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Why need a transport system?
Single-celled organisms, such as bacteria and amoeba
(below), can obtain nutrients and excrete waste simply by
diffusion.
nutrients
waste products
Multi-cellular organisms, such as insects, fish and mammals,
require a more specialized transport system. Why is this?
Surface area to volume ratio
In larger organisms,
diffusion of substances
would occur far too
slowly to enable them
to survive: the rate of
diffusion increases
with the square of the
distance it has to
travel.
This is not just because of its size, however: more important is an organism’s surface
area to volume ratio.
Single-celled organisms have a very large surface area to
volume ratio, because the diffusion path is so short.
Surface area and volume
Components of circulatory systems
Multi-cellular animals overcome the limitations of diffusion by
having a specialized circulatory system. This comprises:

a heart

a fluid in which substances are transported

vessels through which the fluid can flow.
The two types of
circulatory system are
open (e.g. molluscs,
arthropods) and
closed (e.g.
vertebrates, a few
invertebrates).
Top trump cards
• Use your background reading sheets and text
book to fill out the information cards. These
are outlines to top trump cards.
• Decide on which answer is going to be better
than the other and why.
• If you are not sure what the terms mean- look
them up!!!
THE INSECT CIRCULATORY SYSTEMOPEN SYSTEM
Open circulatory systems
An open circulatory system consists of a heart that pumps a
fluid called haemolymph through short vessels and into a
large cavity called the haemocoel.
In the haemocoel, the haemolymph directly
bathes organs and tissues, enabling the
diffusion of substances.
heart
haemocoel
When the heart relaxes, the
haemolymph blood is sucked
back in via pores called ostia.
Haemolymph moves around the haemocoel due to the
movement of the organism.
Closed circulatory systems
In a closed circulatory system, blood is fully enclosed
within blood vessels at all times.
From the heart, blood is
pumped through a series
of progressively smaller
vessels. In the smallest
vessels, capillaries,
substances diffuse in and
out of the blood and into
cells.
heart
capillaries
Blood then returns to the heart via a series of progressively
larger vessels.
Closed circulatory systems
The mammalian circulatory system
Circulation: true or false?
The human heart
The heart is a muscular
organ located between the
lungs in the centre of the
chest (thorax), and is about
the size of a fist.
It pumps blood continuously
around the body. An organism
can lose consciousness within
just a few seconds if the brain
is deprived of blood.
In foetuses, the heart begins
beating about 5–6 weeks after
conception.
Cardiac muscle
The heart mainly consists of cardiac muscle tissue, which
like smooth muscle (but not skeletal muscle), contracts
involuntarily.
Cardiac muscle is
made up of cells that
are connected by
cytoplasmic bridges.
This enables electrical
impulses to pass
through the tissue.
It contains large numbers of mitochondria and
myoglobin molecules.
LABELLING STRUCTURES AND
FUNCTIONS OF THE HEART
Structure of the heart
What structure?
Blood flow through the heart
The cardiac cycle
Interactive heart
Cardiac output
The amount of blood pumped around the body is called the
cardiac output, and depends on two factors:

the stroke volume – the volume of blood pumped by
the left ventricle in each heart beat. A typical value for an
adult at rest is 75 ml.

