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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
...
The Cardiovascular System and the Lymphatic System collectively make up the
Circulatory System
...
Vertebrates have a closed
circulatory system, meaning the blood
is repeatedly cycled throughout the
body inside a system of pipes
...
It was in 1628, when the English Dr
...
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
...

3
...
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
...


THE HEART
1
...
This is a hollow, muscular organ that
contracts at regular intervals, forcing blood through the circulatory system
...
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)
...

3
...

b) The middle layer, comprising the cardiac muscle of
the heart itself, is called the myocardium
...
For obvious reasons, the cardiac muscle is not under the
conscious control of the nervous system, and can generate
its own electrical rhythm (myogenic)
...
Cardiac muscle has a rich supply of blood, which ensures
that it gets plenty of oxygen
...
Since the heart relies on
aerobic respiration to supply its energy needs, cardiac muscle cells are richly supplied with
mitochondria
...
Our hearts beat about once every second of every day of our lives, or over 2
...
The only time the heart gets a rest is between beats
...
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
...

2
...
In that process, carbon dioxide is lost to
the air and oxygen is absorbed
...

3
...

4
...

5
...
Then have thin
walls, so allowing them to be filled easily
...

6
...
This is where
the blood has the highest pressure
...
Vertically dividing the two sides of the heart is a wall, known as the septum
...

8
...
These impulses pass
down specially-modified muscle cells (Purkinje fibres), collectively known as the Bundle of His
...
Deoxygenated blood from the body enters the right side of the heart through two large veins
called the vena cavae
...
Both empty into the right atrium
...

When the heart relaxes (between beats),
pressure in the circulatory system causes the right atrium to fill with
blood
...
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
...
The closing of this valve makes a sound – ‘lub’
...
When the atrium is empty, the pressure inside it falls, and the pressure
inside the ventricle begins to rise
...

5
...

6
...
These are the
only arteries to carry deoxygenated blood
...
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
...
A normal heart-beat is thus ‘lub…dup’
...
Oxygenated blood leaves the lungs and returns to the heart through the
pulmonary veins
...

2
...
The valve which opens is called the left atrioventricular (AV) valve, (or bicuspid or mitral valve)
...

3
...


Ventricles contract

4
...

5
...


THE CARDIAC CYCLE
1
...
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
...
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
...
It can only respire aerobically
...
A heartbeat has two phases:
A
...
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
...
Phase 2 – Diastole is the term for relaxation
...

4
...
The first sound is caused by the contraction of the
ventricles (ventricular systole) closing the A-V valves
...
If any of the valves do not
close properly, an extra sound called a heart murmur may be heard
...
Although the heart is a single muscle, it does not contract
all at once
...
This is
the hearts natural pacemaker, and it initiates each beat
6
...

7
...
2 s
...

8
...
The ventricles then contract simultaneously, from the bottom upwards,
thus allowing them to empty completely with each beat
...
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
...

10
...
The accelerator nerve of the sympathetic
nervous system increases heart rate and the vagus nerve of the parasympathetic nervous system
decreases heart rate
...
For most people, their resting heart rate is between 60 and 80 b
...
m
...
p
...
is about all
we can safely manage!

BLOOD VESSELS (ARTERIES, VEINS and CAPILLARIES)
1
...
The path is the same –
heart (ventricles) → arteries → arterioles → organ (capillaries) → veins → heart (atrium)
2
...
This provides for
both strength and elasticity:
A
...

B
...

C
...


ARTERIES and ARTERIOLES
1
...

2
...
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
...

3
...
This is known as secondary
circulation, and it reduces the load on the heart
...
Other than the pulmonary arteries, all arteries carry oxygenated blood
...
The aorta carries oxygenated blood from the left ventricle to
of the body except the lungs
...

6
...
The smallest
are called arterioles
...
Arterioles can dilate or constrict to alter their diameter and so alter the flow of blood through
the organ supplied by that arteriole
...
Since the volume of blood remains the same, if more blood flows through one organ,
less must flow through another
...
Two organs which always have the same blood flow are the brain and the kidneys
...


CAPILLARIES
1
...
These have a very
large surface area and thin walls that are only one (epithelial) cell thick
...
It is in the capillaries that exchanges take place between the blood and the tissues of the body
...
Capillaries are also narrow
...
In most capillaries, blood cells must flow in single file
...
Tissue fluid is formed in the capillaries, for their walls are leaky (see below)
...
After leaving the capillaries, the blood enters a network of small venules, which feed into veins
...

2
...
The walls
of veins are thinner and less elastic than arteries, but they are also more flexible
...
Veins tend to run between the muscle blocks of the body and nearer to the surface than arteries
...
The larger veins contain valves that maintain the direction of blood-flow
...

5
...
When muscles contract they squeeze against the veins and help to force the
blood back towards the heart
...


PATTERNS OF CIRCULATION
1
...

2
...

3
...

4
...
It carries blood to and from the rest of the body
...
The heart itself receives its supply of blood from the two coronary
arteries leading from the aorta
...

6
...
coronary circulation - supplying blood to the heart muscle
(coronary artery)
...
renal circulation – supplying blood to the kidneys (renal
artery)
...

C
...
This is about 70% of the liver’s blood supply
...
All blood leaves the liver through the hepatic vein
...
Blood moves through our circulation system because it is under pressure, caused by the
contraction of the heart and by the muscles that surround our blood vessels
...

2
...
e
...

3: To measure blood pressure:
a) Ensure the patient is relaxed and has not taken any
exercise for at least 10 mins
...

c) The pressure in the cuff is slowly released – whilst
listening for the first sounds of blood passing through the
artery
...
This is the systolic pressure
...
Average systolic pressure
rises with age so 100+ your age is a safe maximum
...
This is the diastolic pressure, when the
pressure of the blood is sufficient to keep the arteries open
even when the ventricles relax
...

g) Blood pressure readings are given as two numbers –
the systolic (higher) figure over the diastolic (lower) figure e
...
120/80mm Hg
...

