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Animal Structure & Function
The digestive system | What is the rumen? The ruminant digestive system | Ruminant ecophysiology  | Mycotoxins and rumen function | Monogastric digestion | Hindgut fermenters Adaptations to high-fibre diets Human digestive systemReferences

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.
 
     
 

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
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
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).

 
     

The ruminant digestive system

Oesophagus Reticulorumen Omasum |Abomasum |Small intestine |Duodenum | Jejunum | Ileum Caecum |Large intestine

ruminant digestive tract

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, showing folds in omasum

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 side

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

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-rumen-contractions
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

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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Monogastric digestion

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.

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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Human digestive system

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

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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