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GCSE Biology Revision Notes
B.5.Heart & Circulation
HEART AND CIRCULATION
Small organisms have no need for a circulatory system because they naturally have a large
surface area to volume ratio. The distance from the atmosphere to any internal part of the
body is very small and diffusion alone is sufficient to deliver oxygen and remove carbon
dioxide.
Problems begin when the body exceeds a certain size (the surface area to volume ratio
decreases) or when the metabolic demand for oxygen becomes high, perhaps due to high
levels of activity. At this point some kind of circulatory system becomes necessary.
TYPES OF CIRCULATORY SYSTEMS
Single Circulatory System
All vertebrate animals have closed circulatory systems in which the blood is enclosed within
arteries, capillaries or veins at all times. The simplest model is the single circulation system,
as found in fish
Blood is pumped around a circuit (heart – gill – body – heart). The problem with this system
is that pressure from the heart is mostly lost as blood flows through capillaries in the gills, so
transport around the rest of the body is sluggish. Fish can get away with this because their
metabolic rate is relatively low. Nevertheless this system is good enough for some very large
animals, like whale sharks.
Double Circulatory System
Mammals and birds are large and have very high metabolic rates as they need to produce
heat to keep warm. They are also very active. Both mammals and birds have evolved a
sophisticated and highly efficient double circulatory system. The blood flows through two
separate loops known as the pulmonary and systemic circulations.
Blood flows from the body to the heart, then to the lungs, then to the heart and finally back
to the body. Oxygenated and deoxygenated blood are kept separate and the blood flows at
high pressure in arteries.
Efficiency is improved by having a sophisticated pump (the heart), using a system of valves
to prevent backflow, well adapted arteries, veins and capillaries and red blood cells
containing the red pigment haemoglobin which increases the amount of oxygen which can
be transported per unit volume.
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GCSE Biology Revision Notes
B.5.Heart & Circulation
The systemic circulation carries oxygenated blood to all of the major organs in the body.
Blood leaves the left side of the heart in the aorta which then branches to form important
smaller arteries such as the renal artery (to the kidneys).
The liver is unusual in that it is fed by two major blood vessels. The hepatic artery brings
oxygenated blood. The hepatic portal vein flows from the stomach and intestines to the
liver and brings nutrients from digestion to the liver. It also allows the liver to remove any
toxins absorbed from food before the blood from the stomach and intestines is allowed into
general circulation.
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GCSE Biology Revision Notes
B.5.Heart & Circulation
THE HEART
The heart consists of two halves (left and right) each with two chambers (atrium and
ventricle). The halves are separated by a thick septum.
Note: Diagrams of the heart are always drawn as if you were facing the person, so left and
right sides are reversed.
THE CARDIAC CYCLE
The cardiac cycle refers to the sequence of events in the heart that make up one complete
heart beat. The relaxation and contraction of the ventricles is highly synchronized so that the
atria contract before the ventricles contract.
Note that both of the atria contract at the same time, and then both of the ventricles contract
at the same time.
1. The heart relaxes. Blood fills the atria from the vena cava and pulmonary vein.
2. The atria contract. Blood is forced down into the ventricles via the atrio-ventricular
(AV) valves. These valves have various names, including the bicuspid or mitral valve
on the left and the tricuspid on the right.
3. The ventricles contract. The AV valves are forced shut by the rising pressure and
blood escapes past the semi-lunar valves and into the arteries.
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GCSE Biology Revision Notes
B.5.Heart & Circulation
The table shows the separate routes of oxygenated and deoxygenated blood through the
heart.
Right side (deoxygenated
blood)
Left side
(oxygenated blood)
1
Vena cava (from body)
Pulmonary vein (from lungs)
2
Right atrium
Left atrium
3
Tricuspid valve
Bicuspid valve
4
Right ventricle
Left ventricle
5
Semi-lunar valve
Semi-lunar valve
6
Pulmonary artery (to lungs)
Aorta (to body)
Valves
The purpose of the valves is to ensure a one-way flow of blood through the heart or
through veins.
Valves do not open or close by themselves, they are not muscular and cannot move
themselves. They are pushed open or shut by the pressure and movement of blood. Look at
the diagrams carefully and you should be able to figure out which way the blood will be
able to move.
The heart sounds heard with a stethoscope are caused by the closing of the valves. The
sound pattern is “lub–dup, lub–dup, lub–dup.” The first heart sound (“lub”) is created by
the recoil of blood against the closed AV valves. The second sound (“dup”) is the recoil of
blood against the semi-lunar valves.
The diagram below shows an industrial valve but heart valves work the same way.
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GCSE Biology Revision Notes
B.5.Heart & Circulation
Control of Heart Rate
Cardiac output is adjusted in coordination with changes in the rest of the circulatory system.
This coordination of regulatory mechanisms maintains adequate blood flow as the body′s
demands on the circulatory system change. During heavy exercise, for example, the
arterioles in working muscles dilate. This response admits a greater flow of oxygen–rich
blood to the muscles. Arterioles supplying blood to non-essential organs, such as the
digestive system, constrict, reducing blood flow to them.
The heart may increase cardiac output in response to changes in pH, carbon dioxide and
oxygen concentration. Chemoreceptors are receptors that monitor changes in chemicals and
transmit this information to the nervous system.
