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Chapter fifteen. Animal Nutrition and the Digestive System

xv.1 Digestive Systems

Learning Objectives

Past the end of this section, y'all volition be able to:

• Explain the processes of digestion and absorption
• Compare and contrast different types of digestive systems
• Explicate the specialized functions of the organs involved in processing food in the body
• Depict the ways in which organs work together to digest food and blot nutrients

Animals obtain their nutrition from the consumption of other organisms. Depending on their diet, animals tin be classified into the post-obit categories: plant eaters (herbivores), meat eaters (carnivores), and those that consume both plants and animals (omnivores). The nutrients and macromolecules nowadays in nutrient are not immediately accessible to the cells. At that place are a number of processes that change nutrient inside the brute body in society to make the nutrients and organic molecules attainable for cellular office. As animals evolved in complexity of form and function, their digestive systems have also evolved to accommodate their various dietary needs.

Herbivores, Omnivores, and Carnivores

Herbivores are animals whose main food source is plant-based. Examples of herbivores, as shown in Effigy 15.2 include vertebrates similar deer, koalas, and some bird species, besides as invertebrates such equally crickets and caterpillars. These animals accept evolved digestive systems capable of handling large amounts of plant cloth. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).

Figure 15.2.  Herbivores, like this (a) mule deer and (b) monarch caterpillar, eat primarily plant textile. (credit a: modification of work by Bill Ebbesen; credit b: modification of work past Doug Bowman)

Carnivores are animals that swallow other animals. The discussion carnivore is derived from Latin and literally ways "meat eater." Wild cats such as lions, shown in Figure 35.3 a and tigers are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs, shown in Figure 15.3 b . Obligate carnivores are those that rely entirely on beast mankind to obtain their nutrients; examples of obligate carnivores are members of the cat family, such as lions and cheetahs. Facultative carnivores are those that too eat non-animal food in addition to brute nutrient. Annotation that there is no articulate line that differentiates facultative carnivores from omnivores; dogs would be considered facultative carnivores.

Effigy 15.3.  Carnivores like the (a) king of beasts eat primarily meat. The (b) ladybug is also a carnivore that consumes small insects called aphids. (credit a: modification of piece of work past Kevin Pluck; credit b: modification of work by Jon Sullivan)

Omnivores are animals that eat both constitute- and animal-derived food. In Latin, omnivore means to eat everything. Humans, bears (shown in Figure 15.iv a ), and chickens are case of vertebrate omnivores; invertebrate omnivores include cockroaches and crayfish (shown in Figure xv.4 b ).

Effigy fifteen.4.  Omnivores like the (a) deport and (b) crayfish swallow both plant and animal based food. (credit a: modification of work by Dave Menke; credit b: modification of work past Jon Sullivan)

Invertebrate Digestive Systems

Animals take evolved different types of digestive systems to assist in the digestion of the different foods they consume. The simplest example is that of a gastrovascular cavity and is found in organisms with only 1 opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and bounding main anemones) utilise this type of digestion. Gastrovascular cavities, as shown in Figure 15.5 a , are typically a blind tube or crenel with only one opening, the "oral fissure", which also serves as an "anus". Ingested textile enters the mouth and passes through a hollow, tubular crenel. Cells within the cavity secrete digestive enzymes that break down the food. The food particles are engulfed by the cells lining the gastrovascular cavity.

The alimentary canal, shown in Figure 15.5 b , is a more advanced system: information technology consists of 1 tube with a mouth at one stop and an anus at the other. Earthworms are an example of an animate being with an alimentary canal. One time the food is ingested through the oral cavity, it passes through the esophagus and is stored in an organ called the ingather; then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine, the nutrients are captivated, and the waste product is eliminated every bit feces, called castings, through the anus.

Figure 15.5.  (a) A gastrovascular cavity has a single opening through which food is ingested and waste is excreted, as shown in this hydra and in this jellyfish medusa. (b) An comestible culvert has two openings: a mouth for ingesting nutrient, and an anus for eliminating waste material, as shown in this nematode.

Vertebrate Digestive Systems

Vertebrates have evolved more complex digestive systems to adapt to their dietary needs. Some animals have a single breadbasket, while others have multi-chambered stomachs. Birds take adult a digestive system adapted to eating unmasticated food.

