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25.4 Digestive System Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1087
Chapter 26: The Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1107
26.1 Neurons and Glial Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1108
26.2 How Neurons Communicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1116
26.3 The Central Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1130
26.4 The Peripheral Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1137
26.5 Nervous System Disorders
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1142
Chapter 27: Sensory Systems
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165
27.1 Sensory Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1166
27.2 Somatosensation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1170
27.3 Taste and Smell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1175
27.4 Hearing and Vestibular Sensation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1180
27.5 Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1187
Chapter 28: The Endocrine System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209
28.1 Types of Hormones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209
28.2 How Hormones Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213
28.3 Regulation of Body Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217
28.4 Regulation of Hormone Production . . . . . . . . . . . . . . . . . . . . . . . . . . 1230
28.5 Endocrine Glands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232
Chapter 29: The Musculoskeletal System . . . . . . . . . . . . . . . . . . . . . . . . . . . 1257
29.1 Types of Skeletal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1258
29.2 Bone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1270
29.3 Joints and Skeletal Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1278
29.4 Muscle Contraction and Locomotion . . . . . . . . . . . . . . . . . . . . . . . . . . 1287
Chapter 30: The Respiratory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1309
30.1 Systems of Gas Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1309
30.2 Gas Exchange across Respiratory Surfaces
. . . . . . . . . . . . . . . . . . . . . 1318
30.3 Breathing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1324
30.4 Transport of Gases in Human Bodily Fluids . . . . . . . . . . . . . . . . . . . . . . 1332
Chapter 31: The Circulatory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1349
31.1 Overview of the Circulatory System . . . . . . . . . . . . . . . . . . . . . . . . . . 1350
31.2 Components of the Blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1354
31.3 Mammalian Heart and Blood Vessels . . . . . . . . . . . . . . . . . . . . . . . . . 1362
31.4 Blood Flow and Blood Pressure Regulation . . . . . . . . . . . . . . . . . . . . . . 1372
Chapter 32: Osmotic Regulation and Excretion . . . . . . . . . . . . . . . . . . . . . . . . 1393
32.1 Osmoregulation and Osmotic Balance . . . . . . . . . . . . . . . . . . . . . . . . . 1394
32.2 The Kidneys and Osmoregulatory Organs . . . . . . . . . . . . . . . . . . . . . . . 1398
32.3 Excretion Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1407

32.4 Nitrogenous Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410
32.5 Hormonal Control of Osmoregulatory Functions . . . . . . . . . . . . . . . . . . . . 1414
Chapter 33: The Immune System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1437
33.1 Innate Immune Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1438
33.2 Adaptive Immune Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446
33.3 Antibodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1461
33.4 Disruptions in the Immune System
. . . . . . . . . . . . . . . . . . . . . . . . . . 1467
Chapter 34: Animal Reproduction and Development . . . . . . . . . . . . . . . . . . . . . 1487
34.1 Reproduction Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1488
34.2 Fertilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1494
34.3 Human Reproductive Anatomy and Gametogenesis . . . . . . . . . . . . . . . . . 1496
34.4 Hormonal Control of Human Reproduction . . . . . . . . . . . . . . . . . . . . . . 1504
34.5 Fertilization and Early Embryonic Development . . . . . . . . . . . . . . . . . . . . 1510
34.6 Organogenesis and Vertebrate Formation . . . . . . . . . . . . . . . . . . . . . . . 1515
34.7 Human Pregnancy and Birth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1518
Unit 8
Chapter 35: Ecology and the Biosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1539
35.1 The Scope of Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1540
35.2 Biogeography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1545
35.3 Terrestrial Biomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1552
35.4 Aquatic Biomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1560
35.5 Climate and the Effects of Global Climate Change . . . . . . . . . . . . . . . . . . 1568
Chapter 36: Population and Community Ecology . . . . . . . . . . . . . . . . . . . . . . . 1593
36.1 Population Demography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1594
36.2 Life Histories and Natural Selection . . . . . . . . . . . . . . . . . . . . . . . . . . 1600
36.3 Environmental Limits to Population Growth . . . . . . . . . . . . . . . . . . . . . . 1605
36.4 Population Dynamics and Regulation . . . . . . . . . . . . . . . . . . . . . . . . . 1611
36.5 Human Population Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1616
36.6 Community Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1622
36.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior . . . . . . . . . . . 1636
Chapter 37: Ecosystems
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683
37.1 Ecology for Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683
37.2 Energy Flow through Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . 1695
37.3 Biogeochemical Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1702
Chapter 38: Conservation Biology and Biodiversity . . . . . . . . . . . . . . . . . . . . . 1727
38.1 The Biodiversity Crisis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1728
38.2 The Importance of Biodiversity to Human Life . . . . . . . . . . . . . . . . . . . . . 1739
38.3 Threats to Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743
38.4 Preserving Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1754
Appendix A: The Periodic Table of Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1773
Appendix B: Geological Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1775
Appendix C: Measurements and the Metric System . . . . . . . . . . . . . . . . . . . . . . . . 1777
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1779
This OpenStax book is available for free at http://cnx.org/content/col12078/1.6

