Chapter 46: The Digestive System

46.1: Types of Digestive Systems

Learning Outcomes

1. Explain the features that characterize more specialized digestive systems
2. List the components of the vertebrate digestive tract
3. Describe the tissue layers of the gastrointestinal tract

Heterotrophs

Heterotrophs are divided into three groups on the basis of their food sources

1. Herbivores- animals that eat plants
2. Carnivores- animals that eat other animals
3. Omnivores- animals that eat both plants and animals

Invertebrate digestive systems are bags of tubes

Vertebrate Digestive Systems

• Vertebrates, nematodes, and earthworms have one-way digestive tracts
• Vertebrate digestive systems include highly specialized structures molded by diet

Tissues of the Digestive Tract

• Serosa
• Mucosa
• Submucosa
• Muscularis externa

46.2: The Mouth and Teeth: Food Capture and Bulk Processing

Learning Outcomes

1. Identify adaptive variation in vertebrate tooth shape
2. Understand the role of the mouth in the digestive process

Vertebrate Teeth

• Vertebrate teeth are adapted to different types of food items
  • Carnivore
  • Herbivore
  • Omnivore

Mouth

• Mouth- a chamber for ingestion and initial processing

46.3: The Esophagus and the Stomach: The Early Stages of Digestion

Learning Outcomes

1. Describe how food moves through the esophagus
2. Explain what digestive processes take place in the stomach

Esophagus

• Muscular contractions of the esophagus move food to the stomach
• Peristalsis- series of involuntary wave-like muscle contractions which move food along the digestive tract

Stomach

• Acidic breakdown of food occurs in the stomach
• Chyme- the mixture of partially digested food and gastric juice
• Ulcers- overproduction of stomach acid can occasionally eat a hole through the wall of the stomach or the duodenum, causing a peptic ulcer
• Chyme leaves the stomach through the pyloric sphincter to enter the small intestine

Secretory Systems

• Gastric Juice- an acidic secretion of the tubular gastric glands of the mucosa
• Pepsinogen- the inactive form of pepsin
• Pepsin- protein-digesting enzyme
• Intrinsic Factor- a polypeptide needed for the intestinal absorption of vitamin B12

Action of Acid

• The low pH in the stomach helps denature food proteins, making them easier to digest
• Major Functions of Stomach Acid:
  • Sterilizes the Food
  • Protein Digestion
  • Activating Pepsin
  • Activating Intrinsic Factor
  • Stimulating the Delivery of Bile and Enzymes
  • Closing the Esophageal Sphincter
  • Opening the Pyloric Sphincter

46.4: The Intestines: Breakdown, Absorption, and Elimination

Learning Outcomes

1. Compare the structure of the small and large intestines
2. Explain how absorbed nutrients move to blood or lymph capillaries

Small Intestine

• The structure of the small intestine is specialized for digestion and nutrient uptake
• Approximately 4.5 meters long (14 ft)
  • Duodenum
  • Jejunum
  • Ileum
• The epithelial wall is covered with villi
• Each epithelial cell lining the villi is covered by microvilli
• Absorbed nutrients move into blood or lymph capillaries

Large Intestine

• The large intestine, or colon, eliminates waste material
• The small intestine empties directly into the large intestine at the cecum
• No digestion takes place in the large intestine, and only about 4% of the absorption of fluids by the intestines occurs there
• Many bacteria live in and reproduce within the large intestine, and the excess bacteria are incorporated into the feces
• Compacted feces pass through the large intestine into the rectum, and then exit the body through the anus

46.5: Accessory Organ Function

Learning Outcomes

1. Name the accessory organs and describe their role
2. Describe how the liver works to maintain homeostasis
3. Explain how the pancreas acts to control blood glucose concentration

Accessory Organs

• Secrete enzymes into the small intestine

Pancreas

• Trypsin and Chimotripsin- digest proteins
• Pancreatic Amylase- digests starch
• Lipase- digests fat
• Also functions as an endocrine gland

Liver

• The liver is the largest internal organ of the body
• The main exocrine secretion of the liver is bile
  • Bile pigments and bile salts
  • Bile pigments do not aid in digestion
  • Bile salts aid in the digestion of fats
• The liver modifies chemicals to maintain homeostasis
  • Metabolizes drugs
  • Removes toxins
  • Regulates levels of many compounds
  • Produces proteins in blood plasma
• Blood glucose concentration is maintained by actions of insulin and glucagon

46.6: Neural and Hormonal Regulation of the Digestive System

Learning Outcomes

1. Explain how the nervous system stimulates the digestive process
2. Identify the major entergastrones

Hormones

• Stimulates salivary and gastric secretions in response to sight, smell, and consumption of food
• When food arrives in the stomach, proteins in the food stimulate secretion of a hormone called gastrin
• Duodenal hormones secreted into the blood are known as the enterohormones

The Major Enterogastrones

• Cholecystikinin (CCK)- stimulated by high fat content
• Secretin- stimulated by increased chyme acidity
• Gastric Inhibitory Peptide (GIP)- stimulated by chyme with high fat content

46.7: Food Energy, Energy Expenditure, and Essential Nutrients

Learning Outcomes

1. Explain the basal metabolic rate and the effect of exercise
2. List hormones involved in regulating appetite and body weight
3. Name the essential nutrients

Basal Metabolic Rate

• The minimum rate of energy consumption under defined resting conditions is called the basal metabolic rate (BMR)
• The intake of food energy is measured in kilocalories
• Exertion increases metabolic rate
• Food intake is under neuroendocrine
• Leptin- thought to be the main signaling molecule in the afferent portion of the control circuit for energy sensing
• Insulin- has been implicated in signaling satiety as well
• Essential nutrients are those that the body cannot manufacture
  • Vitamins
  • Essential amino acids
  • Long-chain unsaturated fatty acids
  • Trace elements