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The Digestive System. Digestive System. ∞ a group of organs working together to convert food into energy and basic nutrients to feed the entire body .

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The Digestive System

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digestive system
Digestive System

∞ a group of organs working together to convert food into energy and basic nutrients to feed the entire body.

∞ made up of the digestive tract—a series of hollow organs joined in a long, twisting tube from the mouth to the anus—and other organs that help the body break down and absorb food.

∞ contains a layer of smooth muscle that helps break down food and move it along the tract.

∞ From the word digestio which means from, dis means apart and gerere is to carry

Digestive Tract

  • includes all of those processes by which an animal takes in, digests, absorbs, stores and uses food (nutrients) to meet its metabolic needs
  • Nutrients in the food an animal consumes provide the necessary chemicals for growth, maintenance and energy production
  • Autotrophs – green plants and photosynthetic protists can synthesize all their own food
    • From the word auto, self and trophe, nourishing
  • Heterotrophs – those animals that cannot synthesize their own food thus they obtain them by consuming other organisms or their product
    • From the word heteros, another or different and trophe, nourishing
  • Herbivores – animals that subsist entirely on plant materials
    • Herba, plants and vorare, to eat
  • Carnivores – animals that eat only meat
    • Caro which means flesh
  • Omnivores – humans and some animals that eat both plants and animals
    • Omnius means all
  • Insectivores – eat primarily arthropods
nutrients in heterotrophs
  • Macronutrients- needed in large quantities, includes carbohydrates, lipids, and proteins



Neutral fats



Amino acids

Steroids the animal needs

Proteins the animal needs


Fatty acids

Nitrogen waste

Other lipids the animal needs



ATP energy


Krebs cycle

Carbon compounds




Micronutrients- needed in small quantities

    • Essential minerals- are macrominerals needed in relatively large amounts

Vitamins- the general term for a number of chemically unrelated , organic substances that occur in many foods in small amounts and are necessary for normal metabolic functioning

    • Water soluble vitamin- such as vitamin B and C are coenzymes needed in metabolism
    • Fat soluble vitamin- such as vitamins A, D, E, K that are limited to the vertebrates
  • Intracellular digestion-

tiny food particles are taken into the body wall cells by endocytosis,

digestive enzyme in the vacuole then break the small particles into constituent molecules

  • Extracellular digestion-

the enzymatic breakdown of larger pieces of food into constituent molecules, in special organ or cavity and then can take part in energy metabolism

animal strategies for getting and using food
  • Continuous Feeders- sessile animals
  • Discontinuous Feeders- mobile animals
  • Suspension Feeders- removal of suspended food particles from the surrounding water by some sort of capture, trapping or filtration structure
  • Depositor Feeders- omnivorous animals obtain their nutrients from the sediments of soft-bottom habitats (muds and sands) or terrestrial soils
  • Herbivory- consumption of macroscopic plants, it requires the ability to bite and chew large of plant matter
    • Herba, herb and vorare means to eat

Predation- requires the capture of live prey

    • Motile stalkers- actively pursue their prey
    • Lurking predators- sit and wait for their prey to come within seizing distance
    • Sessile opportunists- are not very mobile, they can only capture prey when the prey organism comes into contact with them
    • Grazing carnivores- move about the substrate picking up small organisms
  • Surface Nutrient Absorption
    • free-living and endoparasitic protozoa, cestode worms, endoparasitic gastropods and crustaceans- lack mouths and digestive sytems so they absorb nutrients across their body surface
    • Pogonophoran worms- absorb some nutrients from sea water across their body surface and also supplement their nutrition with organic carbon that symbiotic bacteria fix within the pogonophoran’s tissue
  • Fluid Feeders- animals that feed on biological fluids that are rich in nutrients

Digestive System of Invertebrates

  • The gastrovascular cavity of cnidarians, such as hydra, in an incomplete digestive tract because its mouth serve as the entry and exit point for food and waste.
  • Extracellular digestion occur in gastrovascular cavity while intracellular digestion occur in food vacuole.
  • Gastrovascular cavity – is a gut where digestion takes place in invertebrates.

Incomplete digestive tract is a tract that has one opening for entrance and exit.

Complete digestive tract is a tract with the development of the anus for exit of wastes.


Digestive System of Invertebrates

Platyhelminthes(planarian) has also incomplete digestive tract.

The ingestion of planarian occur in pharynx, sticks it out of its mouth and sucks in food.

Nematodes have a complete digestive tract, it has mouth pharynx and anus.


Digestive System of Invertebrates

  • Some protozoa may be autotrophic, saprozoic, heterotrophic (ingestion processes).
  • Ciliated protozoan uses its cilia to direct food toward the cytostome (mouth).
  • The excess water do not go within the digestion but go in the cytoplasm. The pH is acidic.
  • The lysosomes of the protozoa delivers digestive enzymes and the nutrients are absorbed in the cytoplasm, at this time the pH is alkaline or basic.
  • The waste vacuole moves to the cytopyge (anus).