the heart rate – the number of times the heart beats per
minute. A typical value for an adult at rest is 70 bpm.
cardiac output = stroke volume × heart rate
A typical resting cardiac output is 4–6 litres per minute.
This can rise to as much as 40 litres per minute in highly
trained endurance athletes.
Pacemaker cells of the heart
The heart can beat without any input from the nervous
system as longs as its cells stay alive. This is due to
myogenic contraction.
Muscle cells (myocytes) in the heart have a slight
electrical charge across their membrane. They are
polarized. When the charge is reversed, they are said to
be depolarized and this causes them to contract.
Depolarization is initiated in a region of the heart called the
sinoatrial node (SAN) – also known as the pacemaker –
which is in the wall of the right atrium.
Myogenic stimulation of the heart
Interactive heart
Artificial pacemakers are devices implanted in people
whose heart’s electrical conduction system is not working
properly.
Problems include the SAN not firing, and the blockage or
disruption of impulses between the SAN and AVN, or in the
bundle of His.
Pacemakers monitor the
heart’s electrical activity and
stimulate the ventricles or atria
to contract when necessary.
Impulses are transmitted
down electrodes implanted in
the muscular walls.
The cardiac cycle
The electrical activity of the heart can be monitored by
an electrocardiograph.
Several electrodes are
attached to specific places
on a person’s chest and
limbs. These detect
changes in polarization in
the heart by measuring
current at the skin surface.
The leads are connected to
a machine that draws an
electrocardiogram (ECG).
Components of an ECG trace
ECGs are used to diagnose problems with the heart, as
variations in different components of the trace can
indicate a disease or other abnormality.
An ECG may be taken
while the patient is relaxed
or it may be taken before,
during and after exercise.
This is called a ‘stress test’
and usually involves the
patient exercising on a
treadmill while attached to
an ECG machine.
Abnormal ECGs
Glossary
What’s the keyword?
Multiple-choice quiz
Guide to blood vessels
Identifying blood vessels
Blood flow in veins
Varicose veins
If a vein wall becomes weakened, valves
may no longer close properly. This allows
backflow of blood, causing the vein to
become enlarged and bumpy, and
become varicose.
This usually happens in superficial
veins, near the skin surface in the
lower legs, as opposed to deep
veins, which lie underneath muscles.
Varicose veins can be surgically
removed without affecting blood flow,
as most blood is returned to the heart
by deep veins.
Maintaining high blood pressure
Blood pressure is the main force that drives blood from the
heart around the body.

During systole (heart contraction), blood is pumped
through the aorta and other arteries at high pressure. The
elastic fibres of arteries enable them to expand and allow
blood through.

During diastole (heart relaxation), the blood pressure in
the arteries drops. The elastic recoil of the artery walls
help force the blood on.
As blood moves through smaller arterioles into capillaries,
and then into venules and veins, its velocity and pressure
drop continuously.
Arteries, capillaries and veins
Blood is a specialized
transport medium that is also
considered a special type of
connective tissue. An average
adult has 4–6 litres of blood.
Blood has a range of
functions such as:




transport
defence
thermoregulation
maintaining pH of body fluids.
The composition of blood
What are the specialized features of an erythrocyte?
flattened, biconcave disc shape:
ensures large surface area to volume
ratio for efficient gas exchange
diameter (6–8 µm) larger than
capillary diameter: slows blood
flow to enable diffusion of oxygen
large amount of
haemoglobin: for
transporting oxygen
no nucleus or
organelles:
maximises space for
haemoglobin, so
more oxygen can be
transported
Blood clotting
Substances in blood clotting
Plasma proteins and blood pressure
About 8% of blood plasma consists of plasma proteins, of
which about half may be albumins.
These are a group of small
proteins involved in the
transport of other substances
(e.g. fatty acids, hormones)
and which help regulate the
osmotic pressure of blood.
The balance between the hydrostatic pressure of blood
(‘blood pressure’) and the osmotic pressure of blood is
important in the formation of tissue fluid.
Formation of tissue fluid
Lymph
Not all tissue fluid returns to
the capillaries. The excess
drains into the lymphatic
system, where it forms
lymph.
Lymph is a colourless/pale
yellow fluid similar to tissue
fluid but containing more
lipids.
lymphatic capillaries
The lymphatic system drains into the circulatory system
near the vena cavae via the thoracic duct.
The lymphatic system
The lymphatic system is a secondary circulatory system
and a major part of the immune system. It consists of:

lymphatic capillaries and vein-like
lymph vessels, containing valves

lymph nodes – sac-like organs that
trap pathogens and foreign
substances, and which contain large
numbers of white blood cells

lymphatic tissue in the spleen, thymus
and tonsils – these also contain large
amounts of white blood cells and are
involved in their development.
Composition of body fluids