People have long been fascinated with hearts—how they work, how they change and how they break. The Grinch’s heart grew three sizes. Sgt. Pepper had a band for lonely hearts. And the Doctor on the British science-fiction television series Doctor Who has two hearts, beating in double time.
Hearts serve as a powerful metaphor in Doctor Who, which follows a time-traveler called the Doctor, who belongs to an alien species known as the Time Lords. It’s interesting that someone largely unconstrained by time has not one but two organs dependent upon it. Our “tickers” function as metronomes of life, marking out time inside our chests. This might be why the Doctor needs two of them: their timeline doesn’t march to a single beat.
As an aspiring medical doctor and avid Doctor Who fan, I found myself wondering about the anatomy and physiology of the Time Lord cardiovascular system. How are the two hearts connected, and how are the heartbeats regulated? How does the Doctor survive centuries without developing age-related heart disease? How could a dual cardiac system have evolved? I had to find out. Unfortunately, Time Lords are difficult to study because of their small population and tendency to show up for appointments in the wrong century. So to answer these questions, I analyzed data on cardiac incidents from 13 seasons of Doctor Who (2005 to 2023), pored over the cardiovascular literature on humans and other species, and consulted various experts in these and related fields. Through my extensive studies, I have developed what I think are plausible answers to my questions about the Time Lord’s two hearts.
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Details of the Doctor’s dual cardiac system emerge piecemeal throughout the series. We first learn that Time Lords have two hearts in the season one episode entitled “Dalek,” when the Doctor is body scanned by a collector of alien artifacts. Later, a medical examination of the Doctor by Dr. Martha Jones in season three reveals a dual heartbeat. These offset heartbeats mark four beats per cardiac cycle: “lub-dub-lub-dub (pause) lub-dub-lub-dub,” contrasting with the human “lub-dub (pause) lub-dub.” This rhythm suggests that the two hearts’ contractions are regulated together. The general layout of Time Lord hearts, shown in the episodes “Dalek” and season seven’s “The Power of Three,” indicates one heart on each side of the chest.
The Doctor’s symptoms when one of their hearts fails provide further insight into the layout of their cardiovascular system. Evidently, one heart is enough to temporarily sustain a Time Lord but not without significant pain and difficulty moving—most likely from lack of oxygen reaching their tissues.
By watching the Doctor’s hearts fail, we can understand how they function. In “The Power of Three,” one of the Doctor’s hearts needed an electric shock to the chest. But in “The Shakespeare Code” (season three), it was restarted by striking the sternum with the side of the fist. In consulting my mentor Janak Chandrasoma, an anesthesiologist at the University of Southern California, I learned that these techniques are well-established medical treatments for human ventricular fibrillation and ventricular tachycardia—both dangerous heart rhythms. In “The Stolen Earth” (season four) an even more severe heart problem necessitated the use of extrahuman abilities to repair the damage, probably because of asystole, or a “flatline”—a nonshockable rhythm.
In humans, electric shocks can also stop hearts, depending on an individual’s health and the shock’s location and timing. The Doctor has frequent run-ins with their archenemies the Daleks, who wield a sort of gun that fires death rays rather than bullets. Though we do not yet know the exact mechanism of the Dalek death ray, it presumably delivers a sort of shock. Like humans, the Doctor appears to have varied physiological responses to cardiac stressors, as demonstrated by their inconsistent responses to Dalek attacks. In “The Stolen Earth,” a death ray stops one of the Doctor’s hearts, but in “The Big Bang” (season five) the weapon appears to stop both of their hearts. As in humans, different cardiac injuries in different circumstances lead to different outcomes.
