Digestive Systems

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Digestive Systems
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Digestive Systems

• Herbivore- an animal that feeds on plants
• Omnivore- an animal which may feed on plants and
  animals
• Carnivore- an animal which feeds on animals
• Monogastric- an animal that has only one stomach
• Ruminant- an animal which has a stomach divided
  into multiple compartments and uses regurgitation
  as part of its digestive strategy
• Hindgut fermenter- non ruminant herbivore or
  omnivore in which digestion occurs in the caecum
  and large intestine
• Avian- varies according to diet but has an organ
  to grind food

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Functions of the digestive system

• Collection of food (eating).
• Breaking down of food (digesting).
• Absorption of usable materials (absorbing).
• Disposal of wastes (defaecating)
• Structure of the system will depend on the
  animals diet

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Mouth

• Collecting food (eating) and preparing it for
  swallowing
• Teeth- Breaking food off, and breaking it into
  swallowable lumps
• Tongue -Moving food around mouth and mixing it
  with saliva.
• The tongue has the taste buds which are important
  for food selection.
• Salivary glands -Produce saliva, to lubricate
  food and start digestion

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Salivary Glands

• The parotid gland located cranial to the ear and
  caudal to the cheek muscles
• The sublingual gland under the tongue
• The mandibular gland located at the angle of the
  jaw

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• Saliva consists of a slippery secretion of water
  and mucus with some digestive enzymes.
• The digestive enzyme is the starch digestive
  enzyme called ptyalin
• The smell and sight of food causes the glands to
  produce saliva, which then mixes with the food
  making it slippery and easier to swallow
• The enzymes also start the process of digestion.
• Salivary glands are called exocrine glands
  because the substance they produce passes through
  a duct to the area where it work

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The Teeth

• Incisors
• These are the front teeth, usually flat and
  chisel shaped for cutting and stripping food (eg
  horses ).
• Some animals use them for grooming (eg cats).
• In some animals (rodents and lagomorphs) they
  grow continuously.
• Canines
• Usually sharp and pointed and used for catching
  and holding prey.
• They are well developed in carnivores
• Premolars
• These are the first of the cheek teeth.
• In herbivores they are modified as grinding teeth
  whereas in carnivores they are cutting and
  slicing teeth.
• Molars
• These are the rear cheek teeth and are the same
  as the premolars except there are no deciduous
  molars.

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Dental Formulas

• There are two sets of teeth
• temporary or deciduous teeth which are shed and
  replaced by the permanent teeth.
• The number of each type of tooth varies according
  to the diet (eg ruminants do not have any canine
  teeth).
• The number of teeth in any species can be
  expressed as a dental formula.
• A dental formula counts the teeth in one side of
  the jaw from front to back, top and bottom.  

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The Ruminant Mouth

• The tongue and lips are modified for collecting
  and tearing plant material such as grasses and
  branches,
• The teeth are designed for grinding, and often
  continue growing throughout life.
• There is often a gap, called a diastema between
  the incisors and the premolars, which allows the
  tongue to move the food around.
• Some ruminants lack upper incisors, instead
  having a hard dental pad.
• The salivary glands produce large quantities of
  saliva to help swallow hard fibrous food.

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Simple monogastric digestive system

• No special modifications needed. 
• These animals have a relatively simple diet.
• These digestive systems are found in carnivores
  and omnivores who do not eat large amounts of
  plant matter and therefore do not need the
  special areas for micro-organisms to live in and
  ferment the plant matter.

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Hind and fore gut fermenters

• The digestive system is modified to allow
  microbial fermentation of the herbivorous diet. 
• The modifications are
• the teeth, for grinding rather than tearing.
  canines may be absent or not functional.
• the enlargement of the stomach or large
  intestine, caecum or colon which allows for
  longer passage of food. These areas also provide
  a place for the bacteria and protozoa that do the
  fermentation to live.
• Most herbivores will eat their mother's faeces
  (coprophagy) when young to help them obtain the
  necessary bacteria and protozoa needed by the gut
  to digest plant matter.

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The Ruminant Digestive System

• There are four parts to the stomach
• The rumen or paunch is the largest section and
  occupies the left side of the abdomen. Its lining
  looks like carpet. This is where most of the
  microbial fermentation of the plant matter
  occurs.
• The reticulum or honeycomb tripe is the most
  cranial stomach, and acts as a grinding and
  mixing organ. There is a groove called the
  oesophageal groove that allows food to pass
  directly from the oesophagus to the reticulum.
• The omasum or bible (it looks like pages of a
  book) is the next stomach, which serves to
  squeeze water out of the contents.
• The abomasum or true stomach is the equivalent of
  the monogastric stomach, containing acid and
  digestive juices.

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Oesophagus

• Transfer food to stomach.
• This is simply a muscular tube running from the
  back of the throat down through the chest and to
  the stomach.
• The inner lining of epithelium secretes mucus to
  help keep the food sliding along and the muscles,
  around and along the oesophagus, contract
  rhythmically in waves called peristalsis to move
  food down into the stomach.

