Human anatomy, physiology and basic pathophysiology
Scope
Questions test structure–function links, homeostasis, and the pathophysiology behind common signs. You should predict what happens when a feedback loop fails and calculate basic physiological variables.
Core rules
- Homeostasis depends on negative feedback: receptors detect deviation, a control centre compares with a set point, and effectors restore it. Positive feedback is rare and self-amplifying, as in oxytocin-driven labour and clotting.
- Cardiac output = heart rate × stroke volume, about 5 L/min at rest. Mean arterial pressure ≈ diastolic pressure + one-third of pulse pressure, and is determined by cardiac output × systemic vascular resistance.
- Gas exchange depends on ventilation–perfusion matching. Dead space is ventilated but not perfused; shunt is perfused but not ventilated.
- The nephron regulates volume and electrolytes. Renin–angiotensin–aldosterone conserves sodium and water while wasting potassium.
- Acid–base balance follows Henderson–Hasselbalch: pH is set by the bicarbonate-to-CO2 ratio. Vomiting loses H+ and chloride, causing metabolic alkalosis with hypokalaemia; diarrhoea causes metabolic acidosis.
- Inflammation is a protective vascular and cellular response. Atherosclerosis is chronic endothelial injury with lipid deposition, foam cells and plaque that can rupture and thrombose.
Worked example
A 70 kg man has a heart rate of 80/min, stroke volume of 70 mL and mean arterial pressure of 93 mmHg. Cardiac output is 80 × 70 mL = 5,600 mL/min, or 5.6 L/min. Systemic vascular resistance is MAP divided by cardiac output: 93/5.6 ≈ 16.6 mmHg·min/L, which converts to about 1,330 dyn·s·cm−5 when multiplied by 80. The reasoning: pressure is flow times resistance, so a fall in pressure with unchanged output implies falling resistance (for example sepsis), while a rise in pressure implies vasoconstriction or raised output.
Traps
Do not mix sympathetic and parasympathetic effects; do not confuse hydrostatic and oncotic forces when explaining oedema; do not label all cell death as necrosis (apoptosis is programmed and non-inflammatory); do not assume a normal blood pressure excludes shock.