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Circulation and Gas Exchange
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 Gastrovascular cavity allows exchange of nutrients etc. in
animals with simple body plans (cnidarians, planarians)
 Land-dwellers and aquatic animals with multiple layers of
cells require an internal transport system
 Open circulatory system
 Body fluid= hemolymph
 1+ heart(s) pump this fluid throughout the body through
spaces between organs (sinuses)
 Material exchange directly between cells and
hemolymph
 ex: arthropods, mollusks
 Closed circulatory system
 Blood contained within specialized vessels; distinct from
interstitial fluid
 Material exchange between blood and interstitial fluid
 Ex: earthworms, cephalopods (squid and octopus),
vertebrates
 Heart
 Atria: receiving chambers (like an atrium in a large
building)
 Ventricles: large, muscular chambers that pump blood out
of heart
 Blood and blood vessels
 Arteries: carry blood out of heart; branch into arterioles
and capillaries
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 Veins: carry blood towards heart; capillaries branch into
venules, then veins
 Circulation patterns (Fig 42.3)
 Fish: heart with one atrium and one ventricle; gas
exchange at gills
 Amphibians
 heart with 2 atria and one ventricle
 double circulation: pulmocutaneous circuit leads to gas
exchange in tissues and systemic circuit gets oxygen
from capillaries
 some mixing of O2-rich and O2-poor blood in ventricle
 Reptiles: also 3-chambered heart, but ventricle is partially
divided—this reduces mixing of blood (except—
crocodilians have completely divided ventricle)
 Birds and mammals
 2 atria, 2 ventricles
 double circulation: left side of heart contains O2-rich
blood; right side contains O2-poor blood
 complete separation of blood makes O2 delivery more
efficient—advantage to endotherms
 Atria are small with thin walls; ventricles are much larger with
thick, muscular walls
 Valves maintain unidirectional blood flow
 Atrioventricular (AV) valves: betw atrium and ventricle;
closed when ventricle contracts
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 Semilunar valves: located at exit points—betw aorta/left
ventricle and pulmonary artery/right ventricle; opened
when ventricle contracts
 Heart rate: # of times your heart beats per minute; can be
measured by taking your pulse—which is the stretching of
the arteries as the heart pumps blood through them
 Cardiac cycle: consists of contraction phase (systole) and
relaxation phase (diastole)
 Sinoatrial (SA) node: aka. Pacemaker—sets the pace of
heartbeat; is able to contract independently of nerve
stimulation
 Heart rate is influenced by several factors, including
sympathetic/parasympathetic n.s., exercise, body
temperature
 Three basic tissue layers
 Outside: elastic connective tissue
 Middle: smooth muscle and more connective tissue
 Inside: endothelium—single layer of flattened cells
allowing smooth blood flow
 Capillaries faciliate gas exchange,  made of endothelium
and basement membrane only
 Arteries have thicker middle and outer layers to provide
strength and elasticity as blood is pumped vigorously
through
 Veins are passageways where blood flows with less speed
and pressure; they contain one-way valves directing blood
back to heart
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 Blood flow is far faster in the arteries than in the capillaries
because the total diameter of all the capillary vessiles is
much smaller than the diameters of the largest arteries (the
reverse happens as blood flows from
capillariesvenulesveins
 Blood pressure is the force exerted by blood against vessel
walls
 Blood pressure in veins is not affected by heart; instead,
blood is propelled forward by contraction of smooth and
skeletal muscles
 Plasma
 Liquid solution consisting of 90% water
 Solutes include electrolytes and plasma proteins
 Also contains substances being transported within blood,
i.e. metabolic wastes, hormones, CO2, O2
 Cell components
 Erythrocytes (red blood cells)
 Biconcave disc shape
 No mitochondria (undergoes anaerobic resp. instead) or
nuclei
 Contains hemoglobin, which functions to bind to O2
 Leukocytes (white blood cells)
 Function to fight infection
 Usually found outside circulatory system in interstitial
fluid
 Platelets
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 Bits of cells
 Function in blood clotting
 Stem cells
 RBC’s, WBC’s and platelets are formed from stem cells in
red bone marrow
 Negative feedback system triggers RBC production
 If tissues are not getting enough O2, the hormone
erythropoietin is released, stimulating RBC production
 If tissues are getting more than enough, erythropoietin
production is reduced, thus slowing production of RBCs
 Blood clotting
 Platelets release clotting factor that converts (inactive)
fibrinogen into (active) fibrin
 Hemophiliacs suffer from an inability to naturally heal
wounds and even minor cuts can result in severe bleeding
 When platelets clump up within a blood vessel they create
a thrombus which forms a dangerous barrier to bloodflow.
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Beauchemin 2007