Cardio 8 – Blood Vessels and Blood Flow
Anil Chopra
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1. Understand the design of the circulation and the function of the elements of
the circulation in terms of delivering blood flow and regulating pressure
change throughout the circulation.
Heart is pump that generates a pressure gradient to drive bulk flow through
capillary network.
Gas and nutrients delivered to cells and waste taken from cells all no further than
10μm from a capillary.
2 circuits of the heart both starting and finishing in the heart.
Large arteries have thick muscular walls, smaller arterioles offer the bulk of
peripheral resistance, and capillaries are never more than 1 cell thick in
endothelium to maximise nutrient exchange. Veins and venules act as capacitance
vessels that provide a reservoir for blood.
Diameter of the blood vessels changes from aorta (25mm) to capillary (0.005mm).
The capillaries hold the majority of blood volume along with the veins.
2. Know in approximate terms the distribution of total cardiac output to major
organs.
.75L 1.25L 1L .25L .75L
20L /min
3% 5% 4% 1% 3% <1% 80%
25 L/min
EXERCISE
REST
5 L/min
20% 5% 20% 3% 15% 5% 15%
1L .25L 1L .15L .75L .25L .75L /min
3. Know the definitions of systolic, diastolic, pulse and mean blood pressure and
the relationship between cardiac output, peripheral resistance and blood flow.
Systolic: this is the measurement of blood pressure when the ventricles contract i.e.
the maximum blood pressure in the aorta. Usually around 120mmHg.
Diastolic: measurement of blood pressure when the ventricles relax i.e. the minimum
blood pressure in the aorta. Usually around 80mmHg.
Pulse Pressure = systolic blood pressure – diastolic blood pressure. Usually 120-80=
40mmHg
Mean Blood Pressure = Diastolic blood pressure – 1/3 pulse pressure. Usually 80+13
= 93mmHg.
Mean arterial blood pressure = cardiac output x total peripheral resistance
4. Know Poiseuille’s relationship for blood flow and Laplace’s relationship for
wall tension and blood vessels.
Poiseuille’s Equation: Resistance = 8 L/  r4
Where  = fluid viscosity, L = vessel length, r = vessel radius.
Shear stress = shear rate x fluid viscosity
Shear rate = velocity gradient at any point.
This shows the importance of arterial radius on resistance:
Laplace’s Equation: T = P x R
Where T= wall tension, P=transmural pressure and R=vessel radius.
This shows how thicker walled vessels usually contain higher pressures. If pressure
increases, the vessel must react by increasing wall thickness or reducing diameter (the
latter will increase resistance). An exception is atherosclerosis where the wall
thickening has nothing to do with wall stress but simply affects blood flow.
5. Be able to describe how the blood pressure waveform changes in different
locations of the circulation and understand the basic principals of why this
happens.
o Flow occurs from a region of high hydrostatic pressure to a region of low hydrostatic
pressure.
o As blood leaves the heart with pulsatile trace in aorta and large arteries, pressure is high.
o Systolic pressure rises between
larger and smaller arteries.
o Pressure decreases with increasing
resistance (mainly in small
arterioles)
o Pressure much lower in right heart.
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6. Acquaint with the major
endocrine, neuronal and local
systems influencing systemic
blood pressure and blood flow.
Sympathetic and parasympathetic
action of SAN of heart.
Sympathetic control of vascular tone.
Renin-angiotensin system
Adrenal glands secreting adrenaline and noradrenaline.
Baroreceptors sensing stretch.
7. Understand how blood flow in veins is maintained.
 Pressure in veins is low (5-10mmHg)
 Venous return to heart is assisted by contraction of skeletal muscle.
 Standing causes contraction of the venous smooth muscle so it “stiffens”
(sympathetic stimulation). Use of skeletal muscle also affects venous return.
 They act as a reservoir for blood (contain 2/3 of blood vol).