Open and closed circulation
In humans and in the higher animals, the pulmonary group of arteries and veins are a closed system within the overall system; the overall system being referred to as an open system. It is not an open system, however, when being compared to vertebrates and invertebrates; or with animals with backbones or animals without backbones. In this comparison, humans and all other comparable animals with some form of skeletal makeup depending on a heart and veins and arteries are classified as closed systems. Those with more primitive methods of living are open systems. Closed systems means that the blood vessels are closed at all times and the organs and open cavities do not get filled with blood. The opposite is true with open systems. The organs are virtually enmeshed with blood.
Both systems have hearts. The so-called hearts of insects and other open circulatory system creatures are not true hearts but only blood vessels forcing the blood along. These vessels are open to sinuses that pool the blood allowing it to surround and bathe the organs. Open system creatures have numerous outside openings that make oxygen readily available. Although open circulatory systems definitely are not efficient systems compared to the closed systems, they function well for creatures with smaller needs that have short life spans.
Even closed systems are rudimentary in mollusks and a few other invertebrates. One could say primitive indeed. The most primitive system of the closed systems is that of the lowly, but so very useful earthworm and its kin, the segmented body annelids. In their body they have two main vessels, one in the back, the dorsal system, and the other in the front, the anterior. Blood is forced along with the undulating motion of peristalsis, the same movement propelling them along.
The lowly earthworm has all of us beat, however, when it comes to hearts. In its anterior section it has five. (It also has all of us beat when it comes to taking care of the soil. It's very action is to recycle and reclaim the soil. As dirt is eaten and digested it comes out of its body much cleaner than it went in. I like that thought so much that I like to compare it to writers writing about environmental issues; their five fingers on each hand acting like these tiny little recyclers doing all they can to help the cause along.)
The earthworm has no circulatory system other than the outside air. Their skin is extremely thin - yet tough - allowing them to take in oxygen from the soil. Probably absorbing some of their needed air from the dirt they eat. This type of lifestyle serves them well but as with other creatures, especially those with beginning backbones, a more efficient system developed along with them to supply their needs.
Fish have a much more efficient method of getting nutrients. Their closed system has a two-chambered heart, an atrium and a ventricle. These have muscular walls and valves controlling the flow of blood. This flows from the heart to gills where it gets oxygen and dumps carbon dioxide, then moves to the rest of the body where the exchange of gases take place and then back to the heart. The difference is there is no respiratory system as compared to four chambered hearts and a more efficient closed system as humans have.
Frogs have a three chambered heart. Their heart has two atria and one ventricle. The blood is pushed out the ventricle into a two way aorta where some blood is forced to lungs and some to the rest of the body. Returning from the lungs the blood passes into one of the atria; blood returning from the rest of the body into the other atria. Both empty into the single ventricle where it begins the circuit all over again.
Birds and humans have four chambered hearts. Why are these necessary? Their main purpose is to regulate body heat and to control the many actions of the body. Warm blooded animals have a much more complicated system because their needs are greater. Heating and cooling systems demand intricacy.
Cold blood animals, on the other hand, have no need of such precise regulatory means. They take on the heat or the cold of their environment. When they cannot tolerate the extreme cold, they hibernate until the weather warms. All in all, nature has taken care of all life and doing it quite efficiently. Nothing is wasted and nothing is taken for granted. When environments in which these creatures find life intolerable, mere existence becomes a problem. There are no exceptions. When one type of life is threatened, as with too much carbon dioxide and not thorough gas exchanges - between oxygen and carbon dioxide as one example - all 0f earth's inhabitants suffer.