Editing Anatomy and Physiology – RN Flashcards (section)

== Cellular and Fluid Concepts ==

;What does the sodium-potassium pump do, and why does it matter clinically?
:It uses ATP to move three sodium ions out of the cell for every two potassium ions in, maintaining the resting membrane potential. Every nerve impulse, muscle contraction, and cardiac action potential depends on it, so ATP depletion in shock or hypoxia leads to cellular swelling and loss of excitable tissue function.

;Define preload, afterload, and contractility.
:Preload is the volume stretching the ventricle at end diastole. Afterload is the resistance the ventricle must overcome to eject. Contractility is the intrinsic force of contraction independent of preload and afterload. Stroke volume is determined by all three.

;State the Frank-Starling law and its clinical limit.
:Within physiologic limits, greater ventricular stretch produces a more forceful contraction. Beyond that point, in a failing or overstretched ventricle, further volume decreases stroke volume, which is why fluid loading worsens decompensated heart failure.

;Explain why hypotonic fluids are contraindicated in traumatic brain injury.
:Hypotonic solutions such as 0.45% sodium chloride lower extracellular osmolality, so water shifts into cells, including brain cells. In a fixed cranial vault this increases cerebral edema and intracranial pressure.

;Why can a patient with a low serum albumin be visibly edematous and still be intravascularly volume depleted?
:Starling forces. Capillary hydrostatic pressure pushes fluid out; plasma oncotic pressure, mostly from albumin, pulls it back. Low albumin lowers oncotic pressure, so fluid stays interstitially and the patient can be edematous while intravascularly depleted.