Chapter 23 Digestion and Excretion (chapter 24)
Nutrition encompasses all of the processes by which we consume and digest food, then
absorb its nutrients. The digestive system transports, processes, and stores food. As food
moves through the digestive system, it is physically and chemically broken down into
nutrients, some of which are absorbed, while the unabsorbed residues are excreted.
Adequate nutrition also requires that the body be supplied with vitamins, minerals, and
amino and fatty acids that the body itself does not produce. You should understand that
the digestion and absorption of food are vital to homeostasis (see chapter 19). Interactions
among the digestive and urinary systems (and the circulatory and respiratory systems that
you learned about in chapter 21) supply the body's cells with raw materials, dispose of
wastes, and maintain the volume and composition of extracellular fluid.
Chapter 23 falls down heavily in one respect, it fails to even mention UREA, which is the
major nitrogenous waste product of the human (and other mammals) breakdown of
proteins. A great deal of the primary function of the kidney is in removal of this
Section 23.4 will not be covered in tests or exams. For the understanding of kidney
function you will find figure 23.16 particularly useful to look at as you read the relevant
material in the chapter. Be sure to understand where food goes and what happens to it as
it passes through the digestive system. Also, understand what happens to water and
dissolved waste substances as they are removed from the body via the urinary system.
Digestion and nutrition.
From start to finish the digestive route is;
Mouth - pharynx - esophagus - stomach - small intestine - large intestine - rectum - anus
In Short - Foodstuffs are mechanically broken down in the mouth and passed
downwards, materials such as enzymes and acids are secreted into various parts of the
digestive tract to break down the food and condition it into a consistency which allows it
to be digested - broken down into small enough molecules to be absorbed from the tract
into the body, these molecules, and water, are absorbed so that the end product is semi-
solid undigested material mixed with gut bacteria and bile pigment remnants collectively
called feces which, are expelled (eliminated) from the digestive tract at the anus.
Food is propelled down the digestive tract by coordinated waves of muscular contraction
undertaken by smooth muscle under automatic control (you do not control these wave
actions consciously) these muscle contractions are called peristalsis - they begin in the
esophagus. The lining of the digestive tract contains longitudinal and circularly arranged smooth
muscle that coordinates under autonomic nerve control to produce peristalsis. Variations
of peristalsis also cause backwards movement of the food for very short lengths, which
results in a churning back and forth movement that mixes the food as it passes along the
tract - the general term for the passage of foods along the tract by peristalsis is motility.
Digestion in detail.
A mouthful of food is called a bolus. The teeth mechanically break the food down and
then it is softened with the aid of a few enzymes (such as amylase that breaks down
starches) and a lot of lubricating saliva, secreted by salivary glands (three main pairs
and many minor glands). The saliva contains bicarbonate ions, the aforementioned
amylase enzymes, and glycoproteins that are “slippery” when in solution and that aid the
passage of food from the mouth.
The food passes into the throat or pharynx where a mechanism involving a flap of tissue
called the epiglottis prevents food from getting into the entrance to the lungs. The food
travels down the esophagus - by peristalsis, to the entrance to the muscular stomach -
which is a “ballooning” of the tract which acts to store food temporarily. The head of the
stomach has a ring of muscle called a sphincter that controls entry of food and prevents
backsplash of acid up into the esophagus.
The stomach is strongly muscular and churns the food. Hydrochloric acid and enzymes
(mostly proteases - which degrade proteins) are added to condition the food to a mostly
whitish thick fluid called chyme. Food particles are broken down into small pieces which
thus have maximum surface area, this allows the most rapid action of acid and enzymes.
Pepsinogen is a protease ( an enzyme that breaks down proteins) secreted into the
stomach by the stomach wall. Pepsinogen is an inactive enzyme, it is activated by the
action of the powerful acid hydrochloric acid (HCL) in the stomach into its active form -
pepsin. Cells of the stomach lining produce large amounts of mucus which protects
against the stomach acid.
Very little actual digestion and absorption occurs in the stomach. Most digestion and
absorption occurs in the very long small intestine. Enzymes that further break down
proteins (trypsin, chymotrypsin and others) are released into the small intestine, along
with carbohydrate degrading enzymes, from the pancreas and small intestine lining.
Fat degrading enzymes called lipases are secreted into the small intestine from the
pancreas, and DNA and RNA digesting enzymes called nucleases are released in to the
small intestine from its lining and from the pancreas.
Bile salts produced by the liver and stored in the gall bladder are released into the small
intestine where they emulsify insoluble fats into tiny droplets called micelles which
therefore have a very large surface area that can be attacked by the lipases. Bile, as well
as containing bile salts, also contains excess cholesterol from the body. The lining of the small intestine is structured to provide the maximum surface area for
absorption of digested materials - which are small molecules. The inner surface of the
intestinal lining itself is in the form of many tiny folds called villi that increase surface
area. Each of the millions of absorbing cells which lie on the surface of these folds also
has its own cell membrane extensively folded, these folds are microvilli and also
enormously increase surface area for absorption.
Specific transport proteins in the cell membranes of the lining of the small intestine
transport amino acids, carbohydrates, nucleic acids etc produced by digestion, into the
Fats enter the lymph system from the small intestine at special lymph vessels called
Capillaries in close association with the small intestine lining collect the nutrients, other
than fats, into the hepatic portal vein which transports most of the nutrients absorbed
from the small intestine directly to the liver for processing.