In all three of these cardiovascular crises, the heart with the problem has been on the left. These findings suggest that their cardiovascular system may be asymmetrical—that is to say, individual nonessential organs such as the stomach and intestines don’t necessarily get an equal share of blood from each heart. But although all of our data are based on incidents with the Doctor’s left heart—we know nothing of right heart problems—this pattern may be a coincidence. Other evidence suggests Time Lord circulatory systems aren’t necessarily asymmetrical. When the Doctor experiences left-sided heart collapse, their symptoms other than chest pain aren’t localized to one side. Their mobility loss and ischemic pain—that is, pain that comes from a lack of oxygen reaching the tissues—is systemic, suggesting that each heart is involved in sending blood to both sides of the body.
The show does not specify whether the Doctor’s blood runs in series, flowing through one heart after the other, or in parallel, with two streams flowing side by side. We also don’t know whether the hearts fail when open or closed. But we can make some educated guesses based on the anatomy and functioning of human hearts.
The job of the heart in any animal is to pump blood throughout the body to supply tissues with oxygen and nutrients. To do that, it must create enough pressure to push the blood through the body’s network of arteries, veins and capillaries. The heart generates this pressure by forcefully contracting to squeeze out blood and overcome the resistance of the blood vessels. When two hearts are in parallel, increasing one heart’s resistance can only increase the total resistance by so much. The opposite is true for hearts in series: if either heart has infinite resistance, the resistance of the whole system goes to infinity. Thus, if a heart in series fails while closed, then all blood flow stops. Because we know the Doctor can partially function with one operational heart, we can rule out the possibility that their hearts are arranged in series and fail while closed.
Time Lords have two hearts in the television series Doctor Who. The show does not specify whether the hearts are arranged in parallel or in series, but the available evidence indicates they are in parallel. In this image of the two possible configurations, the resistors represent the internal resistance of the hearts and the total peripheral vascular resistance.
When the Doctor had a one-sided heart crisis in “The Power of Three,” they experienced symptoms of very high blood pressure, including chest pain and shortness of breath. Albert Herrera, a professor of biological sciences at the University of Southern California, helped me understand these symptoms mathematically. Ohm’s law states that the voltage (analogous to blood pressure) is equal to the current (flow rate) times the resistance: V = IR. When a heart in parallel fails closed, the systemic resistance increases. All the blood is shunted through the healthy heart as it attempts to maintain its typical heartbeat. Excessive filling of the heart with blood in individuals with a single heart causes problems in the long term, but in the short term, it helps keep the flow rate approximately stable. If the flow rate is roughly constant and resistance increases, then blood pressure increases—explaining the Doctor’s symptoms. So we can conclude from this observation that the Doctor’s two hearts are arranged in parallel.
To determine whether the Doctor’s hearts fail while open or closed, we have to take a closer look at blood flow. If the hearts were to fail while open, then in accordance with Ohm’s law, there would be a drop in blood pressure proportional to the (low but nonzero) resistance of the hearts. The failed heart would still allow for blood flow, but without a heartbeat, it would be unable to contribute to systemic blood pressure. The resistance of both the failed heart and the system as a whole would remain unchanged, but the systemic flow rate would decrease (only one heart beating instead of two), so the total blood pressure would decrease.
Very low blood pressure, or hypotension, is not immediately fatal—but it can cause dizziness and fainting, not symptoms the Doctor experiences during acute single-heart collapse in “The Power of Three.” Because the Doctor does not exhibit symptoms of hypotension after single-heart collapse, it is reasonable to presume that Time Lord hearts do not fail when open. The only remaining option is that the Doctor’s hearts are arranged in parallel and fail while closed.
At this point you might be wondering, as I have, about the construction of the hearts themselves. Human hearts have four muscular chambers; two pump blood to the lungs to pick up oxygen, and two pump that blood through the rest of the body to deliver oxygen and nutrients. Some animals have different heart configurations: fish hearts have two chambers, for instance; amphibian hearts have three. But assuming the Doctor has two hearts because Time Lord bodies have significant circulatory demands, it follows that each heart has evolved to maximize efficiency. So it is unlikely that the hearts have two or three chambers because these configurations allow for the mixing of oxygenated and deoxygenated blood, reducing efficiency.