Monogastric: Single-chambered Tummy

As the discussion monogastric suggests, this type of digestive organization consists of i ("mono") stomach bedroom ("gastric"). Humans and many animals take a monogastric digestive system as illustrated in Figure 15.6 ab . The process of digestion begins with the mouth and the intake of food. The teeth play an important function in masticating (chewing) or physically breaking down food into smaller particles. The enzymes nowadays in saliva also brainstorm to chemically break down food. The esophagus is a long tube that connects the rima oris to the stomach. Using peristalsis, or wave-like smooth muscle contractions, the muscles of the esophagus push the food towards the stomach. In guild to speed up the actions of enzymes in the stomach, the stomach is an extremely acidic environment, with a pH between ane.5 and ii.five. The gastric juices, which include enzymes in the stomach, human activity on the food particles and keep the process of digestion. Farther breakdown of nutrient takes identify in the small intestine where enzymes produced by the liver, the small intestine, and the pancreas keep the process of digestion. The nutrients are absorbed into the blood stream across the epithelial cells lining the walls of the small intestines. The waste matter material travels on to the large intestine where water is absorbed and the drier waste matter material is compacted into carrion; information technology is stored until it is excreted through the rectum.

Figure 15.vi.
(a) Humans and herbivores, such as the (b) rabbit, have a monogastric digestive organization. Yet, in the rabbit the modest intestine and cecum are enlarged to let more time to digest institute textile. The enlarged organ provides more than surface surface area for absorption of nutrients. Rabbits assimilate their food twice: the first time food passes through the digestive organisation, it collects in the cecum, and then it passes as soft carrion called cecotrophes. The rabbit re-ingests these cecotrophes to farther digest them.

Avian

Birds face special challenges when information technology comes to obtaining nutrition from food. They do non accept teeth then their digestive system, shown in Figure 15.seven, must be able to procedure un-masticated food. Birds accept evolved a variety of pecker types that reflect the vast variety in their nutrition, ranging from seeds and insects to fruits and nuts. Because nigh birds fly, their metabolic rates are loftier in lodge to efficiently procedure nutrient and proceed their body weight low. The stomach of birds has 2 chambers: the proventriculus, where gastric juices are produced to digest the food before it enters the breadbasket, and the gizzard, where the nutrient is stored, soaked, and mechanically ground. The undigested fabric forms nutrient pellets that are sometimes regurgitated. About of the chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca.

Effigy xv.7.  The avian esophagus has a pouch, called a crop, which stores food. Nutrient passes from the crop to the first of two stomachs, called the proventriculus, which contains digestive juices that break down food. From the proventriculus, the food enters the 2nd stomach, chosen the gizzard, which grinds food. Some birds eat stones or grit, which are stored in the gizzard, to aid the grinding process. Birds do not take separate openings to excrete urine and feces. Instead, uric acrid from the kidneys is secreted into the large intestine and combined with waste matter from the digestive process. This waste is excreted through an opening chosen the cloaca.

Parts of the Digestive System

The vertebrate digestive system is designed to facilitate the transformation of food matter into the nutrient components that sustain organisms.

Oral Crenel

The oral crenel, or oral cavity, is the signal of entry of food into the digestive arrangement, illustrated in Effigy 15.9. The nutrient consumed is broken into smaller particles by mastication, the chewing activeness of the teeth. All mammals have teeth and can chew their food.

The extensive chemical process of digestion begins in the mouth. As food is being chewed, saliva, produced by the salivary glands, mixes with the nutrient. Saliva is a watery substance produced in the mouths of many animals. There are three major glands that secrete saliva—the parotid, the submandibular, and the sublingual. Saliva contains mucus that moistens nutrient and buffers the pH of the food. Saliva also contains immunoglobulins and lysozymes, which accept antibacterial activeness to reduce molar decay past inhibiting growth of some bacteria. Saliva also contains an enzyme called salivary amylase that begins the process of converting starches in the nutrient into a disaccharide called maltose. Another enzyme called lipase is produced by the cells in the tongue. Lipases are a class of enzymes that can break downwards triglycerides. The lingual lipase begins the breakdown of fat components in the food. The chewing and wetting activity provided past the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx. The throat opens to 2 passageways: the trachea, which leads to the lungs, and the esophagus, which leads to the tum. The trachea has an opening called the glottis, which is covered by a cartilaginous flap called the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea. This system allows nutrient to be kept out of the trachea.

