THE NEED FOR A
CIRCULATORY
SYSTEM
 What do all living things require to live?
 Unicellular organisms perform all processes using only one cell
 Multicellular organisms require organ systems to function
properly
 Circulatory System: responsible for transporting (circulating)
nutrients, dissolved gases, waters, and chemicals to and from
individual cells in an organism
 Unicellular organisms are in direct contact with their environment
 Gases diffuse across a cell membrane
 Nutrients are engulfed or absorbed through the membrane
Simple Circulation
 Some simple multicellular organisms do
NOT need a circulatory system
 Sponges




Has two layers
O2 is absorbed through the cell
Nutrients are trapped
CO2 and wastes are pushed out through
the top
 Central cavity is flushed every 4.3 seconds
 Can move up to 20,000x is body volume in
water/ day
Circulatory Evolution
 The need for a circulatory system arose
with increase in body size
 Circulatory systems must bring O2 and
nutrients from the external
environment in AND the waste from
the internal environment back to the
external environment
Types of Circulatory Systems
«OPEN»
1)
«CLOSED»
A fluid that circulates materials through the body
2) A network of tubes in which the fluid circulates
3)
A pump that pushes the fluid through the tubes
BONUS!
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