What is homeostasis?
The ability to maintain a relatively stable internal environment (steady state), even though the external environment changed continuously, through the use of homeostatic control systems (feedback loops)
What are some diseases that the accumulation of damaged proteins is responsible for?
Alzheimer’s Disease and Creutzfeldt-Jakob Disease
What does homeostatic control require, and what can accomplish that?
Nervous and endocrine systems, through the use of electrical impulses and hormones to carry information
What are homeostatic mechanisms?
Reflexes that occur subconsciously (feedback loops)
What is homeostasis constantly being disrupted by?
Changes to the internal and external environments
Examples:
Internal
- Drop in blood glucose due to lack of food
External
- Intense heat or lack of oxygen
What is a variable?
The factor or condition being regulated
What are some of the components of a homeostatic control mechanism?
- Receptor/Sensor
- Afferent pathway
- Set-point or reference value
- Integrator/Control center
- Efferent pathway
- Effector
What do receptors do?
- monitors changes in a variable
- Receives the change and sends the input to the control centre through afferent pathways
What does the control centre do?
- Brain (in general the hypothalamus)
- Determines the set point/reference value for a variable (e.g. normal body temperature range)
- Evaluates input received from receptors and generates an output command
- Nerve impulses, hormones
What does the effector do?
- Receives output from the control centre via efferent pathways
- Produces a response or effect that changes the value of the variable (e.g. Shivering to generate heat and thereby increase body temperature)
What are controlled variables?
Variables that are not themselves homeostatically regulated
Ex.
- Arterial blood pressure is regulated by changing heart rate, stroke volume and peripheral resistance
- Heart rate is not controlled around a set point, instead its main role is to contribute to blood pressure regulation
Are homeostatic control mechanisms more often positive or negative feedback loops?
Negative
Positive feedback?
- The response enhances the original stimulus
- The response is positive because it is in the same direction as the
initial change - The value of the variable deviates further from its original set value
e.g. child birth and blood clotting
Negative feedback?
- Reverses a change in the variable by either shutting off the original effect of the stimulus or reducing its intensity
- The mechanisms cause the variable to change in the opposite direction to that of the initial change
- Variable returns back to set point.
e.g. regulation of blood pressure and body temperature
Set point?
- Homeostatic control systems cannot maintain a variable at complete constancy, so any regulated variable will have a range of normal values depending on multiple factors e.g. age, time of day, external environmental conditions
- Set points can be physiologically reset to a new value
e. g. During a fever, the body temperature is raised (≥37°C) in response to infection. This is an adaptation for fighting infection because it inhibits proliferation of some pathogens
Adaptation?
- A characteristic that favours survival in specific environments
- Homeostatic control systems are inherited biological adaptations
Acclimatisation?
- A type of adaptation, where there is improvement of an already existing homeostatic system
- Usually reversible with discontinuation of the environmental stress
Circadian rhythm?
Many body function/processes have rhythmic changes over time, and the most common rhythm is once every 24h, which is called the circadian rhythm
e.g. waking and sleeping, body temperature, growth hormone release
What do biological rhythms do?
Add an anticipatory (or feedforward) component to homeostatic control systems by activating them at times when change likely to occur (but before it actually does)
e.g. Body temperature increases prior to waking in a person on a typical sleep-wake cycle, which allows metabolic machinery of the body to operate most efficiently upon waking, as its partly temp dependant
What are factors that affect performance of homeostatic mechanisms
(causing imbalance)?
– Aging – Nutritional status – Disease causing organisms – Excessive stress – Extreme environmental conditions beyond control
Moderate imbalance?
- Disorder or abnormality of structure and function
- Disease specific for an illness with recognisable signs and symptoms
- Treatment leads to restoration of homeostasis
Severe imbalance?
- Treatment fails to restore homeostasis
- Illness and death
Homeostatic imbalance in type 1 diabetes?
- Beta cells of the pancreatic islets are destroyed.
- Insulin no longer produced
- Blood glucose continues to
rise - The condition is fatal if not treated
What is the function of osteocytes?
- Important role as mechanosensors
- They detect changes in the bone, either chemical or physical changes, and release chemicals in response to either send out osteoblasts (build bone) or osteoclasts (remove bone)
