Homeostasis and negative feedback

· Homeostasis = self-regulating biological processes that maintain a relatively stable internal environment for optimal body function.
· The body constantly responds to internal and external changes to keep key variables near a controlled condition or set point.
· Most homeostatic control uses negative feedback: a change is detected, a response is triggered, and the response reverses the original change.
· Exam phrase: negative feedback mechanisms reverse a change back to a controlled condition.
· Core sequence: stimulus → receptor → control centre → effector → response → return towards homeostasis.

Blood pH, carbon dioxide and ventilation

· Blood pH reflects hydrogen ion concentration and is influenced by carbon dioxide concentration.
· More CO₂ in blood increases acidity, so pH decreases; less CO₂ reduces acidity, so pH increases.
· Chemoreceptors throughout the body and the respiratory control centre of the brain monitor changes linked to CO₂, H⁺ and pH.
· If CO₂ rises during exercise, ventilation can increase to remove more CO₂ and help restore acid–base balance.
· High-performance link: intense exercise increases CO₂ production, so effective respiratory regulation helps maintain internal conditions for muscle function.

The respiratory control centres adjust breathing rate and depth in response to blood chemistry. This supports homeostasis by helping control CO₂ concentration and therefore blood pH. Source

Heart regulation: intrinsic and extrinsic excitation

· Regulation of the heart depends on both intrinsic excitation and extrinsic excitation.
· Intrinsic control = the heart’s own electrical activity initiates and coordinates contraction.
· Extrinsic control = external regulation, especially via the autonomic nervous system, modifies heart rate according to body demands.
· During exercise, extrinsic input helps increase heart rate and blood delivery to active tissues.
· Exam link: know that the heart structure diagram is in the SEHS data booklet; focus here on regulation rather than memorising a new diagram.

Thermoregulation

· Thermoregulation maintains core body temperature at 37 ± 1°C.
· It requires interaction between the cardiovascular, muscular, nervous and integumentary systems.
· If body temperature rises: sweat response and vasodilation increase heat loss.
· If body temperature falls: vasoconstriction, shivering and non-shivering thermogenesis help conserve or produce heat.
· Sweating cools by evaporation; it is less effective in high humidity because sweat evaporates less easily.
· Vasodilation = blood vessels widen near the skin, increasing heat loss.
· Vasoconstriction = blood vessels narrow near the skin, reducing heat loss.

This diagram shows body temperature being regulated by negative feedback. Vasodilation and vasoconstriction act as effectors that help reverse temperature changes back towards normal. Source

Factors affecting thermoregulation

· Training status: trained individuals may regulate heat more effectively through improved cardiovascular and sweat responses.
· Body composition: body fat, muscle mass and body size influence heat storage and heat loss.
· Environment: temperature, humidity, airflow and radiant heat alter how easily the body gains or loses heat.
· Sex differences, including hormonal phases, can affect temperature regulation and exercise responses.
· Exam application: when evaluating performance in heat, mention both physiological strain and environmental conditions.

Blood glucose regulation

· Blood glucose is regulated mainly by the hormones insulin and glucagon.
· When blood glucose is high: insulin promotes glucose uptake and storage, lowering blood glucose.
· When blood glucose is low: glucagon promotes glucose release into the blood, raising blood glucose.
· This is a negative feedback system because the hormonal response opposes the original change.
· During exercise, the body limits insulin release and facilitates glucose uptake into active tissues to regulate blood sugar levels.
· Performance link: stable blood glucose supports energy availability, especially during prolonged activity.

The diagram shows how insulin and glucagon oppose changes in blood glucose. This is a classic negative feedback example because the response restores blood glucose towards normal levels. Source

Applying homeostasis to exercise performance

· Exercise disrupts homeostasis by increasing CO₂ production, heat production, heart rate, ventilation and fuel use.
· The body responds by adjusting breathing, circulation, temperature control and blood glucose regulation.
· Effective homeostatic regulation helps delay fatigue and maintain optimal conditions for enzyme activity, muscle contraction and energy supply.
· Exam answers should link the mechanism to the performance consequence.
· Example: in hot, humid exercise, reduced evaporative cooling increases thermoregulatory strain, which can impair performance.

This graph shows atmospheric pressure decreasing as altitude increases. It helps explain why altitude is an environmental challenge that can alter physiological responses and performance. Source

Exam command-word tips

· Define homeostasis precisely: a self-regulating process maintaining a relatively stable internal environment.
· Explain negative feedback using a clear sequence: stimulus, receptor, control centre, effector and corrective response.
· Apply thermoregulation to sport by linking heat stress to sweating, vasodilation, vasoconstriction and performance.
· Compare mechanisms by stating what variable is controlled: pH, temperature, heart rate or blood glucose.
· Evaluate environmental effects by considering both the activity demands and the athlete’s training status/body composition/sex differences.