Introduction: Why Animals have a Circulatory System

I have endeavored to make the following tract as plain and as intelligible as I can; and if it should appear prolix to those who are already acquainted with the subject, I must beg leave to observe, that it was not written for their information; but if any of those who were unacquainted with it before should from hence gain any useful knowledge, my end will be answered, and I shall be very much pleased

Percivall Pott: Observations on That Disorder on the Corner of the Eye Commonly Called Fistula Lachrymalis, London, 1759

The world of veterinary cardiology has undergone a rapid expansion in the last 50 years. Starting in the early 1960s with Drs. Patterson, Detweiller and Buchanan, who first began looking at congenital disease in dogs, and blossoming in the last few years with four-dimensional echocardiography, electrocardiographically gated computed tomography and new genetic testing research. At no other time has more been understood about the function and dysfunction of the cardiovascular system. As the state of the art grows, so does the need for highly trained paraprofessionals to assist the doctors who are pushing back the frontiers of veterinary cardiology and those putting their discoveries into practical application.

The field of veterinary cardiology has grown so rapidly that each year approximately 10 veterinarians specialize in cardiology. There are so many boarded cardiologists in the USA that there is shortage of highly trained specialized veterinary technicians. It is the hope of the author that this text will be a launch point for those veterinary technicians and nurses who wish to pursue specialization in veterinary cardiology, as well as for veterinary technical students and veterinary students, to explore this fascinating and expanding world. It should be stated at the beginning that this textbook is not designed to be an exhaustive tome of all cardiovascular knowledge, but rather a summary of many other outstanding textbooks, formulated into an easily manageable text to give the reader fundamental knowledge that will prepare them for deeper study into the subjects that interest them. The author also hopes that this text will be a quickly accessible resource to aid in answering common clinical questions faced by the cardiology veterinary technician specialist. The interested reader is encouraged to dig deeper into the particular areas of veterinary cardiology that interest them by accessing the textbooks cited in the references and bibliographies.

Cardiovascular medicine continues to advance and the current horizons will soon be distant memories as new knowledge of the circulation system evolves. As we explore this new world of veterinary cardiovascular medicine we can begin with basic questions.

Why have circulatory systems? How does a circulatory system aid the species? Why is it so complicated? I'd like to think that I was the first to ask these questions, but I know better. These are questions that have been asked by great physicians going back to Hippocrates or earlier. However, Dr Alan C. Burton made quite a delightful description in his classic book Physiology and Biophysics of the Circulation [1] with the invention of the Celestial Committee on the Design of a Mammalian Circulation (CCDMC). This imaginary committee would of course be charged with the task of designing the mammalian circulatory system.

If such a committee was to exist, they would be faced with several daunting tasks. First let us consider the definition of circulation. The CCDMC has put forth the following: "The function of the circulation is to supply oxygen, metabolic fuels, vitamins and hormones, and heat to every living cell of the organism and also to remove metabolic end products (carbon dioxide, water etc.) and heat. The amount of circulation should be in accordance with the needs of each cell."

So an answer to the first question of WHY comes forth. Put in a very simplistic way, if you are a very simple animal, say an amoeba, then diffusion is an acceptable way to move nutrients in and waste material out of the organism. Diffusion, of course is the passive transfer of molecules from an area of high concentration to low concentration across a permeable or semipermeable barrier. Unfortunately, if you are a very large animal, say a horse, then it is much more difficult to move enough nutrients in and waste out by diffusion alone. Diffusion is not efficient enough to work over long distances and multiple cell layers seen in complex organisms. A circulatory system allows the organism to carry nutrients to the cell directly where diffusion can work at the local cellular level and will pick up waste products from the cells and carry them to a disposal area to be eliminated from the body. By use of a circulatory system to move a viscous fluid (blood) to improve diffusion and heat exchange, large and biologically diverse organisms can be sustained.

With this definition as a starting point, we can begin to elaborate some of the challenges that would need to be overcome. How much flow does each tissue need? What determines these requirements? Do the needs of a tissue group change over time? What are the possible methods of making circulation work and what are the physical limitations? What methods are available for controlling the circulation? Of course the answers to these questions are interlocking.

The first thing to consider are the individual needs of the different tissues. It makes sense that different tissues need different amounts of nutrients, and/or waste removal at different times. At the same time, it is inefficient to simply supply the same amount of blood to all the tissues as required by the neediest. Consequently, some form of control or regulation is required. Certain tissues, like the brain and the heart that have critical functions, need some priority so that their blood supply remains constant at all times. Other tissues have varying requirements and different times. For example, during exercise the skeletal muscles would need more perfusion than the gut; after eating the reverse is true. Also we cannot forget the need to balance heat, as indicated in our definition of a circulatory system previously. Muscles and organs under work generate heat that must be removed from the body, to maintain the core body temperature. Temperature also has a large impact on metabolism. Remember that at higher temperatures, metabolism increases and decreases with a reduction in temperature. Maintaining the optimal temperature for cellular metabolism becomes crucial for proper function of the organs.

Another consideration is the function or task of the organ in question. Lungs and kidneys for example are organs that eliminate waste (in the case of the lungs, to pick up nutrients also). Their individual metabolic requirements are actually quite small, but if they do not receive a large amount of flow then they cannot effectively rid the blood of waste products. Although the critical importance of the organ alone is insufficient for dictating its required blood flow, the ability of an organ to continue its work in the face of diminished or even a terminated supply of nutrients or its "functional reserve" also plays into the equation. Tissues such a skin or skeletal muscle can continue to function in a state of "oxygen debt" and are said to have a high functional reserve. They will continue to function on anaerobic metabolism, allowing necessary blood to be shunted to organs with less of a functional reserve, like the brain, which has a low functional reserve. It becomes clear that a system that can change the volume of flow to a specific area at any given time is needed.

One of the limitations is that the tissues require a constant supply of nutrients, which means that the system must also function continuously. A second limitation is the physical properties that govern the movement of fluids. The most important of these is Darcy's Law of Flow. Darcy's Law states that flow (Q), that is volume of liquid moved over a period of time, is equal to the difference in pressure from one space to another in which it moves (P1-P2) divided by the resistance (R) to that flow (expressed mathematically: Q= P1-P2/R). This rule of physics is fundamental to regulating flow in all environs. There are other factors that affect flow, such as viscosity, but an in-depth discussion of these factors is beyond the scope of this text. With these concepts in mind, we can see that a simple solution to these problems is to provide a constant perfusion pressure. This means keeping the P1 and the P2 of Darcy's Law at the same pressure no matter the volume of blood moving to a tissue. By keeping the maximal flow the same to all tissues at all times, a constant perfusion pressure could be maintained, but we have already stated that as inefficient. So how can one provide the right amount of perfusion to the areas in need and yet maintain a constant perfusion pressure throughout the system?

The answer to this question involves two components: a pump and a series of conduction tubes to move the fluid to the cells. The heart in its essence is a very elaborate pump; actually two pumps in parallel. It has the ability to change its rate of pumping (chronotropy) and how hard it pumps (inotropy) providing sufficient output to maintain a constant flow of blood to the whole body or reducing flow when at rest to conserve energy. Additionally the heart has some stretch ability that allows it to increase the volume it will hold and consequently pump out changing volumes (stroke volume).

The conduction tubes are a complex system of...