The dissolved nutrients, to reach the cells, must be transferred from the
alimentary canal to the blood stream. This process is known as
_absorption_. In general, absorption means the penetration of a liquid
into the pores of a solid, and takes place according to the simple laws of
molecular movements. The absorption of food is, however, not a simple
process, and the passage takes place through an _active_ (living)
membrane. Another difference is that certain foods undergo chemical change
while being absorbed.
*Small Intestine as an Organ of Absorption.*--While absorption may occur to
a greater or less extent along the entire length of the alimentary canal,
most of it takes place at the small intestine. Its great length, its small
diameter, and its numerous blood vessels all adapt the small intestine to
the work of absorption. The transverse folds in the mucous membrane, by
retarding the food in its passage and by increasing the absorbing surface,
also aid in the process. But of greatest importance are the minute
elevations that cover the surface of the mucous membrane, known as
*The Villi.*--Each single elevation, or villus, has a length of about one
fiftieth of an inch and a diameter about half as great (_A_, Fig. 76), and
contains the following essential parts:
1. An outer layer of epithelial cells, resting upon a connective tissue
support.
2. A small lymph tube, called a _lacteal_, which occupies the center of
the villus and connects at the base with other lymph tubes, also called
lacteals (_B_, Fig. _76_).
3. A network of capillaries.
The villi are structures especially adapted to the work of absorption, and
they are found only in the small intestine. The mucous membrane in all
parts of the canal, however, is capable of taking up some of the digested
materials.
[Fig. 76]
Fig. 76--*The villi.* _A._ Diagram of a small section of mucous membrane of
small intestine. 1. Villi. 2. Small glands, called _crypts_.
_B._ Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3.
Villus showing termination of the lacteal. 4. Villus showing capillaries.
5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7.
Layer of epithelial cells.
*Work of Capillaries and Lacteals.*--The capillaries and lacteals act as
receivers of material as it passes through the layer of epithelial cells
covering the mucous membrane. The lacteals take up the digested fats,(66)
and the capillaries receive all the other kinds of nutrients. These
vessels do not, of course, retain the absorbed materials, but pass them
on. Their final destination is the general circulation, which they reach
by two well-defined channels, or routes.
*Routes to the Circulation.*--The two routes from the place of absorption
to the general circulation are as follows:
1. _Route taken by the Fat._--The fat is conveyed by the lacteals from the
villi to the receptacle of the chyle. At this place it mingles with the
lymph from the lower parts of the body, and with it passes through the
thoracic duct to the left subclavian vein. Here it enters the general
circulation. Thus, to reach the general circulation, the fat has to pass
through the villi, the lacteals, the receptacle of the chyle, and the
thoracic duct (Fig. 77). Its passage through these places, like the
movements in all lymph vessels, is slow, and it is only gradually admitted
to the blood stream.
[Fig. 77]
Fig. 77--*Diagram of routes* from food canal to general circulation. See
text.
2. _Route of All the Nutrients except Fat._--Water and salts and the
digested proteids and carbohydrates, in passing into the capillaries, mix
there with the blood. But this blood, instead of flowing directly to the
heart, is passed through the portal vein to the liver, where it enters a
_second set of capillaries_ and is brought very near the liver cells. From
the liver it is passed through the hepatic veins into the inferior vena
cava, and by these it is emptied into the right auricle. This route then
includes the capillaries in the mucous membrane of the stomach and
intestines, the branches of the portal vein, the portal vein proper, the
liver, and the hepatic veins (Fig. 77). In passing through the liver, a
large portion of the food material is temporarily retained for a purpose
and in a manner to be described later (page 177).
*Absorption Changes.*--During digestion the insoluble foods are converted
into certain soluble materials, such as peptones, maltose, and
glycerine,--the conversion being necessary to their solution. A natural
supposition is that these materials enter and become a part of the blood,
but examination shows them to be absent from this liquid. (See Composition
of the Blood, page 30.) There are present in the blood, however,
substances closely related to the peptones, maltose, glycerine, etc.;
substances which have in fact been formed from them. During their transfer
from the food canal, the dissolved nutrients undergo changes, giving rise
to the materials in the blood. Thus are the serum albumin and serum
globulin of the blood derived from the peptones and proteoses; the
dextrose, from the maltose and other forms of sugar; and the fat droplets,
from the glycerine, fatty acid, and soluble soap.
While considerable doubt exists as to the cause of these changes and as to
the places also where some of them occur, their purpose is quite apparent.
