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.

Examples in context

Example 1. Eastern bearded dragon thermoregulation in a NSW outback summer. The eastern bearded dragon (Pogona barbata), an ectotherm common across NSW dry sclerophyll woodland, regulates body temperature behaviourally rather than metabolically. At dawn, when air temperature is around 15 degrees C, dragons emerge and bask flat against sun-warmed rocks, with bodies oriented broadside to the sun to maximise solar absorption. As body temperature reaches the preferred 33 to 36 degrees C, the dragon retreats to a shaded crevice. In peak summer heat above 40 degrees C, it gape-pants (opens its mouth, exposing the buccal cavity to evaporative cooling) and lightens skin colour to reflect light. This behavioural thermoregulation maintains body temperature within 3 degrees C of the optimum despite a 25 degree C ambient swing.

Example 2. Red kangaroo licking and saliva cooling during a heatwave. The red kangaroo (Macropus rufus), an endotherm of inland Australia, faces summer temperatures regularly above 45 degrees C. When core temperature begins to rise, the hypothalamus triggers panting and a distinctive behaviour: kangaroos lick their forearms heavily, depositing saliva on a dense superficial vascular network supplying the limbs. Evaporative cooling of saliva removes about 2400 kJ per litre evaporated, reducing forearm skin temperature by up to 6 degrees C and cooling blood returning to the core. This is a negative feedback response: receptors in the hypothalamus detect rising blood temperature, the effector (saliva and panting) cools the animal, and the temperature returns toward set point.

Try this

Q1. Define negative feedback and identify the three core components of any negative feedback loop. [3 marks]

• Cue. A control system that reverses a change to restore set point. Components: receptor (detects change), control centre (hypothalamus, integrates signal), effector (responds).

Q2. A patient's blood glucose rises from 5 mmol/L to 9 mmol/L after a sugary drink, then returns to 5.2 mmol/L over 2 hours. Identify the hormone responsible and describe the negative feedback loop. [3 marks]

• Cue. Insulin from pancreatic beta cells; rising glucose detected, insulin stimulates uptake by muscle and liver, glucose falls, insulin secretion falls.

Q3. Compare ectothermic and endothermic thermoregulation. (a) Identify one Australian example of each. (b) Describe the main thermoregulation mechanism used by each. (c) Justify which strategy is energetically cheaper at moderate ambient temperatures. [2+2+3 marks]

• Cue. (a) Bearded dragon and red kangaroo. (b) Behaviour (basking/shade) vs metabolic (panting/sweating, shivering). (c) Ectothermy is cheaper because metabolic heat production demands constant food intake.