Quick questions on Homeostasis, feedback, thermoregulation and osmoregulation: HSC Biology Module 8

Every homeostatic mechanism has the same five components:

Set point: approximately 37 degrees Celsius. Control centre: the hypothalamus.

The eastern bearded dragon (Pogona barbata) is found across eastern Australia. Lacks internal heat production and relies on behaviour:

Osmoregulation is the control of water and solute balance. The control centre is the hypothalamus, and the effector is the kidney via antidiuretic hormone (ADH, vasopressin).

Notomys alexis, the spinifex hopping mouse, survives in arid Australia without drinking. It produces extremely concentrated urine (osmolarity above 9000 mOsm) due to elongated loops of Henle that establish a steep medullary concentration gradient, maximising water reabsorption.

Skin thermoreceptors detect cold. The hypothalamus triggers:

Skin and hypothalamic thermoreceptors detect heat. The hypothalamus triggers:

Osmoreceptors in the hypothalamus detect rising blood osmolarity.

The posterior pituitary releases ADH. The kidney collecting ducts become more permeable to water through aquaporin-2 insertion. Water is reabsorbed, urine becomes dark and concentrated, and blood osmolarity returns towards normal.

As osmolarity falls, osmoreceptor firing decreases, ADH release slows, and water reabsorption returns to baseline. The walker also experiences thirst (a behavioural response) and drinks, which contributes to recovery.

Negative feedback opposes the change; positive feedback amplifies it. Childbirth, blood clotting and action potentials are positive feedback; everything else in the syllabus is negative.

Negative feedback only works if there is a target value (37 degrees Celsius, 5 mmol/L glucose, 300 mOsm osmolarity).

Heat dissipation = dilation (more blood to skin). Heat conservation = constriction (less blood to skin).