Circulatory systems – general questions
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What is a circulatory system?
Do all animals have circulatory systems?
Why do animals have circulatory systems?
What kinds of animals lack circulatory systems?
Rate of diffusion
• Diffusion is slow over long distances
• Einstein’s diffusion equation
t α x2
Distance
100mm
Time
required
5 seconds
200mm
20 seconds
500mm
2 minutes
1 mm
8 minutes
1m
16 years
Limits of Diffusion
• Unicellular organisms and some small metazoans lack circulatory
systems and rely on diffusion to transport molecules
• Diffusion can be rapid over small distances, but is very slow over
large distances
• Rather than rely on diffusion, large animals move fluid through their
bodies by bulk flow, or convective transport
• Convection mechanisms are needed to move molecules
rapidly over large distances
• Movement of fluid molecules as a group
• Requires input of energy
• Physiologists use the symbol Q to represent flow in circulatory
systems
• You may also see the symbol Q, the dot indicates that this is a rate
process
Convection systems: speed has a cost
• Diffusion is ‘free’, convection is not
• Vertebrate hearts cost ~ 1-4% of resting metabolism
• lung and gill ventilation cost ~ 1-10% of resting metabolism
What are the roles for convection systems?
1. To carry gases (O2, CO2 and others) to and from tissues
2. To carry nutrients, wastes & hormones between organs
3. To provide structural support & locomotion
4. To provide filtration
Fundamental Components
• Fluid: water; cytoplasm; lymph; hemolymph; blood
• Energy: ATP  kinetic energy via potential energy
• Conduits: cellular spaces; sinuses; vessels; circuits
Components of Circulatory Systems
• Circulatory systems move fluids by increasing the
pressure of the fluid in one part of the body
• The fluid flows through the body down the resulting
pressure gradient
• Three main components are needed
• Pump or propulsive structures
• A system of tubes, channels, or spaces
• A fluid that circulates through the system
Different needs for bulk flow
Different types of pumps
• Skeletal muscle
• Pulsating blood vessels 
peristalsis
• Chambered hearts
One way valves help to ensure
unidirectional flow
Figure 9.2
Different types of circulations
• Open - circulatory fluid comes
in direct contact with the
tissues in spaces called
sinuses
• Closed – circulatory fluid
remains within the blood
vessels and does not come in
direct contact with the tissues
Types of Fluid
• Interstitial fluid – extracellular fluid that directly bathes
the tissues
• Blood – fluid that circulates within a closed circulatory
system
• Lymph – fluid that circulates in the secondary system of
vertebrates called the lymphatic system
• Hemolymph – fluid that circulates within an open
circulatory system
Evolution of Circulatory Systems
• First evolved to transport nutrients
• Very early on they began to serve a respiratory function
• Closed systems evolved independently in jawed
vertebrates, cephalopods, and annelids
• Closed systems evolved in combination with specialized
oxygen carrier molecules
Evolution of Circulatory Systems, Cont.
Intracellular convection systems
• All cells & unicellular animals
• Microfilaments & microtubules move organelles & DNA
Animals That Lack Circulatory Systems
• Sponges, cnidarians, and flatworms
• Lack true circulatory systems, but have mechanisms for
propelling fluids around their bodies
• Cilia: sponges and flatworms
• Muscular contractions: cnidarians
Bulk flow using cilia
A choanocyte cell
water +
suspended
nutrients
Law of Bulk Flow
Q= DP/R
Fundamental physical law exploited by
circulatory systems, respiratory systems,
excretory systems, digestive systems etc.
Bulk Flow
Bulk Flow
Invertebrate circulatory fluids
• Hemolymph = plasma (water, ions, proteins, nutrients, hormones, etc.)
