110 Fun Facts About Circulatory System

Blood travels around the human body in which two major circuits?

Pulmonary and systemic loops are joined end to end, so every full lap takes blood through the heart twice, once bound for the lungs and once for the rest of the body.

What is the largest artery in the human body?

The aorta has a stretchy wall that swells with each surge from the heart and then springs back, which keeps blood moving between beats and smooths the heart's spurts into a steadier flow.

Which two large veins return deoxygenated blood from the body to the right atrium?

The superior and inferior vena cava are named for position, not rank: 'inferior' only means lower, and the inferior vena cava, which drains the legs and abdomen, is in fact the largest vein in the body.

Blood on its way out from the heart reaches the capillaries through which small vessels?

Arterioles have muscular walls that tighten or relax to control how much blood reaches each capillary bed, and that squeeze makes them the main place where the body adjusts its blood pressure.

Most capillaries are so narrow that red blood cells can only pass through them how?

In single file is the only way through a vessel barely wider than one red cell, and the narrowest are tighter still, so the flexible cells have to bend out of shape to squeeze along.

A 1920s estimate put human muscle capillaries at 100,000 km end to end; a 2021 recalculation gave roughly what?

10,000 km is the right scale: the recalculation gave 9,000 to 19,000 km. August Krogh's famous figure had assumed a huge man with 50 kg of muscle and far denser capillaries than real muscle has.

Which structures in medium and large veins prevent blood from flowing backwards?

One-way valves got their first clear description from the anatomist Fabricius, who never grasped their purpose; a pupil of his later used them as key evidence that blood travels in a circuit.

A heart bypass often reroutes blood through a vein borrowed from which part of the body?

The leg gives up a length of its great saphenous vein, which is lifted out and stitched in as a detour, so blood can flow around the blocked stretch of a heart artery.

Which vessels break the rule that blood returning to the heart is deoxygenated?

The pulmonary veins carry freshly oxygenated blood from the lungs to the left atrium; a vein is defined by flow toward the heart, not by how much oxygen its blood holds.

Instead of returning gut blood to the heart, the portal vein takes it where?

Into a second capillary bed in the liver: the portal vein sends gut blood there first, so the liver can process absorbed nutrients and detoxify what was swallowed before it reaches the rest of the body.

The heart's own blood supply comes from two arteries that branch off where?

Near the origin of the aorta, just above the aortic valve, the two coronary arteries branch off; they fill mostly while the heart relaxes, because each contraction squeezes the vessels inside its own wall.

The arteries supplying the brain join up in a ring called what?

The circle of Willis lets blood reroute if one feeding artery is blocked; Thomas Willis described it in a 1664 book on the brain illustrated by the future architect Christopher Wren.

Which component of blood clumps at a wound to start forming a clot?

Platelets are not whole cells but fragments shed by giant cells called megakaryocytes, each releasing thousands; they stick to a damaged vessel wall and to each other to plug the gap.

What sets the rhythm of the heartbeat?

Pacemaker cells in the sinoatrial node fire on their own, with no signal from the brain; nerves and hormones only speed up or slow down the rhythm the node sets.

A healthy adult human heart is usually described as about the size of what?

A clenched fist is the usual comparison for a heart of about 250 to 350 grams, and like any muscle it responds to training: a well-trained athlete's heart can be considerably larger.

The heart sits in a double-membraned sac called what?

The pericardium holds a thin layer of lubricating fluid between its membranes, which cuts friction so the heart can slide freely inside the sac with every beat.

How much blood does an average adult have?

About 5 litres, roughly 8% of body weight, is the usual adult figure; a standard blood donation takes about a tenth of it, and the body replaces the lost fluid and cells on its own.

Which protein, the most abundant in plasma, holds water inside the blood vessels?

Albumin is the most abundant protein in plasma, and its pull keeps water inside the blood vessels; when levels drop, fluid seeps into the tissues and the ankles and belly swell.

What colour is blood plasma on its own?

Straw yellow: blood's red comes entirely from the hemoglobin packed into its red cells, so when a sample is spun and the heavy cells sink, the liquid left on top is pale yellow.

After about 120 days in circulation, dying red blood cells are filtered out of the blood by which organ?

