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Evolution of the circulatory system

Evolution of the circulatory system


            The circulatory system is one of the many biological systems in the body.  This paper will focus on the circulatory system- a system that transports blood to the body. There are various parts that work together to ensure a smooth flow of blood, oxygen, nutrients, and other important substances. It is important to note that unicellular organisms use diffusion to transport water, nutrients, and gases.  However, in a multicellular organism such as human beings, animals, and plants, there is a need of a rapid and efficient system that can move blood to the body cells. In a multicellular organism, there must be a bulk flow of the blood and this made it easier through the blood pump. For the blood to flow from the heart, it is important to note that the pulmonary circuit and the systemic circuit play major roles. The pulmonary circuit carries the blood with more carbon dioxide and less oxygen. It then transports the blood to the lungs where it is treated by unloading carbon dioxide and adding oxygen. The systemic circuit carries the oxygenated blood and transports it to the body.  In both circuits, the vessels and capillaries play an important role in carrying blood from and back to the heart, and also to deliver nutrients and other essential materials.

 The evolution of the circulatory system

            Fisher & Berggren (2007) talks about the evolution of the circulatory system and asserts that the supply of oxygen to the multicellular organism evolved several million years ago.   Energy from the sun converted water and carbon dioxide to create sugar molecules, and oxygen provided chemical energy to break the glucose. The oxygen in the earth's atmosphere was high and this resulted to an oxygen crisis. The high concentration of oxygen or oxygen intolerance killed anaerobes organisms (Fisher & Burggren, 2007).  However, some organisms prevented themselves from oxidative stress. The high accumulation of oxygen created a toxic environment but despite the oxygen deprivation or changes in oxygen availability, multicellular organisms used the gene-based system to maintain the supply of oxygen. Hypoxia-inducible transcription factor was used to regulate oxygen in mammalian cells (Fisher & Burggren, 2007).  It is important to note that the Hypoxia-inducible transcriptions factor was a cell-autonomous system and this means that large and complex animals experienced herniation due to reduced energy supplies.  Since the multicellular organism could not survive with simple diffusion, new methods of oxygen delivery were developed. Since the multicellular organisms increased oxygen demand, a high-pressure system was required to supply the oxygen. Rather than relying on environment changes, the multicellular organism had a big body size that affected the metabolic rate (Fisher & Burggren, 2007). These animals also had large organs such as the heart and brain that required an effective system. The circulatory system evolved and it was realized that in vertebrates, the blood circulated in the body through the arterial vascular network.  During the circulatory process, the oxygen-in form of a protein known as hemoglobin is carried into the respiratory cells (Fisher & Burggren, 2007).   The blood that has less oxygen moves from the lungs to the heart for cleaning and further transportation.


Structure and functions of the circulatory system


  Two great arteries proceed from the right ventricles and left ventricle. The Pulmonary artery proceeds from the right ventricle and then forms other branches-right pulmonary arteries and left pulmonary arteries (Marshall, 2010).  The left pulmonary artery splits into other several branches. The function of the arteries is to ensure that all cells of the body have received blood from the heart. The largest arteries are known as the aorta splits into other arteries which transports blood to different body organs.



            Capillaries are thin walls where the exchange of substances such as gas, nutrients, hormones, and more occur (Marshall, 2010). It is important to note that capillaries are found in tissues and this is where the exchange of materials takes place.


  It is important to note that the products in the capillaries that are oxidized and which are waste go into the veins.  Therefore, veins split from the capillary network and their role is to absorb products and returns them to their respective places (Marshall, 2010). In general, veins prevent blood flow back to the capillaries and ensures an effective flow of blood to the heart.

