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The digestive system
The digestive system
is one of the body's major organ systems. All animals - with the
exception of some endoparasites such as tapeworms - have a digestive
system. In this section we're going to look at digestion in ruminants,
and compare their guts with those of other, non-ruminant, mammals.
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What is the rumen?
The
rumen underpins much of our agricultural industry. Without this stomach
chamber, cows and other ruminants would be much less efficient at
turning grass into milk, meat and wool. A cow's rumen has a capacity of
up to 95 litres and contains billions of bacteria and other microbes.
These microbes produce the enzymes that digest cellulose into sugars
and fatty acids for their hosts to use. A less desirable by-product is
the potent greenhouse gas, methane: a single cow can produce up to 280
litres of methane a day.
The rumen is
one of four stomach compartments found in ruminants. Ruminants are animals such as cattle,
sheep, goats and deer. (In comparison, animals such as pigs, dogs and horses have
only a single stomach compartment and are called nonruminants, or monogastric animals.) The rumen allows grazing animals to digest cellulose, a very common carbohydrate
in plants.
Cow's digestive tract, viewed from left side. Image from Nickel et al. 1973
Three of the
four ruminant stomach compartments make up the forestomach. These three
compartments – the rumen, reticulum, and omasum – are an extension
of the lower oesophagus. The rumen, the first of the
forestomach chambers, stores and processes plant material. It can be a very
large structure indeed: in large ruminants, the rumen may store up to 95 litres
of undigested food (Brooker et al. 2008).
The rumen holds plant material until it has been broken down, releasing volatile fatty acids, and fermentation of protein and carbohydrates has begun.
Relative size of ruminant stomach chambers. Image from Nickel et al. 1973
Why is the
rumen so big? Because most plants, especially grasses, have a high cellulose
content. A cellulose molecule is a polymer, made up of a long chain of subunits
called simple sugars, or monosaccharides. Vertebrate animals lack the enzyme, called cellulase, needed to break down
cellulose and release these sugars. Instead, they use symbiotic anaerobic bacteria that do possess
the enzyme cellulase. Huge numbers of these
bacteria are present in the rumen and the reticulum: with each gram of rumen
fluid contains 10 - 50 billion
bacteria.
The ruminant digestive system is a very
efficient adaptation for extracting as much energy as possible from a high
cellulose diet. Because food is held in a ruminant’s gut for a relatively long
time, symbiotic bacteria are able to grow and release nutrient sources that are
not otherwise available to vertebrates.
How does ruminant digestion work?
Ruminant
digestion begins when a cow swallows a mouthful of plants. The food is
partially chewed and mixed into a bolus with saliva, before being swallowed and passing down the
oesophagus into the rumen. When ruminants are grazing they tend to swallow
their food quickly, with only minimal mastication. When the animal is resting after grazing, it
regurgitates this partially chewed food, rechews it, and
then swallows it again (This process is know as “chewing the cud”, or
rumination.) Depending on the amount of fibre in their
food, cattle may spend between 3 – 6 hours per day chewing their cud (Lofgreen et al 1957).
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The ruminant digestive system
The ruminant digestive tract (University of Minnesota, 1996)
The
Oesophagus
The
oesophagus is a muscular tube that connects the mouth with the forestomach. Food
passes down the oesophagus by contraction of the muscles in the walls that push
the food along in a series of waves called peristalsis. The ruminant oesophagus
is also capable of reversed peristalsis or antiperistalsis. This allows food to
be easily regurgitated from the rumen and chewed.
The reticulorumen
The
reticulorumen is composed of the rumen and the reticulum. The reticulorumen is
partially separated from the rumen by the reticular fold, which allows mixing between the two
compartments. The contents of the reticulorumen are mixed by contractions of
the reticulorumen wall. The mixing recirculates undigested material preventing
the rumen becoming clogged and distributing symbiotic bacteria throughout the
ingested material. The reticulorumen becomes colonized by symbiotic bacteria in
the first week after birth. The bacteria help to break down the food and
release nutrients by a fermentation
process.
The
omasum
When food has
been broken down enough, it passes from the reticulorumen through the reticulo-omasal orifice
to the
omasum. The omasum wall is highly folded, giving a large surface area
which allows for the efficient absorption of water and salts released
from the partially digested food. The omasum
also acts as a type of pump, moving the food from the reticulorumen to
the
true stomach, the abomasum, where acid digestion takes place.
Section through cow's stomach, viewed from the front. Note the highly folded wall of the omasum. Image from Nickel et al. 1973.
