Section 15.1 – Objectives
By the end of this section, you should be able to:
• List the building blocks for carbohydrates, proteins, and lipids.
• Draw and label a picture of the digestive system.
• List the names of the salivary glands and describe their functions.
• Describe the sequence of events in swallowing.
• Draw and label a diagram of the stomach and list the function and secretions from
• Draw and label a diagram of the small intestine and the intestinal villi.
• List the different types of carbohydrates.
• Describe the sequence of events in carbohydrate digestion and absorption.
• Describe the sequence of events in protein digestion and absorption.
• Describe the sequence of events in lipid digestion and absorption.
• Describe the site where bile is produced and list its function.
• Describe the similarities between protein and carbohydrate absorption.
• Describe the absorption of vitamins.
• List the amount of water absorbed in the different sections of the digestive tract
and describe the process of water absorption.
• Describe the relationship of the enteric nervous system to the divisions of the
autonomic nervous system and list the enteric nervous system's functions.
• Describe the long loop and short loop reflexes of the enteric nervous system and
their effects on the digestive tract.
• Describe the basal electrical rhythms of the digestive tract and how they can alter
• List the hormones of the intestine and stomach and describe their functions.
• Describe the three phases of gastric acid secretion and how they are regulated.
Compare and contrast the hormones and regulatory mechanism for each.
Section 15.2 – Introduction
• The main function of the digestive system:
Break down organic nutrients so that they can be absorbed into the body.
These organic nutrients can be carbohydrates, proteins, fats, water,
vitamins, and minerals.
Section 15.3 – The Meal
• The meat within the hamburger (assuming it is lean hamburger) is made up of
mostly protein with a little fat (lipid).
• The hamburger bun and fries are made up of carbohydrates (fries also contain fat).
1 • The soda will consist mostly of water, carbohydrates, and some other dissolved
• As we work our way through the digestive tract, we will examine how each of
these substances (proteins, fats, carbohydrates, and water) are broken down
(digested) and then absorbed into the body.
Section 15.4 –Anatomy and Basic Function
• The basic functions of the structures below are summarized below:
1. Mouth: food is broken up by chewing (mastication) and is mixed with
2. Salivary glands: produce saliva to moisten and begin digesting some food
3. Esophagus: a straight muscular tube that connects the mouth and pharynx
(not shown) to the stomach.
4. Stomach: stores, mixes, and digests some food and delivers food to the
5. Liver: has many functions, but for digestion it produces and secretes bile.
6. Gallbladder: stores and concentrates bile.
7. Pancreas: secretes digestive enzymes, hormones, and bicarbonate.
8. Small intestine: allows digestion and absorption of most food particles.
9. Large intestine: stores and concentrates undigested material and absorbs
salt and water.
10. Rectum: the site where the defecation reflex is triggered.
2 Section 15.5 –ACartoon Look
• The illustration below provides a very simple view of the digestive system. If
possible, print this page for future reference.As you proceed through the module,
refer to the diagram to keep track of each digestive enzyme, where it is released
from, where it acts, and what it digests. In addition, keep track of where each food
molecule gets digested.
3 Section 15.6 – Basic Processes
The basic processes of the digestive system include the following:
• Secretion involves the release of digestive fluids into the lumen of the digestive
tract. This fluid may include water, mucus, acid, electrolytes, enzymes, bile salts,
and digestive enzymes.
• Digestion is the process whereby food is broken down into smaller molecules by
the digestive enzymes so they can be taken up by the body.
• Absorption is the process whereby these small molecules are taken up by the
circulatory system and are distributed throughout the body.
• Motility is the movement of the "food" through the digestive system by the
contraction of smooth muscle that lines the walls of the tract.
• Excretion is the removal of the unwanted waste products from the body after
almost all of the wanted material is absorbed.
Section 15.7 – The Mouth
• Digestion of your meal begins in the mouth, where it is broken down by chewing
(or mastication) and is mixed with saliva to form a bolus.
The saliva consists mostly of water (99.5%) with ions and proteins (0.5%)
and is secreted by three glands shown at right.
These glands (parotid, submandibular, and sublingual) can produce
up to 2 liters (0.5 gallon) of saliva each day.
