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CHAPTER 6:
THE CIRCULATORY SYSTEM

THE CIRCULATORY SYSTEM and the LYMPHATIC SYSTEM
Most of the cells in the human body are not in direct contact with the external environment, so rely
on the circulatory system to act as a transport service for them
...
The blood, heart, and blood vessels form the
Cardiovascular System
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The Cardiovascular System and the Lymphatic System collectively make up the
Circulatory System
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Vertebrates have a closed
circulatory system, meaning the blood
is repeatedly cycled throughout the
body inside a system of pipes
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It was in 1628, when the English Dr
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According to him, blood
was pumped out of the heart and into
the tissues through one type of vessel
and back to the heart through another
type of vessel
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3
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Pumped by the heart, blood travels through a network of blood vessels,
carrying nutrients (O2, glucose) and hormones to the cells and removing waste products (CO2
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THE HEART
1
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This is a hollow, muscular organ that
contracts at regular intervals, forcing blood through the circulatory system
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The heart is cone-shaped, about the size of a fist, and is
located in the centre of the thorax, between the lungs,
directly behind the sternum (breastbone)
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3
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b) The middle layer, comprising the cardiac muscle of
the heart itself, is called the myocardium
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For obvious reasons, the cardiac muscle is not under the
conscious control of the nervous system, and can generate
its own electrical rhythm (myogenic)
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Cardiac muscle has a rich supply of blood, which ensures
that it gets plenty of oxygen
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Since the heart relies on
aerobic respiration to supply its energy needs, cardiac muscle cells are richly supplied with
mitochondria
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Our hearts beat about once every second of every day of our lives, or over 2
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The only time the heart gets a rest is between beats
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The heart can be thought of as two pumps sitting side by side – each of which has an upper
atrium and a lower ventricle – a total of 4 chambers
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2
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In that process, carbon dioxide is lost to
the air and oxygen is absorbed
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3
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4
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5
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Then have thin
walls, so allowing them to be filled easily
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6
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This is where
the blood has the highest pressure
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Vertically dividing the two sides of the heart is a wall, known as the septum
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8
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These impulses pass
down specially-modified muscle cells (Purkinje fibres), collectively known as the Bundle of His
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Deoxygenated blood from the body enters the right side of the heart through two large veins
called the vena cavae
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Both empty into the right atrium
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When the heart relaxes (between beats),
pressure in the circulatory system causes the right atrium to fill with
blood
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When the atria contract, pressure inside it rises, the right atrioventricular (AV) valve opens, and blood is squeezed from the right
atrium into the right ventricle
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The closing of this valve makes a sound – ‘lub’
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When the atrium is empty, the pressure inside it falls, and the pressure
inside the ventricle begins to rise
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5
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6
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These are the
only arteries to carry deoxygenated blood
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When the right ventricle is empty, the pressure inside falls below that in the pulmonary artery,
and this causes the semi-lunar valve to snap shut
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A normal heart-beat is thus ‘lub…dup’
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Oxygenated blood leaves the lungs and returns to the heart through the
pulmonary veins
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2
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The valve which opens is called the left atrioventricular (AV) valve, (or bicuspid or mitral valve)
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3
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Ventricles contract

4
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5
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THE CARDIAC CYCLE
1
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In its simplest form, the cardiac
cycle is the simultaneous contraction of both atria, followed a fraction of a second later by the
simultaneous contraction of both ventricles
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The heart consists of cardiac muscle cells that connect with each other – they are branched – and
so when one contracts, they stimulate their neighbours and they all contract
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It can only respire aerobically
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A heartbeat has two phases:
A
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This occurs when the ventricles contract,
closing the A-V valves and opening the Semi-Lunar valves to pump blood into the two major
vessels leaving the heart
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Phase 2 – Diastole is the term for relaxation
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4
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The first sound is caused by the contraction of the
ventricles (ventricular systole) closing the A-V valves
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If any of the valves do not
close properly, an extra sound called a heart murmur may be heard
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Although the heart is a single muscle, it does not contract
all at once
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This is
the hearts natural pacemaker, and it initiates each beat
6
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7
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2 s
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8
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The ventricles then contract simultaneously, from the bottom upwards,
thus allowing them to empty completely with each beat
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The heartbeat is initiated by the Sino-Atrial Node and passes through the Atrio-Ventricular Node,
remaining at the same rhythm until nerve impulses cause it to speed up or to slow down
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10
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The accelerator nerve of the sympathetic
nervous system increases heart rate and the vagus nerve of the parasympathetic nervous system
decreases heart rate
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For most people, their resting heart rate is between 60 and 80 b
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m
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p
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is about all
we can safely manage!

BLOOD VESSELS (ARTERIES, VEINS and CAPILLARIES)
1
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The path is the same –
heart (ventricles) → arteries → arterioles → organ (capillaries) → veins → heart (atrium)
2
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This provides for
both strength and elasticity:
A
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B
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C
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ARTERIES and ARTERIOLES
1
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2
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The smooth muscle and elastic fibres that
make up their walls enable them to withstand the high pressure of blood as it is pumped from the
heart
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3
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This is known as secondary
circulation, and it reduces the load on the heart
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Other than the pulmonary arteries, all arteries carry oxygenated blood
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The aorta carries oxygenated blood from the left ventricle to
of the body except the lungs
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6
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The smallest
are called arterioles
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Arterioles can dilate or constrict to alter their diameter and so alter the flow of blood through
the organ supplied by that arteriole
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Since the volume of blood remains the same, if more blood flows through one organ,
less must flow through another
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Two organs which always have the same blood flow are the brain and the kidneys

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