The spleen threads blood past beds of macrophages that swallow dying red cells; it also fights blood-borne germs, which is why people who lose theirs are more prone to certain infections.

What do mature human red blood cells lack, unlike almost every other cell?

A nucleus is pushed out as each red cell matures, leaving more room for hemoglobin and a flexible disc that can fold over to squeeze through the narrowest capillaries.

Which hormone, a notorious doping agent, orders the body to make more red blood cells?

Erythropoietin (EPO) is released by the kidneys when they sense low oxygen, which is why time at altitude raises red cell counts and why injecting it became a favourite of endurance cheats.

Which gas grips hemoglobin over 200 times more tightly than oxygen?

Carbon monoxide takes the same four sites oxygen uses and holds on far harder, so even a little in the air starves the body of oxygen; it has no smell, which is why homes need detectors.

Which metal sits at the heart of hemoglobin's oxygen-binding heme groups?

Iron at the centre of each heme group is what actually grips the oxygen, which is why too little of the metal in the diet leads to anaemia: less hemoglobin, less oxygen delivered, and constant tiredness.

Arthropods such as insects have what kind of circulatory system?

Open, with hemolymph: the heart pumps this blood-like fluid into the body cavity, where it bathes the organs directly before being drawn back into the heart through pores called ostia.

What is the main oxygen-carrying molecule in the blood of crabs, lobsters and octopuses?

Hemocyanin grips oxygen with copper atoms, which is why the blood of these animals turns blue when it is loaded with oxygen and fades almost to colourless when it is not.

Which of these animals has a closed circulatory system like vertebrates?

Earthworms keep their blood sealed inside vessels, as vertebrates do, which lets it be steered to the hardest-working organs instead of washing loosely through the body cavity.

Where does blood leaving a fish's heart go first?

The gills come first: a fish's heart pushes blood through them to load up with oxygen, and it then flows straight on around the body without returning to the heart in between.

Which of these animals has a three-chambered heart?

Frogs have two atria feeding one ventricle, but mixing is limited: the heart's inner structure steers oxygen-poor blood toward the lungs and oxygen-rich blood out to the body.

Which reptiles have a four-chambered heart, fully separated into two pumps?

Crocodilians have the full four-chambered heart plus a trick of their own: during long spells underwater, valves steer blood away from the lungs and straight back around the body.

Which simple animals get by with no circulatory system at all, relying on a branched gut and diffusion?

Flatworms need no blood system because their shape does the job: a body that flat keeps every cell within easy reach of the branched gut or the skin, so oxygen and food simply diffuse in.

The oldest known writings on the circulatory system appear in which document?

The 16th-century BC Egyptian text linked the heart to the arteries but thought they carried air from the lungs.

Why did ancient anatomists believe the vessels leaving the heart carried air?

They look empty after death, so the first dissectors took arteries for air pipes, and the mistake is still in the name: artery comes from a Greek word for windpipe.

Which 2nd-century physician taught that blood passed through invisible pores in the heart's septum?

Galen needed the pores because his scheme had no return trip: blood was made fresh from food, flowed outward and was used up, so some had to seep across the heart's dividing wall.

In 1242 Ibn al-Nafis described blood's route through the lungs in a commentary on which physician's Canon of Medicine?

Avicenna's Canon was the text Ibn al-Nafis was explaining when he struck out on his own: the heart's dividing wall has no pores, he wrote, so blood must cross from one side to the other through the lungs.

Michael Servetus described how blood passes through the lungs in 1553, in what unlikely kind of book?

A theological treatise: Servetus tucked the passage into a book attacking the Trinity, and was burned at the stake for heresy, as were most copies of it.

William Harvey argued in 1628 that blood must circulate after estimating what about the heart?

How much it pumps out: by Harvey's sums the heart moves more blood in half an hour than the whole body holds, far more than food could replace, so the same blood had to be going round.

William Harvey's De Motu Cordis was printed in 1628 in which German city, home of a famous book fair?

Frankfurt is where the book came out, in Latin and barely 70 pages long; slim as it was, it overturned ideas about the blood that doctors had held since ancient times.

At which Italian university did Harvey study under Fabricius and take his medical degree in 1602?

Padua was where Fabricius had built the first permanent anatomy theatre, so students like Harvey could watch dissections for themselves instead of trusting the old books.

