Digestive Systems

Digestive Systems

• Mammalian Teeth
  Mammals have different teeth depending on what they eat.
  Herbivores have incisors for biting and premolars and molars for chewing grass.
  Carnivores have very large canines for ripping and tearing meat.
  Omnivores have a mixture of all teeth.

Foods
Chemicals found in foods include carbohydrates, fats, proteins, vitamins, minerals and water.
During digestion these foods are broken down into their different chemical components.
These chemicals are broken down further and absorbed.

• The Digestive System

 

The digestive system includes:
–Ingestion – taking in food.
–Digestion – breaking down food.
–Absorption – absorbing the nutrients.
–Assimilation – using the food.
–Egestion – eliminating the undigested food.

• Ingestion
  Physical and chemical digestion occurs in the mouth.
  When animals see or smell food they produce saliva from 3 pairs of salivary glands in the mouth.
  Saliva contains a slimy lubricating substance called mucus and an enzyme salivary amylase that breaks starch down to maltose.
  This mixing of food with saliva is called mastication and results in a bolus being formed which is swallowed.


• The Oesophagus
  food passes down a long tube called the oesophagus to the stomach.
  Rhythmical contractions of the muscles called peristalsis propel the bolus along.
  Birds have a special storage organ at the base of the oesophagus called the crop.


• The Birds Stomach


• 
  Part of the stomach of a bird is called the gizzard and it contains grit which the bird has swallowed.
  This grit along with muscle movement grinds hard foods such as cereal grains.


• Rabbit and Pig Stomachs


• 
  There is only one chamber in these stomachs and so they are called monogastrics.
  Digestive juices are produced such as hydrochloric acid, pepsin and rennin.
  Rennin is very important in young animals as it digests milk protein.
  Food enters the stomach through the cardiac sphincter and leaves through the pyloric sphincter.


• Digesting Cellulose
  Cellulose is a valuable food source if it can be digested.
  Some animals are able to digest cellulose by culturing bacteria, that live in their digestive system, and digest the cellulose for them.
  One of these groups have normal stomachs and long intestines with micro organisms in the hind gut. They are called hind-gut fermenters e.g. rabbits and horses.
  The second group have a specialised stomach with four chambers and are called ruminants e.g. sheep and cows


• Herbivores with monogastric stomachs ferment their food in the large intestine and the appendix.
  This fermentation (anaerobic respiration) digests the cellulose.


• The Ruminant Stomach


• 
  The stomach consists of four chambers:
  –The rumen – contains anaerobic bacteria and protozoa that digest cellulose. Ruminants also digest some of the micro organisms and get protein from them.
  –The reticulum – has a honeycomb wall and digests food further before it is passed back to the mouth for further mastication.
  –The omasum – cud enters here and water is squeezed out and absorbed, solids pass to fourth chamber.
  –The abomasum – like the normal stomach of a monogastric where digestive juices are produced. Calves send milk straight to the abomasum where it is digested by rennin.


• The Small Intestine
  The first part of the small intestine is called the duodenum.
  Bile, produced in the liver, is added to food here and helps break down fat.
  Pancreatic juices, produced in the pancreas, contains a number of enzymes including pancreatic amylase that breaks down starch.
  Pancreatic juices are also added in the duodenum and further break down the food.


• Further along the small intestine intestinal juice, which contains the enzymes sucrase (breaks down sucrose) and lactase (breaks down lactose), are added.
  The internal surface of the small intestine is called the ileum. It has villi that increase its surface area and help absorption of nutrients


• 


• The Liver: Functions
  Production of bile to emulsify fats and lipids
  Storage of glucose in the form of glycogen.
  Storage of Vitamins A,D,E,K
  The breakdown of toxic substances
  –Ammonia is converted to urea
  The breakdown of red blood cells
  Temperature regulation