Circulation

Why we need a circulatory system Diffusion can move substances only a few millimeters and time increases greatly as distance increases Fluid transport can: – Connect cells to organs that exchange gases (respiration) – Absorb nutrients (digestion) – Dispose of waste (excretion)

Invertebrates Most invertebrates have a gastrovascular cavity for dies – Cnidarians have a body wall to enclose their circular gastrovascular cavity

Open circulatory system There’s no differentiation between blood and interstitial fluid, which are called hemolymph collectively – Some organisms (arthropods, mollusks, insects) have their organs bathing directly in blood The heart is an elongated dorsal tube that pumps hemolymph – Heart contracts: hemolymph out of organ sinsuses – Heart expands: Hemolymph enters through pores called ostia

Closed system Blood is confined to vessels and is distinct from interstitial fluids – Large vessels branch into small ones that course through organs – Materials travel by diffusion between blood and interstitial fluid

Comparison Open systems have lower hydrostatic pressures (like blood pressure) so they require less energy expenditure because they have no extensive vessel network – Can also function as hydrostatic skeleton in mollusks and freshly molted aquatic arthropods Closed systems are more effective at transporting circulatory fluids for more active animals

Cardiovascular System Heart: one atrium or two atria (the chambers that receive blood returning to the heart) and one or two ventricles (chambers that pump blood out) Arteries, veins, and capillaries – Arteries carry blood Away from the heart to the organs Branch into arterioles, which convey blood to capillaries – Capillaries with thin, porous walls are called capillary beds and are in every tissue – At the downstream end, capillaries converge into venules, which converge into veins

Heart -> arteries -> capillaries Capillaries -> veins -> heart – It’s all the same blood – Exception: the hepatic portal vein carries blood from capillary beds in the digestive system to capillary beds in the liver Animals with high metabolic rates generally have more complex cardiovascular systems

Aquatic Cardiovascular Systems Fish hearts have one atrium and one ventricle Blood is pumped from the ventricle to the gills, where it picks up oxygen and disposes of carbon dioxide through the capillary walls Gill capillaries converge in a vessel that carries oxygenated blood to capillary beds in other organs and back to the atrium via veins – Passes through gill capillaries and systemic capillaries – Passing through a capillary bed, blood pressure drops substantially so oxygen-rich blood leaves the gills slowly

Amphibian (Frogs!) Cardiovascular System Three chambered heart – two atria, one ventricle – Ventricle pumps blood into a forked artery that splits the ventricle’s output into the pulmocutaneous and systemic circulations – Pulmocutaneous goes to gas-exchange organs (lungs and skin) to pick up oxygen and release carbon dioxide before returning to the left atrium Returning blood into systemic circulation, circles all of the organs