Control of the Internal Environment

Objectives Define the terms homeostasis and steady state Diagram and discuss a biological control system Give an example of a biological control system Explain negative feedback Define what is meant by the gain of a control system

Glossary Maintain – keep up. Constant – the same. Internal – inside the body. Environment – surroundings of the body. Feedback - a cycle in which the output of a system “feeds back” to modify or reinforce the actions of the system in order to maintain homeostasis. Thermoneutral Zone- describes a range of temperatures of the immediate environment in which a standard healthy adult can maintain normal body temperature without needing to use energy above and beyond normal basal metabolic rate

What is Homeostasis? Body cells work best if they have the correct Temperature Water levels Glucose concentration Your body has mechanisms to keep the cells in a constant environment.

Homeostasis Maintenance of a constant internal environment is called homeostasis When the body’s biological control systems maintain physiological variables at manageable constant values at rest, (when the body is not under any stress) it is called homeostasis. Blood pressure–Body temperature–Blood glucose levels

Steady State It is a Balance between the demands placed on a body and the physiological response to those demands Maintenance of an internal environment by a biological control system, where a physiological variable (e.g. body temperature) remains relatively constant yet deviates from its normal value, which occurs when the body is experiencing stress (e.g. exercise).

Body Core Temperature During Exercise Changes in body core temperature during submaximal exercise Body temperature reaches a plateau (steady state)

To distinguishing between these two terms is the case of body temperature during exercise. Graph illustrates the changes in body core temperature during sixty minutes of constant-load submaximal exercise in a thermoneutral environment Core temperature reaches a new and steady level within forty minutes after commencement of exercise. This plateau of core temperature represents a steady state, since temperature is constant; however, this constant temperature is above the normal resting body temperature and thus does not represent a true homeostatic condition. Therefore, the term homeostasis is generally reserved for describing normal resting conditions, and the term steady state is often applied to exercise

In Summary  Homeostasis is defined as the maintenance of a constant or unchanging “normal” internal environment during unstressed conditions.  The term steady state is also defined as a constant internal environment, but this does not necessarily mean that the internal environment is at rest and normal. When the body is in a steady state, a balance has been achieved between the demands placed on the body and the body’s response to those demands. Homeostasis: Dynamic Constancy

Control Systems of the Body The body has literally hundreds of different control systems 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 Control Systems of the Body

Goal To regulate some physiological variable at or near constant value Maintain homeostasis

Non-Biological Control System  if room Temperature below 20 0 C Thermostat set at 20 0 C Heating System Room temperature Returns to 20 0 C ­Room Temperature Signals thermostat To turn off heat

Non-Biological Control System A thermostat-controlled heating/cooling system An increase in temperature above the set point signals the air conditioner to turn on. A decrease in room temperature below the set point results in turning on the furnace. Nature of the Control Systems

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

Components of a Biological Control System

Negative Feedback Most control systems of the body operate via negative feedback Response reverses the initial disturbance in homeostasis feedback is termed negative is that the response of the control system is negative (opposite) to the stimulus. An example of negative feedback can be seen in the respiratory system's regulation of the CO 2 concentration in extracellular fluid. Example: Increase in extracellular CO 2 triggers a receptor Sends information to respiratory control center Respiratory muscles are activated to increase breathing CO 2 concentration returns to normal

Positive Feedback Response increases the original stimulus Feedback is termed positive because the response is in the same direction as the stimulus. Example: Initiation of childbirth stimulates receptors in cervix Sends message to brain Release of oxytocin from pituitary gland Oxytocin promotes increased uterine contractions Nature of the Control Systems

Gain of a control system The precision with which a control system maintains homeostasis is called the gain of the system OR Degree to which a control system maintains homeostasis Gain can be thought of as the “capability” of the control system System with large gain is more capable of maintaining homeostasis than system with low gain Pulmonary and cardiovascular systems have large gains All these systems deal with life-and-death issues. Nature of the Control Systems

In Summary  A biological control system is composed of a sensor, a control center, and an effectors.  Most control systems act by way of negative feedback.  The degree to which a control system maintains homeostasis is termed the gain of the system. A control system with a large gain is more capable of maintaining homeostasis than a system with a low gain. Nature of the Control Systems

Examples of Homeostatic Control Regulation of body temperature Thermal receptors send message to brain Response by skin blood vessels and sweat glands regulates temperature Examples of Homeostatic Control

Regulation of Body Temperature Negative feedback mechanism to regulate body temperature Examples of Homeostatic Control

Regulation of blood glucose Requires the hormone insulin Diabetes Failure of blood glucose control system

Example: Regulation of Blood Glucose The pancreas acts as both the sensor and effector organ

Failure of a Biological Control System Results in Disease Failure of any component of a control system results in a disturbance of homeostasis Example: Type 1 diabetes Damage to beta cells in pancreas Insulin is no longer released into blood Hyperglycemia results This represents failure of “effector” Examples of Homeostatic Control

Exercise Exercise disrupts homeostasis by changes in pH, O 2, CO 2, and temperature Control systems are capable of maintaining steady state during submaximal exercise in a cool environment Intense exercise or prolonged exercise in a hot/humid environment may exceed the ability to maintain steady state. May result in fatigue and cessation of exercise Exercise: A Test of Homeostatic Control

In Summary  Exercise represents a challenge to the body’s control systems to maintain homeostasis.  In general, the body’s control systems are capable of maintaining a steady state during most types of exercise in a cool environment.  However, intense exercise or prolonged work in a hostile environment (i.e., high temperature/ humidity) may exceed the ability of a control system to maintain steady state, and severe disturbances of homeostasis may occur. Exercise: A Test of Homeostatic Control

Submaximal exercise in a cool environment The body’s control systems can maintain steady state Maximal exercise or exercise in a hot/humid environment May not be able to maintain steady state Severe disturbances in homeostasis can occur