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E - Bio @ Horton AP Biology Unit – Animal Diversity Notes – Invertebrate Circulation Invertebrate Transport Mechanisms A. Transport Mechanisms 1. Unicellular protozoa make exchanges directly with environment across the plasma membrane. 2. Larger invertebrates usually have circulatory systems-either an open system or a closed system. B. Invertebrates With a Cavity 1. Hydra and planaria are organisms with a sac body plan; a circulatory system is unnecessary. 2. Hydra cells are part of external layer or gastrovascular cavity; diffusion supplies all nutrients. 3. Planaria have gastrovascular cavity and small, flat body where diffusion meets these needs. 4. The cells of the organism in essence meet the environment on both sides allowing diffusion to take place. C. With an Open or a Closed System 1. In a circulatory system, a pumping heart moves one of two circulatory fluids into blood vessels. a. Blood is a circulatory fluid contained within blood vessels b. Hemolymph is a circulatory fluid which flows into the sinus of certain arthropods and mollusks; it is a mixture of blood and intercellular fluid. 2. Certain arthropods and mollusks have an open circulatory system. a. Hemolymph is pumped by the heart into body cavity or sinuses. Hemolymph bathes internal organs and then drains back to heart. b. In grasshoppers, dorsal heart pumps hemolymph into an aorta, which empties into a cavity. 3. Some invertebrates, including earthworms have a closed circulatory system in which blood never leaves the heart or vessels. a. Valves prevent backward flow of blood as it moves through vessels. b. Earthworms have five pairs of anterior lateral vessels that pump blood to every segment. c. Blood moves in capillaries where exchange with tissue fluid take place before returning in veins. d. With no special cavity for gas exchange, gas must diffuse across body wall. Notes Animal Diversity 1 Vertebrate Circulation A. Closed Circulatory System 1. Vertebrates have a closed circulatory system in a cardiovascular system. 2. Muscular heart keeps blood circulating through the animal body. a. Atrium is chamber of the heart that receives blood. b. Ventricles pump blood into arteries. 3. There are three kinds of blood vessels: arteries carry blood away from heart, capillaries are where exchange with tissue fluid takes place, and veins return blood to heart. (Figs. 41.3-4) a. Arteries: 1. have thick walls and are resilient. 2. elastic tissue allows the expansion to accommodate sudden increase in blood volume that results after heart contraction. 3. divide into small arterioles. b. Arterioles constriction and dilation are regulated by nervous system to regulate blood pressure. c. Capillaries are microscopic blood vessels with a wall formed of one layer simple cells. 1. Capillary beds are so prevalent that, in humans, all cells are within 60-80 µm of a capillary. 2. Only 5% are open at one time; if animal eats, capillary beds of digestive system open. 3. Capillaries are so narrow that red blood cells must pass through in single file. 4. Gas, nutrient, and waste exchange occurs across thin walls. d. Venules are vessels that take blood from capillaries, and join to form a vein. e. Veins transport blood toward the heart. 1. Wall of a vein is much thinner than that of arteries; there is low blood pressure. 2. One-way valves open in direction of heart; close to prevent backflow. Notes Animal Diversity 2 B. Comparison of Circulatory Pathways 1. In vertebrates, there are three different types of circulatory pathways. 2. Fishes have a one-circuit (single-loop circulatory) pathway. a. Heart has a single atrium and ventricle and pumps blood under pressure to gills. b. Blood in gills is oxygenated and then travels to the body.. 3. Other vertebrates have a two-circuit (double-loop circulatory) pathway to breathe air on land. a. Systemic circulation transports blood to tissues. b. Pulmonary circulation pumps blood to lungs. 4. In amphibians and most reptiles, the heart has two atria but a single ventricle. Some mixing of blood occurs in the ventricle taking away from the efficiency of the heart. 5. Hearts of some reptiles and all birds and mammals are divided into two halves. a. With two atria and two ventricles, oxygenated blood is always separate from deoxygenated blood. b. Right ventricle pumps blood to lungs; left ventricle pumps blood to rest of body. c. This arrangement provides adequate blood pressure for both pulmonary and systemic circulations. th Images: Campbell, Neil and Reece, Jane. Biology (6 ed.) San Francisco: Benjamin Cummings th Modified Notes: Mader, Sylvia. Biology (7 ed) New York: McGraw Hill Publishing Notes Animal Diversity 3