4
...

The higher the blood pressure, the more water is forced out in the nephrons; this reduces the volume
of lymph and lowers the blood pressure
...

b) The nervous system, which regulates heart rate
...
This sends impulses down either the accelerator nerve (of the sympathetic nervous
system), which speeds up heart rate, or down the vagus nerve (of the parasympathetic nervous
system), which slows it down
...

c) Stretch receptors in the walls of the heart
...
The heart responds to this by beating faster and
harder
...


TISSUE FLUID and the LYMPHATIC SYSTEM
1
...
This is
known as tissue fluid
...
This fluid carries chemicals such as glucose and
hormones to the cells of the body that are not next to
the capillary, and removes waste products, such as
urea and CO2
...
The mechanism behind the formation of this fluid
is a common question!
a) The high blood pressure (‘hydrostatic
pressure’)at the arteriole end of the capillary bed is
much greater than the solute potential (‘osmotic
pressure’) of the surrounding cells
...

b) at the venous end of the capillary bed, the blood pressure (‘hydrostatic pressure’) is low,
whilst the solute potential (‘osmotic pressure’) of the blood is much stronger, since the blood is
more concentrated
...
This causes some water to be returned to the
blood in the capillaries by osmosis
...

4
...

5
...

6
...

7
...
Examples of lymph nodes are the tonsils, the appendix,
the spleen and the thymus gland (in children only – it disappears from the age of 10 or so)
...
If the blood pressure is too high, or if the person is inactive, the lymph can build up in the
tissues, particularly around the ankles and feet
...
With the blood now thicker, it is more likely
to clot, forming DVT or deep vein thrombosis
...
However, going for a jog is not recommended!

BLOOD
We have between 4 and 6 litres of blood, the liquid connective tissue that is the transport medium
of the circulatory system
...
Blood also transfers heat to
the body surface and plays a role in defending the body against disease
...
Blood is composed of 55% liquid - plasma – and 45% cells, almost all of which are Red Blood
Cells (RBC’s)
...

2
...

3
...


BLOOD PLASMA
1
...

2
...
They are the most common plasma protein
...
Together with the
WBC’s they form the immune system
...


BLOOD CELLS
These comprise Red Blood Cells RBC’s (also known as haemocytes or erythrocytes); White Blood
Cells (WBC’s) of several different types and platelets
...

RED BLOOD CELLS (RBC’s) 1
...
One cubic millimetre
(one microlitre, or 1µl) contains roughly 5 million RBCs
...
The liver destroys excess
RBC’s on returning to sea-level, so training must continue until
immediately before the event, if possible
...
RBC’s are biconcave disks about 8 µ across, thus giving them a larger
surface area (Fick’s Law), and allowing them to fold up and pass through
the smallest capillaries
...
They are produced from stem cells in the bone marrow; are full of haemoglobin; have no nucleus
or mitochondria and their function is to transport respiratory gases
...

4
...
These are outnumbered by RBC’s
approximately 500 to 1 and their numbers
fluctuate, rising during infection and
falling at other times
...

They are larger than RBC’s, almost
colourless,
and
do
not
contain
haemoglobin
...
WBC’s have a nucleus and whilst most live for a few days, others can live for many months or
years, thus providing us with life-long immunity from repeat infections (memory cells)
3
...

Whilst
lymphocytes produce antibodies, the other two types of WBC can also engulf bacteria, in a
process called phagocytosis (a form of active transport!)
...

Description

Site of
production

Mode of action

Lymphocytes
(Cell A)

Large round
nucleus
...


lymph nodes
and spleen

Make antibodies or
kill infected cells
...

bone marrow
Granular
cytoplasm
...

Lymph nodes
Clear
cytoplasm
...
Platelets are not true cells; they are tiny fragments of other cells –
megakaryocytes - that were formed in the bone marrow; their lifespan is 7-11 days
...
Platelets play an important role in blood clotting, by adhering to the
site of the wound and releasing clotting factors known as
prothrombin
...
Clotting factors are part of a cascade reaction which begins with
chemicals released by injured cells and ends with a sticky meshwork of
fibrin stop bleeding by producing a clot
...
A genetic disorder of Factor VII is called haemophilia, suffers (all
male – why?) may bleed extensively from even a small cut or scrape
...
Unwanted clotting of blood within blood vessels can block the flow
of blood – a thrombosis
...


BLOOD TYPES
1
...

2
...

3
...
Each can be either Rhesus +ve
or Rhesus –ve, giving 8 groups in all
...
Blood typing is the identification of the
antigens in a blood sample
...
It classifies blood
by the antigens on the surface of the RBC’s and the antibodies circulating in the plasma
...
An individual's RBC’s may carry an A antigen, a B
antigen, both A and B antigens, or no antigen at all
...

6
...


Rhesus system
1
...
Of the UK population, 85% are Rh+ ve, meaning that Rh
antigens are present
...

2
...

3
...
If the mother is Rhand the father is Rh+, the child may inherit the dominant Rh+ allele (gene) from the father
...

4
...
The
symptoms include damaged liver and so fewer RBC’s, brain (due to lack of oxygen) and skin
...
To prevent this, any Rhmother will automatically be
given an injection of anti-Rh+
antibodies
(known,
confusingly, as anti-D) at
childbirth
...
This tricks her body
into believing she has not had a
Rh+ve child, and so the next
pregnancy will be protected
from attack, since she will have
no antibodies to Rh+ve blood

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