4 Saliva helps to lubricate the bolus of food and begins digesting
carbohydrates because it contains the enzyme amylase.
The production and secretion of saliva is under the control of the
autonomic nervous system.
Section 15.8 – Swallowing
• Once the bolus of food has been formed and sufficiently lubricated with the
saliva, it will be pushed to the back of the mouth by the tongue and the
swallowing reflex will be initiated.
• The uvula of the soft palate closes over the nasopharynx.
• The larynx is lifted by muscles in the neck, and the epiglottis bends back over the
glottis, covering the larynx.
• The bolus moves down the esophagus through the cardiac orifice (also called the
lower esophageal sphincter) and into the stomach by a wave of smooth muscle
contraction called peristalsis.
5 Section 15.9 – The Stomach: Structure
• The stomach is divided into three general areas:
1. The upper, dome-shaped fundus
2. The middle body, which comprises the largest part of the stomach
3. The lower antrum (sometimes called the pyloric region).
• The pyloric sphincter, located at the distal portion, regulates the emptying of the
stomach into the first part of the small intestine, the duodenum.
• The stomach (when empty) is thrown into folds, or rugae, which increase the
surface area and allow for expansion of the stomach as it fills with food.
Section 15.10 – The Stomach: Function
• The stomach liquefies, mixes, and stores each bolus of food from the meal.
This mixture, called chyme, is slowly released into the small intestine,
where most of the digestion and absorption takes place.
In doing so, it regulates the amount of food entering the small intestine so
that it can be fully digested then absorbed.
Some digestion does take place in the stomach.
As we will see, proteins begin to be digested in the stomach by the
Also, very little absorption takes place here.
In fact, only certain substances like alcohol and aspirin can cross
the lining of the stomach to be absorbed into the bloodstream.
The mixing of the chyme is achieved by peristaltic contraction of the
stomach walls that begin at the fundus and end at the antrum.
These contractions also help to move the chyme through the
pyloric sphincter into the small intestine where most of the
digestion and absorption take place.
o This muscle activity that causes the movement of
substances through the digestive tract is called motility and
will be examined in more detail later
Section 15.11 – The Stomach: Function of Gastric Secretions
• The stomach secretes a variety of substances, including:
Mucus, hydrochloric acid (HCl), pepsinogen, gastrin, and intrinsic factor.
Review the list below for a description of each of these substances.
6 Section 15.12 – The Pancreas: Structure
• An important part of the digestive system is the pancreas.
• Although it is not directly part of the small intestine (part of it does drain into it),
it is, nevertheless, very important to discuss its structure and anatomy at this
• The pancreas lies just below the stomach and has both digestive (or exocrine) and
The exocrine products of the pancreas are secreted into a long pancreatic
This duct merges with the common bile duct (from the liver and
gallbladder) just before entering the duodenum.
7 Section 15.13 – The Pancreas: Functions
• The pancreas produces and secretes:
The carbohydrate-digesting enzyme amylase
The protein-digesting enzymes trypsin, chymotrypsin, and proteases
The fat-digesting enzyme lipase.
• The pancreas also secretes sodium bicarbonate into the duodenum to neutralize
the acid from the stomach.
Section 15.14 – The Small Intestine: Structure
• The stomach contents empty through the pyloric sphincter into the small intestine.
This is the longest section of the digestive tract, reaching 9 meters (30 ft)
It ends at the ileocecal sphincter where it empties into the ascending colon
of the large intestine.
It is divided into three segments:
The first segment, the duodenum, is the shortest;
The second segment is the jejunum;
The longest segment, making up 50% of the small intestine, is the
8 Section 15.15 – The Small Intestine: Structure
• The inner wall of the small intestine is thrown into folds
The folds contain finger-like projections called villi (the singular form of
villi is villus).
The end result is a large surface area with which the food comes into
The villi contain a capillary network and lymphatic lacteal to
absorb the digested material.
Alayer of epithelial cells, which secrete digestive enzymes, covers the
These cells have microvilli that face out into the lumen of the
intestine, forming a brush border.
Section 15.16 – The Small Intestine: Function
• Almost all digestion and absorption (except for the small amount that we talked
about in the stomach) takes place in the first two segments of the small intestine –
the duodenum and jejunum.