Chemoreceptors are located in the aorta and carotid artery, which send information on
carbon dioxide levels to the cardiac centre of the medulla oblongata in the brain. Branches
of the nervous system then relay information to the heart, where a specialised region of
heart muscle cells called the pacemaker can increase/decrease cardiac output accordingly.
The hormone adrenaline also affects cardiac output. This hormone is released into the blood
stream from the adrenal glands (located just above the kidneys). Adrenaline increases the
rate and force of contraction. The secretion occurs in response to increased physical activity,
emotional excitement or stressful conditions. Adrenaline takes longer to act on the heart
than nervous stimulation, but the effect lasts longer.
Cardiac output can be increased by either:

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Increasing the heart rate (number of beats per minute)
Increasing the amount of blood pumped per beat (increasing the strength of each
contraction)
Heart rate can be determined by placing two fingers on someone’s wrist and counting the
number of beats in 60 seconds. Alternatively you can use a heart rate monitor.
In the graph below, exercise starts after about 45 seconds and continues until 5 minutes 20
seconds. The heart rate then returns to normal, but gradually. This enables extra oxygen to
be transported to the liver for the breakdown of lactic acid from anaerobic respiration (see
respiration notes). Anaerobic respiration produces an oxygen debt that maintains heart and
breathing rate higher than normal even after exercise has stopped.
You need to be able to describe simple experiments to determine the effects of exercise on
heart and breathing rates. Read the question very carefully to figure out what you are
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GCSE Biology Revision Notes
B.5.Heart & Circulation
actually being asked to do. It may not be necessary to measure heart or breathing rate over a
long period.
“Design a simple experiment to show exercise affects heart rate”
Use 20 volunteers who are similar age, sex, height and weight.
Take the resting pulse (the number of beats per minute) of all the volunteers.
Half the group carry out a standardized amount of exercise (e.g. running 100 m).
The other half (the control group) should sit quietly.
Quickly take the pulse rates of all the volunteers again.
Calculate the change in pulse rate for all volunteers in both groups.
Remove any anomalous results (any that are clearly different from the others).
Take an average for both groups and present as a bar chart of pulse rate with and without
exercise.
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GCSE Biology Revision Notes
B.5.Heart & Circulation
Arteries and Veins
Arteries
Arteries (A in the picture) carry blood under high pressure. They need to have a thick,
elastic wall so that they can stretch slightly as the heart contracts and the pressure rises, in
order to avoid bursting. They also have a thick muscular layer which can contract to control
and direct blood flow around different parts of the body. Contracting the muscle reduces
blood flow to the organ supplied by that artery.
Veins
Veins (B in the picture) carry blood under low pressure. The blood flows much more slowly,
so the space inside the vein (the lumen) needs to be wider than for an artery with the same
capacity. There is no need to be especially elastic or muscular, so the walls are far thinner.
Blood in veins is at low pressure – all the pressure from the heart has been lost due to
friction with the walls of blood vessels, especially in the capillaries. Without assistance from
the heart the blood in veins is slowly squeezed along by the contraction of your muscles
when you move. A system of valves in the veins means the blood can only move towards
the heart.
If you stand still for a long period of time, or sit still in an airline seat, the failure to regularly
contract and relax your leg muscles results in pooling of your blood in the legs. This can lead
to a lack of blood in circulation (which is why guardsmen sometimes collapse after standing
to attention for long periods) or to the blood standing still and clotting (deep vein
thrombosis).
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GCSE Biology Revision Notes
Walls
Lumen
Appearance in
cross section
B.5.Heart & Circulation
Artery
Vein
Thick and muscular
Thin
Relatively narrow
Relatively wide
Round
Irregular or elliptical
Capillaries
Capillary walls are very thin, only one cell thick (note that they do not have a cell wall – cell
walls are only found in plants).
There are gaps between the cells forming pores which blood plasma leaks out of, forming
tissue fluid which bathes the surrounding cells and carries dissolved oxygen to them.
Coronary Heart Disease
The tendency to develop cardiovascular disease is inherited to some extent, but lifestyle
plays a large role, too. Non-genetic factors that increase the risk of cardiovascular problems
include:





smoking
lack of exercise
high blood pressure
a diet rich in animal fat
high concentrations of cholesterol in the blood.
Healthy arteries have smooth inner linings that promote unimpeded blood flow. The
deposition of cholesterol thickens and roughens this smooth lining. A plaque forms at the
site and becomes infiltrated with fibrous connective tissue and still more cholesterol. Such
plaques narrow the artery, leading to atherosclerosis. The rough lining of an atherosclerotic
artery seems to encourage the adhesion of platelets, triggering the clotting process and
interfering with circulation.
Hypertension (high blood pressure) promotes atherosclerosis and increases the risk of heart
attack and stroke. Atherosclerosis tends to raise blood pressure by narrowing the vessels
and reducing their elasticity. According to one hypothesis, chronic high blood pressure
damages the endothelium that lines arteries, promoting plaque formation.
As atherosclerosis progresses, arteries become narrower, and the threat of heart attack or
stroke increases. A heart attack is the death of cardiac muscle tissue resulting from
prolonged blockage of one or more coronary arteries, the vessels that supply oxygen–rich
blood to the heart. Because they are small in diameter to begin with, the coronary arteries
are particularly vulnerable. Such blockage can destroy cardiac muscle quickly, since the
constantly beating heart muscle cannot survive long without oxygen. A stroke is the death
of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the
head.
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