Figure 15.ix.
Digestion of food begins in the (a) oral cavity. Nutrient is masticated by teeth and moistened by saliva secreted from the (b) salivary glands. Enzymes in the saliva begin to digest starches and fats. With the help of the tongue, the resulting bolus is moved into the esophagus by swallowing. (credit: modification of work by the National Cancer Institute)

Esophagus

The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series of wave like movements called peristalsis that push the nutrient toward the stomach, as illustrated in Figure fifteen.10. The peristalsis moving ridge is unidirectional—it moves food from the oral cavity to the breadbasket, and contrary move is not possible. The peristaltic movement of the esophagus is an involuntary reflex; it takes identify in response to the human activity of swallowing.

Figure xv.10.  The esophagus transfers food from the mouth to the tummy through peristaltic movements.

A ring-like musculus called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted past the bolus of nutrient, this sphincter opens, and the bolus enters the stomach. When there is no swallowing activity, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals have a true sphincter; all the same, in humans, at that place is no true sphincter, but the esophagus remains airtight when there is no swallowing action. Acrid reflux or "heartburn" occurs when the acidic digestive juices escape into the esophagus.

Stomach

A big office of digestion occurs in the stomach, shown in Effigy 15.xi. The stomach is a saclike organ that secretes gastric digestive juices. The pH in the stomach is betwixt 1.5 and 2.5. This highly acidic environment is required for the chemical breakdown of nutrient and the extraction of nutrients. When empty, the tummy is a rather small-scale organ; notwithstanding, information technology tin can aggrandize to up to 20 times its resting size when filled with nutrient. This characteristic is particularly useful for animals that demand to eat when food is available.

Figure xv.11.  The human stomach has an extremely acidic environment where nearly of the protein gets digested. (credit: modification of work past Mariana Ruiz Villareal)

Which of the post-obit statements nigh the digestive organization is simulated?

1. Chyme is a mixture of food and digestive juices that is produced in the stomach.
2. Food enters the large intestine earlier the modest intestine.
3. In the small intestine, chyme mixes with bile, which emulsifies fats.
4. The tum is separated from the small-scale intestine past the pyloric sphincter.

The breadbasket is also the major site for protein digestion in animals other than ruminants. Protein digestion is mediated by an enzyme called pepsin in the stomach sleeping room. Pepsin is secreted past the chief cells in the stomach in an inactive form called pepsinogen. Pepsin breaks peptide bonds and cleaves proteins into smaller polypeptides; it likewise helps activate more pepsinogen, starting a positive feedback mechanism that generates more pepsin. Some other cell type—parietal cells—secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the chief acidic component of the tum juices. Hydrochloric acrid helps to convert the inactive pepsinogen to pepsin. The highly acidic surroundings also kills many microorganisms in the nutrient and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning activeness of the tum. Wrinkle and relaxation of smooth muscles mixes the stomach contents most every 20 minutes. The partially digested food and gastric juice mixture is called chyme. Chyme passes from the stomach to the small intestine. Further poly peptide digestion takes identify in the pocket-size intestine. Gastric emptying occurs within 2 to six hours after a meal. Only a modest amount of chyme is released into the modest intestine at a time. The move of chyme from the stomach into the minor intestine is regulated by the pyloric sphincter.

When digesting protein and some fats, the tummy lining must be protected from getting digested by pepsin. There are two points to consider when describing how the stomach lining is protected. First, as previously mentioned, the enzyme pepsin is synthesized in the inactive form. This protects the primary cells, because pepsinogen does not have the aforementioned enzyme functionality of pepsin. 2nd, the tummy has a thick fungus lining that protects the underlying tissue from the action of the digestive juices. When this fungus lining is ruptured, ulcers tin form in the stomach. Ulcers are open wounds in or on an organ caused by bacteria (Helicobacter pylori) when the mucus lining is ruptured and fails to reform.

Small Intestine

Chyme moves from the stomach to the small intestine. The small-scale intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small-scale intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic projections called microvilli. These structures, illustrated in Figure fifteen.12, are lined with epithelial cells on the luminal side and permit for the nutrients to be absorbed from the digested food and absorbed into the blood stream on the other side. The villi and microvilli, with their many folds, increase the surface surface area of the intestine and increment absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the torso and removes toxic substances, including drugs, alcohol, and some pathogens.

Figure fifteen.12.  Villi are folds on the small intestine lining that increment the surface area to facilitate the absorption of nutrients.

Which of the following statements about the small-scale intestine is false?

1. Absorbent cells that line the small intestine accept microvilli, small-scale projections that increase area and aid in the absorption of food.
2. The inside of the small intestine has many folds, chosen villi.
3. Microvilli are lined with claret vessels besides as lymphatic vessels.
4. The inside of the small intestine is called the lumen.

