← All chapters

Field notes · chapter 46

The Digestive System

pp. 1023–1043 · Raven Part VII

Best quiznot started
46.1

Types of Digestive Systems

p. 1024
  • Heterotrophs split three ways by food source: herbivores (plants only — snails, sapsucking insects, cows, horses, rabbits, sparrows), carnivores (crabs, squid, cats, eagles, trout, frogs), and omnivores (humans, pigs, bears, crows).
  • Single-celled organisms and sponges digest intracellularly. All other multicellular animals digest extracellularly, releasing enzymes into a cavity that is continuous with the outside environment.
  • Cnidarians and flatworms have a gastrovascular cavity: one opening acts as both mouth and anus. Because every cell is exposed to every stage of digestion, no regional specialization is possible. Watch The Digestive System: CrashCourse Biology #28 on YouTube, from CrashCourse
  • An alimentary canal has a separate mouth and anus, so food moves one way and each region can specialize. Nematodes show the simplest version — a plain tube. Earthworms already have regions for ingestion, storage, fragmentation, digestion, and absorption. Watch The Digestive System: CrashCourse Biology #28 on YouTube, from CrashCourse
  • Physical fragmentation comes from teeth (most vertebrates) or from pebbles in a gizzard (earthworms, birds). Chemical digestion is hydrolysis, liberating monosaccharides, amino acids, and fatty acids.
  • Digestion products cross the gut epithelium into blood — absorption. What is not absorbed cannot be used and is excreted.
  • The vertebrate GI tract runs: mouth and pharynx → esophagus → stomach → small intestine → large intestine → cloaca or rectum → anus. (from the Chapter Review, p. 1041 — p. 1025 was not photographed)
  • The tract has four tissue layers from the lumen outward: mucosa, submucosa, muscularis, serosa. Watch Digestive System, Part 1: Crash Course Anatomy & Physiology #33 on YouTube, from CrashCourse
46.2

The Mouth and Teeth: Food Capture and Bulk Processing

p. 1026
  • Birds have no teeth; they use a two-chambered stomach where the gizzard churns swallowed pebbles against food. Food is stored in the crop.
  • Chewing is mastication. Carnivores have pointed teeth for cutting and shearing and barely chew, because digestive enzymes act directly on animal cells. Herbivores must pulverize cellulose cell walls first, so they have large flat ridged grinding teeth.
  • Human dentition — "carnivore in the front, herbivore in the back": four chisel-shaped incisors per jaw for biting, pointed canines for tearing, then two premolars and three molars per side with flat ridged surfaces for grinding. Watch Digestive System, Part 1: Crash Course Anatomy & Physiology #33 on YouTube, from CrashCourse
  • The tongue mixes food with saliva. Humans have three pairs of salivary glands emptying through ducts into the mouth. Saliva moistens and lubricates so the bolus does not abrade the esophagus. Watch Digestive System, Part 1: Crash Course Anatomy & Physiology #33 on YouTube, from CrashCourse
  • Saliva carries salivary amylase, which begins hydrolysis of starch into the disaccharide maltose. In humans this is usually minimal because people do not chew long. Watch Digestive System, Part 1: Crash Course Anatomy & Physiology #33 on YouTube, from CrashCourse
  • Baseline salivation is about 0.5 mL per minute with an empty mouth. Taste and smell input drives it higher. Acidic solutions are the most potent stimulus — lemon juice can raise the rate eightfold. Sight, sound, smell, even thinking about food can trigger it.
46.3

