Circulatory System

May 23rd, 2016
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Brief but covers everything under the topic circulation. It explains how blood floes through the heart and also hoe a heart beat is initiated.

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Lecture 18: The Circulatory SystemA large multicellular organism needs a transport system for the following reasons:1.Simple diffusion across the body surface cannot keep pace with the demands of the organism.2.A large part of the external body surface must be kept impermeable to conserve water(particularly for terrestrial organisms).3.Internal distances are too great to be served by diffusion.In these organisms, there are specialized organs for gas exchange, digestion, excretion linked by acirculatory system geared to move substances rapidly from one place to another. The main function ofthe circulatory system then is to transport gases, nutrients, waste material and hormones. It is alsoinvolved in homeostasis and defense of the body (Fig. 1).Fig. 1From animals have an open circulatory system e.g., roundworms and arthropods. The circulatoryfluid in this system percolates slowly through the body cavity in open-ended vessels, bathes the tissuesdirectly and then seeps back to the heart.Most animals including humans have a closed circulatory system where the circulatory fluid travels inclosed tubes or vessels, separate from other body fluids. This allows for high pressures and a rapid flowto be maintained. The tissues themselves are not in direct contact with blood but are bathed byinterstitial/intercellular fluid derived from blood that filters across the vessels.The human circulatory system co

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