Gas Exchange & Circulation Chapter 42 Gas Exchange & Circulation • 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. • Blood or Interstitial Fluid • Most animals have organ systems specialized for exchanging materials with the environment, and many have an internal transport system that conveys fluid throughout the body. • Aquatic Organisms • Structures like 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. • Diffusion is Not Good Enough Diffusion alone is inadequate for transporting substances over long distances in animals • Diffusion is insufficient over distances of more than a few millimeters, because the time it takes for a substance to diffuse to one place to another is proportional to the square of the distance. • For example… • If it takes 1 sec for a given quantity of glucose to diffuse 100 microns • it will take 100 seconds for it to diffuse 1 mm • It will take almost 3 hours to diffuse 1 cm • The circulatory system solves this problem by ensuring that no substance must diffuse very far to enter or leave a cell. Simple Internal Transport Systems • Aquatic Invertebrates • Gastrovascular Cavity (GVC) found in • Cnidarians (Jellies, Hydra, Anemones) • Planarians & other flatworms • Circulatory Systems found in • Mollusks • Crustaceans • InsectsGastrovascular Cavities • 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 and for 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. • 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 are cells are bathed by a suitable medium and diffusion distances are short. Circulatory Systems • Large Animals • For animals with many cell layers, gastrovascular cavities are insufficient for internal distances because the diffusion transports are too great. • Two types of circulatory systems that overcome the limitations of diffusion have evolved: • Open circulatory systems • Closed circulatory systems • Both have a circulatory fluid (blood), a set of tubes (blood vessels), and a muscular pump (the heart). • The heart powers circulation by using metabolic power to push blood through a circuit back to the heart Open Circulatory Systems • Common in… • Insects and other arthropods • Most mollusks • Blood bathes organs directly in an open circulatory system • There is no distinction between blood and interstitial fluid • In this case, they are the same thing a are 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 through pores called ostia. • Body movements that squeeze the sinuses help circulate the hemolymph. • Found in… • Earthworms • Advanced Mollusks (squid, octopuses) • Vertebrates • Blood is confined to vessels and is distinct from the 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 blood and the interstitial fluid bathing the cells. Closed Circulatory System Cardiovascular System (CVS) • Vertebrate Circulatory Systems are often called Cardiovascular Systems • The heart consists of one atrium or two atria, the chambers that receive blood returning to the heart, and one or two ventricles, the chambers that pump blood out of the heart. CVS Blood Vessels • Three primary blood vessel types: • Arteries • Arteries carry blood away from the heart • Veins • Veins carry blood to the heart • Capillaries • Capillaries carry blood to and away from tissues CVS Blood Flow • Heart • Arteries • Arterioles • Capillaries • Venuoles • Veins • Heart Arteries and veins are distinguished by the direction in which they carry blood, not by the characteristics of the blood they carry.Metabolic Rate • Metabolic rate is an important factor in the evolution of cardiovascular systems. • In general, animals with high metabolic rates have more complex circulatory systems and more powerful hearts than animals with low metabolic rates. • Similarly, the complexity and number of blood vessels in a particular organ are correlated with that organ’s metabolic requirements. • Perhaps the most fundamental differences in cardiovascular adaptations are associated with gill breathing in aquatic vertebrates compared with lung breathing in terrestrial vertebrates. Fish • A fish heart has two main chambers, one atrium and one ventricle. • Blood is pumped from the ventricle to the gills (the gill circulation) where it picks up oxygen and disposes of carbon dioxide across the capillary walls. • The gill capillaries converge into a vessel that carries oxygenated blood to capillary beds at the other organs (the systemic circulation) and back to the heart. • In fish, blood must pass through two capillary beds, the gill capillaries and systemic capillaries. • When blood flows through a capillary bed, blood pressure - the motive force for circulation - drops substantially. • Therefore, oxygen-rich blood leaving the gills flows to the systemic circulation quite slowly