Instead I theorize that Time Lord hearts have four chambers. Each heart, operating in parallel, dedicates two chambers to a pulmonary circuit, sending blood through the lungs, and two chambers to a systemic circuit, sending blood through the rest of the body. Both hearts send blood into the lungs via a merged pulmonary artery, where deoxygenated blood from the two systems mix. We know that both hearts supply the vital organs with blood because the Doctor survived several hours between single heart collapse and defibrillation in “The Power of Three.” The Doctor’s symmetrical dysfunction suggests that each heart pumps blood to both sides of the body.
The author proposes that in the Time Lord cardiovascular system, each heart sends blood to the vital organs as well as a subset of the less critical organs.
The rhythm of the four consecutive Time Lord heartbeats has been woven into prophecies, dreams and even songs throughout the TV series. How do those two hearts keep time? Human heart rates are regulated by a small mass of specialized tissue inside the heart known as the sinoatrial, or “pacemaker,” node. The sinoatrial node normally generates a regular electrical stimulus 60 to 100 times a minute. In humans this node can operate without input from the brain, although signals from the brain can tell the heart to speed up or slow down. If this pacemaker fails, a different mass of tissue in the heart, the atrioventricular node, takes over.
I hypothesize that Time Lords, like humans, have a pacemaker node that is primarily responsible for determining heart rate. But the Time Lord pacemaker regulates both hearts: if one heart fails, the other has to keep working. The Time Lord pacemaker must therefore be located outside of the hearts. I surmise that it is located in the brain. This sino-cerebral node, as I call it, coordinates both hearts to ensure that they maintain an appropriate offset heartbeat. Without such coordination, the hearts could easily fall out of step and start pumping simultaneously, a less efficient and potentially dangerous condition.
Much as the human heart has the atrioventricular node to back up the pacemaker if it fails, Time Lord hearts surely also have backups: the sinoatrial and atrioventricular nodes can both stand in for the sino-cerebral node if it falters. If a problem occurs within one heart, the dysfunction remains isolated. If a problem occurs within the brain, then each heart regulates its rate independently, creating an uncoordinated rhythm. We have yet to see the latter type of cardiac crisis on-screen; to date, every case of documented Time Lord heart problems has originated within one of the hearts.
Incidentally, there are a small number of humans with two hearts. Some people who undergo heart transplants have their diseased heart directly connected to a donor heart, so that the two hearts are attached in parallel. Unlike Time Lord hearts, though, the hearts in recipients of these so-called piggyback transplants are regulated separately; each beats to its own rhythm.
In humans who receive heterotopic or “piggyback” heart transplants, the diseased heart is directly connected to the donor heart, such that the two hearts are attached in parallel.
Two hearts might be better than one. But how to keep cardiovascular disease at bay? The Doctor isn’t known for making heart-healthy dietary choices; they have a documented love of foods high in saturated fat—including fish fingers and custard (season five)—which can lead to high cholesterol. The risk of cardiovascular disease increases over the years as cholesterol (specifically, atherosclerotic plaque) builds up in the arteries. Although age is complicated with time travelers, the Doctor is estimated to be thousands of years old. So how does the Doctor regulate cholesterol to protect their hearts?
Humans with high blood cholesterol levels are sometimes treated with bile acid sequestrants. These medications prevent the reabsorption of bile acids in the large intestine for reuse. Bile acids, which aid in the absorption of dietary fat, are made out of cholesterol. By minimizing bile reabsorption, the body has to make most of its bile from scratch—using much of the available cholesterol in the process. If the Doctor has an inefficient bile reabsorption system, perhaps the majority of their cholesterol gets excreted as bile before it can build up in their arteries and potentially cause a heart attack or stroke.
