Part 9 – The Human Circulatory System
Within each cell, we see a system of production, delivery, controls, and waste management. The body as a whole also has a similar system. The circulatory system receives nutrients, delivers nutrients, and removes waste.
What is quite fascinating to me is how every body system is dependent on the circulatory system, while at the same time, the circulatory system cannot operate without the control or input of the majority of the other body systems. Every organ depends on receiving nutrients from the blood. That blood cannot reach the organs without the development of a pump and vessels that connect to every cell. The pump alone cannot successfully control blood flow, but also needs the nervous system to drive the vessel controls for continuous successful flow.
The circulatory vessels cannot retrieve oxygen from the air and must get oxygen from the pulmonary system. The red blood cells that carry the oxygen are not made in the circulatory system, but instead are made in the bones. The bones cannot make the red blood cells without the digestive system. The digestive system requires not just food input, but also an array of enzymes and bacteria entering at different stages to act upon what is ingested to yield the molecules that are required by the bones. The liver, kidneys, and spleen remove waste from the blood which would otherwise kill the organism. The kidneys control the water volume in the blood required for circulation and nutrient perfusion.
A dive into each of the sentences above unlocks a whole new world of chemical cascades, involvement with organelles, anatomical requirements, and control mechanisms. And beyond all these requirements for operation, the blood must be able to clot, or else the organism would quickly bleed to death. And without perfect clot regulation, a clot could form in the wrong location causing organ failure.
This abstract at Clinical Chemistry shows the three main chemical chains required in blood clotting: https://academic.oup.com/clinchem/article-abstract/14/2/97/5674621?redirectedFrom=PDF. This article at the National Library of Medicine introduces the clotting factors involved in the enzymatic cascade that results in a blood clot: https://www.ncbi.nlm.nih.gov/books/NBK507850/.
Evolution of the Circulatory System
Since each evolutionary step in the circulation system had to be beneficial to the organism, the human circulatory system had to develop some components of the circulatory system concurrently. For example, the blood vessels are not beneficial to an organism without the generation of some kind of fluid. When a pump continuously pushed blood in the vessels, the development of a chemical cascade and control mechanism for clot formation was required so that the organism would not bleed out. The development of the heart would not be beneficial without a method of pressure control in the vessels, otherwise the blood would pool. The evolution of red blood cells designed to carry oxygen required the evolution of some type of oxygen retrieving system.
Today the circulatory system is dependent upon the majority of our other body systems. If these distinct but dependent systems simultaneously evolved, changes in one system would have to be accounted for in each other system that depended on it. Changes which affected one system's structural or chemical output, must simultaneously be suitable to the other systems which had depended on the old output. Given the precision seen in every feature of every cell and organ, this again is a tall order.
If on the other hand, the circulatory system was at one time independent, and later evolved dependence, then other questions are introduced. If the circulatory system already had functional independence in a precursor state, then significant changes must have occurred to replace independence with dependence. Efficiency is certainly increased with dependence. However, the steps from independence to dependence must not cause any functional harm on their journey to an improvement in efficiency.
The structures and chemical cascades that were making and operating all the circulatory components would have to be shut down precisely while ramping up the ability to receive the new components and control from the external sources. For example, if the circulatory system made its own fluid at one time, then the body must cease creating the fluid components in this precursor system at a comparable rate to generating ways to make and receive fluid from another system.
If the circulatory system was able to circulate fluid on its own at one time, then the chemical reactions that once controlled the vessel dilation and compression and heart contraction had to smoothly transition to endocrine control and nervous system control. The mechanics of circulation today involves portions of the brain, nerve cells, and muscle cells, all run by electrical impulses, chemical reactions, and hormones. This concept of system turnover would also apply to oxygen retrieval, volume control, waste removal, clotting factors, etc.
Today, small changes in chemical cascades lead to system dysfunction. To imagine stepwise changes sufficient to replace an independent system with a new dependent system is astounding. To begin to learn what is specifically involved in controlling the circulation of blood, check out the articles by the National Library of Medicine (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4404375/) and Advances in Physiology Education (https://journals.physiology.org/doi/full/10.1152/advan.00114.2010).
Today's circulatory system cannot circulate the body's blood without endocrine and nervous system control. And the above two articles show that this control is incredibly complex, precise, and integrated in total body monitoring. Changeover from a precursor circulatory system that could circulate blood itself to the system observed today, would require tremendous development and coordination of new externally assembled, and chemically and electrically matched components.
Summary
A close inspection of the circulatory system's multi-wide system dependence reveals to me that the evolution of this system, whether it developed simultaneously alongside of other systems or independently, would be a very astounding task. In addition to the issue of system dependencies, the number of factors required for precise blood clotting (not too much and not too little) is difficult for me to imagine being created in a successfully stepwise fashion.
Each generated alteration in the circulatory system would change structures and/or the chemical inputs and outputs of increasingly lengthy chemical cascades. These changes would impact not only the way each precursor circulatory system functioned, but also the systems which up to that point depended on it. Each stepwise buildup toward the anatomical and chemical complexity of today, would have to also develop the complete reliance every other system of our body has upon today's circulatory system.
The nervous, endocrine, and skeletal system have developed inputs and controls over the circulatory system while also receiving all their nutrients from the circulatory system. The respiratory, lymphatic, and digestive systems are completely integrated structurally and chemically with the circulatory system. The muscular, integumentary, and reproductive system, as well as every organ in the human body, are dependent upon the delivery system of the blood and its nutrient and waste management. Stepwise changes would have to balance the molecular cascading needs of each and every system listed above. Conceiving a stepwise generation of such integration, with each precursor stage being fully functional itself, is to me, challenging.
In Part 10, we will look a little closer into our DNA to conclude our investigation into the human body.