See also
Digestion and enzymes - section on
human digestive system, metabolism and synthesis
The small intestine is about 7 m long, and is where dissolved digested
food particles are absorbed from the digestive system into the
bloodstream to supply the cells with the necessary nutrients.
The long length and large surface gives plenty
of time for the soluble food molecules to absorbed into the
bloodstream as the food moves slowly along. It takes at least 6-8
hours to travel through the small intestine.
The transfer through the partially permeable
membrane might be by 'natural' diffusion down a diffusion gradient
or by active transport against a diffusion gradient.
The partially permeable membrane regulates the
transfer of substances.
The efficiency of the process is considerably
increased by the structure of the small intestine - adaptations:
(i) a single layer of surface cells - short
diffusion time and distance - fast diffusion through the permeable
membrane,
(ii) long length - increase contact time for
breakdown and absorption of food molecules.
(iii) a large surface area for absorption -
result of many small projections called villi which have microvilli
to increase the surface area even more,
(iv) and a good blood supply from numerous
capillaries that transport the nutrients away efficiently and
maintain the concentration gradient in the direction of absorption.
all of which speed up the process, so read on
for the transport details below in text and on diagram below.
Know and understand that the villi
in the small intestine provide a large surface area with an extensive
network of thin blood capillaries to absorb the products of
digestion by diffusion and active transport.
The tissue lining in the small
intestine is covered with millions of protuberances called villi, which poke
up from the intestine surface into the partially or wholly digested food
/mush'.
The villi consist of a single
very thin
layer of epithelial cells on the very large surface area of the intestine.
Both factors considerably speeds
up the food molecule absorption process.
Each villus (of the millions of
villi) has single layer of surface cells and each villus contains a
multitude of fine blood capillaries into which the small digested food
molecules can rapidly diffuse into and be absorbed into the body.
These molecules include amino acids, sugars like
glucose, glycerol, fatty acids and important ions of sodium, iron and
calcium.
A good blood supply is needed to
efficiently carry the digested food away to where they are
needed.
A lacteal is a lymphatic capillary that absorbs
dietary fats in the villi of the small intestine.
The food molecules can
diffuse into the bloodstream down a normal concentration
gradient,
but sometimes active transport is required.
For example ...
After a meal has been digested, the
concentration of food molecules in the blood can be higher than in
the intestine. In this situation, molecules are conveyed into the
blood by active transport e.g.
When there is a higher concentration of
glucose in the intestine than in the bloodstream, glucose molecules
will naturally diffuse into the blood stream down the
diffusion gradient (concentration gradient from higher to lower
concentration).
However, if there is a lower concentration of
glucose in the intestine, your body still needs glucose for
respiration, therefore active transport must be deployed.
This uses energy in such a way as to transfer glucose molecules from
the intestine against the natural concentration (diffusion)
gradient.
When more food is required simple sugars, amino acids, vitamins
and minerals from digestion are actively transported into the villi
from an area of lower concentration to an area of higher
concentration, against the concentration gradient, and into the
bloodstream of the many micro capillary blood vessels surrounding
the villi.
As well as the uptake of glucose by epithelial cells of villi,
this also applies to kidney tubules.
For more on active
transport see
Diffusion, osmosis and active transport