The materials forming the dissolved foods, although adapted to absorption,
are not suited to the needs of the body, and if introduced in this form
are likely to interfere with its work.(67) They are changed, therefore,
into the forms which the body can use.
*A Second Purpose of Digestion.*--Comparing the digestive changes with
those of absorption, it is found that they are of a directly opposite
nature; that while digestion is a process of tearing down, or
separating,--one which reduces the food to a more finely divided
condition--there is in absorption a process of building up. From the
comparatively simple compounds formed by digestion, there are formed
during absorption the more complex compounds of the blood. The one
exception is dextrose, which is a simple sugar; but even this is combined
in the liver and the muscles to form the more complex compound known as
glycogen. (See Methods of Storage, below.) These facts have suggested a
second purpose of digestion--that of reducing foods to forms sufficiently
simple to enable the body to construct out of them the more complex
materials that it needs. Evidence that digestion serves such a purpose is
found in the fact that both proteids and carbohydrates are reduced to a
simpler form than is necessary for dissolving them.(68)
*The Storage of Nutriment.*--For some time after the taking of a meal, food
materials are being absorbed more rapidly than they can be used by the
cells. Following this is an interval when the body is taking no food, but
during which the cells must be supplied with nourishment. It also happens
that the total amount of food absorbed during a long interval may be in
excess of the needs of the cells during that time; and it is always
possible, as in disease, that the quantity absorbed is not equal to that
consumed. To provide against emergencies, and to keep up a uniform supply
of food to the cells, it is necessary that the body store up nutrients in
excess of its needs.
*Methods of Storage.*--The general plan of storage varies with the
different nutrients as follows:
1. _The carbohydrates_ are stored in the form of _glycogen_. This, as
already stated (page 120), is a substance closely resembling starch. It is
stored in the cells of both the liver and the muscles, but mainly in the
liver (Fig. 78). It is a chief function of the liver to collect the excess
of dextrose from the blood passing through it, and to convert it into
glycogen, which it then stores within its cells. It does not, however,
separate all of the dextrose from the blood, a small amount being left for
supplying the immediate needs of the tissues. As this is used, the
glycogen in the liver is changed back to dextrose and, dissolving, again
finds its way into the blood. In this way, the amount of dextrose in the
blood is kept practically constant. The carbohydrates are stored also by
converting them into fat.
[Fig. 78]
Fig. 78--*Liver cells* where is stored the glycogen. _C._ Capillaries.
[Fig. 79]
Fig. 79--*Stored-up fat.* The figure shows four connective tissue cells
containing small particles of fat. 1. Nucleus. 2. Protoplasm. 3. Fat. 4.
Connective tissue fibers.
2. _The fat_ is stored for the most part in the connective tissue. Certain
of the connective tissue cells have the property of taking fat from the
blood and of depositing it within their inclosing membranes (Fig. 79).
When this is done to excess, and the cells become filled with fat, they
form the so-called _adipose tissue_. Most of this tissue is found under
the skin, between the muscles, and among the organs occupying the
abdominal cavity. If one readily takes on fat, it may also collect in the
connective tissue around the heart. The stored-up fat is redissolved as
needed, and enters the blood, where it again becomes available to the
active cells.
3. _The proteids_ form a part of all the tissues, and for this reason are
stored in larger quantities than any of the other food substances. The
large amount of proteid found in the blood may also be looked upon as
storage material. The proteids in the various tissues are spoken of as
_tissue proteids_, and those in the blood as _circulating proteids_. The
proteids of the tissues serve the double purpose of forming a working part
of the cell protoplasm, and of supplying reserve food material. That they
are available for supplying energy, and are properly regarded as _storage
material_, is shown by the rapid loss of proteid in starving animals. When
the proteids are eaten in excess of the body's need for rebuilding the
tissues, they are supposed to be broken up in such a manner as to form
glycogen and fat, which may then be stored in ways already described.
*General Facts Relating to Storage.*--The form into which the food is
converted for storage in the body is that of _solids_--the form that takes
up the least amount of space. These solids are of such a nature that they
can be changed back into their former condition and, by dissolving,
reenter the blood.
Only energy-yielding foods are stored. Water and salts, though they may be
absorbed in excess of the needs of the body, are not converted into other
substances and stored away. Oxygen, as already stated (page 108), is not
stored. The interval of storage may be long or short, depending upon the
needs of the body. In the consumption of stored material the glycogen is
used first, then as a rule the fat, and last of all the proteids.