The human small intestine is over 6m long and is divided into three parts: the duodenum, the jejunum, and the ileum. The "C-shaped," fixed part of the small intestine is chosen the duodenum and is shown in Figure 15.11. The duodenum is separated from the stomach past the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Pancreatic juices also incorporate several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, too as from gland cells of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats. These digestive juices interruption down the food particles in the chyme into glucose, triglycerides, and amino acids. Some chemical digestion of food takes identify in the duodenum. Assimilation of fatty acids as well takes place in the duodenum.

The 2nd role of the small intestine is chosen the jejunum, shown in Effigy xv.11. Here, hydrolysis of nutrients is connected while well-nigh of the carbohydrates and amino acids are captivated through the intestinal lining. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.

The ileum, too illustrated in Effigy 15.11 is the last function of the modest intestine and here the bile salts and vitamins are captivated into claret stream. The undigested nutrient is sent to the colon from the ileum via peristaltic movements of the muscle. The ileum ends and the big intestine begins at the ileocecal valve. The vermiform, "worm-similar," appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant office in immunity.

Large Intestine

The large intestine, illustrated in Figure 15.13, reabsorbs the h2o from the undigested food material and processes the waste textile. The man big intestine is much smaller in length compared to the small intestine but larger in bore. Information technology has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste material thing. The colon is home to many bacteria or "intestinal flora" that assist in the digestive processes. The colon can be divided into iv regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the h2o and mineral salts from undigested food, and to shop waste matter material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet.

Rectum and Anus

The rectum is the concluding stop of the large intestine, as shown in Figure 15.xiii. The primary role of the rectum is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary.

Accessory Organs

The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs are organs that add secretions (enzymes) that catabolize food into nutrients. Accompaniment organs include salivary glands, the liver, the pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed.

The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fat components of the food in the duodenum. The liver likewise processes the vitamins and fats and synthesizes many plasma proteins.

The pancreas is another important gland that secretes digestive juices. The chyme produced from the breadbasket is highly acidic in nature; the pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the digestion of protein and carbohydrates.

The gallbladder is a small-scale organ that aids the liver by storing bile and concentrating bile salts. When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.

Summary

Different animals accept evolved different types of digestive systems specialized to meet their dietary needs. Humans and many other animals have monogastric digestive systems with a single-chambered breadbasket. Birds take evolved a digestive organisation that includes a gizzard where the food is crushed into smaller pieces. This compensates for their inability to masticate. Ruminants that eat large amounts of plant fabric have a multi-chambered tum that digests roughage. Pseudo-ruminants take similar digestive processes equally ruminants merely do non have the 4-compartment tum. Processing food involves ingestion (eating), digestion (mechanical and enzymatic breakdown of large molecules), absorption (cellular uptake of nutrients), and elimination (removal of undigested waste product as feces).

Many organs work together to digest nutrient and absorb nutrients. The rima oris is the point of ingestion and the location where both mechanical and chemical breakdown of food begins. Saliva contains an enzyme called amylase that breaks down carbohydrates. The food bolus travels through the esophagus by peristaltic movements to the stomach. The stomach has an extremely acidic environment. An enzyme chosen pepsin digests protein in the stomach. Further digestion and absorption have identify in the small intestine. The large intestine reabsorbs water from the undigested food and stores waste matter until emptying.

Exercises

1. Which of the following statements about the digestive system is fake?
   1. Chyme is a mixture of food and digestive juices that is produced in the breadbasket.
   2. Nutrient enters the large intestine before the small intestine.
   3. In the minor intestine, chyme mixes with bile, which emulsifies fats.
   4. The stomach is separated from the small intestine by the pyloric sphincter.
2. Which of the following statements nigh the pocket-sized intestine is fake?
   1. Absorptive cells that line the small-scale intestine have microvilli, small projections that increment surface surface area and aid in the assimilation of food.
   2. The inside of the small intestine has many folds, chosen villi.
   3. Microvilli are lined with blood vessels as well as lymphatic vessels.
   4. The inside of the small intestine is chosen the lumen.
3. Which of the following is a pseudo-ruminant?
   1. moo-cow
   2. pig
   3. crow
   4. equus caballus
4. Which of the following statements is untrue?
   1. Roughage takes a long time to digest.
   2. Birds eat large quantities at one fourth dimension and so that they can fly long distances.
   3. Cows do not have upper teeth.
   4. In pseudo-ruminants, roughage is digested in the cecum.
5. The acidic nature of chyme is neutralized by ________.
   1. potassium hydroxide
   2. sodium hydroxide
   3. bicarbonates
   4. vinegar
6. The digestive juices from the liver are delivered to the ________.
   1. tummy
   2. liver
   3. duodenum
   4. colon
7. How does the polygastric digestive organization assist in digesting roughage?
8. How exercise birds digest their food in the absence of teeth?
9. What is the role of the accessory organs in digestion?
10. Explain how the villi and microvilli assist in absorption.

