4-chambered heart & blood pressure
Nature has difficulty making a rotary, centrifugal pump because the cells of which it must be composed require nutrients which cannot be effectively passed to large aggregates of cells other than by the standard blood transport mechanism. And ... how would you attach an artery or vein to a rotating wheel without it twisting up after a few revolutions?
I once saw such a mechanism where a power source was hooked up to rotating wheel. It twisted up 1800 then untwisted the other 1800 so it got back to untwisted. It can be done but not by nature. Well, that's not altogether true. There was a bacteria some years back that was discovered to have a naturally occurring electric motor which made its corkscrew tail rotate and by this mechanism it was propelled. They proved that it rotated by gluing the tail down to a microscope slide and watching the body then rotate instead. But this was a bacterium and not a huge clump of billions of cells.
You'd also have to create some sort of ball bearings for the pump wheel to ride on with little friction. We have ball joints in our shoulders and hips but they are not in continuous, 3600 motion which would wear them out faster than they could be repaired.
Consequently, nature has opted for the much easier to construct, discrete pumping action. Ultimately, it probably has something to do with the continuous vs the discrete ... something similar to the insoluble problem of making a continuously variable automobile transmission using just gears ... it can't be done ... in principle.
Any discrete pumping mechanism made from living muscle cells requires at least two chambers. One pushes the blood out to the body (ventricle) and the second (auricle) pushes blood into the aforementioned main pumping station. This is required because
So, all we really need is a two-chambered heart. But ... we have four.
Clearly, the other set is for oxygen. After all, the pulmonary circulatory system is isolated from everything else. The body's need for oxygen is "minute critical". It can't wait for an hour. We don't store oxygen in the cells as much as sugar or fat. So, we can wait for food but not air. We can also wait for waste to be removed ... for awhile ... a few hours amybe. Therefore, we have a "helper" pair of heart chambers to ensure high pressure through this major resistance zone.
What if we simply remove the wall from between the left and right ventricle and run the pulmonary artery to the lungs from a "Y" connection ... what do we lose? We can pump just as much blood as before and the rules of hydraulics allows that we can still get the same amount of blood to the lungs per unit time provided that the resistance from the capillaries in the body is the same as the resistance form the capillaries in the lungs. But in that configuration, oxygen depleted blood is not forced to go back to the lungs and oxygenated blood is not forced to go back out to the body ... which is the case with nature's excellent four-door, and necessarily pragmatic design. Note: Some lower animals do have 3-chambered hearts and such. But our style is the top of the heap.
This is the logical cause of the second set of heart chambers.
How about ten or twenty little hearts all along the way to help out with the pumping process? Yes, we might be better served by multiple hearts but nature never does more where less will serve within an individual. More hearts means more information, more errors, more potential breakdowns ... and ... hmmmm ... these aren't things that nature "thinks" about. Nature simply acts on what is presently true. It has no foresight ... no reason. Hence, when the four chambered heart worked well enough to keep an individual unit (animal) going, that's it. Nothing more will be done unless some "unit" accidentally gets an extra heart ... and ... thrives and thus lives long enough to pass on that trait. So far it ain't happened.
Evolution seems to always stop at ...
In order for a fluid to flow in a closed circulation, the output pressure from the pump (ventricle) must exceed the input pressure (at the auricle). Of course, this is obvious. Nothing would flow if there were no direction of flow established (a gradient). There are two extremes worth considering. If you have a completely solid circulatory system of constant volume (like steel pipes) and an incompressible fluid operating against a resistance (like a hierarchy of smaller pipes) and a pump of defined capabilities, you have a difficult situation for the pump. If it pulses at a fixed rate and volume, as in a piston pump, the rate and force at which it works may conflict with the other system parameters which are also fixed. Something has to "give" or the pump strains and slows down against its defined rates or just burns up.
To pump, you need something in the system that "gives" ... something which is adjustable.
In a soft system, movement is easier to express. If infinitely soft though, nothing would circulate because any part of the system would expand like a balloon to the proportion of all the fluid in the system and it would grind to a halt. To get a circular flow, there must be resistance ... but too much or too little upsets the flow/pump parameters, i.e. there is an optimum arrangement for animal circulatory systems.
With blood pressure comes "resistance" and that is from the requirement that all blood go through capillaries about as wide as a single blood cell. This maximizes the surface area through which diffusion can take place (the mechanism by which oxygen, food, waste is transported from cell to cell). It would do our bodies no good if all our arteries were an eighth of an inch wide. The blood would just go round and round, never leaving much of anything behind or taking it up. And ... every cell in the body must be fed, so there must be a road leading to every cell ... or ... at least very close to every cell. Diffusion though a fluid will work by itself without capillaries ... if ... the animal is small enough ... like a spider. For humans you need a faster transport system. [Note: osmosis is the diffusion of nutrients through a semi-permeable membrane like the cell wall which is how the cell actually "eats".]
