60 Fun Facts About Circulatory System

The circulatory system is divided into which two major circuits?

The pulmonary loop carries blood from the right heart to the lungs and back to the left heart; the systemic loop serves everything else.

What is the largest artery in the human body?

It leaves the left ventricle, makes a hairpin turn called the arch and ends in the abdomen by splitting into the two common iliac arteries.

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

From the right atrium the blood is pumped into the pulmonary circuit to pick up oxygen.

Arteries branch into which small passages before reaching the capillaries?

On the way back, capillaries merge into venules, which merge into veins.

Roughly what is the diameter of a capillary?

Their walls are a single layer of squamous endothelial cells, thin enough for gas and nutrient exchange.

The total length of muscle capillaries in a 70 kg human is estimated at between what?

That is enough to stretch a quarter of the way around the Earth even at the low estimate.

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

Fabricius described them in the 16th century without grasping their purpose; his most famous pupil worked out what they were for.

At any moment roughly what share of the body's blood is in the veins?

Veins act as the body's blood reservoir; most of that venous blood sits in small veins and venules.

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

They bring freshly oxygenated blood from the lungs to the left atrium; the fetal circulation is another exception.

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

It collects nutrient-rich blood from around the gut so the liver can process the products of digestion first.

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

The right and left coronary arteries arise from the aortic sinuses just above the aortic valve.

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

The anastomosis means the brain keeps its supply even if one route is blocked.

How many chambers does the human heart have?

One atrium and one ventricle serve each of the two circuits.

What sets the rhythm of the heartbeat?

The adult resting rate is close to 72 beats per minute.

What is the approximate mass of an adult human heart?

It is often described as fist-sized, about 12 cm long, though that description is disputed as slightly small.

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

The sac holds a small amount of fluid and attaches to the mediastinum, the middle compartment of the chest.

How much blood does an average adult have?

Plasma makes up around 55% of it, and the plasma itself is 92% water.

What percentage of blood is plasma?

Red cells make up most of the rest, around 45%, with white cells under 1%.

What colour is blood plasma on its own?

The average person carries 2.7 to 3 litres of it, and it is 92% water.

How long does a human red blood cell circulate before being recycled?

Macrophages break them down in the spleen and liver.

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

It is expelled during development to make room for hemoglobin, leaving each cell as a flexible biconcave sack of the protein.

Roughly how many new red blood cells does an adult produce every second?

Red cells account for about 84% of all the cells in the human body, some 20 to 30 trillion of them.

How many oxygen molecules can one mammalian hemoglobin molecule carry?

Each red cell holds about 270 million hemoglobin molecules; Max Perutz worked out the protein's structure in 1959.

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

In 1825 Engelhart used the iron content to make the first-ever estimate of a protein's molecular mass.

Arthropods such as insects have what kind of circulatory system?

The fluid bathes the organs directly in a cavity called the hemocoel and is drawn back into the heart through pores called ostia.

What is the main oxygen-carrying molecule in arthropod hemolymph?

Free-floating hemocytes in the fluid also play a role in the arthropod immune system.

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

Annelids and cephalopods such as squid and octopus keep their blood inside vessels; sponges and comb jellies have no circulatory system at all.

A fish heart has how many chambers, driving a single-circuit circulation?

Blood goes through the gills and on to the body in one loop, so it is never repressurised after the gills.

Amphibians have a heart with how many chambers?

Reptiles also have an incomplete ventricular septum that lets them divert blood away from the lungs, useful for cold-blooded temperature control.

Besides mammals and birds, which reptiles have a heart fully separated into two pumps?

Their hearts are thought to have reached this design independently of mammals, as did those of birds.

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

Their flattened shape keeps every cell close to the gut or the outside water.

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?

Blood pools in the veins once the heart stops, so dissected arteries appeared to be hollow air pipes.

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

He thought venous blood was made in the liver and arterial blood in the heart, with no return circulation at all.

Which 13th-century Damascus physician first described the pulmonary circulation?

He also predicted small 'pores' between the pulmonary artery and vein, an idea 400 years ahead of its time, and has been called the father of circulatory physiology.

Michael Servetus described the pulmonary circulation in 1553 in what unlikely kind of book?

Christianismi Restitutio was burned along with its author; only a handful of copies survived and stayed hidden for decades.

Who published De Motu Cordis in 1628, demonstrating that blood circulates continuously around the body?

He argued the heartbeat drove blood through minute connections at the extremities, a conceptual leap beyond earlier work on the lungs alone.

In which city was De Motu Cordis published?

The quarto opens with a dedication to King Charles I and lays out the circuit of the blood in 17 chapters.

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

Fabricius's book on the valves of the veins, De Venarum Ostiolis, was a key influence.

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

He accompanied Charles I as 'Physician in Ordinary', and later served as Warden of Merton College during the Civil War.

Harvey never saw the vessels linking arteries and veins. Who found them by microscope in 1661?

His microscope observations completed the circuit Harvey had inferred.

Which two 17th-century scientists independently first described the lymphatic system?

Its largest vessel, the thoracic duct, drains lymph from the left side of the body back into the blood.

Which organs make up the primary lymphoid organs where lymphocytes are produced and matured?

T cells mature in the thymus, and fewer than 5% pass both selection tests.

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

Readings under 90/60 are commonly regarded as hypotension; the world average has hovered around 127/79 in men since 1975.

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

Von Basch had built the first such device in 1881 and Riva-Rocci an easier cuff version in 1896.

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

Mercury versions are still the gold standard because the column never needs recalibrating.

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

It became O in later publications, and many languages still call the group zero or null.

The Rh blood group system takes its name from what?

With about 50 antigens it is the second most important system in transfusion after ABO.

The largest heart ever recorded, weighing about 180 kg, belonged to which animal?

At depth its heart rate can fall to 2 beats a minute, rising to about 37 on surfacing.

Blood vascular systems are thought to have first evolved in a triploblast ancestor roughly how long ago?

They overcame the limits of diffusion; a true endothelium came later, in an ancestral vertebrate 540 to 510 million years ago.

Which medical specialists focus on disorders of blood and lymph vessels rather than the heart?

Cardiothoracic surgeons operate on the heart, while cardiologists manage it medically.

Which valve sits between the right atrium and the right ventricle?

Its three cusps are tethered by chordae tendineae to three papillary muscles that stop the valve blowing back into the atrium.

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

It is also called the bicuspid valve because it has just two cusps, an anterior and a posterior one.

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

The signal starts in the sinoatrial node, passes through the atrioventricular node and the bundle of His, then fans out through these fibres.

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

The operation took place at Groote Schuur Hospital in Cape Town; recipient Louis Washkansky lived 18 days.

The 'lub' of the heartbeat, the first heart sound S1, is made by which structures closing?

The 'dub' of S2 is the semilunar valves shutting; extra sounds S3 and S4 produce so-called gallop rhythms.

Which organ receives the most abundant blood flow, at roughly 1,350 ml per minute?

The kidneys and brain come next at about 1,100 and 700 ml per minute, though per gram of tissue the kidney is the best supplied.

Which Dutch physiologist invented the electrocardiogram and won the 1924 Nobel Prize?

Frank and Starling instead lend their names to the mechanism by which a more stretched ventricle contracts more forcefully.

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

In humans, oxygenated arterial blood is bright red and venous blood a darker red; the blue look of surface veins is a trick of light and skin.

Whose 1906 monograph revealed the atrioventricular node and the heart's conduction system?

That discovery prompted Arthur Keith and Martin Flack to hunt for similar structures, and months later they found the sinoatrial node.