Chap 41

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Dentition and diet.

  1. Carnivores have pointed incisors and canines, and jagged premolars and molars (carnassials in Carnivora).
  2. Herbivores have teeth with broad, ridged surfaces; canines may be absent.
  3. Omnivores have unspecialized dentition: bladelike incisors for biting, pointed canines for tearing, and premolars and molars for grinding and crushing.

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The human digestive system. Humans, like all deuterostomes , have a tube-within-a-tube anatomy where food enters the mouth, passes through a digestive tract , and exits through an anus.

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The human digestive system.

After food is chewed and swallowed, it takes only 5–10 seconds for it to pass down the esophagus and into the stomach, where it spends 2–6 hours being partially digested.

Final digestion and nutrient absorption occur in the small intestine over a period of 5–6 hours.

In 12–24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus.

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The digestive tracts of a carnivore (coyote) and herbivore (koala) compared. Although these two mammals are about the same size, the koala's intestines are much longer, an adaptation that enhances processing of fibrous, protein–poor eucalyptus leaves from which it obtains virtually all its food and water.

Extensive chewing chops the leaves into very small pieces, increasing exposure of the food to digestive juices. The koala's cecum—at 2 m, the longest of any animal of equivalent size—functions as a fermentation chamber where symbiotic bacteria convert the shredded leaves into a more nutritious diet.

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The duodenum. Hydrolytic enzymes from accessory glands mix with acid chyme in the duodenum. Note that bile is produced in the liver but stored in the gallbladder, which releases bile into the duodenum as needed.

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Digestion and absorption of fats.

Hydrolysis of fats is a digestive challenge because fat molecules are insoluble in water.

Bile salts from the gallbladder secreted into the duodenum coat tiny fat droplets and keep them from coalescing, a process called emulsification.

Because the droplets are small, a large surface area of fat is exposed to lipase.

The hydrolyzed fat molecules form micelles, which diffuse into lacteal vessels .

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Fat cells from the abdomen of a human. Strands of connective tissue (yellow) hold the fat–storing adipose cells in place.

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Essential amino acids from a vegetarian diet. Plant proteins are incomplete in amino acid makeup. An adult human can obtain all essential amino acids by eating a meal of corn and beans.

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The four stages of food processing.

  1. Ingestion is the act of eating.
  2. Digestion breaks food down by the enzymatic hydrolysis of polymers into monomers.
  3. Absorption is the uptake of nutrients by body cells.
  4. Elimination occurs as undigested material passes out of the digestive compartment.

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Homeostatic regulation of cellular fuel. The human body regulates the use and storage of glucose by several feedback loops.

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Digestion in a hydra. Digestion begins in the gastrovascular cavity and is completed intracellularly after small food particles are engulfed by the gastrodermal cells.

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The pharynx is a junction that opens to both the esophagus and the trachea (windpipe). Normally the esophageal sphincter muscle is contracted and the epiglottis is up, allowing air to flow through the trachea to the lungs.

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After swallowing, waves of contraction by smooth muscles (peristalsis) move the food down the esophagus to the stomach.

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When swallowing, the esophageal sphincter muscle releaxes and the epiglottis moves down, preventing food from entering the trachea.

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Essential amino acids from a vegetarian diet. Plant proteins are incomplete in amino acid makeup. An adult human can obtain all essential amino acids by eating a meal of corn and beans.

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Protease activation.
The pancreas secretes inactive proteases into the duodenum, where enteropeptidase converts trypsinogen to Trypsin. Trypsin then activates other proteases.

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Ruminant digestion The stomach of a ruminant has four chambers. Because of the microbial action in the chambers, the diet from which a ruminant actually absorbs its nutrients is much richer than the grass the animal originally ate. In fact, a ruminant eating grass or hay obtains many of its nutrients by digesting the symbiotic microorganisms, which reproduce rapidly enough in the rumen to maintain a stable population.

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The structure of the small intestine.
Each villus contains a network of blood vessels to transport amino acids and carbohydrates, and a lacteal vessel of the lymphatic system to transport fats.

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The stomach and its secretions. The gastric glands have three types of cells.


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The bacterium Helicobacter pylori initiates ulcers by destroying protective mucus, then the acidic gastric juice can attack the stomach tissue.