Homeostasis

Roger WatsonTwitter: @rwatson1955

Homeostasis

Homeostasis refers to the body's ability to regulate physiologically its inner environment to ensure its stability in response to fluctuations in the outside environment.

Homeostasis

The concept was first articulated by [Frenchman] Claude Bernard in 1860s

Bernard:

‘I think I was the first to urge the belief that animals have really two environments: a milieu extérieur in which the organism is situated, and a milieu intérieur in which the tissue elements live.’

Homeostasis

Term coined by Walter Cannon (1932)

Cannon:

The coordinated physiological processes which maintain most of the steady states in the organism are so complex and so peculiar to living beings--involving, as they may, the brain and nerves, the heart, lungs, kidneys and spleen, all working cooperatively—that I have suggested a special designation for these states, homeostasis. The word does not imply something set and immobile, a stagnation. It means a condition—a condition which may vary, but which is relatively constant.

Bozeman biology You Tube link

Homeostasis

Fishbowl Human Body FunctionWater Body fluid  Internal environmentGlass bowl Skin Barrier (internal vs. external)

Fish Cells Stay aliveAir pump Lungs Keep O2 level constant (high)

Filter Kidneys Keep nitrogen wastes constant (low)

Heater Muscles Keep temperature constant (high)

Feeder Digestive system

Keep nutrient levels contant (high)

Everything working together Relatively constant conditions

Homeostasis: how does it work?

Feedback:

Homeostasis: how does it work?

Feedback:

• Negative

• Positive

Negative feedback

Example:

• Thermostat

Engineered control system (thermostat) modelExample: thermostatic heating system in a homeComponents of an automatic control system

Variable is the characteristic of the internal environment that is controlled by this mechanism (internal temp in this example)

Sensor (receptor) detects changes in variable and feeds that information back to the integrator (control center) (thermometer in this example)

Integrator (control centre) integrates (puts together) data from sensor and stored "setpoint" data (thermostat in this example)

Setpoint is the "ideal" or "normal" value of the variable that is previously "set" or "stored" in memory

Effector is the mechanism (furnace in this example) that has an "effect" on the variable (internal temperature in this example)

Components of a negative feedback loop

• Stimulus• Condition• Receptors• Control centre• Effector• Response

Example: blood pressure control

Components of blood pressure negative feedback loop

• Stimulus drop in blood pressure• Condition constant blood pressure• Receptors carotid artery receptors• Control centre brain• Effector cardiovascular system• Response increased blood pressure

Positive feedback

Example:

• Audiofeedback

Positive feedback

Physiological example:

• Labour in childbirth

Labour in childbirth

Foetus's head moves into birth canal (vagina) at start of labour, which causes the birth canal to stretch beyond its setpoint amount of stretch, which is detected by sensors (stretch receptors) in the vaginal wall and fed back to hypothalamus of brain, which releases oxytocin, which stimulates stronger and more frequent uterine (womb) contractions, which pushes the foetus, which causes more vaginal stretch, which produces more oxytocin, and so on -- greatly amplifying and speeding up labour contractions…until birth!

Homeostasis: what goes wrong without it?

JUST ABOUT EVERYTHING!

• Hypothermia• Diabetes• Hypo/hyperglycaemia• Hypo/hypertension• Haemorrhage• Acidosis• Ketosis• Hypoxia• Hypercapnia..etc etc

Homeostasis: what compromises it?

• Disease• Neonatal• Ageing• Environmental extremes• Trauma• Malnutrition• Drugs/alcohol

Homeostasis

Homeostasis refers to the body's ability to regulate physiologically its inner environment to ensure its stability in response to fluctuations in the outside environment.

Questions

1. Define 'homeostasis'.

2. What are the components of a homeostatic system?

3. Using blood glucose as an example explain how negative feedback works in the restoration of blood glucose levels after ingesting carbohydrates.

4. Using blood pressure as an example identify the following: sensor or receptor; control centre; effector; variable.

5. Using falling blood pressure as an example explain how negative feedback works.

r.watson@hull.ac.uk

Four things about…(a simple approach to anatomy and physiology)

Homeostasis