So this is derived from a lesson at Khan Academy.

The mind activity assumes that the one ventricle heart pumps blood to the lungs for oxygenation then rest of the body. However, the problem is that there is too much pressure at the lungs leading to bleeding.

This problem, however, can be solved if the lungs come before the one ventricle heart. This way, most of the pressure is used going through the body and pressure would not become a problem.

Thus, is a one ventricle heart that connects to body tissues first then lung feasible? If so, why have organisms not developed this way?

  • $\begingroup$ I removed the "evolution" tag because if this question could be tagged 'evolution', then really any question on this site could be tagged 'evolution'. $\endgroup$ – Remi.b Aug 18 '17 at 16:43

Alternative Heart Morphologies

Amphibians and some reptiles have a three-chambered heart, with 2 atria and a single ventricle. There are still separate circulatory pathways for the lungs and the rest of the body, but the oxygenated and oxygen-depleted blood mix in the ventricle, and are pushed at the same time to the lungs and body.

The disadvantage of this system is that it is less efficient at providing oxygen to the body than a four-chambered system, because the blood pumped to the body is a mixture of blood that has recently been to the lungs and blood that has not.

You seem to be suggesting another system, where there is just one serial pathway from heart to lungs to body, or from lungs to heart to body. This is more like the two-chambered heart of a fish: blood is pumped from the heart through the gills, and then to the body.

Lungs and body in series, and fluid dynamics

So why wouldn't this work for a mammal? It could, but gills and lungs are very different. The vasculature of the lungs requires a lot of pressure to get through, which is generated by the right ventricle.

Pressure is required to push fluids through a constriction (note that even a straight pipe is a constriction). After a fluid passes a constriction, there is always a pressure drop. That pressure drop is a function of the rate of flow and size of the constriction. If you give less pressure at the start, there is less pressure to "drop", and so you get a lower flow rate (note that this is nearly a perfect analogy with electricity, where flow is current, pressure is voltage, and the size of the tube is the conductance).

If you want to pass through a second constriction, you have to use pressure that is "left over" after the first drop: you can't use the pressure you started with because that was lost getting you through the first constriction. In reality, in a closed system, the total flow rate will be determined by the summed series resistance, so adding a second constriction slows the flow rate for the whole system, and you get a partial pressure drop after each.

If you put the lungs and body in series with each other, no matter which comes first, you need to have enough pressure left over when you pass one to get you through the next one, otherwise the flow will slow down. Blood in a human aorta has a mean pressure around 100mmHg, whereas back in the vena cava it is close to 10mmHg. That suggests you need about 90mmHg to get enough flow through the body's circulation. The lungs use a little less, but you still have a pressure drop of around 50-60mmHg. Therefore, you would have to increase the blood pressure by 50%-75% to get circulation through both lungs and body.

Higher pressures lead to more turbulent flow and take more effort to achieve. The four-chambered heart, with 2 ventricles, is simply more efficient.

  • $\begingroup$ @GerardoFurtado I'll edit to say some; only birds and crocodilians have a true fourth chamber, the other reptiles do not, though there is some separation compared to amphibians. $\endgroup$ – Bryan Krause Aug 19 '17 at 2:59
  • $\begingroup$ crocodiles actually have something bordering on a five chambered heart, it allows them to divert oxygenated blood away from non-essential organs during dives. $\endgroup$ – John Aug 19 '17 at 7:35
  • $\begingroup$ @John I agree that crocodiles have a more complex heart than mammals, although I think it's a stretch to call it five-chambered; I looked around and every source I can find describes it as a four-chambered heart and they note the second aorta that allows for diversion of blood away from the lungs. $\endgroup$ – Bryan Krause Aug 19 '17 at 16:04
  • $\begingroup$ there is this extended valve called the cog tooth valve that allows a one way diversion, it is just that that valve is right against the heart and is muscular, and it creates a one way communication between the aorta. hence why some call it five-chambered. $\endgroup$ – John Aug 19 '17 at 23:01

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