Inter-system communication

· Core idea: the body uses the nervous system and endocrine system to detect internal/external conditions and coordinate physiological responses that maintain effective function during rest and exercise.
· Nervous communication = usually fast, electrical/chemical signalling through neurons; best for rapid responses such as changes in heart rate, breathing, and temperature control.
· Endocrine communication = usually slower but longer-lasting, using hormones released by glands into the blood to act on target cells in other parts of the body.
· Exam focus: know the main divisions, required examples of function, and key hormones named in the syllabus.

Nervous system: main divisions

· Nervous system = senses internal and external conditions and coordinates responses of body systems.
· Central nervous system (CNS) = brain and spinal cord; processes information and coordinates responses.
· Peripheral nervous system (PNS) = nerves outside the CNS; carries sensory input to the CNS and motor output away from it.
· Efferent division = motor output from the CNS to effectors.
· Somatic nervous system = voluntary control of skeletal muscle.
· Autonomic nervous system (ANS) = involuntary control of organs such as the heart, lungs, blood vessels, and sweat glands.
· Sympathetic nervous system = prepares the body for activity/stress; generally increases heart rate, ventilation, and energy availability.
· Parasympathetic nervous system = supports rest/recovery; generally reduces heart rate and conserves energy.

This diagram compares the two branches of the autonomic nervous system. It is useful for remembering that sympathetic activity generally prepares the body for action, while parasympathetic activity supports rest and recovery. Source

Autonomic control: required examples

· Cardiac function: heart rate is controlled by intrinsic mechanisms within the heart and extrinsic factors from outside the heart, especially autonomic input and hormones.
· Sympathetic stimulation generally increases heart rate and supports greater cardiac output during exercise or stress.
· Parasympathetic stimulation generally decreases heart rate, especially during rest and recovery.
· Breathing and ventilation: the nervous system adjusts breathing rate and depth to match body demands, especially during exercise when oxygen demand and carbon dioxide production increase.
· Temperature control: nervous system responses help coordinate sweating, skin blood flow, and muscular responses to protect body temperature during changing environmental and exercise conditions.
· High-scoring exam answers should link a stimulus → receptor/CNS processing → effector response → physiological outcome.

This image helps show how autonomic nerves alter heart rate. It supports the IB requirement to understand extrinsic control of cardiac function alongside intrinsic heart mechanisms. Source

Receptors that initiate nervous responses

· Receptors are specialized cells that detect stimuli and trigger nervous system responses.
· Proprioceptors detect body position, movement, and muscle/joint information; important for coordination and movement control.
· Baroreceptors detect changes in blood pressure, helping regulate cardiovascular responses.
· Chemoreceptors detect chemical changes, especially levels linked to blood gases and pH; important for breathing/ventilation control.
· The syllabus requires knowing what these receptors respond to, but the internal function of the receptors is not assessed.
· Exam phrase: receptors help the nervous system sense changes and initiate appropriate responses to maintain effective physiological function.

This labelled figure shows the organization of autonomic pathways. It is useful for connecting receptor input and CNS output to involuntary organ control. Source

Endocrine system: hormones as messengers

· Endocrine system = body system made of glands and hormones that regulate biological processes.
· Hormones = mediator molecules released in one part of the body that regulate activity of cells in other parts of the body.
· Hormones travel through the blood and only affect cells with the correct target receptors.
· Endocrine responses are essential for regulating blood pressure, heart rate, blood sugar, water retention, and reproductive influences on health/performance.
· In exams, always name the hormone, state its main effect, and link it to health or athletic performance when relevant.

This diagram gives a clear overview of the endocrine glands. It supports the idea that hormones are released from glands and act on distant target tissues. Source

Required hormones and key effects

· Epinephrine and norepinephrine: cause changes in blood pressure, heart rate, and blood sugar levels; important in stress/exercise responses.
· Insulin: helps regulate blood sugar concentration, especially by lowering blood glucose when levels are high.
· Glucagon: helps regulate blood sugar concentration, especially by raising blood glucose when levels are low.
· Antidiuretic hormone (ADH): regulates water retention in the kidney, helping conserve body water.
· For exam application, link insulin and glucagon to blood glucose balance and link ADH to hydration/water retention.

This diagram shows insulin and glucagon working in opposite directions to control blood glucose. It is especially useful for exam questions asking how hormones regulate internal conditions during rest or exercise. Source

Reproductive hormones and performance

· Progesterone: has a thermogenic effect, influencing thermoregulation and sleep quality; also affects fuel availability.
· Oestrogen: affects glycogen sparing and joint stiffness, which can influence exercise performance and injury considerations.
· Testosterone: affects bone formation, protein synthesis, and erythropoietin, supporting tissue development and oxygen-carrying capacity.
· High-quality answers avoid overgeneralizing: reproductive hormones can influence performance, but effects vary between individuals and contexts.
· Useful exam link: hormonal effects can influence fuel use, thermoregulation, recovery, musculoskeletal function, and health.

Nervous vs endocrine communication

· Nervous system: rapid signalling; coordinates immediate changes such as heart rate, ventilation, and temperature responses.
· Endocrine system: hormone-based signalling; regulates broader processes such as blood glucose, water retention, blood pressure, and reproductive influences on performance.
· Both systems work together to maintain optimal internal conditions during changing demands.
· Example: during exercise, the nervous system can rapidly increase heart rate and ventilation, while hormones help regulate blood glucose, blood pressure, and fluid balance.
· Best exam structure: identify the system, name the specific division/hormone/receptor, and explain the effect on performance or homeostasis.

Exam traps to avoid

· Do not confuse somatic control of skeletal muscle with autonomic control of involuntary organs.
· Do not write that hormones act only where they are released; hormones can regulate cells in other parts of the body.
· Do not include detailed receptor mechanisms; the syllabus says the internal function of receptors is not assessed.
· Do not add an HL only section for this subtopic: Additional higher level: None for A.1.1.
· Do not discuss unrelated homeostasis content in depth unless the question specifically links to another topic.