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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.
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Images: Campbell, Neil and Reece, Jane. Biology (6 ed.) San Francisco: Benjamin Cummings
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Modified Notes: Mader, Sylvia. Biology (7 ed) New York: McGraw Hill Publishing
Notes
Animal Diversity
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