Cambridge (CIE) AS Biology Your notes
The Circulatory System
Contents
Circulatory Systems
The Main Blood Vessels
Observing & Drawing Blood Vessels
Blood Vessels: Structures & Functions
Cells of the Blood
The Role of Water in Circulation
Blood & Tissue Fluid
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, Circulatory Systems
Your notes
Closed Double Circulatory System
The need for a circulatory system
The cells of all living organisms need a constant supply of reactants for metabolism, e.g.
oxygen and glucose
Single celled organisms can gain oxygen and glucose directly from their surroundings,
and the molecules can diffuse to all parts of the cell quickly due to short diffusion
distances
Larger organisms, however, are made up of many layers of cells, meaning that the time
taken for substances such as glucose and oxygen to diffuse to every cell in the body
would be far too long
The diffusion distances involved are too great
To solve this problem their exchange surfaces are connected to a mass transport
system, for example
The digestive system is connected to the circulatory system
The lungs are connected to the circulatory system
Mass transport is the bulk movement of gases or liquids in one direction, usually via a
system of vessels and tubes
The circulatory system in mammals is a well-studied example of a mass transport
system; the one-way flow of blood within the blood vessels carries essential nutrients
and gases to all the cells of the body
Open & closed systems
Circulatory systems are either described as being open or closed
In a closed circulatory system, blood is pumped around the body and is always
contained within a network of blood vessels
All vertebrates and many invertebrates have closed circulatory systems
In an open circulatory system, blood is not contained within blood vessels but is pumped
directly into body cavities
Organisms such as arthropods and molluscs have open circulatory systems
Humans have a closed double circulatory system: in one complete circuit of the body
blood passes through the heart (the pump) twice
The right side of the heart pumps deoxygenated blood to the lungs for gas exchange;
this is the pulmonary circulatory system
Blood then returns to the left side of the heart, so that oxygenated blood can be
pumped efficiently (at high pressure) around the body; this is the systemic circulatory
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, system
The double circulatory system in mammals
Your notes
The double circulatory system in mammals showing pulmonary and systemic circulation
Main circulatory system structures table
Structure Function
Heart A hollow, muscular organ located in the chest cavity which pumps blood.
Cardiac muscle tissue is specialised for repeated involuntary contraction
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The Circulatory System
Contents
Circulatory Systems
The Main Blood Vessels
Observing & Drawing Blood Vessels
Blood Vessels: Structures & Functions
Cells of the Blood
The Role of Water in Circulation
Blood & Tissue Fluid
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
, Circulatory Systems
Your notes
Closed Double Circulatory System
The need for a circulatory system
The cells of all living organisms need a constant supply of reactants for metabolism, e.g.
oxygen and glucose
Single celled organisms can gain oxygen and glucose directly from their surroundings,
and the molecules can diffuse to all parts of the cell quickly due to short diffusion
distances
Larger organisms, however, are made up of many layers of cells, meaning that the time
taken for substances such as glucose and oxygen to diffuse to every cell in the body
would be far too long
The diffusion distances involved are too great
To solve this problem their exchange surfaces are connected to a mass transport
system, for example
The digestive system is connected to the circulatory system
The lungs are connected to the circulatory system
Mass transport is the bulk movement of gases or liquids in one direction, usually via a
system of vessels and tubes
The circulatory system in mammals is a well-studied example of a mass transport
system; the one-way flow of blood within the blood vessels carries essential nutrients
and gases to all the cells of the body
Open & closed systems
Circulatory systems are either described as being open or closed
In a closed circulatory system, blood is pumped around the body and is always
contained within a network of blood vessels
All vertebrates and many invertebrates have closed circulatory systems
In an open circulatory system, blood is not contained within blood vessels but is pumped
directly into body cavities
Organisms such as arthropods and molluscs have open circulatory systems
Humans have a closed double circulatory system: in one complete circuit of the body
blood passes through the heart (the pump) twice
The right side of the heart pumps deoxygenated blood to the lungs for gas exchange;
this is the pulmonary circulatory system
Blood then returns to the left side of the heart, so that oxygenated blood can be
pumped efficiently (at high pressure) around the body; this is the systemic circulatory
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
, system
The double circulatory system in mammals
Your notes
The double circulatory system in mammals showing pulmonary and systemic circulation
Main circulatory system structures table
Structure Function
Heart A hollow, muscular organ located in the chest cavity which pumps blood.
Cardiac muscle tissue is specialised for repeated involuntary contraction
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 3