After processing in the liver the nutrient molecules are transported out of the liver into
the hepatic vein, to other parts of the body. The liver can form glucose into glycogen for
storage if it is present in excess of immediate requirements.
The liver forms urea to detoxify the ammonia produced by protein degradation in body
cells, and can also detoxify and neutralize many other substances that enter the body
through digestive and absorption processes.
Most of the water is absorbed from the small intestine.
Materials in the digestive tract that are not digested then pass into the large intestine.
Here the non digested material is concentrated, as most of the remaining water and
valuable ions are absorbed into the body. The waste material is now called feces and is
stored in the last part of the large intestine, the rectum, and eliminated.
Huge numbers of bacteria are present in the large intestine (and in feces), some of which
are useful because they synthesize fatty acids and vitamins (such as B and K) used by the
Control of digestion. As with any other body system under homeostatic control, negative
feedback systems regulate digestion. A hormone released by the stomach lining, for
instance, called gastrin, senses the presence of amino acids in food and signals release of
acid into the stomach.
A hormone called somatostatin inhibits acid secretion when it increases beyond
requirements. The highly acidic chyme signals release of secretin when it enters the
small intestine, and this hormone causes release of bicarbonate by the pancreas to
neutralize the acid. Presence of fat in the small intestine causes release of cholecystokinin which stimulates
the pancreas to produce and release secretions, and also causes the gallbladder to empty
bile salts into the small intestine.
The body modulates the appetite, hormonally and via the nervous system in response to
the needs of the entire body for nutrition.
Specific nutritional requirements
Carbohydrates - starch and glycogen, which are storage polymers for glucose. Also high
fiber indigestible carbohydrates that are not absorbed but provide bulk for proper
intestinal function and motility.
Lipids. Needed for cell membrane formation, energy reserves, some fat soluble vitamins
are stored in body fats. There are a few essential fatty acids, the body cannot synthesize
them and they have to come from the diet - linoleic acid is an example.
Proteins. There are twenty common amino acids found in food, eight of them we cannot
synthesize and they must come from the diet. Animal proteins (i.e., meat) have all of the
needed amino acids, plants do not and must be combined if the diet is vegetarian, to
ensure a complete protein mix.
Vitamins are small organic molecules that we cannot synthesize and that we must have
(in small amounts) for survival, we must get them from the diet. Five of the vitamins are
fat soluble (A,D,E,K) the rest are water soluble. Fat soluble vitamins accumulate in fats
and can sometimes reach toxic levels in this way (e.g. vitamin A), water soluble vitamins
do not accumulate.
Minerals are inorganic substances essential for life, often as cofactors for enzymes, and
the iron in blood hemoglobin, and iodine are important examples; by law iodine is added
to table salt in Ontario because the Great Lakes region is low in iodine and this can cause
problems with thyroid gland function - including the benign but disfiguring growth called
All Except material in sections 24.3, 24.4, 24.7 may be in the exam ,
24.1: The Nature of Digestive Systems
Nutritional processes proceed in a digestive system. This body cavity or tube
mechanically and chemically reduces food to particles, and then to molecules small
enough to be absorbed into the internal environment. It also eliminates unabsorbed
Incomplete vs complete digestive systems
Complete digestive system carries out five tasks: 1) mechanical processing and motility- movements that break up, mix, and propel
2) secretion- release of digestive enzymes and other substances into the lumen- the
space inside the tube
3) digestion- breakdown of food into particles, then into nutrient molecules small
enough to be absorbed
4) absorption: passage of digested nutrients and fluid across the tube wall and into
5) elimination: expulsion of undigested, unabsorbed wastes from the end of the gut
carnivores have shorter intestines than herbivores
-incomplete digestive systems are a saclike cavity with only one opening. Complete
digestive systems are a tube with two openings and regional specializations in
Section 24.2: Human Digestive System
-mouth: entrance to system, food is moistened and chewed: polysaacharide digestion
-pharynx: entrance to the tubular part of the system (and to the respiratory system);
moves food forward by contracting sequentially
-esophagus: muscular, saliva-moistened tube that moves food from pharynx to
-stomach: muscular sac; stretches to store food taken in faster than can be processed;
gastric fluid mixes with food and kills many pathogens; protein digestion starts.
Secretes ghrelin, an appetite stimulator
-first part (the duodenum, c shaped, 10 inches long) receives secretions from
liver, gallbladder, and pancreas
-second part (jejunum, about 3 feet long), most nutrients are digested and
-third part (ileum, 6-7 feet long) absorbs some nutrients, delivers unabsorbed
material to large intestines
-large intestine (colon): concentrates and stores undigested matter by absorbing
mineral ions, water; about 5 feet long. Divided into ascending, transverse, and
-rectum: distension stimulated expulsion to feces
-anus: end of system; terminal opening through which feces are expelled
-salivary glands: glands (three main pairs, many minor ones) that secrete saliva, a
fluid with polysaachride-digesting enzymes, buffers, and mucus
-liver: secretes bile (for emulsifying fat); roles in carbohydrate, fat and protein
metabolism -gallbladder: stores and concentrates bile that the liver secretes
-pancreas: secretes enzymes that break down all the major food mole