Homeostasis

• Homeostasis = maintenance of the internal environment of an organism within preset limits despite changes in the external environment.

• In humans, important homeostatic variables are body temperature, blood pH, blood glucose concentration and blood osmotic concentration.

• Core exam idea: homeostasis usually works by negative feedback, not positive feedback.

Negative feedback loops

• Negative feedback returns a variable to its set point when it rises above or falls below the normal range.

• It is used because it stabilizes internal conditions.

• General pattern: stimulus/change → receptor detects deviation → control centre compares to set point → effectors respond → variable returns toward normal.

• Be able to identify the stimulus, receptor, control centre, effector and response in any homeostasis question.

This diagram shows how negative feedback restores body temperature to normal by activating opposite responses when temperature rises or falls. It is useful for spotting the roles of receptors, control centre and effectors in one complete loop. Source

Blood glucose regulation

• Blood glucose regulation is a classic example of hormonal control in homeostasis.

• The pancreas contains endocrine cells that detect changes in blood glucose.

• Beta cells secrete insulin when blood glucose is high.

• Alpha cells secrete glucagon when blood glucose is low.

• Hormones are transported in the blood to target cells.

• Insulin lowers blood glucose by increasing glucose uptake and promoting storage of glucose, especially as glycogen.

• Glucagon raises blood glucose by promoting release of stored glucose, especially from glycogen breakdown.

• Exam focus: explain how opposing hormones produce negative feedback.

This image shows the two opposing pathways that maintain blood glucose homeostasis. It is ideal for revising how high blood glucose triggers insulin and low blood glucose triggers glucagon. Source

Type 1 and type 2 diabetes

• Type 1 diabetes: body produces little or no insulin because pancreatic beta cells are destroyed.

• Type 2 diabetes: target cells become less responsive to insulin and/or insulin supply becomes insufficient.

• Key contrast: Type 1 = lack of insulin production; Type 2 = reduced response to insulin.

• Know the risk factors, prevention and treatment in outline.

• Type 1 treatment commonly includes insulin therapy.

• Type 2 prevention/treatment often includes lifestyle changes; treatment may also include medication and sometimes insulin.

• Exam tip: questions often ask you to compare the physiological basis of the two types, not just list symptoms.

Thermoregulation: control of body temperature

• Thermoregulation is another example of negative feedback.

• Peripheral thermoreceptors detect temperature change.

• The hypothalamus acts as the control centre.

• The pituitary gland and hormones including thyroxin are involved in longer-term regulation of metabolic heat production.

• Muscle tissue and adipose tissue can act as effectors that change body temperature.

• Mammals and birds regulate temperature by both physiological and behavioural means, but for IB you need the human mechanisms.

Thermoregulation mechanisms in humans

• Vasodilation: skin arterioles widen, increasing blood flow near the skin → more heat loss.

• Vasoconstriction: skin arterioles narrow, reducing blood flow near the skin → less heat loss.

• Sweating: evaporation from skin causes cooling.

• Shivering: rapid muscle contractions increase respiration and heat production.

• Uncoupled respiration in brown adipose tissue increases heat generation.

• Hair erection reduces heat loss by trapping a layer of air, though this is less important in humans than in furred mammals.

• Behavioural responses also matter, but IB emphasizes the physiological mechanisms in humans.

This diagram is useful for revising vasodilation, vasoconstriction and other effector responses in a single feedback loop. It directly supports exam questions on how the body restores normal temperature. Source

This illustration shows the main structures involved in ultrafiltration, reabsorption and osmoregulation. It is especially useful for linking nephron structure to function in HL questions. Source

This image is especially useful for revising how the loop of Henle creates a medullary gradient and how the collecting duct changes water reabsorption. It supports HL explanations involving ADH and aquaporins. Source

Blood supply changes with activity

• Blood supply to organs changes depending on activity level.

• You need to know patterns for skeletal muscles, gut, brain and kidneys during sleep, vigorous exercise and wakeful rest.

• During vigorous physical activity, more blood is directed to skeletal muscles.

• During vigorous activity, relatively less blood goes to the gut and kidneys.

• The brain maintains a high blood supply because of its constant demand for oxygen and glucose.

• Exam questions may ask for a comparison across states, so answer comparatively, not as isolated facts.

Common exam links and comparisons

• Blood glucose regulation and thermoregulation are both examples of negative feedback.

• Insulin and glucagon are hormonal antagonists in blood glucose control.

• Thermoregulation uses both nervous and endocrine control.

• Kidneys help maintain water balance and remove wastes.

• ADH changes the permeability of collecting duct cells by controlling aquaporin location.

• In long-answer questions, always connect change in variable → detection → control signal → effector response → restoration of set point.