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WEEK 10 HANDOUT FOR VERTEBRATE ZOOLOGY: NUTRITION AND DIGESTION 3/30/00

Text (7th ed.): Ch. 10. Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)

possible web site: http://www.innerbody.com/htm/body.html

(simplified, animated color diagrams of structures and functions of major organ systems)

The vertebrate digestive tract and its functions: The vertebrate digestive tract is a complete hollow tube that extends from mouth to anus. As befits the active vertebrate lifestyle, there are many specializations of this tube along its length. The ultimate goals of the gut are nutrition (process of taking in foods) and digestion (chemical and mechanical breakdown to the cellular level). The energy value of food is measured in calories (energy needed to raise 1 g of water 1o C; we eat in Calories or kilocalories). The three main organic molecules we eat are carbohydrates, proteins, and lipids. Lipids contain 9 kcal/gm vs. 4 kcal/gm with the other two, so that fats are a concentrated food source. Although digestion is ultimately done at the cellular level, the specialized vertebrate gut allows for extracellular digestion inside the lumen of the digestive tract. As you have already seen in lab dissections, different vertebrates have different specializations in their digestive tracts, depending on diet.

The oral cavity: The mouth aids in obtaining food (think of the different shaped mouths of a lamprey vs. a rattlesnake, for example) and is the first site of breaking down food. A tongue may serve various purposes. Teeth mechanically break down food, and salivary glands produce saliva, which moistens food and contains enzymes (amylase) that begin to break down carbohydrates. Lips help to hold food in. The oral cavity is connected to a pharynx, or common region for both airway and food path. As a vertebrate swallows food, involuntary peristalsis passes the food along. Peristalsis is the result of coordinated (inner) circular and (outer) longitudinal muscle layers in the gut.

The esophagus: The esophagus is a food pipe that takes food from the mouth to the stomach. Its length depends on neck length. One modification at its distal end in birds is a crop or storage area. Lining the gut from here on is an increasingly specialized mucosa or epithelial lining, involved in different digestive purposes.

The stomach: The stomach is useful for handling large, occasional meals, and is a site for continued mechanical and chemical breakdown of food. The stomach muscles help to churn food into a semiliquid chyme as the stomach mucosa secretes acid and pepsinogen (acid-converted into pepsin) to start to break down proteins (and also secretes mucus to protect itself!). The gizzard of a bird for grinding and rumen and other chambers of the cow stomach for bacterial fermentation of the chewed grass1 are notable vertebrate stomach specializations.

The small intestine: Most of digestion and absorption occurs in the small intestine. In addition to its length, the small intestine gets additional surface area from fingerlike villi (s.-villus) that project into the gut. In many animals, the small intestine is divided into a duodenum, a jejunum, and an ileum. The duodenum, right after the stomach, is more involved in digestion, and the jejunum and ileum are more for absorption of nutrients.

The pancreas: The pancreas secretes enzymes into the duodenum that complete carbohydrate, protein, and lipid digestion, as well as secreting bicarbonate-rich juices to help with pH of the chyme. Also, the pancreas secretes insulin and glucagon that control blood sugar levels.

The gallbladder: The gallbladder stores bile produced by the liver. Bile assists in lipid breakdown by emulsification, and secretes it into the duodenum via the bile duct.

The liver: The liver is the largest internal mammalian organ (and not exactly small in sharks!). Blood from the intestine drains to the liver via the hepatic portal vein. The liver cells take this nutrient-rich blood and help to metabolize it. Some nutrients (carbohydrates) may be stored as glycogen, and others may be converted into useful products such as vitamins and blood proteins. The liver is also the major detoxifying center.

The large intestine: The large intestine is the last reabsorber of water and creates feces (much of which are dead bacteria that live in there). The cecum and appendix are at its proximal end. The cloaca or anus (depending on which vertebrate) is the opening for the elimination of feces.

1 CP Hickman Jr, et al., Biology of Animals, 7th ed., (WCB McGraw-Hill, Boston, 1998), pp. 675-676.

 


|main page| |background| |03028: Physiology| |03048: Anatomy|

|03050: Invertebrate Zoology| |03051: Vertebrate Zoology| |03074: Economic Botany|


Please send comments and questions to: cronewil@hvcc.edu

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This web page last updated on April 7, 2000