Summary
Summary Cardiac Cycle
- Module
- Biomedical Science
- Institution
- St Georges
Summary of the cardiac cycle
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Cardiac CycleKey points:
Importance of the cardiovascular system (CVS) – basic functions of
its components; heart, arteries, capillaries, veins
Fundamental laws of the CVS
Electrical activity and conduction pathways of the heart
Mechanical events; phases of cardiac cycle
Focus on the left ventricle – changes in volume, pressure and stroke
work (how much energy we use, each time our heart beats) during
cardiac cycle
Associated sounds of cardiac cycle
Why do we need a CVS? O2 transport via convection
We need a CVS because passive diffusion is fast over short distances (less than
1mm) but very slow over longer distances (over 1mm) so is useless for body O 2
transport.
So we use passive diffusion to transport oxygen at the lungs and at the capillaries
(where we also transport nutrients and other substances).
But in order to get O2 to our tissues quickly and remove CO2 quickly, we use
convection (convective transport). Convection is the “mass movement of fluid
caused by pressure difference” and is fast over long distances.
The heart uses a lot of energy to create blood at high pressure and therefore create
a pressure difference, with the arteries under high pressure and veins delivering
blood back to the heart under low pressure. The pressure difference between these
is what drives blood flow.
Heart – Is the driving force, creating large pressures
Arteries – Involved in distribution of blood and alter blood flow
Capillaries – Involved in exchange, are present in huge numbers and only one cell
thick
Veins – Act as a reservoir, 2/3rds of blood volume, we can access if needed (e.g.
during exercise)
Important Laws of the CVS: Heart
Cardiac output (CO) is the amount of blood ejected from the heart per minute. It is
proportional (changes, will increase if following factors are increased) to how often
the heart beats per minute (heart rate HR) and how much blood is ejected per beat
(stroke volume). This is shown below as an equation
It is important to remember, that output from the right side (via pulmonary artery)
and left side (aorta) are the SAME.
, CO = HR x SV
Cardiac output changes according to demand:
At rest, 70bpm x 70ml = 5 litres/min
During exercise, 180bpm x 120ml = 22 litres/min
Important Laws of the CVS: Circulation
The main law of circulation is Darcy’s law of flow, which is about flow through a
tube, in this case, the tube is our blood vessels.
It states that flow is proportional to the pressure difference across the tube and
inversely proportional to the resistance.
So if we wanted to increase flow, we could increase the pressure at one end and
decrease it at the other, this would increase the pressure drop. We could also
decrease the resistance (e.g. larger radius), which could increase flow.
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