Digestive System of Invertebrates

  • The midgut or the stomach secretes enzymes such as lipases and carbohydrases.
  • Large particles returned to the crop for further processing while the small particles enter at the gastric caeca, where extracellular digestion takes place.
  • Undigested food is moved along the intestine and passes into the rectum, where water and ions are absorbed.
  • The digested food or the solid fecal pellets pass out via anus.
  • Grasshopper is one of the example of the insects with complete digestive tract and extracellular digestion.
  • Mastication – breaking up the food. In this process saliva was mixed.
  • During feeding the food, goes in mouth, esophagus and crop.
  • Crop – function as a food storage organ.

Digestive System of Vertebrates

Mouth ( Oral Cavity)

∞ Food begins its journey through the digestive system in the mouth.

∞ Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx.


Digestive System of Vertebrates






Digestive System of Vertebrates

Pharynx (Throat)

∞ a funnel-shaped tube connected to the posterior end of the mouth.

∞ responsible for the passing of masses of chewed food from the mouth to the esophagus.

∞ plays an important role in the respiratory system, as air from the nasal cavity passes through the pharynx on its way to the larynx and eventually the lungs.

∞ it contains a flap of tissue known as the epiglottis that acts as a switch to route food to the esophagus and air to the larynx.


Digestive System of Vertebrates


∞ a muscular tube connecting the pharynx to the stomach that is part of the upper gastrointestinal tract.

∞ carries swallowed masses of chewed food along its length.

*At the inferior end of the esophagus is a muscular ring called the lower esophageal sphincter or cardiac sphincter. The function of this sphincter is to close of the end of the esophagus and trap food in the stomach.


Digestive System of Vertebrates


∞ a muscular sac, located on the left side of the abdominal cavity, just inferior to the diaphragm.

∞the stomach is about the size of their two fists placed next to each other.

∞ acts as a storage tank for food so that the body has time to digest large meals properly.

∞ contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth.

Structures associated with the


∞Gastroesophageal Opening

∞Pyloric Opening

∞Pyloric Sphincter



Digestive System of Vertebrates

Small Intestine

∞ a long, thin tube about 1 inch in diameter and about 10 feet long that is part of the lower gastrointestinal tract.

∞ located just inferior to the stomach and takes up most of the space in the abdominal cavity.

∞ coiled like a hose and the inside surface is full of many ridges and folds.

*These folds are used to maximize the digestion of food and absorption of nutrients. By the time food leaves the small intestine, around 90% of all nutrients have been extracted from the food that entered it. 


Digestive System of Vertebrates


∞ a roughly triangular accessory organ of the digestive system located to the right of the stomach, just inferior to the diaphragm and superior to the small intestine.

∞ weighs about 3 pounds

∞ the second largest organ in the body.

∞ the main function of the liver in digestion is the production of bile and its secretion into the small intestine.


Digestive System of Vertebrates



∞ a large gland located just inferior and posterior to the stomach.

∞ It is about 6 inches long and shaped like short, lumpy snake with its “head” connected to the duodenum and its “tail” pointing to the left wall of the abdominal cavity.

∞ secretes digestive enzymes into the small intestine to complete the chemical digestion of foods.

∞ a small, pear-shaped organ located posterior to the liver.

∞ used to store and recycle excess bile from the small intestine so that it can be reused for the digestion of subsequent meals.


Digestive System of Vertebrates

Large Intestine (Colon)

∞ a long, thick tube about 2 ½ inches in diameter and about 5 feet long.

∞ located just inferior to the stomach and wraps around the superior and lateral border of the small intestine.

∞ absorbs water and contains many symbiotic bacteria that aid in the breaking down of wastes to extract some small amounts of nutrients.

∞ Feces in the large intestine exit the body through the anal canal.


Digestive System Physiology

Terms that must

be familiar with:

Regulation of

Stomach Secretions

Chyme – semi fluid mixture of food with stomach secretions.

Hydrochloric acid – has a pH of 2.0 in the stomach; a gastric gland that kills bacteria and converts pepsinogen to pepsin.

Pepsin – a gastric gland that digest proteins.

Mucus – mucous cell that protects stomach lining

Intrinsic factor – a gastric gland that binds to B12 and aids in its absorption

  • 21 of gastric secretions (gastric juice) are produced each day.
  • Cephalic Phase- sensation of taste, smell of food, stimulation of tactile receptors during the process of chewing and swallowing and pleasant thoughts of food stimulate centers.
  • Gastric Phase- a period during which the greatest volume of gastric secretion occurs. It is activated by the presence of food in the stomach.
    • *Gastrin
  • Intestinal phase- controlled by the entrance of acidic chyme into the duodenum, which initiates both neural and hormonal mechanisms.
    • *Secretin
    • *Cholecystokinin

Digestive System Physiology

The digestive system is responsible for taking whole foods and turning them into energy and nutrients to allow the body to function, grow, and repair itself.

The six primary processes of the digestive system include:

∞ Ingestion of food – eating

∞ Peristalsis – the involuntary, sequential muscular contractions that move ingested nutrients along the digestive track

∞ Segmentation – mixing the contents in the digestive tract

∞ Secretion – the release of hormones, enzymes and specific ions and chemicals that take part in digestion

∞ Digestion – the conversion of large nutrient particles or molecules into small particles or molecules

∞ Absorption – the passage of usable nutrient molecules from the small intestine into the bloodstream and lymphatic system for the final passage to body cells