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The need for transport systems in multicellular animals
To include an appreciation of size, metabolic rate and surface area to volume ratio (SA:V)
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Metabolic demands of active organisms need to be kept and diffusion over the long distances isn’t enough to supply the
quantities needed
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The different types of circulatory systems
To include single, double, open and closed circulatory systems in insects, fish and mammals
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An effective transport system will include:
 A fluid or medium to carry nutrients and oxygen around the body e
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blood
 A pump to create pressure that will push the fluid around the body e
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the heart
 Exchange surfaces that enable oxygen and nutrients to enter the blood and leave it again where they are needed e
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blood vessels
A mass transport system is a transport system where substances are transported in a mass of fluid
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An open circulatory system is a circulatory system with a heart but few
vessels to contain the transport medium
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In the haemocoel, the
transport medium is under low pressure
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The transport medium returns to the heart through an
open ended vessel
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In
insects, gas exchange takes place in the tracheal system
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It doesn’t carry oxygen and carbon dioxide and transport
food and nitrogenous waste products and the cells involved in the defence
against disease
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The
haemolymph circulates but steep diffusion gradients cannot be maintained by
efficient diffusion
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A closed circulatory system is a circulatory system where the blood is enclosed in blood vessels and doesn’t come directly
into contact with the cells of the body
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The heart pumps the blood around the body under pressure relatively
quickly and returns the blood directly to the heart
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The widening and narrowing of blood vessels adjusts the amount of blood flowing to a particular tissue
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T
A single circulatory system is a circulatory system where the blood flows through the heart once for each cycle and is
pumped out to travel around the body before returning to the heart
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Oxygen and carbon dioxide are exchanged in the first
set and on the second, substances are exchanged between the
blood and the cells
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Fish have a single circulatory system where the blood flows from
the heart to the gills and then on to the body before returning to
the heart
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Their body weight is supported
by the water and they don’t maintain their own body temperature,
reducing he metabolic demands
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In the first circulation, known as the pulmonary circulation, blood is pumped by the heart to the
lungs
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Each circuit only passes through one capillary network meaning a relatively high pressure
and fast flow of blood can be maintained
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To include the distribution of different tissues within the vessel walls
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The blood in the
arteries is under higher pressure than the blood in the veins
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The elastic fibres withstand the force of the blood pumped out of the heart and stretch to take the
larger blood volume
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You can still feel a pulse (surge in blood) when the heart contracts
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The smooth endothelium lining is smooth so the blood flows easily over it
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Collagen helps to maintain its shape and
volume and withstands pressure as it provides strength
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They have more smooth muscle and less elastin in their walls as they have little
pulse surge but can constrict/dilate to control blood flow
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When the smooth muscle relaxes, blood flows through into
the capillary bed – vasodilation
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It has a small lumen
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The gaps between the epithelial cells
making up the capillary walls are relatively large
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Blood entering
the capillaries is oxygenated and is deoxygenated hen leaving (more carbon dioxide than oxygen) except in the lungs and
placenta (deoxygenated to oxygenated)
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They have a large
surface area for diffusion
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Deoxygenated blood flows from the capillaries into venules and then into larger veins
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Venules link the capillaries with the veins
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Several venules form a vein
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The veins hold most of the
blood but don’t have a pulse
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When blood flows backwards, the valves close to prevent this from happening
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Deoxygenated blood must return to the heart but is under low pressure so needs to move against gravity
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Many bigger veins run between the big, active muscles in the body e
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arms and legs
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The valves prevent backflow when the
muscles relax
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The formation of tissue fluid from plasma
To include reference to hydrostatic pressure, oncotic pressure and an explanation of the differences in the
composition of blood, tissue fluid and lymph
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They also contribute to the maintenance of a steady body temperature and acts as a buffer, minimising pH changes
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Plasma also transports red blood cell, white blood cells and platelets (fragments of large
megakaryocytes found in red bone marrow and involved in blood clotting)
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The substances dissolved in plasma pass through fenestrations in the capillary walls except plasma proteins
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They give the blood in the capillaries a relatively
high solute potential (low water potential) compared with surrounding fluid
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This is oncotic pressure and is about 3
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Blood in the arteries has a high hydrostatic pressure generated from the contraction of the ventricle wall
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At the arterial end of the capillary forcing fluid out of the
capillaries, the hydrostatic pressure is relatively high, about 4
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It is higher than the oncotic pressure attracting water in
by osmosis, so fluid is squeezed out filling spaces between the cells (tissue fluid)
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Substances dissolved in blood plasma enter tissue fluid from the capillaries by diffusion, down a concentration gradient
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3kPa as fluid moves out
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The fluid is forced out of the
capillary moving down a pressure gradient
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Lymph has a similar composition to plasma ad tissue fluid but contains fatty acids which have been
absorbed into the lymph from the villi of the small intestine
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Valves
prevent the backflow of lymph
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Feature
Cells

Glucose

Arterial blood
Erythrocytes, Leucocytes
(white) and platelets
Hormones and plasma
proteins
Some transported as
lipoproteins (HDL and
LDL)
The most amount

Amino acids

More than in other fluids
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Carbon dioxide

