homeostasis

physiological variable

a measure of a bodily condition or function

examples of physiological variables

blood glucose concentration, blood pressure, temperature

homeostasis

the dynamic maintenance of physiological variables within a predictable range

set point

the normal ‘basal’ or ‘at rest’ value for a physiological variable

physiological variables are

inter-dependent

osmolality

amount of solute present in the blood -> sodium content mainly determines the plasma osmolality

negative feedback

A change is sensed, and a response is initiated to reverse that change

The effect is therefore to maintain physiological variables within a predicted range

feed-forward

Anticipation of a change brings about the response to that change before the change can be detected by negative feedback sensor

positive feedback

A change in a variable triggers a response that causes further change in that variable

The effect is therefore amplification of the change rather than normalisation

example of positive feedback in homeostasis is the

process of childbirth

how is process of childbirth an example of positive feedback

the pressure of the baby's head on the cervix triggers the release of oxytocin

which causes stronger uterine contractions.

This greater pressure on the cervix leads to the release of even more oxytocin, amplifying the contractions until the baby is born and the stimulus is removed. 

1. Stimulus: The baby's head presses against the cervix, stretching it. 

2. Detection: Nerve signals are sent from the cervix to the brain. 

3. Response: The brain signals the pituitary gland to release oxytocin. 

4. Action: Oxytocin causes the uterus to contract more forcefully. 

5. Amplification: The increased contractions stretch the cervix further, which triggers the release of more oxytocin. 

6. Completion: This positive feedback loop continues to intensify contractions until the baby is delivered, which removes the initial stimulus

example of feed-forward

fight or flight

features of negative feedback

1. a physiological variable drifts away from its set point

2. sensors detect the change in the variable

3. signals are carried from sensors to integrating centre via afferent pathway

4. integrating centre compares inputs from sensors against physiological set point and initiates response

5. signals carried from integrating centre to effectors via efferent pathway

6. effectors produce response that brings variable back to set point

3 types of signalling pathway

neuronal

hormonal

paracrine

many neuronal integrating centres are in the

midbrain or brain stem

neuronal essential for

temperature control

osmolality control

blood pressure control

blood gas/breathing control

paracine homeostatic control features

• sensors, integrating centres and effectors are all located in the same tissue

• efferent pathway involves secretion of diffusible substances from one group of cells to act on another group of cells nearby

• can operate in parallel or independently of neuronal and endocrine control

• diffusible substances may act on cell surface receptors or intracellular targets of effectors

communication with effectors is usually via the

sympathetic and parasympathetic nervous systems

sympathetic and parasympathetic nervous systems have

opposing actions on bodily functions

resulting in fine-tuning of physiological variables

e.g. of fine tuning of physiological variables

cardiac output & blood pressure lung ventilation

• Sympathetic = accelerator (increase HR, BP, breathing)

• Parasympathetic = brake (slow HR, lower BP, calm breathing)

fine tuning meaning

gradual changes (not rapid) to produce the final response

how does temperature control involve negative feedback

1. stimulus = drop in ambient temperature causes drop in body temperature

2. hypothalamus (sensor) detects change

3. neurons communicate with hypothalamus via afferent pathway

4. integrating centre - hypothalamus compares against set point for temperature

5. integrating centre sends nerve signals so from hypothalamus to effectors

6. the effectors are skin blood vessels and muscle

7. so skin shivers for heat production and reduced blood flow to reduce heat loss and blood vessels constrict to increase temperature

human endocrine organs

pancreas

thyroid gland

adrenal gland

ovaries

testes

hypothalamus

pituitary gland

classes of hormones

peptides

polypeptides

glycopeptides

example of peptide hormones

ADH

Oxytocin

example of polypeptide hormones

insulin

growth hormone

examples of glycopeptide hormone

luteinizing hormone

follicle stimulating hormone

thyroid stimulating hormone

paracrine control

Sensors, integrating centres and effectors are all located in the same tissue

Efferent pathway usually involves secretion of diffusible substances from one group of cells to act on another group of cells nearby

May be part of negative feedback or feed-forward pathways

May operate in parallel or independently of neuronal and endocrine control

Diffusible substances may act on cell surface receptors or intracellular targets of effector cells

tyrosine derivatives

thyroxine

adrenaline

derivatives of cholesterol

estradiol

cortisol

aldosterone

testosterone

what is the sensor of increased blood glucose concentration

pancreatic beta cells

what hormone is secreted when blood glucose concentration increases

insulin

examples of paracrine homeostatic control

NO in blood vessels released to diffuse to smooth muscle cells to cause them to relax and dilate

Histamine release in Inflammation act on blood vessels to increase permeability

Neurotransmitter release at synapses to transmit signals

Growth factors in tissue repair released at site of injury for cell division/ matrix regeneration