9 1 Evolution and Physics Evolution of Circulatory Systems Animal life evolved in water where everything their cells needed was dissolved in their surrounding medium The ions oxygen and small molecules like amino acids that are required for cellular respiration and protein synthesis diffused randomly in the water Over time animals became more complex and evolved multiple layers of cells Recall that sponges the oldest living animals develop from only one layer of embryonic cells Placozoans Ctenophores and Cnidarians develop from only two These animals are only a few cell layers thick so each cell is never far from the water Thus these animals do not need organs or mechanisms to transport nutrients to their cells it occurs sufficiently by simple diffusion Diffusion can be rapid over small distances such as the width of a few cells but it is very slow over long distances These animals as well as flatworms do not have circulatory systems Many of them have body cavities that are open at one end and contain fluid that is continuous with the surrounding water The cells lining this cavity are often ciliated to propel the ocean water across the surface of the cavity and keep it circulating As animals evolved more cell layers the distance between some cells and the nutritious ocean water increased While an increase in size and complexity allowed animals to perform more complex behaviors and take advantage of available niches they also required a transport system to keep those cells alive Further as bodies became more complex distant cells needed the ability to communicate with each other and act as a coordinated system These needs acted as selective pressures driving the evolution of circulatory systems Most animals kingdom Metazoa also referred to as metazoans larger than a few cells have circulatory systems These vary in complexity and arrangement but they share the same basic functions Transport oxygen and nutrients from source to cells Transport waste CO2 away from cells to be released from body Transport immune cells and signaling molecules throughout the body Animals with circulatory systems have a well defined body cavity that is filled with fluid The contents of this fluid are tightly regulated a process called osmoregulation which we will learn about in two weeks Because many cells are not in direct contact with a nutritious environmental medium as they are in a very thin marine animal like a jellyfish this internal fluid serves the same purpose providing nutrients and preventing the build up of waste Just as in simple marine animals these molecules move into and out of the cells via diffusion they are completely dependent on concentration gradient Thus regulating the concentration of these molecules in the fluid is crucial to keeping the cells in the body alive Circulatory systems move this internal fluid through the body transporting molecules by bulk flow of the fluid By increasing the pressure on the fluid in one part of the body the animal squeezes the fluid and moves it to another part Using pressure gradients the animal can transport fluid and anything dissolved in it throughout the body Pressure is exerted on the fluid in the body using muscular contractions and flow direction is often ensured using valves as shown below In many invertebrates there is a single fluid called hemolymph that fills the internal cavity and bathes all of the cells In most animals it circulates through tubes called vessels and is pumped from part to part using one or more muscles called hearts This arrangement is called an open circulatory system and it is common in many invertebrates Many molluscs and annelids have open circulatory systems as well as all arthropods and a few chordates only tunicates or sea squirts Some annelids also circulate their fluid using muscular contractions of the body wall which squeezes the body pushing the blood out of that region All vertebrates and some invertebrates have closed circulatory systems Among the molluscs only cephalopods have closed systems In a closed circulatory system the fluid inside the vessels is separate from the fluid that bathes the cells but nutrients and waste diffuse through the layer of cells that separates the two compartments The fluid inside the vessels is blood and the fluid that directly surrounds the cells is interstitial fluid The concentration gradients between the blood vs interstitial fluid and interstitial fluid vs cells are both crucial for the same reason that the gradients between the hemolymph and cells is in open circulatory systems Closed systems exert more pressure on the fluid allowing the animal to increase the flow rate of the blood and more finely control its distribution Closed circulatory systems co evolved with genes that encode efficient oxygen carrying molecules These adaptations allowed animals to efficiently transport gases nutrients and other molecules throughout the body supporting higher metabolic rates and allowing animals to become even larger and more complex Throughout evolution circulatory systems have been shaped by the selective pressures of metabolic demand and anatomy Animals that were pursued by fast swimming predators would be more fit if they could supply their muscle cells with lots of oxygen quickly Animals with large bodies would be more successful if they could efficiently pump their blood over long distances All circulatory systems are limited by the laws of physics that govern how fluid flows The walls of blood vessels will exert some resistance on blood s ability to flow in this way blood vessels act like resistors in an electrical circuit Circulatory Systems as Circuits Recall that resistors arranged in series will add up in the circuit diagram below as electrical current tries to flow clock wise from the battery it will decrease at each resistor it encounters The net effect by the time the current gets back to the battery will be the additive effects of all of the resistors In the same way if we imagine lining up all the blood vessels in an animal s body and try to pump blood through the entire tunnel there would have to be an enormous amount of pressure to get the blood all the way back to the pump By the time it gets through all those vessels the pressure will have dropped dramatically perhaps to the point that the blood is no longer pushed forward and will pool in that part of the body An animal must be able to serve all of the cells in its body regardless of how large it is If it has
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