What if I told you, planet Earth is more similar to a human being than you realize?
Specifically the human circulatory system, a vast network of blood vessels whose purpose
is to transport oxygen, nutrients and other substances around the body, through the flow
of blood.
Earth’s flow of “blood” is the oceans, where water is moved around by a multitude
of factors, including wind, gravity, natural mixing and thermohaline circulation, also
known as the global conveyor belt.
Thermohaline circulation is essentially the global circulation of ocean currents due to
differences in temperature and salinity - the colder the water, the denser it is, and the
saltier the water, the denser it is.
There’s a surprising similarity between the circulation of ocean water and human blood
flow that goes beyond the fact that they both flow in a loop.
Many of the individual components in both mirror each other when it comes to how they
deal with oxygen, nutrients and heat.
Before we get into their similarities, though, the channel is relatively new, so right now
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Now here’s my take on this comparison.
Blood flow in the body is driven by the contraction of the heart, through powerful, rhythmic strokes
that force blood in and out through its valves and blood vessels.
With thermohaline circulation, one of its key drivers also displays similar periodic
action, as if the “heart” of planet Earth – the polar ice caps.
During the coldest times near the Earth’s poles, water gets cold enough to form sea
ice, leaving the salt that was originally dissolved in the water behind.
This makes the surrounding seawater saltier, increasing its density and causing it to sink.
The sinking water forms deep water masses near both the north and south pole, propelling
the deep ocean currents of the global conveyor belt.
In the north we have the Arctic ice pack, which more than doubles in area between summer
and winter, increasing from 7 million square kilometers in summer to 15 million square
kilometers in winter.
This moves a large amount of water from the surface of the ocean into the North Atlantic
Deep Water, mainly around the Greenland Sea and Norwegian Sea.
Compared to the 1.3 million cubic meters per second discharged by all the rivers in the
world combined, the North Atlantic Deep Water is formed at a rate of 15-20 million cubic
meters per second, thanks to the polar ice caps and some other factors such as cooling
and evaporation by the wind.
This is a key contributor to the global conveyor belt, connecting the gulf stream traveling
north and deep current traveling south.
In the south, the Antarctic ice sheet experiences an even more drastic change, from 3 million
square kilometers in summer to 18 million square kilometers in winter – a 6 times
increase.
This feeds into the Antarctic bottom water, a water mass that surrounds the bottom of
Antarctica and is mainly fed into by the Weddell Sea and Ross Sea.
Being the densest and coldest water mass in all of the world’s oceans, the Antarctic
bottom water is formed at an even faster rate of 30 million cubic meters per second.
The seasonal freezing and melting of immense amounts of sea ice makes both polar ice caps
similar to two heart organs of Earth’s oceans – hearts that beat once a year, in their
respective winters.
The lungs are where oxygen enters the blood, with its own loop of blood vessels known as
the pulmonary circulation.
Deoxygenated blood comes out of the right ventricle, travels through hundreds of millions
of tiny air sacs known as alveoli, then re-enters the left atrium of the heart.
This allows oxygen to seep into the many thin capillaries in the lungs, oxygenating the
blood for distribution to the rest of the body.
The equivalent of the lungs in the ocean’s oxygen circulation is the surface.
Here’s a north to south cross section of the middle of the Pacific Ocean, showing oxygen
concentration levels along the longitude of 180 degrees.
As you can see, the top 100 to 200 meters of the ocean has the highest concentration
of oxygen, due to being in direct contact with the atmosphere.
This layer is often referred to as the euphotic zone, for the presence of sunlight, or the
mixed zone, for the ability for water to mix relatively well within this range.
However, because organic matter produced in this layer tends to sink downwards, the surface
layer doesn’t have that many nutrients, just like human lungs.
There are a few key differences between warmer areas near the equator and colder areas near
the poles.
Firstly, water dissolves less oxygen at higher temperatures.
Secondly, stratification is when different layers of the ocean have a hard time mixing
due to drastic differences in density.
In warmer areas, stratification is stronger due to a larger temperature difference between
the warm surface ocean and cold deep ocean.
That also means colder areas of the surface ocean have lower levels of stratification,
which allows different layers to mix and more nutrients to be brought back up to the surface.
This is why higher latitudes often have higher primary production than lower latitudes, which
is the production of energy by organisms.
The pulmonary circulation is the shorter circulation of the human body’s two circulations, only
taking about 1 minute to complete.
Similarly, the travel of water up and down due to this mixing of layers near the surface
also takes a relatively short time of about 10 years.
While the pulmonary circulation loops through the lungs, the systemic circulation is the
other loop, through the rest of the body.
Starting from the left ventricle of the heart, oxygenated blood comes out through arteries
which travel to all over the human body, splitting into capillaries that deliver oxygen to areas
like organs and muscle tissue.
Those capillaries then merge back into veins, which deliver the deoxygenated blood back
into the right atrium of the heart.
Earth’s equivalent of the systemic circulation would be the global conveyor belt – a much
larger loop encompassing all the oceans rather than smaller cycles in the surface layer.
Deep water formation near the poles brings down a large supply of cold, oxygenated water
from the surface to the deep ocean, in which the oxygen is gradually depleted as the currents
flow back towards the equator.
There are barely any nutrients here, as the vast majority of falling organic matter has
already been consumed in the layers above.
This means there is less biological activity to deplete oxygen, and these two factors – the
large supply and small depletion – contribute to the deep ocean actually having quite a
high level of oxygen concentration.
The systemic circulation is the longer of the two circulations, taking about 5 minutes
for a complete loop in the human body.
Similarly, water takes 1000 years to complete a full cycle in the global conveyor belt,
much longer than surface mixing cycles.
The hepatic portal vein is a slight exception to the general behavior of blood vessels,
which involves arteries splitting into capillaries, then merging into veins.
Yes, it’s a vein carrying deoxygenated blood which has merged from capillaries, but the
capillaries come from the small intestine and the blood contains high levels of nutrients.
This vein then splits into capillaries again in the liver, for the liver to process the
absorbed nutrients and toxins, instead of leading directly back to the heart.
Interestingly, this corresponds very well to the thermocline in the ocean – the layer
between the surface and deep ocean, where temperature rapidly drops over a relatively
short distance.
This layer sits between depths of about 200 to 1000m, and just like the hepatic portal
vein, has high levels of nutrients but low levels of oxygen.
80-90% of falling organic matter from the surface layer is consumed in the top 1000m
of the ocean, so the thermocline is where a lot of aerobic bacteria feed on this organic
matter, also using up much of the oxygen in the process.
In this other cross section which shows oxygen utilization instead of concentration, we can
see that around this area is where the most oxygen is being consumed.
That, combined with stratification which prevents oxygen from being brought down from the surface,
means the thermocline layer is one of the least oxygenated areas in the ocean, going
back to the first cross section, forming some of the world’s oxygen minimum zones.
Near the poles, however, oxygen levels are higher due to weaker stratification – sometimes
there isn’t even a clear thermocline where temperature distinctly decreases, due to the
surface water already being so cold.
What an intricate being Earth is.
As a dynamic environment teeming with life, perhaps it should come as no surprise that
our home planet shares some structural similarities to the organisms which walk its surface.
Through these comparisons it should become clear that this diverse world operates just
like us, with its own lungs, hearts and blood channels, so we should take care of it, treating
it the way we would like to be treated.
I hope you enjoyed this novel perspective on ocean currents, and thanks for watching.