• The digestion of the chyme occurs by digestive enzymes from the pancreas and
• Bile, from the liver, helps digest fat although it is not a digestive enzyme.
• Keep in mind the following: There are very specific digestive enzymes for each
part of our meal. Some enzymes digest the protein of the meat, some digest the
carbohydrates of the fries, and some digest the fat found in both the meat and
9 Section 15.17 – The Small Intestine: Carbohydrate Digestion
• Carbohydrates come in a variety of forms.
Some carbohydrates are made up of a single building block called
monosaccharides, like glucose and fructose, shown below, and galactose
• Other carbohydrates, called disaccharides, consist of two monosaccharides.
These include maltose (a carbohydrate found in germinating barley),
lactose (found in milk), and sucrose (table sugar).
• Carbohydrates made up of more than two units are called polysaccharides.
The most common polysaccharides are starch and glycogen.
• In order to absorb the larger carbohydrates, polysaccharides must all be broken
down into monosaccharides.
• Once in this form, they can be absorbed by transport systems in the walls of the
Section 15.18 – The Small Intestine: Carbohydrate Digestion (cont’d)
10 • As mentioned earlier, digestion of carbohydrates begins in the mouth with the
salivary enzyme amylase.
• This enzyme breaks up the large polysaccharides into smaller polysaccharides and
• Once the food reaches the stomach, the digestion of the carbohydrates essentially
stops because of the acidic environment (low pH), which denatures (partially
breaks up) the salivary amylase.
Section 15.18 – The Small Intestine: Carbohydrate Digestion (cont’d)
• When the food reaches the small intestine, digestion of the carbohydrates begins
again because the pancreas secretes amylase into the duodenum.
The pancreas also secretes bicarbonate ions which neutralize the acid from
The neutralized environment permits the pancreatic amylase to
perform its digestive function.
• As shown in vid, the pancreatic amylase digests the polysaccharide starch to the
This disaccharide cannot be absorbed yet because it still must be digested
to a monosaccharide.\
Section 15.20 – The Small Intestine: Carbohydrate Digestion (cont’d)
• Each disaccharide requires a specific enzyme to break it down to its component
These enzymes are located in the brush border of the microvilli located on
the intestinal epithelial cells.
• The enzyme lactase breaks down lactose (found in milk) to galactose and glucose.
• The enzyme maltase breaks down maltose (from starch) to two glucose
• The enzyme sucrase breaks down sucrose (found in table sugar) to glucose and
• Now the carbohydrates are ready for absorption.
Section 15.21 – The Small Intestine: Carbohydrate Digestion – Lactose Intolerance
• Some people are unable to drink or eat foods containing milk because they are
• Remember that lactose is the carbohydrate found in milk.
• When these people ingest food containing milk, they can get serious cramps,
bloating, gas, and diarrhea.
o The cause is the inability to digest lactose because of the inability to
produce the enzyme lactase.
If the lactose is not digested, then it cannot be absorbed into the
body and it will stay in the digestive tract.
This will lead to diarrhea.
11 • If carbohydrates are present in the large intestine, bacteria will feed on it and
produce gas, causing bloating and cramps.
• People who suffer from lactose intolerance can purchase synthetic lactase, which
is generally taken before a meal that may contain lactose.
Section 15.22 – The Small Intestine: CarbohydrateAbsorption
• The process of carbohydrate absorption is almost identical to the process of
glucose reabsorption in the kidneys.
• The intestinal epithelial cells contain Na /K (sodium ions/potass+um ions) pumps
on their basal side that establish a concentration gradient for Na - high on the
outside and low on the inside.
This gradient powers the Na /glucose co-transporter (a type of secondary
active transport mechanism) located on the luminal side of the cell.
This transporter moves glucose into the cell as Na move in, down
their concentration gradient.
Once glucose is inside the cell it will then diffuse out through the
basal side by facilitated diffusion.
Section 15.23 – The Small Intestine: Protein Digestion
• Proteins consist of long chains of amino acids linked together.
There are 20 different amino acids
11 nonessential amino acids that can be produced by the body
9 essential amino acids that must come from the diet.
