L02 Inv to Vert 2011 - COPYRIGHT Mammalian Physiology BIOAP...

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1/8 COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 Spring 2011 THE EVOLUTION OF MULTICELLULAR SYSTEMS (from invertebrate to vertebrate) 1) Design of multicellular animal: the vertebrate plan. The evolution of "bigger and better" multicellular animals via the development of a closed circulatory system among other specializations (divisions of labor). With increasing size of animals, the demands on the circulatory pump (heart) became increasingly larger as the extracellular fluid volume (usually 20% body weight) increased. Thus it became advantageous to divide the extracellular fluid into 1) an interstitial fluid compartment (16% of body weight), and 2) the plasma compartment (4% of body weight). Only plasma is pumped by the heart, thereby reducing its workload. Plasma is circulated at high rates and the interstitial fluid is circulated at low rates. Plasma is the liquid portion of blood under high pressure on the arterial side to the circulatory system. As a result of this hydrostatic pressure, some fluid always leaks from the capillary to the interstitial fluid threatening with edema. Part of the leaked fluid is collected by lymph vessels and returned to circulatory system via the azygous vein (at low pressure) that empties its collections into the vena cava. Fluid can also return to the capillary lumen at the distal end of the capillary (see Starling Law of the capillary). Fig. 1. Functional design of vertebrates (animals with a backbone*) with a closed circulatory system). Claude Bernard was first to recognize the relationship between the external environment (the habitat) and the internal environment (milieu interne, extracellular fluid, plasma and lymph) in which the cells of the multicellular animal live. We reside in our habitat, but we live inside our own private backpack, the extracellular fluid compartment. The extracellular fluid compartment is maintained constant with time (homeostasis) by life- supporting organs such as the lungs, kidneys, liver, gastro-intestinal tract, and other systems. *with the exception of some specimen of Homo sapiens .
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2/8 2) Advantages of the closed circulating system. a) Decreased work load on the pump (heart). Moving plasma volume (1/4 of the extracellular fluid volume) instead of the entire extracellular fluid volume reduces the workload on the heart by 80%. Not quite, because the heart moves blood which is a tissue consisting of cells (45%) and saline (55%). Accordingly, a closed circulatory system reduces the workload of the heart by about 70%. The workload on the heart is furthermore reduced by reducing the total resistance of the closed circulatory system which is accomplished by arranging vascular beds parallel to each other, rather than in series (Fig. 2). Note that the circulatory system of vertebrates can be modeled with an equivalent electrical circuit, where the heart is a DC battery, and where each perfused tissues is represented by a resistor. Flows of water and electric current are analogous. Fig. 2.
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L02 Inv to Vert 2011 - COPYRIGHT Mammalian Physiology BIOAP...

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