Digestive Systems Explained

• Digestion is the process of breaking down food into smaller nutrients for absorption and bodily use
• Digestion is a similar process across all species

Types of Digestive Systems

• Monogastric digestive systems are found in humans and many mammals
• Avian digestive systems are found in chickens and other birds
• Ruminant digestive systems are found in cows and goats
• Pseudo-ruminant digestive systems are found in horses and rabbits

Monogastric Digestive System

• Carnivores and omnivores have monogastric digestive systems
• Humans, swine, dogs, and cats all have monogastric digestive systems
• Monogastric systems contain a simple, single-compartment stomach structure

Mouth

• Mastication, or teeth grinding, occurs here
• Saliva is added from the salivary glands
• The tongue moves food, allows for taste, and prepares food for swallowing
• Prehension is the method by which animals grasp their food

Epiglottis

• A flap or valve, the epiglottis closes the windpipe (trachea) during swallowing

Esophagus

• The esophagus is a muscular tube that passes food from the mouth to the stomach
• The esophagus has a smooth muscle lining
• Peristalsis helps to carry food to the sotmach

Stomach

• The stomach stores and further breaks down food
• Food is digested by hydrochloric acid (HCl)
• pH levels read between 3-4
• Pepsin, an enzyme that breaks down protein, is secreted
• The pyloric valve is a smooth muscle sphincter: a ring of muscle that guards or closes an opening or tube

Liver

• The liver controls the storage and concentration of proteins, fats, carbohydrates, vitamins, and minerals
• The liver makes proteins and clotting factors
• The liver produces bile, a digestive compound

Gall Bladder

• Bile is stored in the gall bladder
• Bile can be concentrated in the gall bladder
• Bile is released from the gall bladder into the small intestine
• The structure and location of the gall bladder varies greatly between species

Pancreas

• The pancreas regulates blood sugar by producing insulin in the endocrine system
• It also produces pancreatic juice in the exocrine system
• The pancreas contains enzymes that break down carbs, fats, and proteins
• It also produces pancreatic juices secreted through the pancreatic

Small Intestine

• Enzymatic digestion and absorption occurs here
• Digestion is aided by proteins, carbohydrates and fats, absorption of the end products of digestion
• The Duodenum is the section of the small intestine where most digestion occurs
• The Jejunum is the section of the small intestine where some digestion and absorption occur
• The Ileum is the section of the small intestine where mostly absorptionoccurs

Villi

• Villi are finger-like projections in the lining of the small intestine that contain blood vessels for nutrient transfer

Duodenum

• The first section of the small intestine
• Connects stomach to the small intestine
• Bile and pancreatic juices are added here

Jejunum

• The middle section of the small intestine and makes up two-fifths of the total structure
• Fully-digested carbohydrates, proteins, sugars, fatty acids, and amino acids are absorbed here
• Nutrients enter the bloodstream, where they distribute to the organs

Ileum

• Means "twisted intestine"
• The final and longest section of the small intestine
• The ileum mainly absorbs any nutrients that got past the jejunum, mainly vitamin B12 and bile acids
• Bile salts are reabsorbed by active transport in the ileum
• Returned by the blood to the liver

Enzymes

• Enzymes enable the breakdown of nutrients within digestive systems
• Different digestive systems have different enzymes
• Some systems use bacteria or other microbes to help break down specific food stuffs

Enzymes in the Small Intestine function

• Trypsin digests proteins and is secreted from the pancreas
• Chymotrypsin digests proteins and is secreted from the pancreas
• Carboxypeptidases digest proteins and is secreted from the pancreas
• Pancreatic amylase digests carbohydrates and is secreted from the pancreas
• Lipases digests lipids and is secreted from the pancreas
• Disaccharidise digests carbohydrates and is secreted from the small intestine
• Dipeptidases digests peptides and is secreted from the small intestine

Ileocecal Valve

• The ileocecal valve is a smooth muscle sphincter
• The valve is where the ileum joins he large intestine

Large Intestine

• Also referred to as the colon
• It is much shorter than the small intestine, but larger in diameter
• Bacterial activity continues the breakdown of indigestible food
• Water is absorbed, thus creating solid waste

Avian Digestive System

Beak

• Avian digestive systems do not contain teeth
• Prehensile action allows pecking food with their beak/bill
• Secretes saliva to soften food and aid in swallowing
• Houses the tongue, which manipulates food and aids in swallowing food whole

Esophagus

• Connects the mouth to the stomach
• Moves food from the mouth to the stomach using wave-like muscle contractions (peristalsis)
• Food is deposited in the crop of many birds before going to the stomach

Crop

• The crop is part of the esophagus
• Serves as a temporary food storage pouch
• Located just outside the body cavity in the neck region
• An evolutionary adaptation that allows birds that need to eat in the open to consume large amounts of food for digestion later
• Stored feed and water are stored in the crop until they are passed to the rest or the digestive tract
• There is very little digestion here

