The heart's job sounds simple — squeeze, and push blood forward. But a muscle that just squeezes has no inherent sense of direction; squeezing a closed chamber of fluid, on its own, could just as easily push blood backward as forward. What actually makes the heart function as a directional pump, rather than just an oscillating bag of fluid, is a set of four one-way valves, positioned at exactly the points where backward flow would otherwise be possible, mechanically enforcing a single direction of travel through every heartbeat.
Four chambers, four valves, one direction
The heart has four chambers: two atria, which receive incoming blood, and two ventricles, which pump blood back out. Between each atrium and its corresponding ventricle sits an atrioventricular valve (the tricuspid valve on the right side, the mitral valve on the left), which opens to let blood flow from atrium to ventricle and then closes to prevent it flowing back the wrong way once the ventricle contracts. At the exit of each ventricle sits a second valve (the pulmonary valve and the aortic valve), which opens to let blood leave the heart during contraction and closes immediately afterward to prevent blood that's just been pumped out from sliding back in as the ventricle relaxes. Four valves, each doing the same basic job at a different point in the circuit: open when flow should go forward, snap shut the instant pressure would otherwise push it back.
Passive valves, driven entirely by pressure difference
Crucially, these heart valves aren't actively controlled by nerve signals telling them individually when to open or close — they're passive structures that respond purely to the pressure difference on either side of them. When pressure on one side exceeds pressure on the other in the "forward" direction, the valve flaps push open; the instant pressure reverses, even slightly, the flaps snap shut, sealed by their own shape and by small supporting structures (in the atrioventricular valves, tendinous cords anchoring the flaps to the ventricle wall) that prevent them from being pushed too far and inverting. This purely mechanical, pressure-driven behaviour is precisely why a failing or leaking valve doesn't require any nerve damage to malfunction — physical damage to the valve tissue itself, allowing some backward leakage (regurgitation) even while the heart's underlying electrical and muscular function remains completely normal, is enough to compromise the pump's forward-only design.
What we're still unsure about
The structure and passive, pressure-driven mechanics of the heart's four valves are thoroughly documented, foundational cardiovascular anatomy, confirmed through both direct observation and centuries of clinical experience. What remains a more genuinely active area of medical research is refining how best to detect and treat valve dysfunction before it becomes severe — subtle valve leakage or narrowing can develop gradually and remain largely asymptomatic for years, and improving non-invasive methods for catching and monitoring these changes earlier, along with less invasive surgical and catheter-based repair techniques, remains an active area of cardiovascular medicine and engineering.
This sits inside Cardiovascular Anatomy, one of seven topics in Anatomy, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.