Detecting & Responding Homeostasis Detecting & Responding

Internal Vs. External Environment External environment can vary greatly Internal environment of the body is kept constant Within a very narrow tolerance range over the lifespan Vital for survival

The Internal Environment This includes: The cells (Intracellular fluid) Extracellular Fluid Tissue Fluid (fluid surrounding cells) Plasma (fluid component of blood in capillaries) Wastes and nutrients are readily exchanged between all of these fluids

Internal Vs. External Environment Range between 0-29C Range between 36.5 and 38C

Environmental conditions Some examples of conditions that need to be kept constant are: Temperature Oxygen concentration Blood pressure/volume pH levels Salt concentration Water balance Glucose concentration

Homeostasis The term homeostasis refers to the maintenance of a stable internal environment within narrow limits despite changes in the external environment Range between 36.5 and 38C

Homeostasis All body systems contribute to the internal environment in some way Monitoring of the internal and external environment is carried out by two different systems: Nervous – nerves/electrical Endocrine – hormones/chemical Both interact to maintain homeostasis Usually the nervous system triggers the endocrine

Homeostasis Control of internal environmental conditions is usually through a negative feedback system When a change is detected in an environmental condition, an action occurs to oppose the change and bring the condition back to normal Any change in environmental conditions that stimulates a homeostatic response such as this is called a stimulus

Negative Feedback In a negative feedback system, there are 2 main stages involved in reacting to a stimulus: Detecting the stimulus: The change is detected by a receptor or sensor which then transmits the message to the control centre which coordinates a response Counteracting the stimulus: The control centre transmits a message to tissues or organs which are able to create a response. These are called effectors.

Definitions Stimulus: A change in the internal or external environment that is able to bring about a response (Stimuli for pl.) Receptor: Specialised cell which is able to detect changes in a certain internal or external environmental condition and transmits the information to the Control Centre Control Centre: Usually a component of the Central Nervous System (CNS). It coordinates the action to be taken and transmits the message to the tissue/organ which will respond Effector: The tissue or organ which responds

Receptors Some examples of receptors include: Chemoreceptors: Detect changes in chemicals (eg. taste, glucose, salt, pH) Mechanoreceptors: Detect changes in pressure and movement (eg vibrations, sound, muscle stretch, blood pressure) Photoreceptors: Detect light (eg vision) Thermoreceptors: Detect changes in temperature

Effectors The types of tissues/organs that normally act as effectors include: Muscles Endocrine glands

Negative feedback Receptor Control Centre Effector STIMULUS (Change in environmental condition) Return of environmental conditions to normal range Negative Feedback Receptor RESPONSE Transmission of message Control Centre Transmission of message Effector

(Decrease in body temperature) Return to normal body temperature An Example STIMULUS (Decrease in body temperature) Return to normal body temperature Negative Feedback Thermoreceptor in skin RESPONSE: Shivering, increased metabolism Transmission of message Control Centre (CNS) Transmission of message Effector:muscles, thyroid

In Summary Stimulus Receptor Control Centre Effector Response Transmission Negative Feedback

Excess Corrective mechanism Negative feedback * Negative feedback is when the response counteracts or cancels out the original stimulus/disturbance Normal (set point) Normal (set point) Negative feedback Corrective mechanism Deficiency

Nervous & Endocrine The nervous and endocrine systems are able to maintain homeostasis by: Detecting changes; and Initiating a response This is made possible because cells of the nervous and endocrine systems are able to communicate with other cells through the use of: Chemical messages – signalling molecules – used by both nervous and endocrine systems Electrical messages – nerve cells only

Signalling molecules Signalling molecules Production in Transport Targets Hormones Endocrine glands In blood circulation and extracellular fluid Receptors on target cells Neurotransmitters Nerve cells – stored in secretory vesicles in axon terminals Across synaptic gap Dendrites of other neurons Pheromones Exocrine glands Released into external environment Other members of same species – usually a scent