Harvey's position as royal physician gave him a steady supply of what for his research?

Deer from the king's herds let Harvey follow how young develop inside the mother; with hens' eggs for comparison, that work became his 1651 book on how animals are generated.

Which Italian anatomist found by microscope in 1661 the tiny vessels linking arteries and veins that Harvey never saw?

Marcello Malpighi's hair-fine vessels closed the loop: blood really did pass from arteries into veins, as Harvey had argued, instead of being used up in the tissues as doctors had long believed.

The 'swollen glands' felt in the neck during a cold or sore throat are really what?

Lymph nodes swell near an infection because that is where it gets fought: fluid draining from the sore area is filtered through them, and the immune cells packed inside multiply to meet the germs.

Which pair are the primary lymphoid organs, where lymphocytes are made and mature?

Bone marrow and thymus: every lymphocyte starts in the marrow, but T cells finish in the thymus, where those that would attack the body's own tissues are weeded out and killed before they can do harm.

Normal resting adult blood pressure is under what, per the American Heart Association?

120/80 mmHg is the ceiling for normal, top number as the heart beats and bottom as it rests; since 2017 US guidelines have counted 130/80 and up as high, a line that takes in nearly half of American adults.

Which Russian physician's 1905 discovery of stethoscope sounds let diastolic pressure be measured?

Nikolai Korotkov noticed that an artery under a slowly loosening cuff starts to tap as blood first squeezes through, then fades once blood flows freely, so two moments of listening give the top and bottom numbers.

The device with an inflatable cuff used to measure blood pressure is called what?

A sphygmomanometer's cuff squeezes the arm's artery shut and its gauge shows the pressure as the artery reopens; the classic gauge is a column of mercury, which is why readings are given in millimetres of mercury.

Karl Landsteiner's original 1901 labels for the blood groups were A, B and which letter, later changed?

C was Landsteiner's label for the group now called O; his finding that one person's serum can clump another's red cells explained why so many early transfusions had gone badly wrong.

The Rh blood group system takes its name from what?

Rhesus monkeys gave the Rh factor its name because 1940s researchers thought antibodies raised against monkey red cells picked out the same marker in human blood; it proved to be a different one, but the name stuck.

Which animal has the largest heart ever measured?

A blue whale's heart, some 180 to 200 kg of muscle, slows to as few as two beats a minute on a deep dive to stretch its oxygen, then races at up to 37 back at the surface to reload.

A human heart starts beating about three weeks after conception, when it is still what shape?

A simple tube is all the heart is when it starts pumping, around day 22, which makes it the first organ in the embryo to work; only afterwards does it fold into an S and divide into chambers.

Which specialists treat diseases of the arteries, veins and lymph vessels, but not the heart itself?

Vascular surgeons split off from general and heart surgery to look after the pipes alone, and much of their work is now done from inside the vessel, threading in balloons and stents to reopen narrowed arteries.

Why is the wall of the heart's left ventricle much thicker than the right's?

It pumps to the whole body, while the right ventricle only pushes blood next door to the lungs; both move the same amount with each beat, so the left needs a far thicker wall to make the extra pressure.

Which valve lies between the left atrium and the left ventricle?

Mitral means shaped like a mitre: closed, the valve's two flaps reminded anatomists of the tall, split-topped hat a bishop wears, and its other name, bicuspid, simply counts those flaps.

What specialised tissue carries the heart's electrical signal through the ventricles?

Purkinje fibers carry the signal faster than any other heart tissue and deliver it to the tip of the ventricles first, so the squeeze starts at the bottom and pushes blood up toward the exits, like toothpaste from a tube.

Who performed the first human-to-human heart transplant in 1967?

Christiaan Barnard's transplant in Cape Town worked, but his patient died 18 days later of pneumonia: the drugs that stopped his body rejecting the heart had also left him defenceless against infection.

The 'lub-dub' sound of a heartbeat is made by what?

Heart valves snapping shut make both sounds, the entry pair first and the two exit valves a moment later, which is why a doctor with a stethoscope can pick up a narrowed or leaky valve as an extra sound, a murmur.

When you take a pulse at the wrist, which artery are your fingertips pressing on?

