An Introduction 13 Biology 2014 Homeostasis An Introduction 13 Biology 2014

Homeostasis The term is derived from the Greek word meaning ‘to stay the same’ It is used to describe the remarkable ability of organisms to maintain relatively constant internal conditions in the face of very different and often changeable external conditions.

In everyday life, an organism must regulate respiratory gases, maintain fluid and salt balance, regulate nutrient supply and energy, maintain body temperature, protect itself against disease and repair damaged tissues. All these responses must be coordinated.

Principles of Homeostasis Organisms need to maintain a cellular environment that is optimal for cell function. Homeostatic control systems have 3 functional components: A receptor – to detect change A control center An effector – to direct the appropriate response

Components of a Homeostatic System For homeostasis the body must be able to detect changes in the environment and bring about a response. stimulus response Control centre Receptor Effector Communication System

The Communication System There are 2 kinds of communication system: Nervous Endocrine

The Communication System Nervous System Endocrine System Route Travelled Specific pathways (nerve axons) Hormones via blood Time to reach target cell Milliseconds Seconds Cells reached Only those at the end of axons Entire body (only some cells respond) Strength of the signal indicated by Frequency of impulses Concentration of hormone Duration of Signal Millisecond for each impulse Minutes to hours or days

Principles of Homeostasis There are 2 feedback systems in homeostasis: Negative Feedback Mechanisms More common. A movement away from the steady state triggers a mechanism to counteract the change. Positive Feedback Mechanisms Rare. Leads to a response escalating in the same direction. Used in labour, lactation, fever and blood clotting.

Negative Feedback Mechanisms When a receptor detects a change from the set point, this is communicated to effectors, which respond by bringing about changes tending to correct the disturbance. The greater the disturbance from normal, the greater the tendency to correct it. This is negative feedback.

Negative Feedback Mechanisms E.g. the maintenance of O2 and CO2 levels in the tissue fluid during exercise. …the more the blood CO2 concentration rises above the normal, the more strongly the breathing muscles are stimulated to respond, thus the greater the rate of CO2 excretion.

Negative feedback Control of Breathing During Exercise Increased rate of CO2 excretion Increased breathing Rise in CO2 in blood Increased muscular activity Normal blood CO2 (set point)

Negative Feedback Mechanisms NB homeostatic systems cannot prevent any change in the internal environment. What happens is that conditions fluctuate above and below the set point. This is because there must be a delay between the detection of a change and the response that corrects it. The longer the delay, the greater the departure from the set point and the stronger the corrective response, leading to “overshoot”.

Over-correction in feedback control brought about by delayed response

Positive Feedback Mechanisms Positive feedback is described as a self-amplifying cycle in which one change, leads to even more significant changes in the same direction. Positive feedback is not a corrective mechanism activated by the body when it strays from the set point, but rather, it is a way for the body to produce significant changes in a relatively short period of time.

Positive Feedback Mechanisms E.g. a woman in labour. The baby's head pushes on the cervix, sending signals to the brain, which then produces oxytocin, which stimulates contractions, which then push the baby's head even more tightly against the cervix, and the loop then repeats and becomes more and more intense until the baby has been born.

Positive Feedback Mechanisms

Summary of Key Ideas Living organisms have the ability to keep their internal conditions different from their external environment. The regulation of internal conditions is called homeostasis. In more complex organisms cells are bathed in a liquid that forms an internal environment.

Summary of Key Ideas In animals with closed blood systems (e.g. vertebrates) the internal environment is the tissue fluid, which is separated from the blood by the capillary walls. Tissue fluid and blood are collectively called extracellular fluid. Disturbances in the internal environment are detected by receptors, and corrections are made by effectors.

Summary of Key Ideas In most animals there are 2 communication systems linking the receptors and effectors: nervous and endocrine. Homeostasis involves negative feedback, in which the greater the disturbance from the set point the greater the tendency to correct it. Positive feedback is where one change, leads to even more significant changes in the same direction.