Chapter 47

The Digestive System

Types of Digestive Systems

• Heterotrophs are divided into three groups based on their food sources1. Herbivores are animals that eat plants

exclusively

2. Carnivores are animals that eat other animals

3. Omnivores are animals that eat both plants and other animals

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Types of Digestive Systems

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Wastes

Food

Tentacle

Body stalk

Gastrovascularcavity

Mouth

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.• Single-celled organisms and sponges digest their food intracellularly

• Other multicellular animals digest their food extracellularly– Within a digestive cavity

• Cnidarians and flatworms have a gastrovascular cavity – Only one opening, and no

specialized regions

Types of Digestive Systems

• Specialization occurs when the digestive tract has a separate mouth and anus– Nematodes have the most primitive digestive tract

• Tubular gut lined by an epithelial membrane, pseudocoelomate– More complex animals have a digestive tract specialized in

different regions

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Types of Digestive Systems

• Ingested food may be stored or first subjected to physical fragmentation

• Chemical digestion occurs next– Hydrolysis reactions liberate the subunit

molecules• Products pass through gut’s epithelial

lining into the blood (absorption) • Wastes are excreted from the anus

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Vertebrate Digestive Systems• Consists of a tubular

gastrointestinal tract and accessory organs

• Mouth and pharynx – entry

• Esophagus – delivers food to stomach

• Stomach – preliminary digestion

• Small intestine – digestion and absorption

• Large intestine – absorption of water and minerals

• Cloaca or rectum – expel waste

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Vertebrate Digestive Systems

• Accessory organs–Liver

• Produces bile–Gallbladder

• Stores and concentrates bile–Pancreas

• Produces pancreatic juice• Digestive enzymes and bicarbonate buffer

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Vertebrate Digestive Systems• Gastrointestinal tract is layered

– Mucosa – innermost • Epithelium that lines the

interior, or lumen, of the tract

– Submucosa• Connective tissue

– Muscularis• Circular and longitudinal

smooth muscle layers– Serosa – outermost

• Epithelium covering external surface of tract

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Mouth and Teeth

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• Many vertebrates have teeth used for chewing or mastication

• Birds– Lack teeth– Break up food in a two-

chambered stomach– Gizzard – muscular chamber

that uses ingested pebbles to pulverize food

• Carnivores – pointed teeth that lack flat grinding surfaces

• Herbivores – large flat teeth suited for grinding cellulose cell walls of plant tissues

• Humans have carnivore-like teeth in the front and herbivore-like teeth in the back

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Horse Lion Human

Molars

Premolars

Canines

Incisors

Herbivore Carnivore Omnivore

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Mouth and Teeth

• Inside the mouth, the tongue mixes food with saliva

• Moistens and lubricates the food• Contains salivary amylase, which initiates the

breakdown of starch• Salivation is controlled by the nervous system

– Tasting, smelling, and even thinking or talking about food stimulate increased salivation

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Mouth and Teeth

• Swallowing– Starts as voluntary action

• Continued under involuntary control – When food is ready to be swallowed, the tongue moves it to the

back of the mouth– Soft palate seals off nasal cavity– Elevation of the larynx (voice box) pushes the glottis against the

epiglottis• Keeps food out of respiratory tract

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The Esophagus

• Muscular tube connecting the mouth to the stomach• Actively moves a bolus through peristalsis

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The Stomach • Saclike portion of tract• Convoluted surface allows

expansion • Contains 3rd layer of smooth

muscles for mixing food with gastric juice

• 3 kinds of secretory cells– Mucus-secreting cells– Parietal cells

• Secrete HCl and intrinsic factor (for vitamin B12 absorption)

– Chief cells• Secrete pepsinogen

(inactive form of pepsin)

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The Stomach

• Starts to break down proteins• No significant digestion of carbohydrates or fats occurs• Absorption of some water (aspirin and alcohol)

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The Small Intestine

• About 4.5 m long – small diameter• Consists of duodenum, jejunum, and ileum• Receives

– Chyme from stomach– Digestive enzymes and bicarbonate from pancreas– Bile from liver and gallbladder

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Accessory Organs

• Pancreas – Pancreatic fluid is secreted into the

duodenum through the pancreatic duct – Enzymes to break down proteins, carbs

and fats– Bicarbonate neutralizes acidic chyme

from stomach– Exocrine and endocrine gland

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• Liver – Body’s largest internal organ– Secretes bile

• emulsifies fats

• Gallbladder– Stores and concentrates bile – Arrival of fatty food in the

duodenum triggers the gallbladder to contract, causing bile to be transported through the common bile duct and injected into the duodenum

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Accessory Organs

Absorption

• Amino acids and monosaccharides are transported through epithelial cells to blood– Blood carries these products to the liver via the

hepatic portal vein

• Fatty acids and monoglycerides diffuse into epithelial cells– Enter the lymphatic system and later join the

circulatory system

• Almost all fluid reabsorbed in small intestine

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The Large Intestine (colon)

• Much shorter than small intestine, but has larger diameter

• Small intestine empties directly into the large intestine at a junction where two vestigial structures, cecum and appendix, remain

• No digestion occurs• Function to reabsorb water and remaining electrolytes• Prepare waste for expulsion

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The Large Intestine

• Many bacteria live and reproduce within the large intestine. Provide humans with Vitamin K

• Feces compacted and passed to rectum• Feces exit anus

– Smooth muscle sphincter (involuntary)– Striated muscle sphincter (voluntary)

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Variations in Digestive Systems• Digestive tracts of some

animals contain bacteria and protists that convert cellulose into substances the host can absorb– Minor in humans– Essential to some animals

• Herbivores have longer digestive tracts– Greater time for digestion of

cellulose– Modifications to enhance

digestion of plant material

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• Ruminants have a four-chambered stomach– Contents can be

regurgitated and rechewed• Rumination

– Evolved only once

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ReticulumEsophagus

RumenSmall intestine

Abomasum Omasum

Variations in Digestive Systems

• Rodents, horses, deer, and rabbits digest cellulose in the cecum – Regurgitation of contents is not possible

• However, some such animals practice coprophagy– Eat their feces to absorb nutrients on the

second passage of food– Cannot remain healthy if prevented from

eating feces

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Accessory Organ Function• Liver

– Chemically modifies the substances absorbed from the digestive tract before they reach the rest of the body

– Ingested alcohol and other drugs are taken into liver cells and metabolized

– Removes toxins, pesticides, and carcinogens, converting them to less toxic forms

– Regulates levels of steroid hormones – Produces most proteins found in plasma

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Accessory Organ Function

• Regulation of blood glucose– After a carbohydrate-rich meal

• Insulin stimulates removal of excess blood glucose by liver and skeletal muscles (glycogen)

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Food Energy

• Ingestion of food serves two primary functions1. Source of energy

2. Source of raw material (to make things the body needs)

• Basal metabolic rate (BMR)– Minimal amount of energy consumed under

defined resting conditions– Continued ingestion of excess food energy

results primarily in accumulation of fat 27

Essential Nutrients

• Animal cannot manufacture these for itself but are necessary for health and so must be obtained in the diet

• Vitamins– Humans, apes, monkeys, and guinea pigs have lost

the ability to synthesize ascorbic acid (vitamin C)• Amino acids – humans require 9• Long-chain unsaturated fatty acids

– Vertebrates can synthesize cholesterol, a key component of steroid hormones, but some carnivorous insects cannot

• Minerals

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