The Esophagus and the Stomach: The Early Stages of Digestion

pp. 1027–1029
46.4

The Intestines: Breakdown, Absorption, and Elimination

pp. 1029–1031
46.5

Accessory Organ Function

pp. 1032–1033
46.6

Neural and Hormonal Regulation of the Digestive Tract

p. 1034
46.7

Food Energy, Energy Expenditure, and Essential Nutrients

pp. 1035–1038
  • Food does two things: supplies energy, and supplies raw materials the animal cannot make itself.
  • Basal metabolic rate (BMR) is the minimum energy consumption under defined resting conditions. It is fairly constant per individual, depending on age, sex, and body size.
  • Exercise does not change BMR, but it adds to total daily expenditure. Excess intake is stored first as glycogen, but glycogen reserves are limited, so sustained excess becomes fat.
  • Energy is measured in kilocalories (1 kcal = 1000 calories; the nutritionists' "Calorie" with a capital C). Measured by burning food in a calorimeter. Human daily expenditure ranges roughly 1300–5000 kcal.
  • Obesity results when intake exceeds expenditure for a sustained period. About 39% of U.S. adults aged 40–59 are classified obese; including 20–40 year olds it is about 35%.
  • Rodent work: the ob gene encodes leptin. Mice homozygous for the recessive mutant become obese; injecting leptin makes them stop overeating and lose about 30% of body weight in two weeks. The db gene encodes the leptin receptor, expressed in hypothalamic neurons.
  • Leptin is produced by adipose tissue in proportion to body fat and is the main afferent satiety signal into the CNS. Fasting lowers it (signalling "eat"); refeeding raises it fast (killing appetite).
  • Critical human twist: obese people have higher leptin and their leptin is normal. Human obesity mostly reflects reduced sensitivity to leptin in the brain, not a shortage of it.
  • Insulin also signals satiety; its levels fall with fasting and rise with obesity, though its primary job is glucose homeostasis.
  • Gut hormones give short-term control: GIP and CCK have hypothalamic receptors and inhibit feeding like leptin; ghrelin does the opposite and stimulates feeding. Ghrelin rises before meals and may initiate eating. Gastric bypass surgery lowers ghrelin, one proposed reason appetite drops afterward.
  • In the hypothalamus two antagonistic neuropeptides act: neuropeptide Y (NPY) induces feeding, and α-melanocyte-stimulating hormone (α-MSH) suppresses it. Leptin stimulates α-MSH and suppresses NPY.
  • High leptin and insulin → more α-MSH, less NPY → less appetite, more energy expenditure, reproduction and growth allowed. Persistently low leptin inhibits reproduction and growth.
  • Essential nutrients are substances the body needs but cannot synthesize. Humans need at least 13 vitamins. Watch Introduction to vitamins and minerals | Biology foundations | High school biology | Khan Academy on YouTube, from Khan Academy
  • Humans, apes, monkeys, and guinea pigs cannot make ascorbic acid (vitamin C); deficiency causes scurvy, degeneration of connective tissue. Watch Introduction to vitamins and minerals | Biology foundations | High school biology | Khan Academy on YouTube, from Khan Academy
  • Humans require 9 essential amino acids. Strict vegetarians must combine foods so one complements another, and may need supplements for some B vitamins.
  • All vertebrates need certain long-chain unsaturated fatty acids from food. Interestingly vertebrates can make cholesterol while some carnivorous insects cannot.
  • Minerals (calcium, magnesium, phosphorus) and trace elements (zinc, molybdenum) come ultimately from plants. Watch Introduction to vitamins and minerals | Biology foundations | High school biology | Khan Academy on YouTube, from Khan Academy
46.8

Variations in Vertebrate Digestive Systems

pp. 1039–1041
  • No animal makes cellulase. Animals that live on cellulose host bacteria and protists that do — a mutually beneficial symbiosis. Watch Digestive Physiology of the Ruminant on YouTube, from Mike Hutjens
  • Because cellulose resists digestion, herbivores have much longer digestive tracts than carnivores, buying time.
  • Ruminants (cows, deer) have a four-chambered stomach: rumen, reticulum, omasum, then the true stomach, the abomasum — the only chamber that secretes gastric juice. Watch Digestive Physiology of the Ruminant on YouTube, from Mike Hutjens
  • The rumen can hold up to 50 gallons and works as a fermentation vat. Because it sits at the front, contents can be regurgitated and rechewed — rumination, "chewing the cud" — which shreds fiber and increases surface area for microbes. Watch Digestive Physiology of the Ruminant on YouTube, from Mike Hutjens
  • Foregut fermentation evolved convergently many times: hippopotamuses, langur monkeys, sloths, kangaroos, and the hoatzin all enlarged the stomach, though the four-chambered version evolved only once.
  • Striking molecular convergence: lysozyme, normally an antibacterial enzyme in saliva and tears, was repurposed to digest stomach bacteria in both ruminants and the leaf-eating langur monkey — via the same five amino acid changes. Their lysozymes resemble each other more than those of closer relatives.
  • Rodents, horses, and lagomorphs instead ferment in a greatly enlarged cecum. Because the cecum is beyond the stomach, regurgitation is impossible. Watch Digestive Physiology of the Ruminant on YouTube, from Mike Hutjens
  • Rodents and lagomorphs solve this with coprophagy — eating nutrient-rich feces so food passes through twice, absorbing what cecal microbes produced. Prevented from doing so, they cannot stay healthy.
  • Insectivores and carnivores have little or no cecum.
  • All mammals rely on intestinal bacteria to synthesize vitamin K, needed for blood clotting. Birds lack these bacteria and must eat it. Prolonged antibiotics can require vitamin K supplementation. Watch Introduction to vitamins and minerals | Biology foundations | High school biology | Khan Academy on YouTube, from Khan Academy