The radial artery runs just under the skin on the thumb side of the wrist, close enough to the surface that fingertips can feel its wall bulge and spring back as each heartbeat's surge of blood passes.

Which Dutch physiologist built the first practical ECG machine and won the 1924 Nobel Prize for it?

Willem Einthoven's machine weighed some 600 pounds, yet its heart traces were sharp enough to diagnose from; Augustus Waller, who had recorded the first human trace in 1887, doubted hospitals would find much use for them.

Skinks of the genus Prasinohaema have green blood because of a buildup of what waste product?

Biliverdin, a green bile pigment made when old red cells are broken down, builds up in these New Guinea lizards to levels that would mean severe jaundice in a person, turning even their muscles, bones and tongues green.

Why does the heart's electrical signal pause briefly between the upper and lower chambers?

So the ventricles can fill: the atrioventricular node passes the signal on slowly, holding it about a tenth of a second while the atria squeeze the last of their blood down before the big pump fires.

Which Bee Gees hit do first-aid trainers use to teach the rhythm of CPR chest compressions?

Stayin' Alive runs at about 103 beats a minute, inside the 100 to 120 compressions a minute that CPR calls for. Learners who practise to it remember the pace better months later.

Which African mammal needs about double a human's blood pressure to keep blood reaching the brain?

Giraffe hearts weigh about 11 kg, with walls up to 7.5 cm thick, to drive blood all the way up the neck. Thick, tight skin on the lower legs stops it pooling down there.

Which bleeding disorder, passed on by Queen Victoria, became known as the 'royal disease' of Europe?

Hemophilia reached the royal houses of Spain, Germany and Russia through two of Victoria's daughters, Alice and Beatrice. Tests on the Romanovs' remains suggest it was the rarer type B.

Which widely prescribed blood thinner first went on sale in 1948 as a rat poison?

Warfarin grew out of a 1920s mystery: cattle were bleeding to death after eating mouldy sweet clover hay. The compound behind it was identified in 1940, then refined into a stronger rodent killer.

Veins look blue through the skin, but what colour is the oxygen-poor blood flowing inside them?

Dark red is the colour of blood that has handed over its oxygen, as any blood donation shows. Veins only look blue because of the way skin scatters light and the brain reads the result.

The heart drug digoxin comes from which poisonous flower, used against dropsy since the 1780s?

Foxglove, the genus Digitalis, gave medicine its first drug for the swelling of heart failure, described by William Withering in 1785. Digoxin itself was isolated from a Grecian species in 1930.

Which explosive has been prescribed since 1878 to ease the chest pain of angina?

Nitroglycerin relaxes blood vessels, easing the load on the heart. Alfred Nobel, who tamed it into dynamite, was prescribed it months before he died and called that 'the irony of fate'.

Which tiny bird's heart has been clocked at 1,260 beats a minute, over ten times a human's at rest?

Hummingbird hearts hit that rate in a blue-throated species, yet on cold nights the birds sink into torpor and a hummingbird's heart can slow to about 50 beats a minute.

Which ancient treatment did the dying George Washington request for a throat infection in 1799?

Bloodletting rested on the old idea that illness came from unbalanced 'humors', and Washington asked to be bled heavily. It was surgeons' most common practice from antiquity until the late 19th century.

Carrying one copy of the sickle cell gene helps protect people against which tropical disease?

Malaria parasites fare badly in red cells that sickle, so carriers die of the disease far less often. That is why maps of the gene and of the infection in Africa almost overlap.

People with which blood type are called universal donors, as their red cells suit almost anyone?

O negative red cells carry no A, B or RhD markers for a recipient's immune system to target. Guidelines reserve them for O negative patients and for emergencies when there is no time to test a patient's type.

Which everyday painkiller do many heart patients take in low daily doses to prevent blood clots?

Aspirin is used long term to cut the risk of clots in people at high risk. Its forerunner, salicin from willow bark, has been used as medicine for at least 2,400 years.

Which French doctor invented the stethoscope in 1816 after listening through a roll of paper?

René Laennec rolled a quire of paper into a cylinder to examine a young woman with heart symptoms, and heard far more than with his bare ear. A keen carver of wooden flutes, he soon built a proper one.

