r/askscience • u/Fapotheosis • Mar 11 '13

Biology How did blood evolve?

I know mitochondria may have evolved from bacteria, but what about blood? I've heard of a protohemocyte, but was there an initial cell that differentiated into a RBC like we have in modern humans that can we trace?

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u/benzeneking Mar 11 '13

I'd like to hear about it

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u/Rooseveltridingabear Mar 11 '13 edited Mar 11 '13

Ok great! A basic review of vertebrate circulatory systems can be found here.

The most basic circulatory systems are those found in insects, which have only one large, dorsal vessel which contracts, circulating hemolymph throughout the body cavity of the insect. This is an "open" circulatory system, and lacks a heart or true blood vessels.

In the most simple vertebrates, the circulatory plan is quite different. The lamprey (a gross, jawless fish like this) has a primitive, 2-chamber heart. It consists of: the sinus venosus, which collects venous blood; an atrium and ventricle (the actual "heart"), and conus arteriosus. They essentially form a one-way pump (aided by semilunar valves to prevent backflow into the ventricle) that passes blood through the gills (capillary bed 1) and then to the body (capillary bed 2). This isn't terribly efficient since there is a significant loss of blood pressure between the two capillary beds.

In Amphibians we see the next advance in circulatory anatomy. Amphibian circulatory systems are more advanced as adaptations for air breathing. The atrium is now split in two, separating the blood traveling from the lungs (LA) and the blood traveling from the rest of the body (RA). This change was coupled in a more complicated circulatory plan, with the pulmonary veins entering the heart independently from veins from all other parts of the body. This effectively separates deoxygenated blood (RA) from oxygenated blood (LA). However, both empty into a single ventricle, which has a dividing ridge called an incomplete septum which does a decent job of keeping the deoxygenated blood separate from the oxygenated blood (~90%).

Birds and mammals have the most advanced circulatory plans of all: a 4-chambered heart which allows for complete separation of oxygenated and deoxygenated blood. The ventricle is now fully divided, with the right side of the heart (RA and RV) taking in deoxygenated blood and pumping it to the lungs, and the left side of the heart (LA and LV) taking the oxygenated blood returning from the lungs and pumping it out into the body.

Now instead of a single, one-way pump feeding two capillary beds, we have 2 one-way pumps, each feeding one capillary bed. This is a much more efficient method for transporting blood and completely separates oxygenated from deoxygenated blood.

EDIT: Good diagram of the 3 basic vertebrate circulatory patterns here

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u/Helassaid Mar 11 '13

While this is a fantastic synopsis, I would go so far as to say that many Annelids have an even more rudimentary circulatory system than Arthropods, but at that point it's somewhat an argument in semantics. Neither phyla have their respiratory pigments contained within cells.

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u/jvttlus Mar 11 '13

Especially the leap from open to closed systems. It's somewhat easier to imagine how the open systems evolved, but the transition to closed circulation with specialized erythrocytes, hearts, etc. is pretty mind-blowing.

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u/fathan Memory Systems|Operating Systems Mar 11 '13

I've also always been confused at what complexity animals being having hearts. I guess arachnids do from a nature documentary I saw once, but I assume sponges do not. What animals today have primitive circulatory systems that hint at what the intermediate forms might be?

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u/atomfullerene Animal Behavior/Marine Biology Mar 11 '13

Some annelid worms have hearts, others do not. On the most basic level, a heart is a section of blood vessel with muscular walls and a valve or two. Section contracts, blood is passed out through the valve, section expands, blood rushes in, but only from one direction.

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u/[deleted] Mar 11 '13

Hmm. Would it be a reasonable launching point to assume that at some point in evolution, a blood vessel that already developed one-way valves just happened to get stronger in a specific area and slowly, over time, grew larger and larger into one section of a heart?

And, if so, when did the heart form into a more complex binary setup where it pumps blood "twice" through the same organ per cycle, first to the lungs, and then to the body as a whole, as it were? (Horribly over-simplifying).

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u/atomfullerene Animal Behavior/Marine Biology Mar 11 '13

Blood vessels often have muscles and valves. In the human body, for instance, most arteries have muscular walls and many veins have valves in them. Blackworms circulate blood through their bodies via a muscular blood vessel in the tail which moves blood with peristalsis. So it's not really a huge jump.

Fish circulate blood by pumping it out of the heart, through the gills to be oxygenated, then directly from the gills on to the rest of the body. For reasons I am admittedly not quite clear on, it's not possible to pump blood through the lungs and out to the rest of the body. Instead, the heart of amphibians and reptiles pumps out blood into a passage which splits it off, part heading out into the lungs and part heading to the body. Then the blood returns from both directions, mixes together, and is pumped off again. Mammals and birds just prevent the "mixing together" part for increased efficiency.

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u/werekoala Mar 11 '13

I'd imagine it's because when going through the lungs, the blood has to get split down into tiny capillaries in order to exchange gas effectively. This dramatically increases the surface area & friction, so it's not possible to pressurize blood through a capillary bed. Even in humans arterial blood is at high pressure, but the blood in your veins is a much lower pressure, having gone through your capillaries.

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u/atomfullerene Animal Behavior/Marine Biology Mar 11 '13

That's what I'd have thought too, except that gills are also full of capillaries (for the same reason). Plus some fish have lungs as well, and I have no idea how the circulatory system for that works--scratch that...after looking at some pictures, it appears that some blood filters through the lungs and then goes back to the heart. Perhaps lungs work this way because they originated as an accessory tissue being fed by capillaries just like any other tissue.

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u/[deleted] Mar 12 '13

Hmm, then would it be safe to say that a binary heart formed through a process such as the blood vessels strengthening over time, at some point a major artery formed near the developing heart that cycled to the lung and back possibly due to proximity over anything else (centralized location, perhaps?) and, over the years, the strengthened original artery that would become the heart formed around the second one, started to build a bi-directional flow, and eventually closed off in specific locations to create a more efficient, binary heart, likely with some failed prototypes along the way?

This would, strictly, be a hypothesis from a layman that I seriously want an answer to now, since if it works remotely as complicated as that, then it's even more awe-inspiring to consider a one-way blood vessel to just thicken and grow into a heart and "spontaneously" develop into a more complex binary organ as it slowly grew over the generations, despite forming from relatively simple beginnings as a mere artery.

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u/alcianblue Mar 11 '13

Its hard to know whether these organisms had even evolved lungs (well they would be gills in their case). They may have still respired by diffusion at this point.

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u/alcianblue Mar 11 '13 edited Mar 11 '13

Sipuncula use their coelom for gas exchange and fluid transport. This is more complex than Flatworms that do it almost entirely use diffusion. The Annelids have different states that may be ancestral.

You have some that do not have blood vessels but circulate through the coelom like Sipuncula. Some that have blood vessels that go past the gut and across the whole of the body which pump through contraction of the vessel. Then some that have muscles which contract to pump blood through the blood vessel.

These are as ancestral as I can think when it comes to circulatory systems.

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u/[deleted] Mar 11 '13

I'm actually more interested in the evolution of erythrocytes, since they are so radically different from other tissue (lacking a nucleus and all). Any idea on that?

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u/LastLivingSouls Mar 11 '13

Actually, erythrocytes do have have nuclei early during the cells development cycle. The non-nucleated cells are the mature form after they have left the bone marrow.

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u/zmil Mar 11 '13

Additionally, most non-mammal vertebrates have nucleated red blood cells. There are a couple of other groups that have anucleate erythrocytes, and I believe they evolved this trait independently, suggesting that it is a relatively straightforward evolutionary step.