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Homeostasis and the Organization of the Animal Body Chapter 31 Dynamic Constancy • The cells of the animal body cannot survive if the internal environment deviates from a narrow range of acceptable states • Homeostasis refers to the constancy of the body’s internal environment • The internal environment is actually in a state of dynamic constancy • The body actively adjusts to ongoing internal and external changes to maintain constant conditions Dynamic Constancy • Homeostatic mechanisms regulate a variety of conditions in the fluids surrounding cells – Temperature – Water and salt levels – Glucose – pH – Oxygen and carbon dioxide Body Temperature Regulation • Animals are grouped according to source of body warmth – Endotherms – Ectotherms • Endotherms generate body heat through metabolic reactions • Maintain constant body temperature • Birds, mammals Body Temperature Regulation • Ectotherms derive body heat from environment • Maintain heat by occupying a constant environment or behaviorally, e.g. basking in sun • Reptiles, amphibians, fishes, invertebrates Feedback Systems • Homeostatic mechanisms that maintain internal constancy are collectively known as feedback systems • There are three parts to feedback systems – Control center (with set point) – Sensor – Effector Feedback Systems • There are two types of feedback systems – Negative feedback: more common; counteracts the effects of changes in the internal environment to maintain homeostasis – Positive feedback: rarer; drive rapid, selflimiting changes, e.g. the birth process Negative Feedback • In negative feedback the response to change is to counteract the change resulting in a return to the original condition – “Negative” because it negates the initial change Negative Feedback • Negative feedback maintains a set point by detecting a deviation from the set point (stimulus) using a receptor • The receptor signals a control center that activates an effector mechanism that counteracts the stimulus Positive Feedback • Positive feedback intensifies the original change – Tends to be self-limiting – Occurs during labor • Labor contractions force baby’s head against the cervix causing, causing it to dilate • Stretch-receptor neurons in the cervix signal the hypothalamus • Hypothalamus releases oxytocin, which stimulates stronger uterine contractions • Delivery relieves pressure on the cervix, halting the + feedback cycle Systems Act in Concert • Homeostasis is maintained by body systems working together as a team • Numerous mechanisms constantly respond to various (chemical) stimuli that change as a result of an animal’s activities and changes in environment Hierarchy of Animal Organization Cells Tissues Organs Organ Systems Animal Tissues • An animal tissue is composed of cells that are similar in structure and perform a specialized function • There are four major categories of animal tissues – Epithelial tissue – Connective tissue – Muscle tissue – Nervous tissue Epithelial Tissue • Continuous sheets (membranes) that cover the outside body and line its cavities – Adapted to various functions, e.g. protection, absorption, gas exchange, etc… – Continually lost and replaced by cell division • Some epithelial tissues form glands (clusters of cells that are specialized to release substances) – Exocrine glands – Endocrine glands Epithelial Tissue • Exocrine glands release secretions using ducts – Examples: sweat glands and sebaceous glands • Endocrine glands typically release secretions into blood – Examples: adrenal glands and thyroid gland Connective Tissue • Connective tissues exist mostly to support and bind other body tissues • They secrete large quantities of extracellular substances, often including tough collagen protein fibers, among living cells • Three main categories of connective tissue – Loose connective tissue – Fibrous connective tissue – Specialized connective tissue Loose Connective Tissue • Attaches to epithelium to form membranes • Contains protein fibers and syrup-like extracellular fluid • Surrounds, cushions, and supports most organs – Example: the dermis Fibrous Connective Tissue • Contains densely packed collagen fibers that provide strength • Examples – Tendons: connect bones to muscles – Ligaments: connect bones to bones Specialized Connective Tissue • Have diverse functions and structures • Examples – Cartilage - function in support, friction-reduction, and shock absorption – Bone – Fat (adipose tissue) - used for energy storage and insulation – Blood - tissue composed of cells suspended in extracellular fluid – Lymph - Composed of fluid leaked out of blood at capillary blood vessels Muscle Tissue • Contract (shorten) when stimulated • Three types – – – – – Cardiac muscle - Located in heart Spontaneously active and not under conscious control Smooth muscle - Located in tubular organs Produces slow, sustained, involuntary contractions Skeletal muscle - Produces voluntary contractions, usually to move skeleton – Cells are cylindrical muscle fibers Nerve Tissue • Nerve tissue enables animals to sense and respond to the world • Makes up the brain, spinal cord, and nerves that travel to all parts of the body • Composed of two cell types – Neurons, that transmit electric signals – Glial cells, that surround, support, and electrically insulate neurons Organs • Organs are formed from at least two types of tissues that work together • Example: the skin Organ Systems • Organ systems are two or more individual organs that work together, performing a common function • Human organ systems are represented in Table 31-1, p. 645