Not all cholesterol is created equal. HDL cholesterol is considered to be good cholesterol because it clears the harmful LDL cholesterol from peripheral tissues and brings it to the liver for degradation. Though there are currently no medications (on Earth, at least) to increase HDL cholesterol, low HDL levels can be improved with diet and exercise modifications. The Doctor certainly has an awful lot of running to do as they travel through time and space to save worlds, rescue civilizations and defeat terrible creatures. Still, regular exercise isn’t going to be enough to counteract thousands of years of plaque buildup. Maybe there’s some intrinsic biochemical adaptation in the cholesterol of Time Lords that allows them to survive centuries without developing coronary artery disease.
The Doctor’s remarkable cardiac health might result from high levels of very effective HDL cholesterol, very low levels of LDL cholesterol, or both. Coronary artery disease risk in humans has been shown to be proportional to the ratio of LDL cholesterol to apolipoprotein A1 (ApoA1), a protein found on the surface of HDL cholesterol particles. Theoretically, the Doctor can reduce their risk of heart disease by either decreasing LDL, increasing ApoA1, or both.
ApoA1 is responsible for accepting fats into HDL particles, decreasing the risk of hardened arteries (atherosclerosis). HDL particles that contain high levels of ApoA1 are extremely effective at picking up LDL cholesterol from arteries and organs, so people with high ApoA1 density have decreased risk of heart disease. Conversely, people with low ApoA1 in their HDL particles are at increased risk of premature coronary artery disease. If Time Lord HDL particles have a high density of ApoA1, they would be at lower risk of heart disease from fatty plaque buildup. Another possibility is that Time Lord HDL particles are bigger on the inside and thus able to remove more cholesterol from tissues than their human equivalents.
LDL particles deliver cholesterol around the body, including the walls of arteries, causing coronary artery disease. High LDL is associated with an increased risk of coronary artery disease, whereas lower LDL decreases risk. Humans with high LDL can be treated with statins, a type of medication that inhibits the critical enzyme involved in producing LDL cholesterol, HMG-CoA reductase. If Time Lords somehow have low HMG-CoA reductase activity, they would effectively have a built-in statin to keep their cholesterol levels in check.
Heart disease risk is largely influenced by the levels and efficacy of HDL and LDL cholesterol. I propose that high ApoA1 density and low HMG-CoA reductase activity optimize both of these metrics to allow Time Lords to live long lives with a low risk of heart disease.
Having a single heart is the norm in vertebrate creatures, so it’s hard to imagine how Time Lords evolved their dual cardiac system. But in talking with marine biologist Edie Widder of the Ocean Research and Conservation Association, I learned that there are some real-world examples of animals with multiple hearts. For example, most cephalopods, a group that includes squids and octopuses, have three of them: one main “systemic” heart and two supplementary “branchial” hearts. The three-chambered systemic heart does most of the work, whereas the one-chambered branchial hearts support higher blood flow through the gills.
Why do cephalopods have three hearts? Their mollusk ancestors, which evolved more than 500 million years ago, were probably slow-moving creatures that fed on sedentary prey, including sea sponges and algae. As cephalopods evolved to feed on mobile prey, such as fish and crustaceans, they developed higher metabolic rates and higher-capacity circulatory and respiratory systems that allowed them to move faster. Evolving multiple hearts helped support this more active way of life.
Perhaps the dual cardiac system of Time Lords evolved to meet similar energetic demands. But in that case, why not evolve one large heart instead? Redundancy. Heart disease is the global leading cause of death in humans. Cardiac function is a major limiting factor on lifespan. With two hearts, Time Lords can survive cardiac injuries that would otherwise prove fatal.
Nature asks, “What if?” incrementally through evolution. As humans, we can ask, “What if?” at scale by using our imaginations to, among other things, engage with science fiction. That’s the beauty of sci-fi: the ability to explore deeply human questions and experiences in novel contexts. Pondering alien species frees our minds to see ourselves anew. As the Doctor said in season five, “We’re all stories in the end. Just make it a good one.” Perhaps “a good one” is one that helps us understand something about ourselves and each other—something that moves our hearts.
