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Fueling Body Activities: Digestion

Chapter 43

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Outline

• Types of Digestive Systems– Vertebrate Digestive Systems

The Mouth and Teeth Esophagus and Stomach The Small Intestine The Large Intestine Accessory Organs

• Neural and Hormonal Regulation of Digestion• Food Energy and Energy Expenditure

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Types of Digestive Systems

• Single-celled organisms digest their food intracellularly.

• Other animals digest their food extracellularly within a digestive cavity.

– digestive enzymes released into a cavity Specialization occurs when the

digestive tract, or alimentary canal, has a separate mouth and anus.

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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 subunit

molecules.Products pass through the epithelial

lining of the gut into the blood (absorption).

Waste products are excreted.

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Vertebrate Digestive Systems

• Consists of tubular gastrointestinal tract and accessory digestive organs.

– mouth and pharynx – esophagus - delivers food to stomach– stomach - preliminary digestion– small intestine - absorption– large intestine - water absorption– cloaca or rectum - waste storage

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Human Digestive System

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Vertebrate Digestive Systems

• Accessory digestive organs include:– liver

produces bile– gallbladder

stores and concentrates bile– pancreas

produces pancreatic juice

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Vertebrate Digestive Systems

• Tubular gastrointestinal tract has a characteristic layered structure.

– mucosa - epithelium– submucosa - connective tissue– muscularis - double layer of smooth muscle– serosa - connective tissue

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Gastrointestinal Tract Layers

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The Mouth and Teeth

• Vertebrate teeth– Carnivorous mammals have pointed teeth

that lack flat grinding surfaces.– Herbivores must pulverize cellulose of cell

walls of plant tissue before digestion. have large, flat teeth suited to grinding

– Humans are essentially carnivores in the front, and herbivores in the back.

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Generalized Vertebrate Dentition

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The Mouth and Teeth

• Mouth– The tongue mixes food with saliva.

moistens and lubricates food secretions controlled by nervous system

Taste-sensitive neurons in the mouth send impulses to the brain, which responds by stimulating the salivary glands.

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The Mouth and Teeth

• When food is ready to be swallowed, the tongue moves it to the back of the mouth.

– elevated by soft palate– pressure against pharynx triggers an

automatic, involuntary reflex larynx contracted and raised

glottis pushed against epiglottis keeps food out of respiratory tract

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Human Pharynx, Palate, and Larynx

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Esophagus and Stomach

• Structure and function of the esophagus– Swallowing center stimulates successive

waves of contraction that moves food along esophagus to stomach.

controlled by ring of smooth muscle (sphincter)

• Structure and function of the stomach– Surface is highly convoluted, enabling it to

fold when empty and expand as it fills with food.

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Esophagus and Stomach

• Secretory systems– Exocrine glands contain two cell types:

parietal cells - secrete hydrochloric acid chief cells - secrete pepsinogen

• Action of acid– Human stomach produces about 2 liters of

HCl and other gastric juices everyday. helps denature food proteins

chyme

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Stomach and Duodenum

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Esophagus and Stomach

• Ulcers– Gastric ulcers are rare because epithelial

cells in the mucosa are protected by a layer of alkaline mucus.

Susceptibility increased when mucosal barriers are weakened by Helicobacter pylori.

– Chyme leaves the stomach through the pyloric sphincter.

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The Small Intestine

• Digestion– approximately 4.5 m long, and divided into

duodenum, jejunum and ileum– epithelial wall covered with villi

covered by microvilligreatly increase surface area

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Small Intestine

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Accessory Organs

• Secretions of the pancreas– Pancreatic fluid is secreted into duodenum

through the pancreatic duct. host of enzymes: trypsin, chymotrypsin,

pancreatic amylase, and lipaseDigest proteins into smaller

polypeptides, polysaccharides into shorter sugar chains, and fat into free fatty acids.

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Pancreas

• Pancreas also functions as endocrine gland, secreting hormones to control blood glucose.

– produced in islets of Langerhans

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Accessory Organs

• Liver and gallbladder– Liver is largest internal organ of the body.

Main secretion is bile, a fluid mixture of bile pigments and bile salts delivered into the duodenum during digestion.

Bile pigments are waste products.Bile salts act as detergents.

emulsification of fat stored in gallbladder

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The Small Intestine

• Absorption– Glucose and amino acids enter the

bloodstream via the hepatic portal vein.– Fat enters the lymphatic system.– Approximately 9 liters of fluid passes

through the small intestine daily. Only about 50 g of solids and 100 ml of

liquid leave the body as feces.

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The Large Intestine

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

– no digestion takes place, and only about 4% of absorption occurs there

undigested material, primarily bacterial fragments and cellulose, compacted and stored

compacted feces driven by peristaltic contractions pass into rectum

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Variations in Vertebrate Digestive Systems

• Digestive tracts of some animals contain bacteria and protists that convert cellulose into substances the host can digest.

• Ruminants have large, divided stomachs.– rumen and reticulum– omassum and abomasum

rumination– Rodents and lagomorphs practice

coprophagy.

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Four-Chambered Ruminant Stomach

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Variations in Vertebrate Digestive Systems

• All mammals rely on intestinal bacteria to synthesize vitamin K.

– necessary for blood clotting prolonged treatment with antibiotics

greatly reduces bacterial populations in the body

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Neural and Hormonal Regulation of Digestion

• Gastrointestinal activities are coordinated by the nervous system and endocrine system.

– Stomach secretions are regulated by food and gastrin.

– The passage of chyme into the duodenum inhibits stomach contractions.

– Duodenum secretes other hormones that inhibit stomach emptying and promote bile release and bicarbonate secretion.

enterogastrones

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Hormonal Control of Gastrointestinal Tract

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Accessory Organs

• Liver regulatory functions– Liver chemically modifies substances

absorbed in the gastrointestinal tract before they reach the rest of the body.

also removes toxins and poisons, and converts them into less toxic forms

– Liver regulates many compounds such as steroid hormones, and produces most proteins found in blood plasma.

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Accessory Organs

• Regulation of blood glucose concentration– After a carbohydrate-rich meal, the liver and

skeletal muscles remove excess glucose from blood and store it as glycogen.

stimulated by insulinWhen glucose levels decrease, the

liver secretes glucose in the blood. breakdown of glycogen gluconeogenesis - process of

converting other molecules into glucose

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Actions of Insulin and Glucagon

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Food Energy and Energy Expenditure

• Ingestion of food serves two primary functions:

– provides source of energy– provides raw materials that cannot be

manufactured by the organism Basal metabolic rate (BMR) refers to the

minimum rate of energy consumption under defined resting conditions.

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Food Energy and Energy Expenditure

• If the amount of food energy taken in is greater than the energy consumed per day, the excess energy will be stored in glycogen and fat.

– As glycogen reserves are limited, ingestion of excess food energy results primarily in the accumulation of fat.

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Food Energy and Energy Expenditure

• Regulation of food intake– Recent human studies show activity of ob

gene and blood concentrations of leptin (satiety factor) are higher in obese people than in lean people.

Leptin produced by obese people appears to be normal.

Most cases of human obesity may result from reduced sensitivity to action of leptin in the brain.

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Essential Nutrients

• Essential nutrients are substances an animal cannot manufacture for itself but which are necessary for health must be obtained in the diet.

– essential amino acids– unsaturated fatty acids

• Essential minerals– trace elements

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Summary

• Types of Digestive Systems– Vertebrate Digestive Systems

The Mouth and Teeth Esophagus and Stomach The Small Intestine The Large Intestine Accessory Organs

• Neural and Hormonal Regulation of Digestion• Food Energy and Energy Expenditure

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