Unit 6 of Campbell Biology in Focus (3rd Edition) examines how animals are structured and how their organ systems maintain homeostasis, acquire energy, reproduce, and respond to environments. It begins with the internal environment of animals, where hierarchical organization of tissues, organs, and systems underpins form and function. Homeostasis is regulated through feedback mechanisms, with the endocrine and nervous systems coordinating signals. Thermoregulation, osmoregulation, and excretion are discussed with adaptations for aquatic and terrestrial life, including the nephron’s countercurrent system and hormonal regulation of kidney function.
The unit then covers nutrition, circulation, and gas exchange. Animals obtain essential nutrients—amino acids, fatty acids, vitamins, and minerals—through diverse feeding strategies, with digestive systems ranging from gastrovascular cavities to complete alimentary canals. Circulatory systems vary between open and closed designs, while vertebrates evolved single and double circulation with specialized four-chambered hearts in birds and mammals. Gas exchange surfaces, such as gills, tracheae, and lungs, maximize diffusion efficiency, with adaptations like countercurrent exchange and negative pressure breathing.
Next, the immune system is explored, contrasting innate immunity with adaptive immunity. Innate defenses include barriers, phagocytosis, and inflammatory responses, while adaptive immunity relies on B cells, T cells, and memory that underpins vaccines and immunological memory. Disruptions such as autoimmune disease, allergies, and immunodeficiency (HIV/AIDS) illustrate the balance between protection and pathology.
The unit also details reproduction and development, covering asexual and sexual strategies, human reproductive anatomy, gametogenesis, hormonal control, and cycles. Fertilization, cleavage, gastrulation, and organogenesis explain how single cells become multicellular organisms. Human gestation, childbirth, and reproductive technologies (IVF, contraception) highlight modern applications.
Finally, the unit examines nervous and sensory systems, motor mechanisms, and behavior. Neurons transmit signals through action potentials and synapses, integrating into nervous systems with specialized brain regions. Sensory systems transduce mechanical, chemical, and electromagnetic stimuli into perceptions like vision, hearing, taste, and smell. Muscles (skeletal, cardiac, smooth) interact with skeletons (hydrostatic, exoskeleton, endoskeleton) to produce locomotion. Behavior is studied through proximate and ultimate causes, communication, learning, mating systems, foraging strategies, and altruism, with evolutionary and ecological contexts shaping animal actions.
Together, Unit 6 shows how anatomy, physiology, and behavior are integrated, enabling animals to survive, reproduce, and adapt across ecosystems.
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