamet) etc.
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Constipation - motility problems in the intestine
- not enough water, high fat - low fiber diet, etc
- change in diet can help alleviate the problem
Hemorrhoids - also called piles
- swollen veins of the rectum and anus
- may develop unnoticed until a strained bowel movement and then they become
inflamed and painful
• - caused by a number of different conditions but largely related to constipation
• - best treatment is prevention - diet
• NOTE: Many digestive problems can be treated and/or controlled by diet
modification. The best treatment is prevention - eating a well-balanced diet, high in
fiber and low in fat.