*Storage in the Food Canal.*--Not until three or four hours have elapsed
are all the nutrients, eaten at a single meal, digested and passed into
the body proper. The undigested food is held in reserve, awaiting
digestion, and is only gradually absorbed as this process takes place. It
may properly, on this account, be regarded as _stored material_. That such
storage is of advantage is shown by the observed fact that substances
which digest quickly (sugar, dextrin, "predigested foods," etc.) do not
supply the needs of the body so well as do substances which, like starch
and proteids, digest slowly. Even substances digesting quite slowly
(greasy foods and pastry), since they can be stored longer in the food
canal, may be of real advantage where, from hard work or exposure, the
body requires a large supply of energy for some time. These "stay by" the
laborer, giving him strength after the more easily digested foods have
been used up. Storage by the food canal is limited chiefly to the stomach.
*Regulation of the Food Supply to the Cells.*--The storage of food
materials is made to serve a second purpose in the plan of the body which
is even more important than that of supplying nourishment to the cells
during the intervals when no food is being taken. It is largely the means
whereby the rate of supply of materials to the cells is regulated. The
cells obtain their materials from the lymph, and the lymph is supplied
from the blood. Should food substances, such as sugar, increase in the
blood beyond a low per cent, they are converted into a form, like
glycogen, in which they are held in reserve, or, for the time being,
placed beyond the reach of the cells. When, however, the supply is
reduced, the stored-up materials reenter the blood and again become
available to the cells. By this means their rate of supply to the cells is
practically constant.
We are now in a position to understand why carbohydrates, fats, and
proteids are so well adapted to the needs of the body, while other
substances, like alcohol, which may also liberate energy, prove injurious.
It is because foods are of such a chemical nature that they are adapted in
all respects to the body plan of taking up and using materials, while the
other substances are lacking in some particular.
[Fig. 80]
Fig. 80--*Diagrams illustrating the relation of nutrients* and the
non-relation of these to alcohol. _A._ Inter-relation and convertibility
of proteids, fats, and carbohydrates (after Hall).
_B._ Diagram showing disposition of alcohol if this substance is taken in
quantity corresponding to that of the nutrients (F.M.W.). The alcohol
thrown off as waste is unoxidized and yields no energy.
*Why Alcohol is not a Food.*--If the passage of alcohol through the body is
followed, it is seen, in the first place, that it is a simple liquid and
undergoes no digestive change; and in the second place, that it is rapidly
absorbed from the stomach in both weak and concentrated solutions. This
introduces it quickly into the blood, and once there, it diffuses rapidly
into the lymph and then into the cells. Since the body cannot store
alcohol or convert it into some nutrient that can be stored (Fig. 80),
_there is no way of_ _regulating the amount that shall be present in the
blood, or of supplying it to the cells as their needs require_. They must
take it in excess of their needs, regardless of the effect, at least until
the organs of excretion can throw off the surplus as waste. Compared with
proteid, carbohydrates, or fats, alcohol is an _unmanageable_ substance in
the body. Attempting to use it as a food is as foolish as trying to burn
gasolene or kerosene in an ordinary wood stove. It may be done to a
limited extent, but is an exceedingly hazardous experiment. Not being
adapted to the body method of using materials, alcohol cannot be classed
as a food.
*Assimilation.*--Digestion, absorption, circulation, and storage of foods
are the processes that finally make them available to the cells in the
different parts of the body. There still remains another process for these
materials to undergo before they serve their final purposes. This last
process, known as _assimilation_, is the appropriation of the food
material by the cell protoplasm. In a sense the storage of fat by
connective tissue cells and of glycogen by the liver cells is
assimilation. The term is limited, however, to the disposition of material
with reference to its final use. Whether all the materials used by the
cells actually become a part of the protoplasm is not known. It is known,
however, that the cells are the places where most of the oxidations of the
body occur and that materials taking part in these oxidations must, at
least, come in close contact with the protoplasm. Assimilation, then, is
the last event in a series of processes by which oxygen, food materials,
and cell protoplasm are brought into close and _active_ relations. The
steps leading up to assimilation are shown in Table II.
TABLE II. THE PASSAGE OF MATERIALS TO THE CELLS
MATERIALS DIGESTION ABSORPTION ROUTE TO STORAGE CONDITION
THE GENERAL IN THE
CIRCULATION BLOOD
Proteids Changed In passing Through the Become a As proteids
into into the portal vein part of the in
proteoses capillaries, to the protoplasm colloidal
and the liver and of all the solution.
peptones by proteoses from there cells.
the action and through the
of the peptones hepatic
gastric and change into veins into
pancreatic the the
juices. proteids of inferior
the blood. vena cava.
