Organs work together in organ systems to perform essential life functions. Organ systems include the circulatory, respiratory, digestive, excretory, endocrine, lymphatic/immune, nervous, integumentary, musculoskeletal, and reproductive systems. Larger complex animals rely on specialized surfaces and organ systems for exchanging materials with the environment due to having less surface area relative to volume compared to smaller animals. Homeostasis is the active maintenance of steady internal conditions and depends on negative feedback mechanisms that sense changes and direct responses to counteract deviations from set points.

5. Figure 20.10_L Blood vessels Heart Circulatory system Respiratory system Nasal cavity Pharynx Larynx Trachea Bronchus Lung Bone Cartilage Skeletal system Muscular system Skeletal muscles Integumentary system Hair Skin Nails Urinary system Digestive system Urinary bladder Small intestine Large intestine Kidney Ureter Urethra Mouth Esophagus Liver Stomach Anus

6. Figure 20.10_R Endocrine system Thymus Adrenal gland Pancreas Testis (male) Hypothalamus Pituitary gland Thyroid gland Parathyroid gland Ovary (female) Lymphatic and immune systems Lymph nodes Appendix Bone marrow Thymus Spleen Lymphatic vessels Reproductive system Female Oviduct Ovary Uterus Vagina Male Seminal vesicles Prostate gland Vas deferens Penis Urethra Testis Nervous system Brain Sense organ (ear) Spinal cord Nerves

12. External environment Mouth Food CO 2 O 2 Animal Respiratory system Digestive system Circulatory system Nutrients Interstitial fluid Body cells Excretory system Intestine Anus Unabsorbed matter (feces) Metabolic waste products (urine) Heart Blood

19. Thermostat in brain activates cooling mechanisms Temperature rises above normal Temperature increases Blood vessels in skin dilate and heat escapes Thermostat shuts off warming mechanisms Homeostasis: Internal body temperature of approximately 36 − 38  C Thermostat shuts off cooling mechanisms Blood vessels in skin constrict, minimizing heat loss Skeletal muscles rapidly contract, causing shivering, which generates heat Sweat glands secrete sweat that evaporates, cooling body Temperature decreases Temperature falls below normal Thermostat in brain activates warming mechanisms