Integrated Physiology & Exercise IN

Exercise raises ATP demand, increasing muscle blood flow, ventilation, cardiac output and substrate mobilization. Responses depend on intensity, duration, training and environment.

Trace demand → local muscle changes → cardiovascular/respiratory responses → fuel and heat handling.

Exercise intolerance may reveal cardiac, pulmonary, haematological, metabolic or neuromuscular limitation.

A single system rarely explains normal exercise adaptation; integrated reserve determines performance.

Core temperature reflects balance between heat production and heat loss. Hypothalamic control recruits skin blood-flow changes, sweating, shivering and behavioural responses.

Identify heat load, sensors/integrator and available effectors.

Heat illness and hypothermia occur when environmental or physiological demands exceed regulatory capacity.

Fever and hyperthermia are not identical: fever involves regulated set-point change, whereas hyperthermia reflects excessive heat accumulation without that same reset.

Standing shifts blood toward dependent veins, reducing central filling unless venous valves, muscle pump and autonomic reflexes compensate. Baroreflex responses support arterial pressure and cerebral perfusion.

Follow posture change → venous return → stroke volume/pressure → baroreceptor firing → autonomic response.

Orthostatic symptoms can reflect volume depletion, autonomic failure, medication effects or impaired cardiovascular compensation.

A fall in venous return can reduce stroke volume before compensatory tachycardia restores pressure.

Haemorrhage, dehydration, hypoxia and other stresses recruit overlapping neural, cardiovascular, respiratory, renal and endocrine mechanisms. Compensation preserves vital-organ perfusion but may fail when stress is severe or prolonged.

Build a timeline: immediate neural response → hormonal/renal support → longer adaptation or decompensation.

Shock physiology illustrates the difference between compensation and restoration of the underlying cause.

Compensatory tachycardia or vasoconstriction can temporarily support pressure without correcting blood loss.

Dynamic exercise rapidly increases ATP consumption in active skeletal muscle. Local metabolites dilate resistance vessels in working muscle, while sympathetic and cardiovascular adjustments redistribute flow and raise cardiac output. Heart rate and stroke volume increase according to intensity and training state, and ventilation rises to support gas exchange and acid-base demands.

Fuel selection changes with intensity, duration, nutritional state and training. Heat production also rises, requiring increased skin blood flow and sweating when environmental conditions permit. Exercise is therefore a whole-body integration problem involving muscle, circulation, respiration, endocrine regulation, temperature control and fluid balance.

Clinical reasoning: exercise intolerance can result from limited cardiac output, impaired ventilation/gas exchange, anaemia, neuromuscular disease, metabolic limitation or deconditioning.

Teach it back: trace the physiological consequences of suddenly increasing skeletal-muscle ATP demand during running.

Core temperature depends on the balance between metabolic heat production and heat exchange with the environment. Heat can be transferred by radiation, conduction, convection and evaporation. The hypothalamus integrates thermal information and recruits autonomic, endocrine, somatic and behavioural responses.

When heat load rises, skin vasodilation increases heat transfer to the surface and sweating permits evaporative cooling. In cold conditions, vasoconstriction conserves heat and shivering increases heat production. Evaporation becomes less effective when ambient humidity is high.

Fever differs from hyperthermia. In fever, thermoregulatory control operates around an elevated set point; in hyperthermia, heat accumulation overwhelms regulation without the same regulated set-point shift.

Teach it back: explain why a person can sweat profusely yet cool poorly in a hot, very humid environment.

On standing, gravity increases venous pooling in dependent regions. Central venous return and ventricular filling transiently fall, reducing stroke volume and potentially arterial pressure. Reduced stretch of arterial baroreceptors alters afferent firing to cardiovascular centres, increasing sympathetic and reducing parasympathetic influence.

The response raises heart rate and contractility and constricts selected vessels, supporting venous return and arterial pressure. Skeletal-muscle pumping and venous valves further assist return during movement.

Clinical reasoning: orthostatic hypotension can occur when circulating volume is low, autonomic reflexes are impaired, medications limit compensation or venous pooling is excessive.

Teach it back: describe the sequence from standing up to restoration of arterial pressure by the baroreflex.

Acute haemorrhage illustrates integrated compensation. Loss of circulating volume reduces venous return, stroke volume and arterial pressure. Baroreflex-mediated sympathetic activity raises heart rate, contractility and vascular tone. Hormonal and renal responses then promote sodium and water conservation and support vascular function.

These mechanisms buy time; they do not replace lost red cells or definitively stop bleeding. A patient can therefore maintain arterial pressure during early compensation while tissue perfusion reserve is falling. With severe or prolonged stress, compensatory mechanisms can become insufficient and organ dysfunction can follow.

The same reasoning applies broadly: physiological compensation reduces the consequences of a disturbance but is not necessarily treatment of its cause.

Teach it back: explain why a near-normal blood pressure does not prove that circulating volume has been restored after blood loss.