Which tiny air sacs in the lungs, about 480 million in a typical pair, pass oxygen into the blood?

Alveoli give the lungs a gas-exchange surface of 70 to 80 square metres, wrapped in a mesh of capillaries. Each is a thin-walled 'little cavity', which is what the Latin name means.

Which small expandable device is left inside a narrowed artery to hold it open?

A stent is usually made of a metal alloy or polymer and is often fitted during an angioplasty. Drug-coated versions, the commonest type in heart arteries, help stop the vessel narrowing again.

Syncope is the medical name for what, usually caused by a brief drop in blood flow to the brain?

Fainting at the sight of a needle is among the commonest kinds: vessels widen and the heart slows at just the wrong moment. Onset is fast, the spell is short and recovery comes unaided.

Which digit should never be used to take someone's pulse, because its own pulse can mislead?

The thumb has a strong pulse of its own, so anyone using it may end up counting their own heartbeat. The wrist and the side of the neck are the usual places to feel for someone else's.

Capillaries, the body's narrowest blood vessels, take their name from the Latin word for what?

Hair is the root: Latin capillaris means 'of hair', a nod to how fine these vessels are. The word entered English in the mid-17th century.

Which animal's blood did Jean-Baptiste Denys transfuse into a 15-year-old boy in 1667?

A sheep supplied the blood, and the boy survived, as did a labourer treated next. Denys was physician to Louis XIV of France.

Hirudin, a natural anticoagulant, comes from the saliva of which bloodsucking animals?

Leeches secrete hirudin so a host's blood keeps flowing while they feed. The medicinal leech, Hirudo medicinalis, is the classic example.

Which nutrient, routinely injected into newborns, does the body need to make blood clot?

Vitamin K takes its letter from koagulation, the Scandinavian and German spelling of the word. Newborns are given a shot of it to prevent a bleeding disorder caused by having too little.

A myocardial infarction, when a blocked artery starves muscle of oxygen, is better known as what?

A heart attack usually begins when a fatty plaque in an artery wall ruptures and a clot seals the vessel. It is not the same as the heart stopping outright, though one can lead to the other.

Which Greek philosopher taught that the heart did the thinking and the brain just cooled the blood?

Aristotle reasoned from his animal dissections, in which the veins all seemed to spring from the heart. That made it, for him, the centre of both emotion and intellect.

The coronary arteries that feed the heart muscle take their name from the Latin for which object?

Crown is corona in Latin: the vessels ring the heart like a garland. Anatomists have used the word for them since the 1670s.

Which dome-shaped sheet of muscle beneath the heart and lungs does most of the work of breathing in?

The diaphragm contracts to enlarge the chest cavity, and the drop in pressure draws air into the lungs. It also walls off the heart and lungs from the abdomen below.

Which disease, often silent for decades, is the build-up of fatty plaque inside artery walls?

Atherosclerosis plaque is a mix of fat, cholesterol, immune cells and calcium. Arteries widen around it at first, which is why blood flow stays normal and the person notices nothing for years.

Which fight-or-flight hormone makes the heart beat faster and harder within moments of a fright?

Adrenaline, also called epinephrine, pours out of the adrenal glands, driving the heart harder and sending extra blood to the muscles. The same compound is injected to treat severe allergic reactions.

Replacement human heart valves are often made from the tissue of cows or which other farm animal?

Pigs and cows supply the tissue for these 'bioprosthetic' valves, which is treated to prevent rejection. The alternative is a mechanical valve of metal or hard carbon that can last 20 to 30 years.

Which medical term describes lips and fingernails turning bluish when blood is short of oxygen?

Cyanosis appears when too little oxygen is bound to the hemoglobin in blood near the skin's surface. Large birthmarks and even blue food dye are sometimes mistaken for it.

Which class of drugs, the first of which came from a fungus, do millions take to lower cholesterol?

Statins block the enzyme the body uses to make cholesterol. Lovastatin, the first to reach the market, was isolated from the mould Aspergillus terreus.

Which common disorder, marked by tiredness and pale skin, cuts the blood's ability to carry oxygen?

Anemia can creep up so slowly that its vague signs, such as weakness, headaches and breathlessness on exertion, go unnoticed. It means too few red cells or too little hemoglobin.

