Homeostasis and Control Systems

define, with examples, what is meant by homeostasis •outline the main components required for a homeostatic control system • explain how thermoregulatory mechanisms exemplify homeostasis • identify factors which limit homeostasis • understand the concept of disease as failure of homeostasis

Homeostasisis the maintenance of a relatively constant or stable internal environment

• Concentration of nutrient molecules • Concentration of O2& CO2 (carbonic acid) • Concentration of waste products • pH ( 7.35-7.45- enzyme denaturation) • Concentration of water, salt & electrolytes (cell lysis/ crenation) • Temperature (amino acid denaturation) 7 .Volume & pressure

• Intrinsic: • Local control: Regulatory mechanism initiated within an organ and inherent to that organ • Accomplished by release of chemicals such as paracrines& autocrines • Example: hypoxic vasodilation of blood vessels 2.Extrinsic/Reflex • Long-distance control: Regulatory mechanism initiated outside an organ • Accomplished by nervous and endocrine systems • Example: blood pressure regulation

• A receptor • A control centre • An effector • receptor senses change • control centre evaluates change & initiates a response • effectors return conditions toward normal • deviation from set-point lessens • effectors are shut off

• Are responsible for many of the body’s homeostatic controls e.g. blood pressure control, body temperature • Consist of pathways where the response opposes or removes the stimulus (graph in lecture slides) • Stabilise the variable that is being regulated

Receptors: • Peripheral and central thermal receptors- signals sent to ant. hypothalamus Control centre: • Hypothalamus- causes vasodilation in skin (radiant heat loss) hair follicles flatten hair and sweat glands open- opposite when body temp too low. Effectors: • skin blood vessels, sweat glands, arrector pili muscles, skeletal muscles, endocrine organs

Positive feedback is a process in which the end products of an action cause more of that action to occur in a feedback loop. This amplifies the original action. -Occurs in fever- in this case it can cause a life-threatening loss of homeostatic control - It enhances the initial change- Does NOT help maintain stability and is very rare under normal circumstances 2 examples are in labour and in blood clotting.

Fever or pyrexia is a process where normal body temperature is raised over homeostatic conditions- caused by infection: can stimulate pyrogens which increase the set point in the hypothalamus Receptor: • chemoreceptors Control centre: • Hypothalamus Effectors: • sweat glands, skin blood vessels, skeletal muscle

Extremes of Age Examples of altered control mechanisms: • Fluid balance- lower thirst, less water conservation, more risk • Thermoregulation- more dysfunctional than in younger people, vasodilation/constriction Babies have high surface area/mass ratio, less white fat, large head (large s.a)- need to be kept warm (can’t shiver to preserve heat) Presence of a pathologic condition • The usual negative feedback mechanisms become overwhelmed.

Most disease is caused by a disturbance of homeostatic mechanisms known as “homeostatic imbalance” • Occurs when a factor in the internal environment is controlled inadequately; its level falls outside the normal range • Example 1: the cardiovascular system • chronic hypertension • atherosclerosis • congestive heart failure • Example 2: the endocrine system • Type 1 diabetes