Stomach

• Is divided into two parts
• The Proventriculus is a Glandular part of the stomach where food is partially digested
• Hydrochloric acid and digestive enzymes, such as pepsin are added
• The Ventriculus/gizzard is part of the digestive tract of birds, reptiles, earthworms, and fish
• It's a Muscular portion of the stomach which grinds food, often with the help of ingested stones or grit (a supplement given to chickens that eat whole grains)

Small Intestine

• Made up of the duodenum, jejunum, and ileum
• The Duodenal Loop surrounds the pancreas
• It's length varies depending on diet
• Longer in carnivorous birds
• Shorter in herbivorous birds
• The remainder of digestion takes place here
• Is the main place of absorption of nutrients
• Bile aids in the absorption of fat-soluble vitamins (A, D, E, and K)

Ceca

• Ceca is the plural form of cecum
• Two pouches located where the small and large intestines meet
• Remaining water is absorbed here
• Fermentation and ay remaining coarse materials

Large Intestine

• Is also known as the colon
• It absorbs water, dries out indigestible items, and eliminates waste products
• Contains bacteria, which allows birds to metabolize remaining nutrients
• Connects to the cloaca

Intestinal Microbiome

• Both the small and the large intestines contain beneficial organisms
• Microflora are the 'micro' meaning small and the "flora' meaning plants
• Aids in digestion
• Birds are born with sterile digestive tracts and need to consume the microflora
• Receives the benefit from the mother's fecal material
• Probiotics in feed also provides support

Cloaca/Vent

• The location where digestive wastes mix with wastes from the urinary system
• Chickens void fecal material as digestive waste with uric acid crystals on the outer surface
• Chickens do not, therefore, urinate
• Eggs also pass out of this vent

Ruminant Digestive System

• Ruminants acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions

Mouth

• Food is moistened to aid in chewing by the teeth
• Lips, tongue, teeth, and saliva start digestion
• Ruminants only have front teeth in the lower jaw, which cuts the grass against the dental pad
• Upper and lower molars are used for grinding food

Esophagus

• Transports food to and from the mouth and stomach
• Food makes multiple trips
• Ruminants can regurgitate, re-masticate, and re-swallow their food ("chewing their cud")
• May do this for up to 8 hours a day

Stomach

• Ruminants have one stomach with four parts
• Each compartment has a specific function
• The first and third compartments are considered forestomachs
• These aid digestion through microbial fermentation
• Microbes help break down fibrous material

Rumen

• The rumen, also known as “the paunch" is the largest stomach compartment
• It can hold up to 40 gallons in a cow
• Makes up 80% of the stomach
• The top third is gas, the middle third is solid feedstuffs, and the bottom third is digested feedstuffs
• The rumen contains a large population of microorganisms
• Helps digest feed and provides energy for the animal
• Produces the majority of amino acids

Papille

• Papille contains finger-like structures that texture the inner lining
• It provides more surface area

Reticulum

• The reticulum also known as honeycomb or "hardware" is relatively small
• Contains 2 gallons in a cow
• Makes up 5% of the stomach
• Contractions cause movement for the rumen to mix feed
• Pumps food back up the esophagus for rumination
• Acts as a sieve

Omasum

• The Omasum, also known as “the butcher's Bible," is round
• It provides a muscular section with many folds to grind and squeeze the feed
• Holds 4 gallons in a cow
• Makes up 8% of the stomach
• Includes many folds in the interior walls' structure--looks like a book
• Absorbs water

Abomasum

• The abomasum is the "true stomach"
• Functions are similar to the monogastric stomach
• Bile helps the breakdown of proteins and lipids
• The only compartment that produces enzymes and mucous

Small Intestine

• Most nutrients are absorbed into the bloodstream here
• Is about 20 times the length of the animal
• A 6-foot long cow contains a 120 foot small intestine
• Includes three main parts
  • Duodenum
  • Jejunum
  • Ileum

Large Intestine

• Unused food material is prepared for removal from the body
• Includes three main parts
  • Cecum: has a minor role in further breakdown of roughages
  • Colon: absorbs water and forms undigested wastes into feces
  • Rectum: stores feces until it's passed out of the body

Anus

• The opening in which waste exits the body

Pseudo-Ruminants

• Animals that eat large amounts of roughage
• Do not have a stomach with several compartments
• A pseudo-ruminant digestive system performs some functions similar to those of ruminants, exemplified by the cecum fermenting forages in horses.
• They can digest large amounts of roughage, because of greatly enlarged cecum and large intestine (many areas for microbial digestion of fiber)
• Pseudo-ruminants often eat forages

Explore the diverse world of digestive systems of animals, from monogastric to avian systems. Learn about the key processes and anatomical differences that facilitate nutrient processing across species. Understand the role of organs like the stomach, liver, and esophagus.