The
abomasum
Unlike a ruminant's three
forestomachs, the abomasum is a 'secretory stomach'. This means that
cells in the abomasum wall produce enzymes and hydrochloric
acid which hydrolyse
proteins in the food and also in the microbes mixed in with the food.
Hydrolysis breaks the proteins into smaller sub-units (e.g. dipeptides
and amino acids), ready for further digestion and absorption in the
small intestine.
Because
ruminants eat such large amounts of plant material, there is an almost
continuous flow of food through the abomasum.
In comparison, activity in the stomach of monogastric animals generally
has a circadian rhythm associated with food intake (Djikstra, 2005).
Cow's stomach viewed from the right-hand side. Image from Nickel et al. 1973
The small
intestine
The small
intestine is an elongated tube running from the abomasum to the large
intestine. In ruminants, the small intestine is about 20 times longer than the
length of the animal - so a cow two metres in length would have a small
intestine 40 metres long!
A
large proportion of the digestion and
absorption of nutrients and water occurs in the small intestine.
Enzymes in the small intestine break nutrient molecules down into their
building blocks. Carbohydrates are broken down to simple sugars (monosaccharides), fats into fatty acids and monoglycerides, nucleic acids into nucleotides and proteins into amino acids. Some of these enzymes are on the surfaces of intestinal cells,
while others are secreted into the small intestine, primarily from the liver
and pancreas.
The
small intestine has three regions: the duodenum,
the jejunum and the ileum. Partially digested food passes from the
duodenum
along the small intestine by way of peristaltic muscle contractions
that start at the part where the abomasum is joined to the duodenum.
Duodenum
The liver and pancreas both secrete materials through ducts
into the duodenum. The common bile duct carries bile salts, a
greenish fluid that is
manufactured in the liver, stored in the gall bladder (the ruminant
gall bladder does very little to concentrate the bile), and
released into the duodenum to digest fats. The main pancreatic duct
carries digestive secretions, which are rich in enzymes and bicarbonate.
The bicarbonate neutralises acid from the stomach, which would otherwise
inactivate many of the duodenum's digestive enzymes.
Jejunum
The lining of the jejunum is specialised for the absorption of
carbohydrates and proteins. Its inner surface is covered in finger-like
projections called
villi, which increase the surface area available to absorb
nutrients
from the gut contents. The villi in the jejunum are much longer than in
the
duodenum or ileum. The epithelial cells which line these villi possess
even
larger numbers of microvilli, known collectively as the brush border.
The
combination of villi and microvilli increases the surface area of the
small
intestine, increasing the chance of a food particle encountering a
digestive
enzyme and being absorbed across the epithelium and into the blood
stream.
Nutrients can cross the intestine wall by either passive or active transport. In passive transport molecules
diffuse into the intestinal cells down a concentration gradient (i.e.
they move from a region where they are in high concentration to an area
of low concentration.) The sugar xylose enters the blood by
passive transport. Active transport
requires energy. Amino acides, small peptides, vitamins, and most
glucose are moved across the intestine lining by active transport. Once
nutrients have moved through the epithelial cells, they are taken
up by either capillaries or lacteals and then
transported around the body.
Ileum
The ileum's main function is absorption of vitamin B12, bile salts and whatever nutrients that were not absorbed by the jejunum. At the point where the ileum
joins the large intestine there is a valve, called the ileocaecal valve, which
prevents materials flowing back into the small intestine.
The
caecum
The caecum is
a pouch connected to the large intestine and the ileum. It is separated from
the ileum by the ileocaecal valve, and is considered to be the beginning of the
large intestine. In herbivores the caecum is greatly enlarged and serves as a
storage organ that permits bacteria and other microbes time to further digest
cellulose. Partially digested food enters the caecum through the ileoacecal valve, which is
normally closed. The valve occasionally opens to allow food material in. As there is
only one opening to the caecum, digesta must move in and out to the caecum
through the same opening.
The large
intestine
In addition to the caecum the large
intestine is made up of the ascending colon, transverse colon, sigmoid
colon, rectum, and anus.
Much of the large intestine comprises the colon, which is shorter in length but larger in
diameter than the small intestine. The colon is involved in the active
transport of sodium, and absorption of water by osmosis, from the
digested material that it contains. It also provides an environment for
bacteria to grow and reproduce. These symbiotic bacteria produce
important vitamins such as vitamin K, thiamine, and riboflavin,
required by the animal for proper growth and health. Finally, the large
intestine eliminates wastes. Undigested and unabsorbed food, as well as
other body wastes, leave the intestine in the form of faeces, via the
rectum & anus.