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Stomachs
Simple
Complex
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Stomach

• Continue breaking down the food and mixing it
  with digestive enzymes and acid. some absorption.
  
• The stomach stores food and is the next step in
  the actual digestion process.
• It is a muscular sac with three parts
• fundus
• cardia
• pylorus.
• The stomach lies on the left hand side of the
  abdomen towards the midline (ie left medial).  
• Water, glucose, salt and alcohol are absorbed
  directly through the stomach wall into the
  bloodstream
• Of the digested food in the stomach starch and
  liquids pass rapidly from the stomach into the
  intestine. Protein moves slower and fat stays in
  the stomach the longest.
• The passage of food into the small intestines
  under the control of the pyloric sphincter which
  is a ring of muscle around the opening of the
  stomach. The pasty mixed food at this stage is
  called chyme.

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Gastric Pits

• The  stomach's muscular wall churns the food
  mixing it with the secretions produced by the
  exocrine gastric glands in the epithelial lining.
• These secrete
• hydrochloric acid which make the stomach contents
  acidic
• the enzymes amylase (digests starch), pepsin
  (digests (protein), lipase (digests fat).
• In young animals renin is also produced for the
  digestion of milk protein.

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Small Intestine

• This is where most of the digestion and
  absorption takes place.
• Breaking down the food continues by mixing with
  alkali, bile and enzymes.
• It is divided into 3 parts
• The duodenum
• The jejunum 
• The ileum

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Duodenum

• This is where most of the digestion takes place.
• The main organ that supplies the digestive
  enzymes is the pancreas and the juice from it
  contains the following enzymes
• amylase  
• lipase 
• trypsin to digest protein
• These empty into the duodenum through the
  pancreatic duct.
• The bile duct also empties into the duodenum and
  bile (produced by the liver) is used to emulsify
  fat (break it down into smaller globules) which
  helps in its digestion. These secretions are
  alkaline and help to neutralise the acid from the
  stomach.

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Jejunum

• The food is broken down into its simple parts and
  these are absorbed into the bloodstream in this
  part of the intestine.
• The lining is thrown up into large villi (finger
  like projections that increase the surface area
  for absorption).
• The digested protein and carbohydrates are
  absorbed into the bloodstream and the fats into
  the lacteals

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Ileum

• This is the last part of the small intestine and
  the bile is absorbed here and transported back to
  the liver so it can be reused.

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Pancreas

• Produces digestive enzymes (as well as insulin).
•   Each food group starts out as a large molecule
  made up of many small units joined together by
  chemical bonds. The enzymes function by breaking
  these bonds.
• proteins are broken down to amino acids
• carbohydrates (polysaccharides-literally many
  sugars) are broken down to monosaccharides (one
  sugar eg glucose)
• fats are broken down to fatty acids and
  cholesterol.

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The Liver

• The liver is a large, multi-lobed organ, lying
  just caudal to the diaphragm. 
• Produces digestive enzymes as well as bile.
• After absorption, helps process the products of
  digestion.
• Converting glucose from digestion, into glycogen
  to be stored until needed.
• Making many important proteins (eg plasma
  proteins, or the proteins in the blood which help
  blood clot fight disease).
• Breaking other proteins down to produce energy,
  and producing urea which is then removed by the
  kidneys.
• Converting fatty acids from digestion into fats
  for storage.
• Storage of glycogen, fats, fat-soluble vitamins
  and iron.
• Detoxifying (getting rid of poisons). The liver
  is the main organ to get rid of poisons and drugs
  in the body.

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Large Intestine

• Main site of water absorption, also manufacture
  of some vitamins
• Some bacterial fermentation depending on
  species.

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Caecum

• At the junction of the small and large intestines
  is a blind sac called the caecum. The size and
  importance of the caecum varies enormously
  between animals
• In man it is called the appendix. It is only a
  small swelling and serves no purpose.
• In horses it is huge and is used along with the
  colon for the fermentation of grass.

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• The colon
• The functions of the colon are to absorb water,
  water soluble vitamins, form the faeces and to
  secrete mucus and hind gut fermenters use their
  colon to ferment plant matter.
• The rectum
• This is the last part before the anus and is the
  last area of water absorption and the formation
  of pellets.
• It also stores faeces prior to elimination.
• The anus
• Elimination of wastes (defaecation).
• This is the exit and is controlled by the anal
  sphincter which helps prevent inappropriate
  defecation and the entry of dirt and
  micro-organisms.

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Avian Digestive System
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Diets for Different Species
Rats Meat insects cereals grains fruit
Mice Insects cereals grains nuts
Rabbits Grass fresh green vegetables carrots
Guinea pig Grass grains fruit
Dogs Dogs eat the whole carcass of animals they catch
Cats Will often leave stomach and fur from prey killed
Fowl Cereals grain seeded vegetable
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Time for a break