Control Systems & Homeostasis

Question 1

What is Homeostasis?

Answer 1

Homeostasis is the maintenance of a constant internal environment within the body. It relies on Control Systems in order to accomplish this. The control systems involved can be cellular, organ or body systems.

Question 2

Explain the operations of the body’s control systems?

What is:
Regulated variable?
Sensor?
Set point?
Comparator?
Effector?

Answer 2

The control systems generally control the internal environment through negative feedback, “what goes up must come back down”. There are certain elements of control:

REGULATED VARIABLE: the thing that is to be controlled
SENSOR: the means of measuring the regulated variable
SET POINT: the ideal value for the regulated variable to be at
COMPARATOR: means of comparing the regulated variable with the set point
EFFECTOR: means of restoring the regulated variable to that set point

Question 3

How is heat lost from the body?

Answer 3

At rest in a neutral thermal environment, the body loses heat; mostly through radiation, conduction and convection.
Thermoregulation copes better with lower environmental temperatures.

Question 4

Explain the differences in temperature from different clinical sites of measurement?

Answer 4

At 10 degrees Celsius, the body limits heat loss through cutaneous vasoconstriction, shivering, activity, curling up, hyperphagia, piloerection & sympathetically mediated chemical thermogenesis.

Thermoregulatory control centre regulates sympathetic outflow to skin arterioles and sweat glands and the stimulation of motor nerves controlling shivering.

In hot climates, sweating occurs far sooner, sweat volume increases and the concentration of sodium in sweat decreases.

Question 5

Explain temperature-regulating systems and homeostasis under integrated hypothalamic control?

Answer 5

THERMOREGULATION

• Humans are homeothermic in that their metabolic enzymes have a narrow operational temperature range.
• The regulatory control centre is within the hypothalamus where there is both a heating and cooling centre and integrated input from central and peripheral temperature receptors.
• Temperature sensors are situated mostly centrally to maintain warmth (with control coming from the hypothalamus and spinal cord) but also within the periphery for both cold and warmth (control for which is located in the skin to give early warning of changes in ambient temperature)
• Human Temperature set point is a core temperature of 37 degrees Celsius. (temperature can be taken sublingually, via the ear canal or rectally)
• Normal variation for human temperature is between 36 - 37.5 degrees (greater variation in the young and very old). There is also a diurnal variation of around 0.6 degrees between morning and afternoon and a variation after ovulation of around 1 degree known as menstrual variation.

Question 6

Explain the pathophysiology of fever?

Answer 6

Fever is an increase in the set-point temperature induced by pyrogens. These pyrogens can be either exogenous (endotoxins from bacteria) or endogenous (cytokine induced).

During Fever, prostaglangin (PGE2) is synthesized. Non steroidal anti-inflammatory drug such as aspirin have anti-pyretic qualities and can inhibit this.

Fever can induce chills because the heating mechanism becomes activated in response to the set-point rising (shivering).
Crisis then occurs as the cooling mechanism is activated as the set point lowers again, expressed through sweating.