Blood is basically an incompressible fluid (it's mostly water), so the amount circulating is a constant. As much goes into the capillaries as comes out. The large arteries can't carry any more volume than is presently going through the capillaries. So the outflow per unit time in the figure below is the same for the large artery as for the two smaller arteries taken together.
One might think that there should be no direct resistance from gravity in blood circulation. This is clear from the following illustration. If blood vessels were solid pipes, and as much goes up as comes down ... there's no gravitational resistance. To expend energy against gravity, the heart would have to pump blood higher in the gravitational field while getting nothing back when the blood is lowered.
But blood vessels are not solid ... they are like rubber ... they can collapse. So, change the figure to a soft hose going up and then down and back to the pump. Now, the pressure on one side of the pump is greater than on the other. So, the artery expands and now holds more blood on one side than the other ... thus, the other side collapses somewhat ... so now you've got a wheel with heavier weights on one side and it wants to spin in a direction opposite of what the blood is flowing in. Hence, because blood vessels are rubbery, pumping blood through them is like keeping a wheel rotating which always has weight hanging on the side that resists rotation.
Why are blood vessels like rubber?
So that they can ... "accept the pulse".
Imagine what would happen is an artery were not able to expand. Then, the blood pressure from a contraction of the ventricle would be instantaneously transmitted to the capillary resistance which would impede the progress of the blood. Now, recall that the flow of blood must be constant in volume through any crossection in the circulatory system (blood is an incompressible fluid). So, as the heart pumps, it immediately encounters a force of resistance which limits the amount of blood that can flow through it. Because the heart must pump all at once by the mechanism of muscle contraction, it can't slow down and wait in mid-pump for the flow to travel through the capillaries. The blood must have an "overflow buffer" to temporarily take up some volume.
Thus, the artery bulges (a pulse) to accommodate the excess flow from the heart, allowing the heart to complete it's contraction in normal time. It is therefore maximally efficient given it's mechanics which nature is pretty much stuck with ... unless it invents something different ... which isn't likely at this stage of the game ... and, it's probably an example of "integrated simplicity" anyway.
Blood vessels have muscle cells which allow them to expand and ... contract ... like a "re-pump" (here are the other hearts we might like to have). They expand to accommodate that extra blood ... then they contract and help move it along to the capillaries. These muscles also reduce blood flow to places where it is not needed as much by constricting the flow forcing it to go elsewhere.
Veins have check valves along their length so that blood, once pumped to a level, won't fall back down. It's like a rung in a ladder. In humans, prolonged standing is a problem because the blood returning to the heart is assisted by our motor muscles. As we move, they put pressure on the walls of veins and tend to push it along as though working a gasoline siphon pump. (Integrated simplicity often means getting double work out of the same action.)
EBTX' theory of why inactivity can lead to hypertension
As I understand it, all the bodies cells (except brain cells) are replaced over the course of about seven years. Now, since the blood vessels are like rubber ... well, I guess they ARE rubber of a sort ... they must behave like other rubber devices. Let's consider balloons. You know what happens when you try to blow up a new balloon. It's difficult ... much easier if you first stretch the thing. After it's been blown up, it's easier to blow up the second time ... and after blowing it up several times it reaches minimum difficulty.
I posit the same for blood vessels.
As they are gradually replaced, lack of use keeps them tight. Exercise (especially aerobic exercise) gets the heart pumping fast and big and this stretches the vessels making them better able to "accommodate the pulse". When they are harder to stretch, they are more like a solid pipe, so the heart must work harder to push blood since it's pushing directly against the resistance of the capillaries instead of the vessel wall where it's designed to push.
What do you think? Does that make sense or does that make sense?
You can see then that as the arteries branch into successively smaller diameters, more surface area is presented over which blood must slide ... so there is more friction here. In fact, the accumulated friction in arteries, veins and capillaries is so great that an M1 Abrams tank couldn't force blood through our circulatity system as fast as it really goes. If it were not for the millions of pumping stations along the route helping out (arterial walls constricting), nothing would advance through the system. The heart doesn't actually pump blood throughout the body ... it just forms the initial "pulse" and maybe a bit more. The remainder of the transport is carried out by all the other "pumps" which simply advance the pulse to the next pump along the line. Well, wait a minute ... maybe the arterial walls just pump enoubh to negate all that friction ... then ... the heart pumps blood through the whole system as though it were frictionless. That sounds aesthetically purer ... "The heart pumps the blood through the body while the arteries negate 100% of the friction". Yup, that's more like what nature would do ... even it up.
When the pressure in a section is low, the arterial walls relax (the balloon is deflated). When the pressure is high, the balloon is inflated and the muscles in the walls contract and force the blood through to the next section of blood vessel in conjunction with the "check" valves at either end of the section. Very importanat: Each section is oblivious of what any other is doing as far a pumping goes. This saves having to compute when to send an electrical signal that says "contract". There are some "overrides" however, else a vessel couldn't constrict to reduce flow to an area which the body can do under some circumstances.