Control of the Internal Environment

The Cellular Stress Response

1) Stress (ex: high heat) 2) Damaged protein 3) Synthesis of stress proteins 4) Repair of damaged protein 5) Release of stress proteins within the cell Represents the method by which biological control systems are able to combat stress (disturbances in homeostasis)

Steady State

A steady and unchanging (relatively constant) level of some physiological variable - Denotes a constant internal environment as well (physiological variable is not changing), but the physiological variable of interest is not at normal homeostatic resting state Represents a balance between demands placed on the body and the body's physiological response to meet these demands ex: body temperature - Body temperature reaches a new and steady level (a plateau) within 40 minutes of commencement of exercise

Acclimatization (Acclimation)

Adaptation to environmental stresses - ex: heat stress in a hot environment

Stress Proteins

Cells synthesize "stress proteins" when homeostasis is disrupted - ex: heat shock proteins repair damaged proteins in cell - Those who are better adapted to training have higher levels of heat shock proteins Stresses include: - High temperature - Low cellular energy levels - Abnormal pH - Alterations in cell calcium - Protein damage by free radicals Exercise induces these stresses and causes a disturbance in homeostasis

Adaptation

Change in structure or function of cell or organ system - Results in improved ability to maintain homeostasis during stressful conditions

Gain of a Control System

Degree to which a control system maintains homeostasis System with large gain is more cable of maintaining homeostasis (correcting a disturbance in homeostasis) than system with low gain - Pulmonary and cardiovascular systems (most important systems) have large gains

Non-Biological Control System Model

Example: a thermostat-controlled heating/cooling system - An increase in temperature above the set point signals the air conditioner to turn on - A decrease in temperature below the set point signals the furnace to turn on - Response by the control center is to correct the condition that initially turned it on An increase in temperature above the set point is detected by a sensor, which sends data to the thermostat (control center) This data signals the thermostat (control center) to turn the AC on - Negative feedback and return to normal temperature (homeostasis) A decrease in room temperature below the set point is detected by a sensor, which sends data to the thermostat (control sensor) This data signals the thermostat to turn the furnace on - Negative feedback and return to normal temperature This is analogous to temperature control of the body; - Too much cold (stimulus) is detected by thermoreceptors in the body, which elicits the skeletal system to respond by shivering (produces heat - Too much heat (stimulus) is detected by thermoreceptors in the body, which elicits sweat (cools the body down)

Exercise

Exercise disrupts homeostasis by changes in pH, O2, CO2, and temperature Control systems are capable of maintaining steady state during submaximal exercise in a cool environment - Body's control systems must respond rapidly (ex: increased pulmonary ventilation) Intense exercise or prolonged exercise in a hot/humid environment may exceed ability to maintain steady state - May result in fatigue and cessation of exercise

Overview of Cellular Protein Synthesis

Exercise-induced protein synthesis improves ability of cells to maintain homeostasis Process: 1) Exercise activates cell signaling pathways 2) Activates transcriptional activator molecule 3) Transcriptional activator binds to gene promoter region 4) DNA transcribed to mRNA 5) mRNA leaves nucleus and binds to ribosomes 6) mRNA is translated to protein Resistance and endurance exercise promote different cell signaling pathways

Failure of a Biological Control System

Failure of any component of a control system results in a disturbance from homeostasis (disease) Example: Type 1 diabetes - Damage to beta cells in pancreas - Insulin is no longer released to blood - Hyperglycemia results This represents failure of "effector"

Positive Feedback

Feedback control mechanisms whose response increases the original stimulus - Response is in the same direction as the initial stimulus (disturbance in homeostasis) Example: - Initiation of childbirth stimulates receptors in the cervix - Sends message to the brain - Release of oxytocin from pituitary gland - Oxytocin promotes increased uterine contractions

Exercise Physiology

Internal environment of the body remains remarkably constant despite a changing external environment The body is able to maintain a relatively constant internal environment in spite of stressors (ex: heat, cold, exercise) as the result of many biological control systems

Control Systems of the Body

Intracellular control systems - Protein breakdown and synthesis - Energy production - Maintenance of stored nutrients Organ systems - Pulmonary and circulatory systems (replenish oxygen and remove carbon dioxide and maintain relatively normal levels even during strenuous exercise) The body has hundreds of different control systems, and the overall goal is to regulate some physiological variable at or near constant value - Maintain homeostasis

Homeostasis

Maintenance of a relatively constant (and normal) internal environment during "normal" (resting) conditions - Although the internal environment is unchanging, this does not mean it is constant - Most physiological values vary around some "set" value and represent a rather dynamic constancy ex: changes in arterial blood pressure at rest - Although arterial blood pressure oscillates over time, mean pressure remains constant (around 93 mm Hg) - Oscillation occurs because homeostasis is characterized by negative feedback through biological control systems

Negative Feedback and Regulatory Control Systems

Negative feedback is the primary method responsible for maintaining homeostasis int he body - Most control systems of the body operate via negative feedback - Response of the control system is negative (opposite) to the initial stimulus (disturbance in homeostasis) Involves: 1) A stress or disturbance takes the internal environment away from the optimal value 2) Stress is detected by receptors and corrective biological control systems are activated 3) The corrective mechanisms act to restore conditions back to the set value (maintain homeostasis) Example: increase in extracellular CO2 triggers a receptor - Sends information to respiratory control center - Respiratory muscles are activated to increase breathing - CO2 concentrations are returned to normal

Cell Signaling

Regular exercise promotes cellular changes that result in improved ability of cells and organ systems to maintain homeostasis (adaptation) - Occurs via variety of cell signaling mechanisms Communication between cells using chemical messengers Important for maintaining homeostasis

Examples of Homeostatic Control

Regulation of body temperature: - Thermal receptors (sensors) send message to brain (control center) that it is too hot - Response by skin blood vessel dilation and sweat glands (effectors) to regulate temperature by promoting heat loss - When body system returns to normal, the control center is inactivated - Thermal receptors (sensors) send message to brain (control system) that it is too cold - Response by skin blood vessel constriction and shivering in the skeletal muscles to conserve and promote heat - When body system returns to normal, the control center is inactivated Regulation of blood glucose: - Function of the endocrine system; requires the hormone insulin - Elevated blood glucose after eating is detected by the pancreas (sensor) signals the pancreas (sensor/effector organ) to release insulin - Insulin causes cellular uptake of glucose and glucose metabolism - Returns the blood glucose level to normal

Biological Control System

Series of interconnected components that maintain a physical or chemical parameter at a near constant value Composed of 3 elements: - Sensor (receptor): detects the changes in a variable - Control center: integrates the response; assesses input and initiates response - Effector: organs that produce the desired effect; changes internal environment back to normal 1) The signal to begin the biological control system is the stimulus that represents a change in the internal environment (ex: too much or too little of a regulated variable) 2) The stimulus excites a sensor that is a receptor in the body capable of detecting change of the variable in question 3) The excited sensor then sends a message (data) to the control center 4) The control center integrates the strength of the incoming signal from the sensor and sends a message to the effectors to bring about the appropriate response to correct the disturbance (desired effect) 5) The return of the internal environment to normal (ex: homeostasis) results in a decrease in the original stimulus that triggered the control system into action via negative feedback