Just like carbohydrates, the different groups of amino acids require
different enzymes to break them apart.
• Likewise, the proteins must be broken down into the amino acid building blocks
before they can be absorbed by transport systems in the small intestine.
Section 15.24 – The Small Intestine: Protein Degradation (cont’d)
• As mentioned earlier, digestion of proteins begins in the stomach.
Here, hydrochloric acid (HCl) converts the inactive pepsinogen to the
active enzyme pepsin.
The HCl also helps to uncoil the long, twisted strands of proteins.
This unfolding of the protein gives the pepsin access to the long
protein chains in order to digest them into smaller chains called
o These polypeptides then pass through the pyloric sphincter
into the small intestine where they continue to be digested
and absorbed into the body.
Section 15.25 – The Small Intestine: Protein Digestion (cont.)
12 • The pancreatic enzymes that will continue protein digestion must have an
environment with a neutral pH in order to work optimally.
Therefore, the chyme from the acidic stomach must be neutralized.
• This is achieved by bicarbonate, which is secreted from the pancreas.
• As a result of the neutral pH environment, the pepsin, which was secreted by the
stomach to begin the process of digestion, now becomes inactivated.
• Now, the enzymes trypsin and chymotrypsin from the pancreas continue the job in
the small intestine that was begun by pepsin in the stomach.
Section 15.26 – The Small Intestine: Protein Digestion (cont’d)
• Aclass of enzymes called proteases can continue digesting proteins into amino
These enzymes are produced in the pancreas and are secreted into the
Likewise, some proteases are found along the brush border of the
intestinal epithelial cells.
There are two different classes of protease enzymes, each of which is
responsible for breaking apart amino acids located in different parts of the
1. The endopeptidases break the bonds between amino acids in the
inner part of the protein.
2. Exopeptidases break the bonds between amino acids at the ends of
Section 15.27 – The Small Intestine: Protein Absorption
• Now that the proteins are broken into single amino acids with a few remaining
very small polypeptides (dipeptides or tripeptides), absorption can take place.
• Much like carbohydrates, the absorption of amino acids is through secondary
active transport requiring the presence of a Na concentration gradient.
13 • As Na move into the intestinal epithelial cell and down their concentration
gradient, they power a co-transporter that also moves the amino acids into the
• The remaining small peptides are absorbed by endocytosis.
Section 15.28 – So Far…
• So far we have seen how the body digests and absorbs the carbohydrates and
proteins of our lunch.
• Review the important reminders below.
Section 15.29 – The Small Intestine: Fat and Lipids
o Also called lipids
o Include substances such as triglycerides, phospholipids, cholesterol, and
long-chain fatty acids.
These substances are not water soluble and do not mix well with
• Therefore, a complex set of steps is necessary to digest fats
and then absorb them into our "watery environment."
14 Section 15.30 – The Small Intestine: Fat and Lipids (cont’d)
• The churning action of the stomach helps break up the large drops of fat into
smaller ones, a process called emulsification.
The process of emulsification is extremely important because it is much
easier for the lipid-digesting enzymes to do their work on very small
droplets than on very large ones.
The main problem is keeping the small droplets from forming back into
Therefore, it is very important to keep the lipids emulsified throughout the
whole digestion process.
Section 15.31 – The Small Intestine: Fat Digestion – Bile
Produced in the liver and is transported to the gallbladder where it is
stored and concentrated.
Bile is not a digestive enzyme; rather, it is a substance that contains,
among other things, water, bile salts (the most abundant component),
cholesterol, fatty acids, and many ions.
The gallbladder releases bile into the duodenum of the small intestine
during a meal.
The bile salts keep the lipid droplets emulsified, preventing them from
forming back into large droplets.
Section 15.32 – The Small Intestine: Fat Digestion – Colipase
• At this stage, there is one small problem.
• Because the lipid droplets are coated in bile salts, the enzyme lipase cannot reach
and digest the lipids.
• This problem is solved by another secretion of the pancreas called colipase.
15 Colipase is also not a digestive enzyme.
It is a protein that allows the lipase to get access to the lipid interior of the
• Now the pancreatic lipase can digest the lipids.
Section 15.33 – The Sm