Answers

1. B
2. C
3. D
4. B
5. C
6. C
7. Animals with a polygastric digestive system have a multi-chambered stomach. The iv compartments of the tummy are chosen the rumen, reticulum, omasum, and abomasum. These chambers comprise many microbes that pause downwardly the cellulose and ferment the ingested nutrient. The abomasum is the "truthful" tum and is the equivalent of a monogastric stomach bedchamber where gastric juices are secreted. The four-compartment gastric chamber provides larger infinite and the microbial support necessary for ruminants to digest institute material.
8. Birds have a stomach bedroom chosen a gizzard. Here, the food is stored, soaked, and ground into effectively particles, ofttimes using pebbles. In one case this process is consummate, the digestive juices take over in the proventriculus and proceed the digestive process.
9. Accessory organs play an important office in producing and delivering digestive juices to the intestine during digestion and assimilation. Specifically, the salivary glands, liver, pancreas, and gallbladder play important roles. Malfunction of whatever of these organs can lead to disease states.
10. The villi and microvilli are folds on the surface of the pocket-sized intestine. These folds increase the surface area of the intestine and provide more surface area for the absorption of nutrients.

Glossary

gastrointestinal tract: tubular digestive system with a mouth and anus
anus: get out indicate for waste material
bile: digestive juice produced by the liver; important for digestion of lipids
bolus: mass of food resulting from chewing activeness and wetting by saliva
carnivore: creature that consumes animal flesh
chyme: mixture of partially digested food and breadbasket juices
digestion: mechanical and chemical break downward of food into pocket-sized organic fragments
duodenum: first part of the minor intestine where a large function of digestion of carbohydrates and fats occurs
endocrine organisation: organization that controls the response of the diverse glands in the body and the release of hormones at the advisable times
esophagus: tubular organ that connects the mouth to the stomach
essential nutrient: food that cannot be synthesized by the body; it must be obtained from nutrient
gallbladder: organ that stores and concentrates bile
gastric inhibitory peptide: hormone secreted by the small intestine in the presence of fat acids and sugars; information technology too inhibits acrid production and peristalsis in order to slow down the rate at which food enters the small intestine
gastrin: hormone which stimulates muriatic acid secretion in the tum
gastrovascular cavity: digestive arrangement consisting of a unmarried opening
gizzard: muscular organ that grinds nutrient
herbivore: animal that consumes strictly plant nutrition
ileum: final role of the small intestine; connects the pocket-size intestine to the large intestine; important for absorption of B-12
ingestion: act of taking in food
jejunum: second part of the pocket-sized intestine
lactase: enzyme that breaks down lactose into glucose and galactose
big intestine: digestive system organ that reabsorbs water from undigested textile and processes waste matter thing
lipase: enzyme that chemically breaks downwardly lipids
liver: organ that produces bile for digestion and processes vitamins and lipids
maltase: enzyme that breaks down maltose into glucose
mineral: inorganic, elemental molecule that carries out important roles in the body
monogastric: digestive organization that consists of a single-chambered stomach
omnivore: animate being that consumes both plants and animals
pancreas: gland that secretes digestive juices
pepsinogen: inactive form of pepsin
pepsin: enzyme constitute in the stomach whose main role is protein digestion
peristalsis: moving ridge-like movements of muscle tissue
proventriculus: glandular office of a bird'due south tummy
rectum: area of the body where feces is stored until elimination
roughage: component of food that is low in energy and high in fiber
ruminant: brute with a stomach divided into iv compartments
salivary amylase: enzyme found in saliva, which converts carbohydrates to maltose
small intestine: organ where digestion of protein, fats, and carbohydrates is completed
somatostatin: hormone released to stop acid secretion when the stomach is empty
sphincter: band of muscle that controls movement of materials throughout the digestive tract
stomach: saclike organ containing acidic digestive juices
villi: folds on the inner surface of the small intestine whose role is to increase absorption area
vitamin: organic substance necessary in pocket-size amounts to sustain life

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