The ventricles pump blood out of the heart. These are the two lower chambers of the heart, and they do the heavy lifting of circulation. The left ventricle pushes oxygen-rich blood out to the entire body, while the right ventricle sends oxygen-depleted blood to the lungs. Together, they pump about 5 to 6 liters of blood per minute when you’re at rest.
How the Ventricles Work
The ventricles are thick-walled chambers made of specialized muscle called myocardium. When this muscle contracts, it squeezes the chamber smaller and forces blood out through large arteries. The left ventricle ejects blood into the aorta (the body’s largest artery), and the right ventricle ejects blood into the pulmonary artery leading to the lungs.
Each heartbeat pushes out a surprisingly specific amount of blood. In men, the left ventricle ejects roughly 75 milliliters per beat. In women, it’s about 66 milliliters. This is called stroke volume. Multiply that by your heart rate, and you get total cardiac output. At a resting heart rate of 70 beats per minute, the math lands right around that 5-liter-per-minute range. During intense exercise, cardiac output can increase several times over through a combination of faster heart rate and larger stroke volume per beat.
The left ventricle doesn’t shrink evenly when it contracts. It primarily narrows side to side rather than shortening top to bottom. When the heart needs to pump harder, it uses three strategies: squeezing more completely, filling with more blood between beats, and getting an extra push of blood from the atrium (the upper chamber) just before contraction begins.
Why the Left Ventricle Is So Much Thicker
The left and right ventricles don’t do equal work. The left ventricle has to generate enough pressure to push blood through every artery, capillary, and vein in your body, from the top of your head to the tips of your toes. The right ventricle only needs to push blood the short distance to your lungs. This difference in workload is reflected in their walls: the left ventricle is 8 to 12 millimeters thick, while the right ventricle is only 3 to 5 millimeters. That’s roughly a 3-to-1 ratio, directly matching the difference in resistance between the two circulation loops.
The Electrical Signal That Triggers Each Pump
The ventricles don’t decide on their own when to contract. A small cluster of cells in the upper right chamber of the heart, called the sinus node, generates an electrical impulse that starts every heartbeat. That impulse travels to a relay point between the upper and lower chambers, where it pauses for a fraction of a second. This brief delay is important because it gives the upper chambers time to finish pushing blood down into the ventricles before the ventricles start to squeeze.
From the relay point, the signal races down a specialized pathway that splits into two branches, one for each ventricle. These branches fan out into a network of fibers that spread the electrical signal across the entire ventricular muscle almost simultaneously. This coordinated activation is what allows the ventricles to contract as a unified pump rather than rippling unevenly.
What Happens Inside the Muscle Cells
At the cellular level, each electrical impulse triggers a chain reaction involving calcium. When the signal reaches a heart muscle cell, it opens tiny channels in the cell membrane that let a small amount of calcium flow in. That small influx triggers a much larger release of calcium from storage inside the cell, a process sometimes described as a “calcium spark” setting off a “calcium explosion.” The calcium then binds to proteins within the cell’s contractile machinery, causing microscopic filaments to slide past each other. This sliding shortens the cell, and when billions of cells shorten together, the ventricle contracts and blood is ejected.
For the heart to relax and refill, the calcium has to be cleared away. Pumps on the cell membrane and inside the cell actively pull calcium back into storage or push it out of the cell entirely. This cycle of calcium release and removal happens with every single heartbeat, tens of millions of times per year.
Valves That Direct the Flow
Pumping blood out would be pointless if blood could simply flow backward. The heart solves this with four one-way valves. Two of them sit at the exits of the ventricles: the aortic valve (left ventricle exit) and the pulmonary valve (right ventricle exit). These valves have cup-shaped flaps that swing open when the ventricle contracts and snap shut the moment blood tries to flow back.
Two more valves sit between the upper and lower chambers: the mitral valve on the left and the tricuspid valve on the right. These prevent blood from being pushed back up into the upper chambers when the ventricles squeeze. Keeping these valves sealed during the high-pressure phase of contraction requires internal support structures. Small muscles inside the ventricles, called papillary muscles, connect to the valve flaps through thin, cord-like tethers. These cords pull the valve flaps taut during contraction, preventing them from bowing upward under pressure. Some of these cords anchor the edges of the flaps, while others attach to the underside of the flaps to distribute tension more evenly.
The Full Pumping Cycle
A complete pump cycle has two phases. During the contraction phase (systole), the ventricles squeeze, the exit valves open, and blood is ejected into the arteries. During the relaxation phase (diastole), the ventricles expand, the exit valves close, the inlet valves open, and the ventricles refill with blood from the upper chambers. This entire cycle takes less than a second at a normal resting heart rate.
Both ventricles contract at the same time and eject the same volume of blood per beat. If they didn’t, blood would pool on one side of the circulation. The right ventricle sends the same volume to the lungs that the left ventricle sends to the body, keeping the two loops of circulation perfectly balanced.