Chapter 42 Circulation and Gas ExchangeLecture OutlineOverview: Trading with the Environment- Every organism must exchange materials and energy with its environment, and this exchange ultimately occurs at the cellular level.- Cells live in aqueous environments.- The resources that they need, such as nutrients and oxygen, move across the plasma membrane to the cytoplasm.- Metabolic wastes, such as carbon dioxide, move out of the cell.- Most animals have organ systems specialized for exchanging materials with the environment, and many have an internal transport system that conveys fluid (blood or interstitial fluid) throughout the body.- For aquatic organisms, structures such as gills present an expansive surface area to the outside environment.- Oxygen dissolved in the surrounding water diffuses across the thin epithelium covering the gills and into a network of tiny blood vessels (capillaries).- At the same time, carbon dioxide diffuses out into the water.Concept 42.1 Circulatory systems reflect phylogeny- Diffusion alone is not adequate for transporting substances over long distances in animals—for example, for moving glucose from the digestive tract and oxygen from the lungs to the brain of a mammal.- Diffusion is insufficient over distances of more than a few millimeters, because the time it takes for a substance to diffuse from one place to another is proportional to the square of the distance.- For example, if it takes 1 second for a given quantity of glucose to diffuse 100 microns, it will take 100 seconds for it to diffuse 1 mm and almost three hours to diffuse 1 cm.- The circulatory system solves this problem by ensuring that no substance must diffuse very far to enter or leave acell.- The bulk transport of fluids throughout the body functionally connects the aqueous environment of the Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 42-1body cells to the organs that exchange gases, absorb nutrients, and dispose of wastes.- For example, in the mammalian lung, oxygen from inhaled air diffuses across a thin epithelium and into theblood, while carbon dioxide diffuses out.- Bulk fluid movement in the circulatory system, poweredby the heart, quickly carries the oxygen-rich blood to allparts of the body.- As the blood streams through the tissues within microscopic vessels called capillaries, chemicals are exchanged between blood and the interstitial fluid that bathes the cells. Most invertebrates have a gastrovascular cavity or a circulatory system for internal transport.- The body plan of a hydra and other cnidarians makes a circulatory system unnecessary.- A body wall only two cells thick encloses a central gastrovascular cavity that serves for both digestion andfor diffusion of substances throughout the body. The fluid inside the cavity is continuous with the water outside through a single opening, the mouth. Thus, both the inner and outer tissue layers are bathed in fluid.- In cnidarians such as Aurelia, the mouth leads to an elaborate gastrovascular cavity that has branches radiating to and from the circular canal.- The products of digestion in the gastrovascular cavity are directly available to the cells of the inner layer, and it is only a short distance to diffuse to the cells of the outer layer.- Planarians and most other flatworms also have gastrovascular cavities that exchange materials with the environment through a single opening.- The flat shape of the body and the branching of the gastrovascular cavity throughout the animal ensure that cells are bathed in a suitable medium and that diffusion distances are short.- For animals with many cell layers, gastrovascular cavities are insufficient for internal distances because the diffusion transports are too great.- In more complex animals, two types of circulatory systems that overcome the limitations of diffusion have evolved: open circulatory systems and closed circulatory systems.- Both have a circulatory fluid (blood), a set of tubes (blood vessels), and a muscular pump (the heart).Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 42-2 The heart powers circulation by using metabolic power to elevate the hydrostatic pressure of the blood (blood pressure), which then flows down a pressure gradient through its circuit back to the heart.- In insects, other arthropods, and most molluscs, blood bathes organs directly in an open circulatory system.- There is no distinction between blood and interstitial fluid, collectively called hemolymph.- One or more hearts pump the hemolymph into interconnected sinuses surrounding the organs, allowing exchange between hemolymph and body cells.- In insects and other arthropods, the heart is an elongated dorsal tube.- When the heart contracts, it pumps hemolymph through vessels out into sinuses.- When the heart relaxes, it draws hemolymph into the circulatory system through pores called ostia.- Body movements that squeeze the sinuses help circulate the hemolymph.- In a closed circulatory system, found in earthworms,squid, octopuses, and vertebrates, blood is confined to vessels and is distinct from interstitial fluid.- One or more hearts pump blood into large vessels that branch into smaller ones coursing through organs.- Materials are exchanged by diffusion between the bloodand the interstitial fluid bathing the cells.- The fact that open and closed circulatory systems are both widespread in the animal kingdom suggests that both systems offer advantages.- The lower hydrostatic pressures associated with open circulatory systems make them less costly than closed circulatory systems.- Because they lack an extensive system of blood vessels, open systems require less energy to build and maintain.- In molluscs and freshly molted aquatic arthropods, the open circulatory system functions as a hydrostatic skeleton.- What advantages are associated with closed circulatorysystems?- Closed systems with their higher blood pressure are more effective at transporting circulatory fluids to meet the high metabolic demands of the tissues and cells of larger and more active animals.Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 42-3- Among the molluscs, only the large and active squid and octopuses have closed circulatory systems. Vertebrate phylogeny is reflected in adaptations of the cardiovascular system.- The closed circulatory system of humans and other vertebrates is often