Body System Interactions
Why interactions matter
The exam tests whether you can predict how a problem in one system changes another. Immobility, for example, reduces muscle protein synthesis, lowers venous return, decreases tidal volume, promotes insulin resistance, and increases pressure-injury risk. Exercise is a shared stress test: the pulmonary system limits oxygen uptake, the cardiovascular system delivers it, and muscle extracts it; failure at any link shows up as the same symptom - dyspnea, fatigue, or reduced work capacity.
Core concepts to name
- Oxygen transport chain: ventilation → diffusion → cardiac output → capillary delivery → mitochondrial use.
- Neurohormonal coupling: sympathetic activation raises HR, BP, and respiratory rate; the renin-angiotensin-aldosterone system retains sodium and water.
- Metabolic-endocrine links: cortisol and catecholamines raise glucose; diabetes impairs wound healing and increases infection risk.
- Renal and hepatic clearance determine drug half-life and bleeding risk.
- Integumentary and immune systems mediate pressure injury and infection.
Traps
Questions often give a single-system diagnosis and ask for the most likely systemic finding; candidates pick the local finding. Watch medications: beta-blockers blunt heart-rate response, so use RPE or symptom scales. Fluid overload from heart failure can look like pneumonia (crackles, dyspnea); anemia can mimic deconditioning.
Worked rule
If a patient with COPD and heart failure desaturates with ambulation, both ventilation and cardiac output are candidates. Check respiratory rate, breath sounds, HR response, and Borg rating together; a rise in HR with stable SpO2 points to cardiac limitation, while falling SpO2 with rising respiratory rate points to pulmonary limitation. Treat the limiting system, not the symptom alone.