How did the cardiovascular system evolve?

Question

How has evolution created our blood, lungs and the heart?We can't exist without blood, which transports the oxygen to all areas of our body. However, the blood needs a lung, which gives it the oxygen to transport. The blood also needs something which lets it flow through the whole body, which are our veins. And in order to allow the blood to flow through our veins, an organ is needed to pump the blood, which is our heart. We also need a brain which controls all that, and the brain in turn needs the blood in order to function right.Evolution makes very slow steps....."it just doesn't jump". So, How did evolution manage to create all that?

Answer 1

Believe it or not, we can simply look at extant organisms on the planet today to observe the slow evolution of our circulatory system (generally considered one of the most complex ones, one of the "latest stages" in evolution).

Early organisms obviously had no circulatory system as they had to have been evolved first. Flatworms do not have one; instead, they have a large muscle called a pharynx that pushes nutrients throughout their body cavity to reach all of their cells (not to be confused with our pharynx, a more complex structure in the back of our throats).

Later organisms like insects, mollusks, and some crustaceans have what's called an open circulatory system. In an open circulatory system, blood is pumped through large cavities where organs are bathed in blood. The heart was first was first developed at this stage in the system's evolution, albeit a much simpler version, for the sole purpose of regulating this pump. You mentioned the brain -- this is why the innermost "reptilian" brain is in charge of regulatory processes like the heart. They simply evolved first.

After this came the evolution of closed circulatory systems, where blood is pumped through a closed system of arteries, veins and capillaries rather than a body cavity. Some invertebrates and higher organisms evolved these to support their larger size and need to oxygenate cells farther from the environment. The closed system allows for greater regulation of blood flow and increased speed and pressure because vessels are so narrow.

Fish have the most primitive of the closed circulatory systems: a single cycle system, which is just one circuit of circulation where the blood is pumped through the gills and body tissues. This simple system creates the need for only one valve in the heart to separate the oxygenated and deoxygenated heart, thus creating a two-chambered heart. This system, while functional, suffers from low blood pressure and no thermal regulation.

Next comes the evolution of the double circulatory system, in which two circuits are used. There is still one heart, but it has two pumps in order to serve the greater length of circuits. As you may have guessed, this is also designed to support larger organism sizes such as most reptiles and all amphibians. According to Wikipedia:

"In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart...

In reptiles, the ventricular septum of the heart is incomplete and the pulmonary artery is equipped with a sphincter muscle. This allows a second possible route of blood flow. Instead of blood flowing through the pulmonary artery to the lungs, the sphincter may be contracted to divert this blood flow through the incomplete ventricular septum into the left ventricle and out through the aorta. This means the blood flows from the capillaries to the heart and back to the capillaries instead of to the lungs. This process is useful to ectothermic (cold-blooded) animals in the regulation of their body temperature."

Finally, birds and mammals evolved a four chamber heart with 2 atria and 2 ventricles in order to keep oxygenated and deoxygenated completely separate (amphibians in particular had problems with mixed oxygenated/deoxygenated blood due to having 2 atria but only 1 ventricle causing them to have somewhat diluted O₂ in their blood). The separation is key for thermal regulation and allows sustained muscle movement. As usual, with the addition with the addition of another chamber, blood pressure is increased.

I should note that that the four-chambered heart of birds and crocodilians is thought to have evolved independently from that of mammals, which reinforces our theory of their evolution to serve specific purposes.

Hope I was able to help and provide some more insight on the process.