Little

Protein
Fats

Tissue fluid
Phagocytes

Lymph
Lymphocytes

Hormones and proteins
secreted by body cells
None

Few proteins

Less than in the blood
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Gets
absorbed by cells
Less than in the blood
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Gets
absorbed by cells
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Least amount
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Most
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Muscle
contraction or systole creates pressure inside the heart chambers
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The heart is surrounded by
inelastic pericardial membranes helping to prevent the heart from over-distending with blood
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The atria have thin muscular walls
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When both the atrium and ventricle are filled with blood, the
atrium contracts forcing all the blood into the right ventricle and stretching the ventricle walls
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The tendinous cords prevent the
valves from turning inside out by the pressures exerted when the ventricle contracts
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The
semilunar valves prevent backflow of blood into the heart
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As the pressure in the left atrium builds
up, the atrioventricular valve (bicuspid/mitral valve) opens between the left atrium and the left ventricle so the ventricle also
fills with oxygenated blood
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The left ventricle then contracts and pumps oxygenated blood through the
semilunar valves into the aorta and around the body
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The wall of the left ventricle is thicker than the wall of the right atrium because the left ventricle contains more muscle to
create more force for a higher pressure
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The left ventricle pumps blood
further to all parts of the body, supplying the systemic circulation
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The right and left side of the heart fill and empty
together
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The cardiac cycle describes the events in a single heartbeat
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The atria and ventricles fill with blood increasing the blood volume and pressure
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In systole, the atria and ventricles contract
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The volume and pressure of the blood
in the heart is low at the end of systole and the blood pressure in the arteries is at a maximum
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During atrial systole and ventricular
diastole, the atria contracts and the ventricle
relaxes
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This keeps the semilunar valve
closed and the atrioventricular valve open
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During ventricular systole and atrial
diastole, the ventricle contracts which
relaxes the atrium
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The left atrioventricular
valve closes and the ventricular pressure
rises above the aortic pressure as blood is
forced
past
the
semilunar
valves
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During atrial and ventricular diastole, the atrium and ventricle relaxes
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The recoil produces a temporary rise in pressure at the starts of the relaxation phase
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Ventricular pressure falls as the ventricles empty and the walls relax
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How heart action is initiated and coordinated

To include the roles of the sino-atrial node (SAN), atrio-ventricular node (AVN), purkyne tissue and the myogenic
nature of cardiac muscle (no detail of hormonal and nervous control is required at AS Level)
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The electrical activity from the SAN is picked up by the atrio-ventricular node (AVN)
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The delay between the excitation of the atria and excitation of the ventricles during electrical stimulation is essential
because it allows time for the atria to fully contract allowing the atria to empty and the ventricles to fill so that the ventricles
don’t contract too early
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The cardiac heart muscle walls are myogenic meaning it doesn’t need electrical impulses from a nerve to make it contract
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g
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An ECG measures the electrical activity of the heart
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Tachycardia – Very rapid heartbeat (>100bpm)
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Abnormal if caused by
problems in the electrical control of the heart and may need to be treated by medication or by surgery
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Very fit people have slower heartbeats
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Atrial Fibrillation – An example of arrhythmia – abnormal rhythm of the heart
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Contract very fast (fibrillate) up to 400 times a minute
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As a result the heart doesn’t pump blood very effectively
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Most people have at least one a day
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The role of haemoglobin in transporting oxygen and carbon dioxide
To include the reversible binding of oxygen molecules, carbonic anhydrase, haemoglobinic acid, HCO₃¯ and the
chloride shift
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Oxygen binds to haemoglobin in the alveoli in the lungs forming
oxyhaemoglobin
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When the erythrocytes enter the capillaries, oxygen levels are relatively low
making a steep concentration gradient between the inside of the erythrocytes
and the air in the alveoli
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The arrangement of the haemoglobin molecules means one oxygen molecule binds to a haem group,
changing the shape of the molecule, making it easier for the next oxygen molecules to bind
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The free oxygen concentration stays low in the erythrocytes because the oxygen is bound to the
haemoglobin, so a steep diffusion gradient is maintained until all of the haemoglobin is saturated with oxygen
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Oxygen moves out of the erythrocytes down a concentration gradient
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Most of the carbon dioxide is transported as hydrogencarbonate ions in the plasma
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Carbonic acid dissociates to form hydrogen and hydrogencarbonate ions
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The reaction
takes place slowly in the blood plasma
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The
hydrogencarbonate ions move out of the erythrocytes into the plasma by diffusion down a concentration gradient
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This is known as the chloride shift
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When the blood reaches the lung tissue, where there is relatively low concentration of carbon dioxide, carbonic anhydrase
catalyses the reverse reaction, breaking down carbonic acid into carbon dioxide and water
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Chloride ions diffuse out of the
erythrocytes into the plasma down an electrochemical gradient
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This alters the structure of haemoglobin so more oxygen is released where more respiration
occurs or where it is needed
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To include the significance of the different affinities for oxygen and the changes to the dissociation curve at
different carbon dioxide concentrations (the Bohr effect)
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This occurs because it reduces affinity of haemoglobin for oxygen
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Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin so takes up oxygen in low partial pressure of
oxygen
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The placenta has a low partial pressure of oxygen so the adult oxyhaemoglobin
will dissociate and release oxygen

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