William Harvey and the Circulation of Blood
For over a thousand years, Western medicine accepted the account of blood movement described by the second-century physician Galen: that blood was continuously produced by the liver, consumed by the body’s tissues, and did not circulate in any closed loop. English physician William Harvey overturned that consensus in 1628 with a work titled De Motu Cordis, built on a deceptively simple calculation: Harvey estimated the volume of blood the heart pumped with each beat, multiplied it by an average heart rate, and found that the amount of blood moving through the heart in an hour vastly exceeded the total blood volume the body could plausibly produce or consume in that time. The only explanation that made sense was that the same blood was circulating in a closed system, pumped continuously by the heart — a genuinely radical claim at the time, since it directly contradicted centuries of accepted medical authority, and it took decades for Harvey’s model to gain full acceptance.
Harvey’s model described the pump and the loop, but he lacked the microscope resolution to see how blood actually passed from arteries to veins at the tissue level — that missing piece, the capillary, wasn’t observed until 1661, four years after Harvey’s death, when Italian physician Marcello Malpighi examined frog lung tissue under a microscope and saw the tiny vessels connecting arterial and venous circulation. MCAT questions on the cardiovascular system build directly on both discoveries: pulmonary circulation carries deoxygenated blood from the heart’s right side to the lungs and back oxygenated, while systemic circulation carries that oxygenated blood from the heart’s left side out to the rest of the body’s tissues and back — two closed loops within the single circulatory system Harvey first correctly described.
Gas Exchange, Filtration, and the Physiology the MCAT Tests Beyond Memorization
Respiratory physiology on the MCAT centers on gas exchange occurring across the extremely thin walls of the alveoli, the tiny air sacs where oxygen diffuses into the blood and carbon dioxide diffuses out, driven entirely by differences in partial pressure between the air in the alveoli and the blood in the surrounding capillaries — a passive process requiring no active transport, governed by the same basic diffusion principles the exam tests in a chemistry context. The renal system performs an analogous filtration task for the blood itself: the nephron, the kidney’s functional filtering unit, first identified in detail by Italian anatomist Marcello Malpighi in the same era as his capillary work, filters blood plasma and then selectively reabsorbs water, ions, and nutrients while excreting waste products as urine.
What distinguishes MCAT-depth anatomy and physiology questions from a simpler labeling exercise is the expectation that test-takers understand these systems as interconnected feedback loops rather than isolated diagrams — how a drop in blood oxygen affects respiratory rate through chemoreceptors, how kidney filtration rate responds to blood pressure changes, how the cardiovascular and respiratory systems adjust together during exertion. Harvey’s original insight, that circulation is a single continuous system rather than a series of disconnected events, remains the organizing principle behind nearly every physiology passage the exam includes.
Source: National Institutes of Health (NIH) and Gray’s Anatomy.