Chapter 2:

Control of the Internal Environment

Objectives

Define the terms homeostasis and steady stateDiagram and discuss a biological control systemGive an example of a biological control systemExplain negative feedbackDefine what is meant by the gain of a control system

Homeostasis: Dynamic Constancy

Homeostasis– Maintenance of a constant internal

environmentSteady state– Balance between demands placed on body

and the physiological response to those demands

Body Core Temperature During Exercise

Fig 2.2

Blood Pressure at Rest

Fig 2.3

Control Systems of the Body

Goal– To regulate some physiological variable at or

near constant value– Maintain homeostasis

Non-Biological Control Systemin room

Temperature below 200 C

Thermostat set at 200 C

Heating System

Room temperature

Returns to 200 C

Room TemperatureSignals thermostat

To turn off heat

Fig 2.4

Biological Control Systems

Series of interconnected components that serve to maintain a physical or chemical parameter at or near constant Receptor– Capable of detecting changes

Integrating center– Assesses input and initiates response

Effector– Corrects changes to internal environment– Removing stimulus

Components of a Biological Control System

Fig 2.5

Control Systems

Most biological control systems – Response reverses the initial disturbance in

homeostasis– Negative feedback

Negative Feedback

Receptors– Chemoreceptors

Specialized nerve endings → CNSRespond to changes in O2 and CO2

Exercise – Adrenergic (adrenaline) (α1 and α2)

Epinephrine (hormone) and Norepinephrine (SYM)Inhibit or excite

Receptors

Receptors (cont)– Proprioceptors (sensory)

Golgi, muscle spindles, joint receptorsBody positionMechanoreceptors (muscle) – sensitive to force and speed

– Body temperature Skin (thermal receptors)Core (stimulates sweat glands)

Positive Feedback

Vicious cycle– Hemorrhage

Heart ↓ pumping abilityDecreased coronary blood flowFurther weakens heart

Clotting (useful)– Clotting factors activate enzymes to clot blood

Clotting (vicious cycle)– Heart attacks

Gain of a Control System

Gain of the system– Degree to which the control system maintains

homeostasis– System with large gain is more capable of

maintaining homeostasis– System with small gain is less capable of

maintaining homeostasis

Gain of a control system

Small gain– Baroreceptors

Normal pressure – 100 mmHgBlood infusion – 175 mmHg (no baroreceptors)Blood infusion – 125 mmHg (with baroreceptors)

Large gain (greater control)– Body temperature– 98 to 98.8 oC

Example:Regulation of Blood Pressure

Fig 2.6

Example:Regulation of Blood Glucose

Fig 2.7

Stress Proteins

Disturbance beyond gainProteins help maintain homeostasis– Intracellular transporters– Enzymes

Stress Proteins

Stress proteins– Synthesized to repair damaged proteins– Restore homeostasis

Stressors– Hyperthermia– Hypoxia– Alkalosis– Free radical production

Example:Cellular Stress Response

Fig 2.8

Exercise: A Test of Homeostatic Control

Submaximal exercise in a cool environment– The body’s control systems can maintain

steady state– Lactic acid produced = lactic acid utilized– Oxygen delivery = oxygen required for ATP

Body Core Temperature During Exercise

Fig 2.2

Exercise: Homeostatic Control

Maximal exercise or exercise in a hot/humid environment– May not be able to maintain steady state– Severe disturbances in homeostasis can

occur

Exercise

Heavy exercise– Lactic acid – acidosis– ↑ carbon dioxide production– ↑ oxygen requirements

Resulting in– ↑ in pulmonary ventilation– ↑ in blood flow