The Evolution of the Heart
The Evolution of the Heart
The evolution of the heart has enabled oxygen and nutrients to be pumped efficiently to body tissues, enabling the oxygen greedy brain to develop….
Invertebrate animals have a simple circulatory system, as opposed to a heart. Many do not even have blood, but rather are filled with fluids that receive its nutrients through body cells. More complex invertebrates use an open circulatory system, which has a few, if any blood vessels. A pumping mechanism (muscle contraction..) pumps the blood and fluids throughout the tissues, and filters back to the pumping mechanism, for example earthworms have small muscular areas that contract and pump blood throughout the earthworms body. The origins of the heart pump are believed to have started with the pumping of a jellyfish.
The next evolution of a heart saw the development of a fish heart, the simplest type of heart. Primitive fish do have the beginnings of a four-chambered heart, but the chambers are arranged sequentially, unlike mammal and bird four chambered hearts. In the adult fish, the four chambers are arranged in an S-shape, with the latter two chambers lying above the former two. These examples saw the development of a closed circulatory system, with two chambers to separate the blood being pumped away, and the blood returning for reoxygenation. The top chamber is called the atrium, the bottom chamber is called the ventricle. It has one vessel that directs blood into gills for reoxygenation.
A double circulatory system saw the ability for more efficient oxygen distribution throughout the body. This change in morphology coincided with the development of lungs and can be found in terrestrial organisms.
In living amphibians, a three chambered heart has developed. Frogs have two atria, yet still only one ventricle. Two atria ‘s allow oxygenated and deoxygenated blood to be separated. The single ventricle is large and strong, so it is able to pump oxygenated blood throughout the body. The less efficient three chambered heart is adequate for these organisms to survive, as they still respire through the skin, allowing a less degree of oxygenation mechanisms within the blood system.
Reptile hearts vary, with some, such as turtles, having a variation of three and a half chambers. There is a septum that goes halfway into the ventricle. Blood still mixes in the ventricle, but the timing of the pump minimizes mixing of the blood. There has been observations that this pump timing and septum size is actually means that there is no mixing of oxygenated and deoxygenated blood.
Archosaurs (crocodilians and birds) and mammals all portray a complete separation of the heart into two pumps and four heart chambers. It is believed that archosaur hearts, and mammal hearts developed independently of each other. The crocodilians have a small opening, (the foramen of Panizza) at the base of the arterial trunk, so some degree of mixing can occur, usually during an underwater dive. Only in birds and mammals are the two streams of blood, permanently kept separate by a barrier.
The mammalian heart has a fully formed septum that separates the atria and the ventricles. Deoxygenated blood enters the right atrium, which is then pumped to right ventricle, of which is then pumped to the pulmonary artery to be reoxygenated by the lungs. It then returns to the left atrium and leaves through the left ventricle, and pumped using the largest artery in the body, the aorta.
In mammals, the right side of the heart collects de-oxygenated blood, into the right atrium from the body and pumps it via the right ventricle into the lungs. This is called the pulmonary circulation. that Carbon dioxide is dropped off and oxygen picked up by a passive process of diffusion allowing the gas exchange to occur. The left side collects oxygenated blood from the lungs into the left atrium, where it is pumped throughout the body via the aorta.
The heart is effectively a syncytium, a multinucleate cell that results from multiple cell fusions of unicellular cells, using an electrical messaging system to control the complex yet efficient circulatory system.