 Lymphatic system

 The lymphatic system also plays a role in the circulatory system as they transport a clear fluid to the heart.  The system assist the circulatory system by maintaining blood and tissue volume, it helps in the absorption of dietary lipids, and also in immune cell trafficking (Santambrogio, 2013). The lymphatic system has vessels that regulated immune response by allowing the passage of extravagated leukocytes. The system also contains lymphatic capillaries to allow the flow of interstitial fluid. Impairment can occur in the lymphatic system due to genetic mutations, or other conditions such as chylothorax, autoimmune diseases, filariasis, and more (Santambrogio, 2013). In general, as part of the circulatory system, the lymphatic system acts as a defense mechanism, ensures the transportation of molecular compounds to the bloodstreams, the transportation of digested fats to the bloodstreams, and the transports of tissue fluids into the blood system.


 Heart evolution in vertebrates

 The heart is an important part that ensures a smooth flow of blood during the circulatory process. The heart evolved 500 million years ago and since it ovulation, many changes, and adaptations have occurred (Stephenson, et al. 2017). Multicellular life appeared about 750 million years ago and later Phylum Porifera evolved. The ancient multicellular organisms had asymmetric shape. For example, the sponge is a multicellular organism that has only two germ layers. They do not have the circulatory system and they use the motions of water currents for oxygen and nutrients supply (Stephenson, et al. 2017).  However, 700-600 million years ago, bilateral symmetry evolved in about 1.5 million species.  The bilateralisms contained the third germ layer known as mesoderm.  It was found that the heart developed in the mesoderm.  Besides, the bilaterians have primordial gene patterns (Stephenson, et al. 2017). The first circulatory primordial occurred in the bilaterian.  They had a single-layered tube with an open circulatory system. However, an open circulatory system lacked heart chambers and therefore there was no proper one-way flow of blood or in other words, only a little blood traveled to the body organs. Later, all chordates including the vertebrates had a multicellular chambered heart and a circulatory system. This led to the anatomy and physiology of the heart in vertebrates (Stephenson, et al. 2017).  The study of the heart was done using primitive vertebrates such as hagfish, agnathans, and others.  The anatomical studies found that the vertebrates have a circulatory system after finding evidence of the movement of oxygen in the circulatory system.  This marked the vertebrate's evolutional history.



Chambers of the heart


  The right atrium and left atrium- the intertribal septum divides the atriums and makes the right atrium lie in front of the left atrium.  However, the two chambers are separated by an intertribal septum (Lippincott et al. 2007).  Both left and right atrium play a similar role in holding the blood which is then transported to the ventricles. However, the right atrium holds the blood with more carbon dioxide while the upper chamber on the left side holds the blood with less carbon dioxide

Right Ventricle and left ventricles- both ventricles serves a similar role of pumping blood. However, the blood with much carbon dioxide and which is transported by the right atrium is received by the right ventricle and pumped into the pulmonary arteries (Lippincott et al. 2007). On the other hand, the blood with rich in oxygen and less in carbon dioxide and which is transported by the left atrium is received by the left ventricle where it pumped into the systematic circulation.  Both ventricles are separated by the interventricular septum.





  Multicellular organism benefits from the circulatory system. First, all processes in the circulatory system ensure effective exchange and transportation of nutrients and oxygen. Also, the process ensures healthy cells. For example, the heart is an amazing organ that pumps blood when one as asleep and while working. Having understood how the heart works and the benefits of the circulatory system, it is important to protect our hearts from illness. Human beings can avoid risk factors such as poor diet stress, high cholesterol level, and smoking among other factors. It is also important to ensure regular physical activity to prevent illnesses such as heart attack, high blood pressure, and others.










Fisher, S. A., & Burggren, W. W. (2007). Role of hypoxia in the evolution and development of

the cardiovascular system. Antioxidants & redox signaling9(9), 1339-1352.


Marshall, J., (2010). A description of the human body: Its structure & functions. the Bavarian

State Library


Santambrogio, L. (2013). Immunology of the lymphatic system. L. Santambrogio (Ed.). New

York: Springer.


Lippincott Williams & Wilkins., & Ovid Technologies, Inc. (2007). ECG strip ease.

Philadelphia: Lippincott Williams & Wilkins.


Stephenson, A., Adams, J. W., & Vaccarezza, M. (2017). The vertebrate heart: an evolutionary

perspective. Journal of anatomy231(6), 787-797.




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