+ respiratory pigment
+ white blood cells
• Blood =
plasma
+ red/pink blood cells
+ white blood cells
Invertebrate respiratory pigments: intracellular & extracellular
1. Red blood cell hemoglobins (Fe):
Some molluscs, annelids, echinoderms
2. Pink blood cell hemerythrins (amino acids):
Some marine worms: annelids, sipunculids
3. Extracellular hemoglobins & chlorocruorin (Fe):
Some annelids; bivalve molluscs, arthropods
4. Molluscan extracellular hemocyanin (Cu):
Cephalopods, gastropods & some bivalves
5. Arthropod extracellular hemocyanin (Cu):
Decapod crustaceans
All arthropods have an open circulatory system
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Insects: Highly successful terrestrial animals & capable of high
metabolic rates
Role of circulatory system:
• To deliver nutrients, immune cells & hormones
• Tracheal system moves respiratory gases directly to tissues)
• Multiple, segmental hearts
• Large dorsal vessel
• Sluggish flow rates
• Accessory pumps: wings, limbs
All arthropods have an open circulatory system
• Crustaceans: Highly successful aquatic animals
& decapods are capable of high metabolic rates
Decapod circulatory system:
• Respiratory function, respiratory pigments, gills
• Large ostial heart
• Many branching outflow vessels
• Small tissue sinuses
The decapod crustacean heart
• The ostial heart sits in a pericardial sac that contains hemolymph
Cardiac cycle
• Cardiac muscles contract in unison,
ejecting hemolymph into arteries &
stretching suspensory ligaments
• Cardiac muscles relax, recoil of
suspensory ligaments expands the
heart chamber, drawing hemolymph
from the sinus via the ostia into the
heart
Cardiac control
• Neurogenic: CNS controls heart rate &
force of contraction (stroke volume)
• Neural control of arterial sphincters
Crayfish cardiac cycle - systole
• Heartbeat is initiated in
neurons of cardiac ganglion
• (neurogenic heart)
• Neural signal closes ostia
• Causes cardiomycocytes to
contract
• Volume of heart chamber
decreases, pressure
increases
• Blood exits via arteries
• Suspensory ligaments are
stretched
Crayfish cardiac cycle - diastole
• When neural signal is
absent, cardiomyocytes
relax
• Releases tension on
suspensory ligaments
• Ligaments spring back
• Pulls on the walls of the
heart, increasing volume of
heart chamber
• Reduces pressure inside
heart
• Ostia open, blood is sucked
into the heart
Annelids have open & closed circulatory systems
Three Main Classes
• Polychaeta – tube worms
• Oligochaeta – earth worms
• Hirudinea – leeches
• Most also have vessels that circulate fluid with oxygen carriers
• Can be an open (polychaetes) or closed (oligochaetes) systems
• Aquatic and terrestrial animals that have low metabolic
rates & can live in hypoxic environments
Pumping systems
• Chambered heart
• Body wall movements ie muscular contractions
• Cilia
Earthworms
• Five, segmental contractile tubes
• Segmental connecting vessels
Molluscs have open & closed circulatory systems
• Bivalves: Sedentary & low metabolic rates
• Cephalopods: High locomotory abilities (except Nautilus)
Cephalopod circulatory system
• Myogenic, chambered heart
• Systemic heart pumps oxygenated
blood to tissues
• Paired branchial hearts pump
deoxygenated blood to gills
• Coronary arteries
• Distributional arteries
Open vs closed circulatory systems in invertebrates
Common features
• Muscular ‘hearts’
• Unidirectional flow, one-way ‘valves’
• Large distribution vessels away from (artery) & to (veins) the heart
• Hemolymph in which respiratory pigments may be dissolved
Benefits
• No building and maintenance costs
• Low resistance to flow b/c there are no small distribution vessels
• Lower pump pressures needed for flow  saves energy
• Hemolymph readily bathes most cells
• Large hemolymph volume
Disadvantages
• Limited control of blood flow distribution except at the arterial level
• Slow circulation/response times
• Body movements greatly influence blood pressure & flow