Digestive System

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The digestive systems of animals are quite diverse. Sponges, the simplest animals, do not have specialized digestive systems. Food particles filtered from the water are simply digested within individual cells (intracellular digestion). One of the first steps toward a complex digestive system in animals, and the processing of larger prey, is the evolution of a gastrovascular cavity, a digestive sac with a single opening to the external environment. The gastrovascular cavity serves as a protected space for extracellular digestioninside the animal, and at the same time allows distribution of the digested material to most cells of the body. Following extracellular digestion in the gastrovascular cavity, the digestion products from carbohydrates, proteins, and fats are taken up by cells lining the gastrovascular cavity, where digestion is completed intracellularly.

Cnidarians and flatworms have a gastrovascular cavity. Cnidarians, such as the hydra, use their tentacles to move food through their mouth into their gastrovascular cavity. Then the cells lining this cavity excrete digestive enzymes that will start extracellular digestion and break the prey into smaller pieces. Any undigested remnants of the prey are expelled through the mouth opening. Like cnidarians, flatworms have a gastrovascular cavity with a single opening, but the cavity itself is highly folded. These folds greatly increase the surface area and extend throughout the body, bringing nutrients within the reach of all cells.

The gastrovascular cavity of cnidarians and flatworms allows them to digest larger prey than they could with intracellular digestion. However, the effectiveness of a gastrovascular cavity in supplying the animal with nutrients is limited. Because there is only one opening to the external environment through which prey is taken in and remnants are expelled, the animals have to complete digestion of the first prey and expel its remnants before taking in another prey. With the evolution of a second opening in the digestive system, the digestive system became a digestive tract, or alimentary canal, making it a one-way system between mouth and anus. Food could now be taken in and processed continuously, providing the animal with more nutrients. Most animals—including vertebrates, arthropods, mollusks, round worms and earthworms—have this form of digestive tract.

A one-way digestive tract is efficient because it allows the food to pass through a series of specialized regions. Such regions may be specialized for protein, fat, or carbohydrate digestion, making each step more efficient. Other regions may be used for food storage or for preparing the food for chemical digestion by physically grinding it into smaller pieces, which exposes more surface area to the action of digestive enzymes. These specialized regions eventually evolved into organs as parts of a complex digestive organ system. However, because nutrient dispersal, by the digestive system itself to all cells of the body, was no longer feasible with such a specialized digestive system (and animals became larger and bulkier) a separate cardiovascular system evolved to serve that function.

Simple animals such as earthworms suck soil into the mouth with the pharynx, pass it through the esophagus into the crop, where it is moistened and stored. From there it is moved into the gizzard, which contains small grains of sand that help grind down the food. The actual digestion and absorption of nutrients takes place in the intestine, and anything that remains is excreted through the anus. Insects also move food from the mouth through the esophagus into a crop (all parts of the foregut) for food storage and moistening. From there it is moved to a midgut where digestion and nutrient absorption through specialized extensions, or ceca, takes place. The hindgut functions mainly to reclaim water and ions from the gut content that would otherwise be lost in the feces.

During the evolution of vertebrates, two trends can be recognized. First, animals became larger, requiring a more efficient digestive system to meet their nutritional needs. Second, some animals moved from living in water to living on land. This meant they required more energy for locomotion and a more efficient digestive system to provide that energy.

In vertebrates, the physical digestion of food begins in the mouth. Birds crunch seeds with their beaks, and mammals use their powerful jaws and specialized teeth to chew food into smaller pieces, increasing the surface area for digestive enzymes to work on. Salivary glands in the mouth coat the food with saliva to make it slippery for swallowing. After swallowing, the food is moved along the digestive tract with the help of involuntary smooth muscle contractions, called peristalsis. Sphincters regulate the passage of food from one chamber of the digestive tract into the next. First the food passes through the esophagus into the stomach. In the stomach, the food is stored and mixed with gastric juice. The gastric juice kills most swallowed bacteria, breaks down most food into individual cells (increasing the surface area for enzyme attack), and begins the digestion of proteins. Birds may store food in a crop without digesting it before passing it into the stomach. This allows parent birds to regurgitate food from their crops for their nestlings. Some birds move food from the stomach into a muscular gizzard containing swallowed stones that grind down seeds before digestion continues in the small intestine. The small intestine is the major site of digestion and absorption in vertebrates, and has three distinct regions—the duodenum, the jejunum, and the ileum. Accessory glands such as the pancreas and liver secrete digestive enzymes and other products into the duodenum. The jejunum also releases digestive enzymes. These enzymes digest carbohydrates, proteins, nucleic acids, and fat, and the products of the digestion are absorbed by cells lining the small intestine, especially the ileum. The large intestine is connected to the small intestine. The major function of the large intestine is to reabsorb water that was added to the gut content in the small intestine, and to absorb inorganic ions from the digested food. As a result the feces become more solid. The large intestine also contains many bacteria, which may produce gases asbyproduct of their metabolism, but also vitamins, such as vitamin K, that are absorbed into the blood. Feces are stored in the rectum until they can be eliminated through the anus.

Many grazing animals (e.g., deer, cattle, sheep, giraffes) who swallow grass hastily without chewing while watching out for predators, have a two-part stomach. In the first part of the stomach, the unchewed grass is fermented and predigested by bacteria before it is regurgitated back up into the mouth. There it can be chewed more thoroughly when the animal is in a safe place. After chewing, the food is swallowed again and passes into the second section where digestion takes place. Herbivores generally have a longer and more complex intestine than carnivores. This allows them to get as many nutrients as possible out of their more nutrient-poor food.

Digestion.

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