Investigate the responses of a named Australian ectothermic and endothermic organism to changes in the ambient temperature, and explain how these responses assist in maintaining homeostasis, including negative feedback, positive feedback, thermoregulation and osmoregulation

What this dot point is asking

NESA wants you to define homeostasis, distinguish negative from positive feedback, and apply the feedback model to thermoregulation and osmoregulation in named organisms. Expect a 4 to 8 mark question that requires you to walk through a feedback loop step by step.

The answer

Homeostasis is the maintenance of a stable internal environment despite changes in the external environment. The internal variables regulated include core body temperature, blood glucose, blood pH, blood pressure and water and solute balance.

Feedback loops

Every homeostatic mechanism has the same five components:

1. Stimulus. A change in the internal variable away from the set point.
2. Receptor. A sensor that detects the change.
3. Control centre. An integrator (often the hypothalamus) that processes the signal.
4. Effector. A muscle, gland or behaviour that produces a response.
5. Response. The action that restores the variable.

Negative feedback reverses the original change. The response moves the variable back towards the set point, then switches itself off. Almost all human homeostatic loops are negative feedback (temperature, glucose, blood pressure, osmolarity).

Positive feedback amplifies the original change. The response pushes the variable further from the starting point. Examples include uterine contractions during childbirth (oxytocin increases contractions, which increases oxytocin release), the clotting cascade, and the action potential in neurons.

Thermoregulation in endotherms: humans

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

Cold response. Skin thermoreceptors detect cold. The hypothalamus triggers:

• Vasoconstriction of skin arterioles to reduce heat loss.
• Shivering: rapid involuntary muscle contractions that generate heat.
• Pilo-erection (raising body hair) to trap an insulating air layer.
• Release of thyroxine and adrenaline to increase metabolic rate.
• Behaviour: putting on clothing, moving to warmth.

Heat response. Skin and hypothalamic thermoreceptors detect heat. The hypothalamus triggers:

• Vasodilation of skin arterioles.
• Sweating: evaporation cools the skin.
• Reduced muscle activity.
• Behaviour: removing clothing, seeking shade.

Thermoregulation in endotherms: red kangaroo

The red kangaroo (Macropus rufus) inhabits arid central Australia. Adaptations include:

• Forearm licking. A dense capillary network in the forearms is exposed by licking; saliva evaporates, cooling the blood before it returns to the core.
• Panting. Increases evaporative cooling from the respiratory surfaces.
• Behavioural avoidance. Resting in shade during the hottest part of the day.
• Reflective fur. Light-coloured fur reflects solar radiation.

Thermoregulation in ectotherms: eastern bearded dragon

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

• Basking on rocks in the morning to absorb solar radiation.
• Posture changes. Flattening to maximise surface area to the sun; lifting the body off hot ground.
• Colour change. Darkening in cool conditions to absorb more radiation, lightening when hot.
• Sheltering. Retreating into burrows or shade when temperatures exceed the preferred range (around 35 degrees Celsius).

Osmoregulation

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).

Dehydration (high blood osmolarity).

1. Osmoreceptors in the hypothalamus detect increased solute concentration.
2. The posterior pituitary releases ADH.
3. ADH binds to receptors on the collecting ducts of the nephron, inserting aquaporin-2 channels into the membrane.
4. Water is reabsorbed from the filtrate into the blood, producing concentrated urine.
5. Blood osmolarity falls back to the set point. Negative feedback switches off ADH release.

Overhydration (low blood osmolarity).

1. Osmoreceptors detect reduced osmolarity.
2. ADH release is suppressed.
3. The collecting ducts are impermeable to water, producing dilute urine.
4. Blood osmolarity rises back to the set point.

Australian osmoregulation example: the spinifex hopping mouse

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.

Worked example

A bushwalker becomes severely dehydrated during a hot day.

Receptors. Osmoreceptors in the hypothalamus detect rising blood osmolarity.

Response. 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.

Negative feedback. 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.

Common traps

Confusing negative and positive feedback. 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.

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

Calling thermoregulation in ectotherms "non-existent." Ectotherms regulate temperature through behaviour; they just do not generate heat metabolically.

Mixing up vasodilation and vasoconstriction. Heat dissipation = dilation (more blood to skin). Heat conservation = constriction (less blood to skin).

In one sentence

Homeostasis maintains a stable internal environment through negative feedback loops in which receptors detect change, the hypothalamus integrates the signal, and effectors such as sweat glands, skin arterioles and the kidney collecting ducts produce a response that returns the variable to its set point.