Fat Changed In passing Through the As fat in Chiefly as
into fatty into the lacteals to the cells minute oil
acid, lacteals, the of droplets.
glycerine, the thoracic collective
and glycerine duct, by tissue.
soluable unites with which it is
soap by the the soluable emptied
bile and soap and into the
pancreatic fatty acid left
juice. to form the subclavian
oil droplets vein.
of the
blood.
Starch Reduced to Enters the Through the As glycogen As dextrose
some of the capillaries portal chiefly by in
different as dextrose. vein, the liver, solution.
forms of liver, but to some
sugar, as hepatic extent by
maltose, veins, into muscle
dextrose, inferior cells.
etc. vena cava.
Water Undergoes Taken up by Both Is not As the
no change. both the routes, but stored in water which
lacteals and mostly by the sense serves as a
capillaries, way of the that energy carrier of
but to the liver. foods are. all the
greater other
extent by constituents
the of the
capilaries. blood.
Common salt Undergoes Taken up by By way of Not stored. In solution.
no change. the portal
capillaries vein,
without liver, and
undergoing hepatic
apparent veins into
change. inferior
vena cava.
Oxygen Taken up by Already in Is not United with
the the general stored. the
capillaries circulation. hemoglobin
at the and to a
lungs. small extent
in solution
in the
plasma.
*Tissue Enzymes.*--The important part played by enzymes in the digestion of
the food has suggested other uses for them in the body. It has been
recently shown that many of the chemical changes in the tissues are in all
probability due to the presence of enzymes. An illustration of what a
tissue enzyme may do is seen in the changes which fat undergoes. In order
for the body to use up its reserve fat, it must be transferred from the
connective tissue cells, where it is stored, to the cells of the active
tissues where it is to be used. This requires that it be reduced to the
form of a solution and that it reenter the blood. In other words, it must
be _redigested_. For bringing about these changes a substance identical in
function with the steapsin of the pancreatic juice has been shown to exist
in several of the tissues.
Although this subject is still under investigation, it may be stated with
certainty that there are present in the tissues, enzymes that change
dextrose to glycogen and _vice versa_, that break down and build up the
proteids, and that aid in the oxidations at the cells. The necessity for
such enzymes is quite apparent.
*Summary.*--The digested nutrients are taken up by the capillaries and the
lymph vessels and transferred by two routes to the circulation. In passing
from the alimentary canal into the circulation the more important of the
foods undergo changes which adapt them to the needs of the body. Since
materials are absorbed more rapidly than they are used, means are provided
for storing them and for supplying them to the cells as their needs
require. _Capability of storage is an essential quality of energy-yielding
foods_; and substances, such as alcohol, which lack this quality are not
adapted to the needs of the body. For causing the chemical changes that
occur in the storage of foods, as well as the oxidations at the cells, the
presence of active agents, or enzymes, is necessary.
*Exercises.*--1. In what respects does the absorption of food materials
from the alimentary canal differ from the absorption of a simple liquid by
a solid?
2. In what different ways is the small intestine especially adapted to the
work of absorption?
3. What are the parts of a villus? What are the lacteals? Account for the
name.
4. What part is played by the capillaries and the lacteals in the work of
absorption? How does their work differ?
5. What changes, if any, take place in water, common salt, fat, proteids,
and carbohydrates during absorption?
6. What double purpose is served by the processes of digestion?
7. Trace the passage of proteids, fats, and carbohydrates from the small
intestine into the general circulation.
8. What is the necessity for storing nutrients in the body? Why is it not
also necessary to store up oxygen?
9. In what form and at what places is each of the principal nutrients
stored?
10. How is the rate of supply of food to the cells regulated? Why is the
body unable to regulate the supply of alcohol to the cells when this
substance is taken?
11. Explain Fig. 80, page 181. What becomes of the alcohol if this is
taken in any but very small quantities?
12. State the general purpose of enzymes in the body. Name the enzymes
found in each of the digestive fluids. What ones are found in the tissues?
PRACTICAL WORK
Illustrate the ordinary meaning of the term "absorption" by bringing the
end of a piece of crayon in contact with water, or a piece of blotting
paper in contact with ink, noting the passage of the liquid into the
crayon or the paper. Show how absorption from the food canal differs from
this kind of absorption.
Show by a diagram similar to Fig. 77 the two routes by which the foods
pass from the alimentary canal into the blood stream.