Basic Physiology

The trick to remembering this is to line the questions up in order, so that every mode of transport files itself into rank automatically. Question one: is a membrane protein required? → If not, it is simple diffusion → the substance crosses the lipid bilayer freely down its gradient, as oxygen, carbon dioxide, and lipid-soluble molecules do; this eliminates simple diffusion first. Question two: down or against the gradient? → Down means facilitated diffusion → the carrier merely lowers the activation energy and accelerates the crossing → no energy consumed, saturable, and specific (because the number of carriers is finite, the rate can climb no further once the concentration is high enough, and this is precisely what distinguishes it from simple diffusion). Question three: how does transport against the gradient pay the bill? → Two ways of paying: primary hydrolyzes ATP itself, secondary borrows an existing ion gradient (usually Na⁺). One sentence gathers it all: facilitated diffusion is rowing with the current and costs nothing; active transport is rowing against the current — primary pays out of its own pocket, secondary buys on credit.

Why does almost every secondary active transporter stand on the shoulders of the Na⁺-K⁺ ATPase (Na⁺-K⁺ adenosine triphosphatase)? Follow the sequence and it becomes clear. The Na⁺-K⁺ ATPase hydrolyzes ATP first → drives Na⁺ out of the cell and pulls K⁺ in → establishes a steep extracellular high-Na⁺ gradient → this gradient is a "savings deposit of potential energy" → the moment SGLT opens, Na⁺ rushes down the steep gradient into the cell → and drags glucose in along with it, so that glucose keeps pace even though it is moving against its own gradient. Hence, although SGLT itself does not consume ATP directly, the one truly paying behind the scenes is still the Na⁺-K⁺ ATPase. Its stoichiometry is also a high-frequency exam point: for every 1 ATP hydrolyzed, 3 Na⁺ are pumped out and 2 K⁺ pumped in, and this 3:2 ratio exports one net positive charge → making the pump electrogenic → contributing a modest −2 to −4 mV of its own to the resting membrane potential; ouabain and digoxin (digitalis) are its inhibitors, and the rise in extracellular K⁺ seen in toxicity stems from precisely this.

The membrane potential is the weighted average of each ion's equilibrium potential, weighted by its membrane permeability (this is the Goldman-Hodgkin-Katz equation). Follow this thread to the resting membrane potential: at rest the membrane holds a great many K⁺ leak channels open → permeability to K⁺ is highest → the weighted average is pulled under K⁺'s dominance → the resting membrane potential lies closest to E_K (≈ −90 mV). Other commonly cited equilibrium potentials: E_Na ≈ +60 mV, E_Cl ≈ −70 mV, and E_Ca strongly positive. The direction of the action potential is decided by the same principle — whichever channel opens, Vm is pulled toward that ion's equilibrium potential: open Na⁺ channels → Vm heads toward +60 → depolarization, the rising phase; open K⁺ channels → Vm heads toward −90 → hyperpolarization, repolarization. The Nernst equation E = (61/z) × log([out]/[in]) computes the equilibrium potential of a single ion, but questions usually do not ask you to calculate the number — they ask you to judge the direction.

The detail most often reversed on exams is that "osmolarity ≠ tonicity," and the difference between them lies in one causal thread — "whether the water can be held in place." Tonicity counts only the effective osmoles that "cannot cross the membrane" → these substances remain on one side of the membrane and hold the water fast → only then can a sustained water flow arise. Permeant solutes (such as urea, or glucose that will eventually be metabolized) count toward osmolarity yet generate no sustained water flow, because they either follow the water or are metabolized away. The classic example is 5% dextrose: initially iso-osmotic (≈278 mOsm/L) → but once the glucose is taken up and metabolized by cells → what remains is equivalent to an infusion of pure water → the final effect is hypotonic, and cells swell. Getting this direction backward is suicide on a licensing-exam giveaway question.

Body temperature is governed by the hypothalamus, which sets a "target temperature (set point)," compares it with the actual temperature, and then activates heat production or heat loss to return the body to that set point. Follow the two sensing zones and the picture becomes clear: the anterior hypothalamus (preoptic area, POA) houses warm-sensitive neurons → chiefly senses heat → activates cutaneous vasodilation, sweating, and behavioral heat loss → governs heat dissipation, so if the anterior region is destroyed the person cannot shed heat and drifts toward hyperthermia. The posterior hypothalamus chiefly senses cold → activates cutaneous vasoconstriction, shivering, heightened thyroid and sympathetic activity, piloerection and the donning of clothing → governs heat production and conservation, so if the posterior region is destroyed the person cannot generate heat and drifts toward hypothermia. One in front, one behind — and their failures point in opposite directions. Every body-temperature question becomes clear as soon as you return to this set-point logic.

Why can fever be treated with antipyretics while heat stroke cannot? Follow the two mechanistic chains and the answer becomes clear. Fever: infection or inflammation → cells release the endogenous pyrogens IL-1, IL-6, and TNF → these act on the vascular endothelium of the hypothalamus → COX is switched on → PGE₂↑ → the set point is raised → the body believes it ought to be warmer, and therefore shivers to generate heat → antipyretics (NSAID, acetaminophen) inhibit COX and suppress PGE₂ → the set point returns to normal → the shivering stops. Heat stroke: the set point was normal from the outset → the problem is simply environmental overheating coupled with a collapse of the heat-dissipating machinery → internal heat cannot be expelled → antipyretics have nothing to act upon, and direct physical cooling is mandatory, external or internal, to draw the heat away. This is why the elderly man at 42 °C cannot be rescued with Panadol — his set point was normal to begin with, the drug has nothing to act upon, and direct physical cooling is required; the child's fever, by contrast, originates in cytokines that have raised the set point through PGE₂, so an antipyretic can push it back down and the chills will cease.

Muscle contraction requires ATP, but the ATP reserve is limited, so the body always spends the fastest source first and only then switches to the largest. Follow the three stages: the first 0–10 seconds → the ATP already stored in muscle is consumed almost immediately → creatine phosphate (PCr) is mobilized → creatine kinase (CK) catalyzes PCr + ADP → creatine + ATP → ATP is replenished in an instant, with no oxygen required → this is the "instant battery." From roughly 10 seconds to 2 minutes → PCr too is nearly spent → anaerobic glycolysis takes over → glucose to pyruvate and onward to lactate → rapid, but it piles up a lactate debt → suited to short-to-medium bouts of high intensity. Beyond 2 minutes → the heart and lungs catch up and the oxygen supply steadies → only now is aerobic metabolism (oxidative phosphorylation) called upon → the largest yield and the most enduring.

The entire causal chain of pernicious anemia is remarkably clean — follow it step by step: autoimmune destruction of gastric parietal cells (or production of anti-intrinsic factor antibodies) → deficiency of intrinsic factor (IF) → vitamin B12 (cobalamin) loses its escort → in the terminal ileum it can neither bind IF nor be absorbed → serum B12↓ → two downstream pathways fail simultaneously: (a) DNA synthesis is impaired → in red-cell precursors the nucleus matures more slowly than the cytoplasm → megaloblastic anemia (MCV >100 fL); (b) myelin synthesis is impaired → subacute combined degeneration (SCD) → the posterior columns (impaired proprioception and vibration sense) together with the lateral corticospinal tracts (spasticity, hyperreflexia) are damaged at the same time → hence the word "combined." Folate deficiency travels only pathway (a) and never (b), so it produces the same megaloblastic anemia yet no neurological symptoms — and that is precisely the cleanest point of distinction between the two.