Anatomy and Physiology: homeostasis, compartments, and integrated control
Anatomy and Physiology in the PLE tests how structure supports function and how the body defends steady states. Expect questions on fluid compartments, excitable tissue, cardiovascular and respiratory mechanics, renal handling, and autonomic control; the exam rewards mechanism over memorized labels.
Core rules
- Homeostasis is maintained by negative feedback; positive feedback is normally self-limiting (clotting, labor) or pathologic (hemorrhagic shock).
- Compartments obey osmotic and Starling forces: total body water is about 60% of lean weight; ICF is two-thirds of TBW, ECF one-third; plasma is about one-quarter of ECF.
- Flow depends on gradients and resistance: Q = ΔP/R; airway resistance falls with lung volume, while arteriolar tone dominates systemic vascular resistance.
- Structure predicts function: the thick ascending limb is water-impermeable and drives countercurrent multiplication; alveolar type II cells produce surfactant.
Worked example
A 70-kg adult receives 1 L of isotonic saline. Estimate TBW, ICF, ECF, and plasma distribution. TBW = 0.60 × 70 = 42 L; ICF = 2/3 × 42 = 28 L; ECF = 14 L; plasma = 1/4 × 14 = 3.5 L. Isotonic saline stays in ECF, so ICF is unchanged. The 1-L ECF load distributes roughly 3:1 between interstitium and plasma, adding about 0.75 L interstitial and 0.25 L plasma before renal excretion. Reasoning: tonicity determines water shifts; volume and capillary permeability determine the plasma/interstitial split.
Traps
Do not equate osmolarity with tonicity: urea crosses membranes and is an ineffective osmole. Do not reverse afferent/efferent arteriolar effects on glomerular filtration. Do not assume all IV fluid distributes through TBW; isotonic crystalloid expands ECF first.