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Digestive system and liver
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The liver
Position and structure:
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The liver has a large volume of blood flowing through it
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This is the remains of foetal blood vessels from the umbilicus
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Lies in the cranial abdomen
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The caudal aspect is concave and in contact with the stomach, duodenum,
and right kidney
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Microscopically the liver contains hepatocytes
They are arranged in hexagonal liver lobules surrounded by connective
tissue
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The products of digestion are carried from the S
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The Hepatic Portal System:
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Between the hepatocytes are channels called bile canaliculi, into which
bile is secreted
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There is no direct connection between the sinusoids and the canaliculi
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The central vein flows into the hepatic vein
This takes the deoxygenated blood back to the caudal vena cava
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Microscopic Structure of the Liver:

Functions Carbohydrate metabolism:
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Glucose is a source of energy for the body
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Insulin converts the glucose to glycogen to be stored in the liver
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When glucose levels are low, glucogon is secreted from the pancreas
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This process is known as glycogenolysis
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Protein metabolism:
Proteins pass via the hepatic portal vein to the liver as amino acids
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Fibrinogen and Prothrombin (involved
in the blood clotting mechanisms)
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Regulation of amino acids and transamination; There are 20 amino acids
essential in forming body protein
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AA’s that are not essential may be converted to useful
ones by a process called transamination
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So if they are not
synthesised into plasma proteins or needed by the cells any surplus are
destroyed in the liver, this process is called deamination
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As the amino acid is oxidised there is a
release of energy (heat)
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The orthinine cycle converts
the ammonia to urea which can be excreted by the kidneys
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Fat metabolism; The liver converts fatty acids and glycerol into phospholipids
for the formation of cell membranes, and into cholesterol for bile salts
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When the
fat is transferred from the fat deposits around the body it may accumulate in
the liver
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If excessive
cholesterol is produced it can lead to gall stones or deposits in the blood
vessels and heart disease
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They are
produced particularly when blood glucose levels are low
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Formation of bile; Synthesised continuously by the liver and stored in the
gall bladder
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Excess accumulation of bilirubin can cause
jaundice as there is an over destruction of red blood cells
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Formation of new red blood cells; The liver can form new blood cells in the
foetus
Storage of blood
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Storage of iron
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Detoxification; The liver acts on poisons, toxins or drugs to convert them into
harmless substances ready for excretion
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The digestive system (Ruminants)
Ruminants:
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Ruminants are a class of herbivores who are able to digest fibre in their food
by utilising microbes to ferment cellulose
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Often said to have 4 stomachs – although this is probably more accurately
stated as having 4 chambers to their stomach
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Reticulem (Honeycomb stomach)
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Abomasum
(True stomach)
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Food which has been rolled into boli before swallowing is brought back
up the oesophagus into the mouth for chewing on the huge molar teeth
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Rumen:
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The largest of the 4 chambers is the rumen
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It houses a vast population of
microbes
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Muscles ensure the contents are regularly mixed
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These are important in
starting rumen contractions and also absorb some products of fermentation
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Heavy objects collect in this region
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In terms of digestion, the process is similar to the rumen
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The inner surface contains leaves or ‘pages’ which act as a press or filter
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Abomasum:
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Digestion in the abomasum is identical to the stomach in the non-ruminant via
enzymes
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Proteins are digested via pepsin under acid conditions
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Carbohydrates are reduced to glucose
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Fats (lipids) are reduced to fatty acids & glycerol
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The pancreas releases a range of enzymes and hormones
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The large intestine:
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This is also divided into the Caecum, colon & rectum
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B vitamins are also made by bacteria and these are absorbed in the colon
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Colon and rectum:
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The colon absorbs water and minerals arriving from the ileum
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Those without produce ‘dollops’
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Faeces:
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These are undigested components of feeds
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Dry Matter is variable but typically 25 – 30%
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Structure of the rumen:
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The structure of the rumen changes in the pre weaning period
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Advantages of Rumination:
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Animals are able to digest fibre & use foods other animals can’t use
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Ruminants have a large rumen so they can consume large amounts of feed
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Animals can digest food in ‘safe’ environments
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This is not necessarily the most efficient mode of digestion
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Gases are a loss of energy to the animal & potential harm to the environment
Foods take up to 72 hours to be digested
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The digestive system (hind gut fermenters)
The nutrition model that is used for hind gut fermenters (HGF) is the horse –
since apart from the rabbit, it is the most widely researched HGF
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To process fibre, HGF’s have variations to ‘normal’ gut
structure
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g
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g
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The horse:
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Foregut; Stomach, Small intestine
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Can’t vomit or belch strong cardiac sphincter muscle
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Continuous feeders
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Stomach never completely empty
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Digestion is via enzymes under acid pH in the stomach & alkaline pH in the
small intestine
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Role of the caecum & colon NH3 – Ammonia:
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The processing of materials is essentially the same as in in the rumen & is
carried out by bacteria, protozoa & fungi
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Cellulose is fermented to Volatile Fatty Acids
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Hindgut Fermentors:
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Fermentation depends on materials (mainly fibre) that escape enzymatic
digestion before fermentation occurs
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However, overfeeding of soluble carbohydrates can cause excessive gas
production
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May not be able to absorb all the products of fermentation produced posterior
to the Small Intestine (vitamins, VFA’s & protein)
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Furthermore, as in ruminants changes in pH occur which upsets the balance
of microbes which in turn upsets the pattern of fermentation
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They consume large volumes of relatively indigestible high fibre foods
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Digestive System
Structure of the Digestive System:
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Consist of;
Oral cavity
Pharynx
Oesophagus
Stomach
Small intestine – duodenum, jejunum and ileum
Large intestine – caecum, colon, rectum
Anus

Associated with the tract are several accessory glands:
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Salivary glands
Pancreas
Gall bladder
Liver

Digestion:
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Defined as the breakdown of complex food substances into constituent parts
for use by the animal in maintenance processes and for production
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Two modes of breakdown are possible, depending upon whether the animals
are ruminants (digastric) or non - ruminant (monogastric)
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Mastication -Food is chewed and mixed with saliva
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Digestion - The process of breaking down food by digestive enzymes into
smaller soluble units
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Absorption - The smaller units pass through the wall of the small intestine, into
the blood and are carried to the liver
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This takes place mainly in the liver
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Enzymes:
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In mammals the digestive process takes place with the use of enzymes
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Each enzyme acts on a specific material and not on any other e
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Lipase acts
on lipids

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