PREFACE
Welcome to Biology for AP® Courses, an OpenStax resource. This textbook was written to increase student access to high-
quality learning materials, maintaining highest standards of academic rigor at little to no cost.
About OpenStax
OpenStax is a nonprofit based at Rice University, and it’s our mission to improve student access to education. Our first
openly licensed college textbook was published in 2012, and our library has since scaled to over 25 books for college
and AP® courses used by hundreds of thousands of students. OpenStax Tutor, our low-cost personalized learning tool, is
being used in college courses throughout the country. Through our partnerships with philanthropic foundations and our
alliance with other educational resource organizations, OpenStax is breaking down the most common barriers to learning
and empowering students and instructors to succeed.

25.4 Digestive System Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1087

Chapter 26: The Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1107

26.1 Neurons and Glial Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1108

26.2 How Neurons Communicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1116

26.3 The Central Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1130

26.4 The Peripheral Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1137

26.5 Nervous System Disorders

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1142

Chapter 27: Sensory Systems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165

27.1 Sensory Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1166

27.2 Somatosensation

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1170

27.3 Taste and Smell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1175

27.4 Hearing and Vestibular Sensation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1180

27.5 Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1187

Chapter 28: The Endocrine System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209

28.1 Types of Hormones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209

28.2 How Hormones Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213

28.3 Regulation of Body Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217

28.4 Regulation of Hormone Production . . . . . . . . . . . . . . . . . . . . . . . . . . 1230

28.5 Endocrine Glands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232

Chapter 29: The Musculoskeletal System . . . . . . . . . . . . . . . . . . . . . . . . . . . 1257

29.1 Types of Skeletal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1258

29.2 Bone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1270

29.3 Joints and Skeletal Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1278

29.4 Muscle Contraction and Locomotion . . . . . . . . . . . . . . . . . . . . . . . . . . 1287

Chapter 30: The Respiratory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1309

30.1 Systems of Gas Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1309

30.2 Gas Exchange across Respiratory Surfaces

. . . . . . . . . . . . . . . . . . . . . 1318

30.3 Breathing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1324

30.4 Transport of Gases in Human Bodily Fluids . . . . . . . . . . . . . . . . . . . . . . 1332

Chapter 31: The Circulatory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1349

31.1 Overview of the Circulatory System . . . . . . . . . . . . . . . . . . . . . . . . . . 1350

31.2 Components of the Blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1354

31.3 Mammalian Heart and Blood Vessels . . . . . . . . . . . . . . . . . . . . . . . . . 1362

31.4 Blood Flow and Blood Pressure Regulation . . . . . . . . . . . . . . . . . . . . . . 1372

Chapter 32: Osmotic Regulation and Excretion . . . . . . . . . . . . . . . . . . . . . . . . 1393

