The Circulatory System

Chapter 31: The human
circulatory system
Leaving Certificate Biology
Higher Level
The Circulatory System
• Organisational complexity of the human
involves having multiple organs and
tissues each with their own individual
functions and an extensive network of
tubes carrying oxygen and nutrients to
every living cell of the human body and the
efficient disposal of wastes produced by all
the living cells of the human
The Circulatory System
• The circulatory system of the human is a closed
system whereby blood continually flows around
the body inside a network of blood vessels and
oxygen, carbon dioxide, nutrients, and wastes
diffuses into and out of these vessels
– The closed system involves blood flowing through
structures in the following order: strong muscular
heart, arteries, arterioles, capillaries, venules, veins
and finally back to the heart
The Circulatory System
• Advantages of the closed circulatory
system are:
– Blood can be pumped around the body fast
for fast and efficient delivery of oxygen and
nutrients and fast and efficient removal of
– Blood can be directed to different areas of the
body through constriction and dilation of the
blood vessels where needed
Blood Vessels
• Arteries: carry blood away from heart
• Arterioles (small arteries): carry blood from the
arteries to the capillaries
• Capillaries: carry a single-file of red blood cells
through their lumens delivering oxygen and
nutrients and removing wastes from local cells
and tissues
• Venules (small veins): carry blood from the
capillaries to the veins
• Veins: carry blood towards the heart
Arteries and Veins
Thick wall
Thin wall
Differences between arteries and
Carries blood away from
Carries blood toward heart
Blood under high pressure
Blood under low pressure
Thick walls
Thin walls
Pulse flow
Smooth flow
Narrow lumen
Large lumen
No valves
Valves present
Blood rich in oxygen (except Blood poor in oxygen
pulmonary artery)
(except pulmonary veins)
Smooth Muscle, Skeletal Muscle
and Valves
• Smooth muscle is involuntary (not under
conscious control): surrounds arteries and
veins and help to push blood along
• Skeletal muscles in the legs and arms help
to push blood along veins back to the
• Valves present in veins help to prevent
blood flowing backwards thus preventing
pooling of blood in the extremities
The Heart
• Located slightly to the left of the sternum
between the lungs above the diaphragm
• Composed of cardiac muscle (doesn’t
fatigue as easily as skeletal or smooth
• Size of clenched fist
(Fig. 27.9
p 261)
Double Circulation
• In the heart there is two separate pumps – one
on the right and one on the left – separated by
the septum
• Double circulation in the human is necessary in
order to keep deoxygenated (right-side) blood
separate from oxygenated blood (left-side)
• The two circuits are called:
– Pulmonary circuit
– Systemic circuit
Blood Flow Through the
Pulmonary Circuit
• Deoxygenated blood enters right atrium from the
superior and inferior vena cavae which have carried
blood from the upper and lower bodies, respectively
• Right atrium contracts and blood flows through the
tricuspid valve
• Blood then enters right ventricle which in turn contracts
• Right ventricular contraction causes the tricuspid valve to
close and the semi-lunar valve to open and blood flows
in the pulmonary artery
• The pulmonary artery carries blood to the lungs to
become re-oxygenated as the blood flows through the
capillaries that surround the alveoli
Blood Flow Through the
Systemic Circuit
• Oxygenated blood is carried from the lungs via the
pulmonary veins to the left atrium
• Blood enters the left atrium and the left atrium contracts
forcing blood through the bicuspid valve
• Blood then enters left ventricle which in turn contracts
more strongly than the right ventricle but at the same
• Left ventricular contraction causes the bicuspid valve to
close and the semi-lunar valve to open and blood flows
into the aorta
• The aorta carries the blood to the head, upper body, and
lower body to deliver oxygen and nutrients
Coronary Artery
• Cardiac muscle itself needs oxygen and
nutrients in order to continue pumping
• Therefore, the heart itself is supplied with blood
via a small branch from the aorta
• The coronary arteries splits up into many
