Animal Circulatory Systems

true circulatory systems share:

• a circulatory fluid to facilitate nutrient and/or gas exchange

• set of tubes/vessels for fluid transport (arteries, capillaries, veins)

• a muscular pump (heart) to move the circulatory fluid

can be open or closed

gastrovascular cavity

functions in both digestion and distribution of substances throughout the body

• small organisms with only two cell layers; flattened body with high surface area/volume ratio

• sponges, cnidarians, and flatworms lack a distinct circulatory system

• exchange of fluids is assisted by the pulsing of the animal’s body

open circulatory system

circulatory fluid, called hemolymph (blood), is pumped by the heart through vessels and exits the vessels into a body cavity where it directly bathes the organs directly for nutrients and/or gas exchange- the circulatory fluid is the interstitial fluid

• heart has arteries but no veins

• may or may not exchange oxygen; has oxygen-carrying pigment but not contained within cells- free in the hemolymph

• low pressure = less energy cost

• common among invertebrates, including arthropods and most mollusks

• no oxygen exchange in insect circulatory system

closed circulatory system

circulatory fluid (blood) is pumped by a muscular heart and:

• is confined to vessels

• is distinct from interstitial fluid

• transports oxygen and carbon dioxide

• common among many invertebrates including annelids (earthworms) and cephalopods (octopi and squids)

• higher pressure = more energy cost

• multiple single-chambered hearts and no lungs in the annelid circulatory system

circulatory system in vertebrates

• system consisting of blood vessels and a two(+) chambered heart

  • blood enters heart thru an atrium and is pumped out through a ventricle

• arteries carry blood away from heart toward capillaries

• capillaries are site of gas and nutrient exchange between blood and interstitial fluid (exchange doesnt happen anywhere else)

• veins return blood from capillaries to heart

single circulation

• bony fishes, rays, and sharks have single circulation with a two-chambered heart

• blood leaving the heart passes through two sets of capillary beds before returning to the heart:

  • gills

  • body tissues (systemic)

variations in double circulation

• amphibians, reptiles, mammals and birds have double circulation

  • blood leaving heart passes through only one set of capillary beds before returning to heart- either pulmonary or system

• oxygen-poor blood flows through pulmonary circuit to pick up oxygen through the lungs

  • amphibians have a pulmocutaneous circuit (lungs + skin)

• oxygen-rich blood delivers oxygen through the systemic circuit

evolution of vertebrate circulatory systems

• in mammals and birds, the heart is divided completely into four chambers: two atria and two ventricles

  • oxygenated blood is fully separated from deoxygenated blood, which improves the efficiency of double circulation and is probably required for supporting the warm-blooding lifestyle of mammals and birds

• four-chambered heart of birds and mammals evolved independently from a three-chambered heart- convergent evolution

blood vessel types

• veins

• arteries

• capillaries

veins

carry blood into the heart under low pressure from capillaries

arteries

carry blood away from heart under high pressure toward capillaries

capillaries

have slower blood flow and lower blood pressure and very thin walls → permit gas/nutrient exchange between blood and interstitial fluid

the pressure of blood flow is highest to lowest in:

arteries > capillaries > veins

the velocity of blood flow is highest to lowest in:

arteries > veins > capillaries

the total cross-sectional area of blood flow is highest to lowest in:

capillaries > veins > arteries

blood flow and pressure can be regulated within the capillaries by the muscles through:

• vasoconstriction- narrowing of blood vessels → increases blood pressure

• vasodilation- widening of blood vessels → reduces blood pressure

• open systems: contraction of body muscle helps regulated blood pressure and velocity

• closed systems: contraction of arterial and venous smooth muscle helps regulate blood pressure and velocity

gas, nutrient, and fluid exchange across capillaries:

• in addition to exchange of gas and nutrients, plasma (watery connective tissue in blood) also leaves the capillaries and contributes to the interstitial fluid (extracellular fluid surrounding body tissues)

• most of the plasma-derived interstitial fluid returns to the capillaries before the capillaries converge into venules bc of an osmotic and pressure gradient across the length of the capillary:

  • plasma leaves the capillaries dt relatively higher pressure at the arteriole end of capillary

  • as plasma leaves, the pressure in the interstitial fluid increases and the conc of proteins and other large solutes remaining in the capillaries increases

  • loss of watery plasma creates hyperosmotic solution within the capillaries, especially near the venules

  • as a result, most of the plasma diffuses back into the capillaries near the venules

  • remaining plasma drains out from the interstitial fluid into nearby lymphatic vessels, where it passes through lymph nodes before it returns to the heart through lymphatic system

osmotic pressure

the pressure which must be applied to a solution to prevent inward flow of pure solvent across a semipermeable membrane

• water moves out of capillaries where blood pressure is higher than the osmotic pressure, and into capillaries where blood pressure is lower than the osmotic pressure

composition of blood

blood = the liquid that moves through the circulatory system

• plasma: liquid portion of the blood

  • contains water, proteins, salts, lipids, and glucose

• red blood cells: contain hemoglobin- transports oxygen + co2 in the circulatory system of vertebrates and many invertebrates

  • invertebrates that utilize hemolymph rather than blood have alternatives to hemoglobin to bind and transport oxygen:

    • hemocyanin- blue-green, copper (mollusks, crustaceans, some arthropods)

    • chlorocruorin- green, iron (four families of polychaete tubeworms)

    • hemerythrin- red, iron (some polychaete worms and annelids)

• white blood cells: immune response; formed continually

• platelets: work with other factors to promote blood clotting at sites of tissue damage