* Maintaining of a stable internal environment Homeostasis * Maintaining of a stable internal environment Homeostasis must be maintained for normal body functioning and to sustain life Homeostatic imbalance – a disturbance in homeostasis resulting in disease “physiological normal” involves maintaining the volume and composition of body fluids intracellular (ICF) extracellular (ECF)

Homeostatic Control Mechanisms COMPONENTS receptor monitors changes input to control center control center evaluates input and generates output effector receives output from control center produces a response

Homeostatic Control Mechanisms continually disrupted by internal and external environments balance regulated by nervous action potentials rapid endocrine hormones slow

Homeostatic Control Mechanisms Control center The hypothalamus detects the deviation from the set point and signals effector organs. Receptors Thermo receptors send signals to the control center. Effectors Skin blood vessels dilate and sweat glands secrete. Stimulus Body temperature rises above normal. Response Body heat is lost to surroundings, temperature drops toward normal. too high Normal body temperature 37°C (98.6°F) too low Response Body heat is conserved, temperature rises toward normal. Stimulus Body temperature drops below normal. Receptors Thermoreceptors send signals to the control center. Effectors Skin blood vessels constrict and sweat glands remain inactive. Effectors Muscle activity generates body heat. Control center The hypothalamus detects the deviation from the set point and signals effector organs. If body temperature continues to drop, control center signals muscles to contract Involuntarily. 4

Homeostatic Control Mechanisms Negative feedback summary: Prevents sudden, severe changes in the body Reduces the actions of the effectors Corrects the set point Causes opposite of bodily disruption to occur, i.e. the ‘negative’ Limits chaos in the body by creating stability Most common type of feedback loop Examples: body temperature, blood pressure & glucose regulation

Homeostatic Control Mechanisms Positive feedback summary: Increases (accelerates) the actions of the body. Positive feedback mechanisms are short-lived . Examples: blood clotting and child birth

The Body’s Environment Interior of body separated from external environment by a layer of epithelial tissue Lumen of respiratory system, gastrointestinal system, and urinary system are part of external environment Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Body fluid compartments Two main compartments Intracellular Extracellular. Extracellular composed of two sub compartments. Blood plasma Interstitial fluid (between cells/ within tissues). Also considered part of the interstitial fluid are lymph, cerebrospinal fluid, humors of the eye, synovial fluid, serous fluid in pericardial, pleural and peritoneal spaces and secretions of the gastrointestinal tract. These are referred to as Transcellular fluids

Body fluid compartments

Body Fluid Compartments Figure 1.5 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Fluid movement between compartments. There is a continuous exchange and mixing of body fluids between fluid compartments. This is regulated by osmotic and hydrostatic pressures. Water can move freely between compartments along osmotic gradients. Solutes are unequally distributed because of their size, electrical charge, or dependence on transport proteins.

Water balance Homeostasis is the combination of processes occurring within the body facilitating maintenance of the composition of interstitial fluid . For the body to remain properly hydrated, water intake must equal water output. These processes ensure a constancy within the body internal environment. Maintenance of homeostasis includes two major tasks: Maintaining water balance within the cells (ICF). This ensures adequate functioning of cells. Maintaining water balance in the plasma. This ensures the tissues of the body are adequately perfused. Both are achieved by maintaining the correct level of water in the ECF.

Water balance Feedback mechanisms that facilitate maintenance of a stable internal environment.

Water balance The influence of ADH

Disorders of water balance: Dehydration. Results in body water which leads to plasma osmolarity. Water moves from intracellular space to extracellular spaces. Cells dehydrate. Cell functioning impaired

II. Homeostasis: A Central Organizing Principle of Physiology Negative Feedback Controls and Thermoregulation Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Negative Feedback Control in Homeostasis Primary mechanism for maintaining homeostasis. External change  triggers change in regulated variable in internal environment  triggers reaction to oppose the change and return regulated variable toward normal (set point) Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Negative Feedback Mechanisms Set point = desired level of regulated variable Sensors detect level of regulated variable and provide input to integrating center Integrating center compares set point to actual level of regulated variable Error signal = difference between actual level and set point Integrating center sends output to effectors to return regulated variable toward set point Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Negative Feedback Control of Body Temperature Figure 1.6c, d Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Negative Feedback Loop Figure 1.7 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Positive Feedback Loop Positive feedback loops cause a rapid change in a variable. Figure 1.8 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Thermoregulation Homeothermic animals – regulate body temperature within a narrow range Poikilothermic animals – do not regulate body temperature Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Homeothermic Animals Normal body temperature = set point Differs in animal species Humans: 37oC (98.6oF) Hypothermia = decrease in body temperature Hyperthermia = increase in body temperature above 41oC, dangerous above 43oC, deadly Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Mechanisms of Heat Transfer Between Body and External Environment Radiation - thermal energy as electromagnetic waves Conduction - thermal energy through contact Evaporation - heat loss through evaporation of water insensible water loss sweating Convection - heat transfer by movement of fluid or air Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Components of Thermoregulatory System Thermoregulatory system maintains core body temperature Detectors - thermoreceptors (central and peripheral) Integrator - hypothalamus Effectors - sweat glands, blood vessels in skin, skeletal muscles Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Thermoneutral Zone Range of outside temperature where alterations in blood flow alone regulates body temperature 25-30oC Body temperature increase:blood flow to skin increases Body temperature decrease:blood flow to skin decreases Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Thermoregulation Figure 1.9c Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.

Fever Fever accompanies infections White blood cells secrete pyrogens Body temperature set point increases Fever enhances immune response Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.