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Mycotoxins and rumen function
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It's important for the rumen
to function effectively as this results in maximal digestion and
absorption of nutrients from the animal's feed, and this in turn
maximises production of milk, meat and wool.
Grass is often infected with a range of fungi (called endophytes because they live within the plant's tissues). Some of these fungi produce a range of toxic chemicals, or mycotoxins,
which can affect the animals that eat the infected grass. Some of these
chemicals protect plants from insect attack and have little effect on
ruminants, while others have marked effects on ruminants and other
herbivores. A research
group based at the University of Waikato has studied the impact of
mycotoxins on rumen function.
The team looked specifically at the mycotoxins that result in muscle
tremors - such toxins are called 'tremorgens'. Tremorgens result in the
condition called 'ryegrass staggers', because the increased
excitability of skeletal muscle causes animals to stagger when they
move. These toxins can both
excite and inhibit smooth muscle contractions. Because the wall of the
rumen contains smooth muscle tissue, tremorgens have the potential to
upset normal rumen function.
The rumen and the reticulum normally undergo regular rhythmic
contractions, which constantly mix and stir the contents of both
stomach chambers. These contractions are controlled by the release of a
neurotransmitter chemical called acetylcholine. The research team (McLeay et al. 1999)
studied the impact of several different tremorgens on acetylcholine
release, & thus on contraction of the muscles in the reticulum and
rumen.
Effect of ergovaline on reticulum contractions.
Image courtesy of Lance McLeay.
They found that at least
some of the mycotoxins they tested disrupted muscle contractions in the
rumen and reticulum. This disruption was sometimes severe and lasted up
to 12 hours. They concluded that "severe disruption of digestion may
occur in animals grazing endophyte-infected pasture" (McLeay et al. 1999),
affecting both milk/meat production and the animals' health. The
endophytic fungi also produce another set of compounts - the ergots
(similar to adrenalin) that increase body temperature, induce heat
stress, and cause marked effects on contractions of the rumen and
reticulum (as shown in the above image). This
mycotoxin may cause scours (diarrhoea) in affected animals.
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Ruminant ecophysiology
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Ruminant
animals range in size from 10-600 kg i.e. they are in the mid size-range of
vertebrate animals. Why are there no ruminants with mass <10kg? Smaller animals
have higher energy requirements per unit of body weight. The small body size
means that their small guts would not be able to cope with the high retention
times and throughput of a ruminant digestive system. In comparison, large
nonruminants such as giraffes and elephants have comparatively lower energy
requirements than ruminants per unit of body weight. Therefore, they are able
to extract enough energy from plants without the need for rumination. These
non-ruminant herbivores have a somewhat different digestive anatomy. The three
forestomach chambers are absent, replaced with a single secretory stomach, and plant
material is fermented in the caecum and large intestine. This process is known
as hindgut fermentation.
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Humans,
horse, dogs and rabbits all have monogastric digestion – they have a single
stomach chamber. Animals with monogastric digestion are still able to digest
some of the cellulose in their diet, by way of symbiotic gut bacteria. However,
their ability to extract energy from cellulose digestion is less efficient than
in ruminants. Monogastric animals have evolved a number of behavioural and
anatomical adaptations that improve their ability to use plants as a food source.
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Hindgut
fermenters
Horses, rabbits,
rodents and other herbivores use cellulose and other fermentable plant material
in much the same way as ruminants. However, as they only have a single stomach
compartment, fermentation and digestion of cellulose primarily occurs in the
large intestine and cecum.
Digestive tract
anatomy of hindgut fermenters
The stomach
and small intestine of hindgut fermenters are similar in form and function to
other non-ruminant herbivores. Food passes down the oesophagus and into the
stomach (which does much the same job as the ruminant abomasums). However, monogastric
species do not regurgitate their cud so all mastication occurs when food is taken into the
mouth. Acid and enzymatic digestion begins in the stomach, and then partially
digested food moves from the stomach into the small intestine where further
breakdown and absorption of nutrients occurs. Like the ruminant system, the
small intestine empties its contents into the caecum through the ileocecal
orifice.
However,
unlike other monogastric herbivores, the caecum and large intestine of hindgut
fermenters is very large and anatomically complex. The caecum and colon in much
the same way as the reticulorumen of ruminant animals;
slowing down the throughput of food and allowing time for the fermentation of cellulose and
other plant material by symbiotic bacteria. Food leaves the caecum through an opening called the cecocolic orifice and moves into the ascending
colon. Here regular muscle contractions in the colon wall efficiently mix the
digested food and allow absorption of water, salts and the nutrients produced
through fermentation. Food moves only slowly through the colon – in horses it
takes 2-3 days to pass the length of the colon. The lower colon absorbs water
and concentrates waste material before this is egested as faeces through the
rectum.
