Function

• Digestive system


  The organ system in which food substances are broken down in the body
• Gastrointestinal (digestive) tract of most vertebrates

  • Includes the headgut (mouthparts and pharynx)
  • Foregut (esophagus and stomach)
  • Midgut (small intestine)
  • Pancreas
  • Biliary system (liver and gallbladder)
  • Hindgut (large intestine, cecum, colon, and rectum or cloaca)
• Nervous and endocrine systems

  Control the activities of the digestive system
• Major physiological activities of the digestive system

  • Motility
  • Secretion
  • Digestion
  • Absorption
• Proper digestion could not take place without substantial numbers of indigenous bacteria, which are found throughout the digestive tract
• Vertebrate digestive system


  • Consists of the digestive tract (alimentary canal) and ancillary organs that serve for the acquisition of food and the assimilation of nutrients required for energy, growth, maintenance, and reproduction
  • Food is ingested, reduced to particles, mixed with digestive fluids and enzymes, and propelled through the digestive tract
  • Enzymes produced by the host animal and microbes indigenous to the digestive tract destroy some harmful agents and convert food into a limited number of nutrients, which are selectively absorbed
  • Some characteristics of the vertebrate digestive system are common to all classes of vertebrates and appear to have been conserved in their evolution
  • In other aspects, the digestive systems of vertebrates show numerous structural and functional adaptations to their diet, habitat, and other physiological characteristics
• Digestive tract complexity by diet

  • Carnivores (feed exclusively on other animals) and species that feed on plant concentrates (seeds, fruit, nectar, and pollen) tend to have the shortest and simplest digestive tract
  • Omnivores (feed on both plants and animals) have a more complex digestive tract
  • Herbivores (feed principally on the fibrous portions of plants) have the most complex digestive tract
• Because of wide species variations, the digestive system of vertebrates is best described in terms of the headgut, foregut, midgut, pancreas, biliary system, and hindgut. The various structures and organs associated with the human digestive system
• Headgut


  Consists of the mouthparts and pharynx, which serve for the procurement and the initial preparation and swallowing (deglutition) of food
• Jaws


  • Movable, articulated jaws are found in all vertebrates, except the cyclostomes (lampreys and hagfish), which are primitive, parasitic fish
• Teeth


  Used for grasping, cutting, or tearing of food, are located in the jaws or other mouthparts of most fish, and adult amphibians are carnivores with a weak dentition that is used only for grasping and positioning prey
• Reptile teeth


  • Most reptiles have teeth that are used for the procurement, cutting, tearing, or crushing of food, but turtles and birds use a beak for these purposes
• Mammalian teeth


  • Incisors and canines for grasping, cutting, and tearing, and premolars and molars with uneven occluding surfaces
• Mammalian jaws


  • Articulation and musculature allow the vertical movement of the lower jaw and either its lateral movement, as seen in most species, or a forward and backward movement, as seen in rodents and elephants
• Mammalian grinding and crushing


  Unique ability to use their premolar and molar teeth for the grinding and crushing of food
• Tongue


  • Movable tongue aids in the procurement of food (for example, this is seen in frogs, toads, woodpeckers, and anteaters) and for the placement of food between the molars of mammals
• Oral glands


  Secrete fluids that aid in the swallowing of food, as well as adhesive materials or even toxins in a number of species
• Midgut


  The small intestine, the principal site for the digestion of food and the absorption of nutrients
• Midgut


  • Lined with a single layer of cells that secrete mucus and fluids, contain enzymes that aid in the final stages of the digestion of carbohydrates and proteins, and absorb nutrients from the lumen into the circulatory system
• Increasing surface area of midgut lumen

  1. Presence of villi (macroscopic projections of the epithelial and subepithelial tissues)

  2. Brush border of microvilli on the lumen-facing surface of the midgut absorptive cells
• The lumenal surface area of the human small intestine is increased 10-fold by the presence of villi and an additional 20-fold by the presence of microvilli, resulting in a total surface area of 2,000,000 cm2 (310,000 in.2)
• Harmful agents are absorbed through the intestinal epithelium


  Can cause acute toxicities along the digestive tract, including inflammation, ulceration, and necrosis of the alimentary canal and accessory organs
• Heavy metal toxicity


  Can disrupt gut microbiota, change the composition of microbes present in the gut and lead to inflammation, and interfere with digestion by interacting with digestive enzymes, leading to ulceration
• Diet low in fiber, lack of exercise, stress, and eating large amounts of dairy products


  Can upset the gastrointestinal tract and its motility, leading to chronic conditions such as constipation, irritable bowel syndrome, nausea, gas, bloating, and diarrhea
• Pancreas


  Secretes digestive enzymes and fluid to aid digestion in the midgut
• Biliary system


  Liver secretes bile to emulsify lipids and increase their surface area for digestion by lipase
• Pancreatic tissue


  • Represented by primitive ceca (blind sacs) along the midgut of cyclostomes
  • Distributed along the intestinal wall, and even into the liver, of some species in the more advanced classes of fish
  • Compact organ in sharks, skates, rays, many teleosts (ray-finned fishes), and all other vertebrates
• Liver


  • Compact organ in all vertebrates
  • One of its many functions is the secretion of bile
  • In most vertebrates, the bile is stored in the gallbladder and released into the intestine as needed, but a gallbladder is absent in some species of fish and mammals
• Bile salts


  Serve to emulsify lipids and increase their surface area available for digestion by the water-soluble lipase
• Hindgut


  The final site of digestion and absorption prior to defecation or evacuation of waste products
• Hindgut of fish, amphibian larvae, and a few mammals

  • Short and difficult to distinguish from the midgut
• Hindgut of adult amphibians and reptiles, birds, and most mammals

  • A distinct segment, separated from the midgut by a muscular sphincter or valve
  • Larger in diameter
• Midgut and hindgut


  Often referred to as the small intestine and the large intestine, respectively
• Hindgut of some reptiles and many mammals

  • Includes a blind sac (cecum) near its junction with the midgut
• Hindgut of many birds and a few mammalian species

  • Includes a pair of ceca
• Remainder of the hindgut


  Consists of the colon and a short, straight, terminal segment, which is called the rectum in mammals
• Digestive and urinary tracts

  • Exit separately from the body of most species of fish and mammals
• In adult amphibians and the reptiles, birds, and some mammals

  • The hindgut or, if present, the cloaca terminates in the anus
• Hindgut or cloaca


  • Also serves as an exit for the urinary and reproductive systems
• Hindgut lining


  • Single layer of absorptive and mucus-secreting cells that originate in crypts
  • Lacks villi
  • Absorptive cells lack digestive enzymes and the ability to absorb most nutrients