arterioles and capillaries that perfuse through
the heart muscle
• These capillaries then drain into venules and
into the coronary veins which eventually drain
back directly into the right atrium
The Hepatic Portal System
• The hepatic portal system begins in the
capillaries of the digestive system and ends in
the portal vein – it does not connect directly to
the heart
• Consequently, portal blood contains substances
absorbed by the stomach and intestines
• Portal blood is passed through the hepatic
lobules where nutrients are absorbed and toxins
are excreted or detoxified
Detailed Study of the Heart Beat
• The heart beat is controlled entirely by the
pacemaker – which is a bundle of specialised
nervous tissue located in the top of the right
• The pacemaker is also called the sino-atrial (SA)
• Pacemaker sends out regular nervous impulses
which travel along the axons of the nerve cells
out through the walls of the atria – this causes
the atria to contract
• Atrial contraction is also called atrial systole
Detailed Study of the Heart Beat
• The impulse from the SA node travels via
axons to the atrio-ventricular (AV) node –
located further down in the right atrium
• This sets up an impulse that travels from
the AV node down nerve fibres through
the ventricular muscle causing the
ventricles to contract – also called
ventricular systole
Detailed Study of the Heart Beat
• The rate at which the SA node fires can be
controlled by various factors:
– Exercise
– Temperature
– Emotion
– Shock
Heart Sounds
• Sound of the heart beat is caused by the
opening and closing of the valves of the
• The “lub” sound is caused by the closing
of the tricuspid and bicuspid valves closing
and the semi-lunar valves opening as the
ventricles contract
• The “dub” sound is caused by the semilunar valves closing as the ventricles relax
• A pulse is the pressure the flowing blood
exerts on the walls of an artery causing
the walls of the artery to expand
• Pulse can be felt most easily on the wrist
or on the neck
• Pulse rate (pulses/min) is an indicator of
heart rate (heart beats/min)
• Average heart rate is 72 bpm in an adult
Blood Pressure
• Blood pressure is given as a large number over a usually
smaller number, e.g. 120/80 mmHg would be a normal
blood pressure reading for a healthy adult
• The larger number is the systolic pressure – which is the
pressure the blood exerts on the walls of an artery the
moment the blood passes through
• The smaller number is the diastolic pressure – which is
the pressure the blood exerts on the walls of the artery
the moment there is no movement of blood
• Blood pressure is measured using a
Blood Pressure (continued)
• Drugs that reduce blood pressure and hence
heart rate are:
– Anti-hypertensives: stimulate vasodilation where the
smooth muscle surrounding all the blood vessels
relax and the blood vessels widen thereby reducing
blood pressure
– Diuretics: stimulate loss of water from the blood by
closing the pores present in the collecting duct of
nephrons – therefore water is NOT reabsorbed into
the blood – if there is reduced blood volume this
reduces blood pressure and also heart rate
– Beta-blockers: act on the pacemaker of the heart
directly reducing its rate
Effects of Smoking, Diet, and
Exercise on the Circulatory System
• Smoking:
– Nicotine, present in all cigarettes, is an extremely
addictive chemical that causes increased heart rate
and blood pressure by causing arteries and arterioles
to narrow (vasoconstriction)
• Diet:
– Diet high in saturated fat and cholesterol causes
increased risk of blockages in the arteries and
arterioles (caused by atherosclerosis [fatty deposits in
blood vessels]) – the main cause of heart attacks and
Effects of Smoking, Diet, and
Exercise on the Circulatory System
• Exercise:
– Causes the heart to beat faster and stronger thereby
causing the heart muscle to increase in size
– Improves circulation by stimulating formation of more
capillaries – thereby indirectly reducing blood pressure
– Increases the body’s ability to use oxygen – therefore
the heart does not have to work as hard at rest
– Reduces body weight – thereby reducing blood
pressure as the heart does not have to work as hard to
pump blood

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