32.1 Osmoregulation and Osmotic Balance . . . . . . . . . . . . . . . . . . . . . . . . . 1394

32.2 The Kidneys and Osmoregulatory Organs . . . . . . . . . . . . . . . . . . . . . . . 1398

32.3 Excretion Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1407

32.4 Nitrogenous Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410

32.5 Hormonal Control of Osmoregulatory Functions . . . . . . . . . . . . . . . . . . . . 1414

Chapter 33: The Immune System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1437

33.1 Innate Immune Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1438

33.2 Adaptive Immune Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446

33.3 Antibodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1461

33.4 Disruptions in the Immune System

. . . . . . . . . . . . . . . . . . . . . . . . . . 1467

Chapter 34: Animal Reproduction and Development . . . . . . . . . . . . . . . . . . . . . 1487

34.1 Reproduction Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1488

34.2 Fertilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1494

34.3 Human Reproductive Anatomy and Gametogenesis . . . . . . . . . . . . . . . . . 1496

34.4 Hormonal Control of Human Reproduction . . . . . . . . . . . . . . . . . . . . . . 1504

34.5 Fertilization and Early Embryonic Development . . . . . . . . . . . . . . . . . . . . 1510

34.6 Organogenesis and Vertebrate Formation . . . . . . . . . . . . . . . . . . . . . . . 1515

34.7 Human Pregnancy and Birth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1518

Unit 8

Chapter 35: Ecology and the Biosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1539

35.1 The Scope of Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1540

35.2 Biogeography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1545

35.3 Terrestrial Biomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1552

35.4 Aquatic Biomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1560

35.5 Climate and the Effects of Global Climate Change . . . . . . . . . . . . . . . . . . 1568

Chapter 36: Population and Community Ecology . . . . . . . . . . . . . . . . . . . . . . . 1593

36.1 Population Demography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1594

36.2 Life Histories and Natural Selection . . . . . . . . . . . . . . . . . . . . . . . . . . 1600

36.3 Environmental Limits to Population Growth . . . . . . . . . . . . . . . . . . . . . . 1605

36.4 Population Dynamics and Regulation . . . . . . . . . . . . . . . . . . . . . . . . . 1611

36.5 Human Population Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1616

36.6 Community Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1622

36.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior . . . . . . . . . . . 1636

Chapter 37: Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683

37.1 Ecology for Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683

37.2 Energy Flow through Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . 1695

37.3 Biogeochemical Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1702

Chapter 38: Conservation Biology and Biodiversity . . . . . . . . . . . . . . . . . . . . . 1727

38.1 The Biodiversity Crisis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1728

38.2 The Importance of Biodiversity to Human Life . . . . . . . . . . . . . . . . . . . . . 1739

38.3 Threats to Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743

38.4 Preserving Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1754

Appendix A: The Periodic Table of Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1773

Appendix B: Geological Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1775

Appendix C: Measurements and the Metric System . . . . . . . . . . . . . . . . . . . . . . . . 1777

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1779

This OpenStax book is available for free at http://cnx.org/content/col12078/1.6

PREFACE

Welcome to Biology for AP® Courses, an OpenStax resource. This textbook was written to increase student access to high-

quality learning materials, maintaining highest standards of academic rigor at little to no cost.

About OpenStax

OpenStax is a nonprofit based at Rice University, and it’s our mission to improve student access to education. Our first

openly licensed college textbook was published in 2012, and our library has since scaled to over 25 books for college

and AP® courses used by hundreds of thousands of students. OpenStax Tutor, our low-cost personalized learning tool, is

being used in college courses throughout the country. Through our partnerships with philanthropic foundations and our

alliance with other educational resource organizations, OpenStax is breaking down the most common barriers to learning

and empowering students and instructors to succeed.