Homeostasis

Claude Bernard (1813 – 1878)

• French Physiologist

• Called the environment of cells the internal environment

• Bernard made the observation that body cells survived in a healthy condition only when the temperature, pressure, and chemical composition of their fluid environment remained relatively constant.

Walter B. Cannon

• Coined the term homeostasis in 1932 for the relatively constant state maintained by the body.

• Comes from two Greek words– homoios = the same – stasis = standing

Set Point

• The set point, or the set point range, is the normal reading or range of normal.

• Example: Normal body temperature is around 98.6° F

Homeostasis of Blood Glucose

Note that the concentration of glucose fluctuates above and below a normal set point (90 mg/ml) within a normal set point range (80 – 100 mg/ml).

Homeostatic Control Mechanisms• Processes for maintaining and restoring

homeostasis are known as homeostatic control mechanisms.

• They involve virtually all of the body’s organs and systems.

Exercise as an Example:• Exercise increases our need for oxygen

and results in accumulation of the waste product carbon dioxide.

• In response, our body increases the respiration rate which maintains an adequate oxygen supply and increases the elimination of carbon dioxide.

Feedback Control Loop

Highly complex and integrated communication control network, classified as negative or positive; negative feedback loops are the most important and numerous control mechanisms.

Positive and Negative Feedback

Negative Feedback – feedback control system in which the level of a variable is changed in the direction opposite to that of the initial stimulus.

Positive Feedback Mechanism – feedback control system that is stimulatory; tends to amplify or reinforce a change in the internal environment.

Basic Components Of Control Mechanisms• There is a minimum of four basic

components in every feedback control loop:– Sensor Mechanism– Integrating, or control, center– Effector mechanism – Feedback

Directional TermsAfferent means that a signal is traveling towards a particular center or point of reference.

Efferent means that a signal is moving away from a center or other point of reference.

Sensors• Sensory nerve cells or hormone– producing

(endocrine) glands frequently act as homeostatic sensors

• Must be able to identify the element being controled

• Must be able to respond to any changes that may occur from the normal set point range

• The sensor generates an afferent signal (nerve impulse or signal) to transmit that information to the integration, or control center

Contol Center

• The control center is often a discrete area of the brain.

• When it receives input from a sensor, that information is analyzed and integrated with information from other sensors

• An efferent signal travels from the center to some type of effector mechanism, where a specific action is initiated

Efferent Mechanism and Feedback• Organs such as muscles or glands, that

directly influence controlled physiological variables

• The activity of effectors is ultimately regulated by feedback of information regarding their own effects on a controlled variable