Which organ supplies an unborn baby's blood with oxygen, since the lungs are not yet in use?

The placenta hands oxygen and nutrients from the mother's blood to the baby through the umbilical cord. Because the lungs are idle, the fetal circulation has shunts that steer blood past them.

Which term describes blood vessels widening, as happens in the skin to shed heat on a hot day?

Vasodilation in vessels near the surface diverts warm blood to the skin, where the heat can escape. Its opposite, vasoconstriction, narrows them to hold warmth in when it is cold.

Which condition is a balloon-like bulge at a weak spot in a blood vessel wall, liable to rupture?

An aneurysm can form in any blood vessel, and the bigger it gets, the greater the risk that it ruptures and bleeds uncontrollably.

What do doctors call a whooshing heart sound caused by turbulent blood flow?

A murmur is often harmless: an estimated 66% of children have one at some point, and many of those are judged 'innocent' by a specialist.

Which cancer of the blood-forming tissues takes its name from the Greek words for 'white blood'?

Leukemia was named by the German pathologist Rudolf Virchow for the flood of abnormal white cells seen in most patients' blood before treatment.

Which term describes a resting heart rate below 60 beats a minute, often normal in trained athletes?

Bradycardia is not always a problem: rates under 50 are often normal in athletes, and in healthy young adults while they sleep.

Which measure of the heart's pumping is found by multiplying heart rate by stroke volume?

Cardiac output is the volume of blood the heart pumps each minute. It must rise in exercise to feed working muscle, and aerobic training lifts it by increasing how much each beat delivers.

Which long cranial nerve, part of the 'rest and digest' system, acts as a brake on the heart?

The vagus is the longest nerve of the autonomic system, running from the brainstem through the neck and chest to the colon. It calms the body after stress, lowering the heart rate.

Which American surgeon set up large-scale blood banks early in World War II?

Charles Drew ran the Blood for Britain project, having found that plasma keeps far longer than whole blood. He protested against the Red Cross policy of separating donations by race, which lasted until 1950.

Which hole between the upper chambers of a fetal heart lets blood bypass the lungs before birth?

The foramen ovale passes blood straight from the right upper chamber to the left. In most people it closes at birth, and the walls fuse to leave a remnant called the fossa ovalis.

Which white blood cells, the most abundant in humans, are among the first to reach an infection?

Neutrophils make up roughly half to 70% of all white cells, and the body turns out about 100 billion of them every day.

Which stringy protein forms the mesh that binds a blood clot together and completes the plug?

Fibrin is made on the spot from fibrinogen, a soluble protein that circulates harmlessly until the enzyme thrombin links it into strands at an injury.

Which fish are the only known vertebrates with colourless blood, lacking hemoglobin as adults?

Icefish live in the Southern Ocean around Antarctica, where the water is so cold and rich in dissolved oxygen that their plasma alone can carry enough. A low metabolic rate helps too.

Which central compartment of the chest, flanked by the lungs, holds the heart and windpipe?

The mediastinum is bounded by the chest wall in front and the spine behind, with major nerves and the heart's great vessels threading through its loose connective tissue.

Which fibrous cords, nicknamed the heart strings, keep the valve flaps from flipping inside out?

Chordae tendineae anchor the flaps to small muscles in the chamber wall and pull taut as pressure builds, holding them in place.

Which German doctor pushed a catheter up an arm vein into his own heart in 1929?

Werner Forssmann then walked downstairs to the X-ray department so the catheter could be recorded on film inside his heart. The self-experiment brought him a share of the 1956 Nobel Prize in medicine.

Which blood test figure is the percentage of blood volume taken up by red cells?

Hematocrit normally runs at about 41 to 50% in men and 36 to 44% in women. Since red cells do the oxygen carrying, the figure is a quick guide to how much oxygen the blood can deliver.

Which artificial heart, the first implanted permanently, sustained Barney Clark for 112 days?

Jarvik-7 was also known as the Utah Heart, after the university that developed it. Robert Jarvik had joined the team behind it in 1971, eleven years before the Clark operation.

Which Massachusetts town gave its name to a heart study that has tracked residents since 1948?

Framingham beat Paintsville, Kentucky, in the final choice of site, and 5,209 adults signed up. The study has since followed their children and grandchildren too.