Scientists
believe that hind-gut fermentation is comparatively less efficient than
rumination when it comes to extracting nutrients from cellulose-rich plant material.
This has driven the evolution of a number of behavioural and physical
adaptations in monogastric animals that maximise the energy they extract from their
high-fibre diets.
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Adaptations
to high fibre diets
Horses
Horses
lose a lot of the proteins produced by microbes living in their colons.
This is because there is no opportunity for the animal to absorb many
of the amino acids that the microbes generate. To compensate for this, horses consume
comparatively more food than ruminant animals (Duncan et al. 1990).
This is helped by the fact food travels faster through a horse's
digestive system than it does through a cow's gut..
Rabbits & Hares
Rabbits
and
hares (lagomorphs) are all small animals and have a high metabolic
rate. This means that they need to obtain energy from their food
quickly, eating little and often. Rather than retaining food in the gut
for long periods of time, lagomorphs have evolved another process
to allow them to extract more nutrients from their
food. Plant material is digested in the caecum by symbiotic bacteria.
This produces volatile fatty acids,
which are absorbed through the lining of
the caecum and provide approximately 30% of the animal's energy needs.
The caecum contents then pass through the large intestine and are
egested as caecal pellets - which are promptly eaten by the animal as
they leave the anus! (This process is called caecotrophy, and usually
happens while the animal is in its burrow.) The pellets pass
through the digestive system a second time, allowing more nutrients to
be
extracted, and the remaining undigested material leaves the body
as normal faecal
pellets.
Rodents and omnivores
Many rodents
are either partly or wholly herbivorous. Their generally small size means that
they have high metabolic energy requirements but little physical capacity for
retentive digestion of vegetative matter. Therefore, ingestion of plant
material is generally restricted to high energy sections of the plant such as
fruit, nectar and pollen or seeds; or to sections that are more easily
digestible such as growing tips, seedlings and flowers. This selectivity is
also practiced by omnivorous animals such as bears, pigs, possums and humans.
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An adult human's digestive tract is approximately 6.5 meters long
and consists of the pharynx, oesophagus, stomach, small intestine and large
intestine. Digestion in humans is similar to that of other monogastric animals. However, unlike
most herbivorous animals, humans have a relatively small caecum with a vermiform
appendix. The
appendix is a blind-ended tube connected to the caecum near the point
where the
small intestine joins the large intestine. The appendix appears to be a
vestigial structure, reduced in size and function when compared to the
same structure in other animals. One explanation for this is that the
human appendix was once much larger
and served a similar function to the caecum of hind gut fermenters. Over
time, the diets of early humans
changed to include more meat and less high-fibre plant material. This
meant that there was no selective advantage in having a large appendix
(and in fact there would be an energy cost in maintaining it), and
individuals with a smaller appendix became more common over time .
Modern humans would have difficulty extracting enough nutrients if they
were restricted to a
diet similar to that of ruminant animals. While we are encouraged to
eat a diet high in vegetables and fruit, that diet is generally
restricted to easy-to-digest material that is relatively low in
cellulose: fruit, flowers and new stems and leaves. In other words, our
diet is restricted by our inability to extract sufficient nutrients
from high-cellulose plant material.
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References
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Brooker R.J.,
Widmaier E.P., Graham L.E. & Stiling P.D. 2008: Biology. McGraw-Hill. New York..
Nickel, R., Schummer, A., & Seiferle, E. (1973) The Viscera of the Domestic Mammals. Verlag Paul Parey, Berlin.
Dijkstra J. 2005: Quantitative
Aspects of Ruminant Digestion and Metabolism (2nd Edition). CABI
Publishing. Wallingford.
Duncan P., Foose T.J., Gordon I.J., Gakahu C.G. & Lloyd
M. 1990: Comparative nutrient extraction from forages by grazing bovids and
equids: a test of the nutritional model of equid/bovid competition and
coexistence. Oecologia 84: 411-418.
Lofgreen G.P., Meyer
J.H. & Hull J.L. 1957: Behavior patterns of sheep and cattle being fed
pasture or soilage. Journal of Animal
Science 16: 773-780.
L.M. McLeay, B.L. Smith & S.C. Munday-Finch (1999) Tremorgenic
mycotoxins paxilline, penitrem and lolitrem B, the non-tremorgenic
31-epilolitrem B and electromyographic activity of the reticulum and
rumen of sheep. Research in Veterinary Science 66: 119-127.
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