Transcript Document

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Components of the CVS
• The CVS consists of
a double pump (the
heart) and a complex
system of blood
vessels to transport
oxygen carrying
blood around the
body.
Blood Vessels
There are 3 types of blood vessel:
• Arteries (and arterioles)
• Capillaries
• Veins (and venules)
Arteries
Arteries (and arterioles) carry blood away
from the heart. The largest arteries e.g. the
Aorta, have thick elastic walls which can
stretch to accommodate the surge of blood
after each contraction of the heart. Arteries
branch many times, forming smaller and
smaller vessels, the smallest of which are
arterioles.
Contraction of the smooth muscle lining
the walls of the arterioles allows them to
open or close to varying degrees to adjust
blood flow to different parts of the body
e.g. during vasodilation and
vasoconstriction.
Veins
Veins (and venules) carry blood back to
the heart. Blood flows out of the
capillaries into the smallest of the
veins – venules – which in turn reunite
to form larger veins. The walls of
veins are thinner than those of
arteries as blood pressure is far lower
as it travels through veins.
Consequently, veins have valves to
prevent the back flow of blood.
Oxygenated blood leaves the heart from
the left ventricle via the aorta,
moves through arteries to arterioles
to capillaries to venules and returns
to the right atrium by way of veins.
Capillaries
Capillaries are tiny vessels where the
exchange of substances with the
tissue occurs. They also connect
the arterioles to the venules.
Their walls are only one cell
thick, allowing nutrients and
waste to diffuse through with
ease.
Capillaries form extensive branching
networks (capillary beds)
throughout the body tissues, but
only certain beds are open at any
one time. This allows the
‘shunting’ of the blood from one
region to another.
Arteri Capillaries Venule
ole
(capillary
bed)
• The flow of blood through the
capillaries is controlled by the
contraction of a ring of muscle
called a sphincter. This is
important as the body does not
contain sufficient blood for all
vessels to be filled with blood at
one time.
This is understandable when we
consider that that an individual
could have between 25,000 to
60,000 miles of capillaries!
The CVS consists of 2 distinct circuits
• The pulmonary circuit carrying
deoxygenated blood from RV to lungs
• The systemic circuit carrying
oxygenated blood from LV to the aorta
and then the rest of the body
The cardiac cycle
• Each heartbeat is called a cardiac cycle and
consists of the following
• Atria contract simultaneously
• Ventricles contract simultaneously
• All chambers relax
• Lasts about 0.8 secs (0.3systole,0.5 diastole)
Two phases of the cardiac cycle
• Systole: contraction of the heart (Atrial
first, the ventricular)
• Diastole: relaxation of the heart
Cardiac Output
Cardiac Output is the volume of blood pumped
by each ventricle per minute and is the
function of two factors:
• Heart rate (beats per minute)
• Stroke volume (the volume of blood ejected
by each ventricle during each contraction)
CO = HR x SV
At rest: HR = 72bpm
SV = 70ml
i.e.
CO = 72 x 70
= 5040 ml/min
= 5 litres/min
Cardiac Output varies between individuals and depends
on their physical fitness and level of activity. For
example, the heart of a highly trained athlete can
pump 30-35 l/min while most non-athletes can only
achieve a max of 20 litres.
Some typical values for cardiac output at varying levels of
activity
Activity
Level
Heart rate
(bpm)
Stroke
Volume (ml)
Cardiac
Output
(l/min)
Rest
72
70
5
Mild
100
110
11
Moderate
120
112
13.4
Heavy
(athlete)
200
150
30
As work load increases, HR increases to a
maximal value of about 180 – 200 bpm (220
minus age), while SV increases proportionally
less (70-150ml). The increase in cardiac
output with exercise is achieved principally by
increasing the heart rate.
Blood Pressure
The force exerted by the blood
against the walls of the blood
vessels is known as blood
pressure
Measurement of BP
Both systolic and diastolic BP can be
measured by an inflatible instrument
called a sphygmomanometer which is
wrapped around the upper arm.
• BP is measured in millimeteres of
mercury (mm Hg)
• Normal reading for a healthy adult is
about 120/70 mm Hg (SBP/DBP)
• As we age, BP rises due to
atherosclerosis. A 65 yr old man may
have a BP of 140/90