CTB4: Blood pressure and flow Flashcards
Define compliance
The ability to distend in response to applied pressure
Define resistance
The ability to oppose a change
Large elastic arteries are ______
conduits
Muscular arteries and arterioles have smooth muscle which provides ______ to blood flow
resistance
What do veins ability to be highly compliant mean they are able to do?
allows them to act as reservoirs of blood as they are responsible for capacitance
What does capacitance mean?
Storing blood volume
Make the appropriate connections between the following:
Vein, capillaries, small arteries, large arteries
and
distribution, exchange, capacitance, resistance
Vein = capacitance
Capillaries = exchange
Small arteries = resistance
Large arteries = distribution
What is the main aortic pressure in healthy humans?
~90 mmHg
Why does pressure fall across circulation?
Due to viscous (frictional) pressure losses
Where does the large fall in mean blood pressure occur? What is the percentage drop?
Between large arteries and small arteries/arterioles
- approximately 50-70% drop
What is the mean pressure in capillaries?
25-30 mmHg, depending on the organ
When does blood pressure fall further?
As blood travels into the veins and back to the heart
What are the greatest systolic and diastolic pressures?
systolic: 120mmHg
diastolic: 80mmHg
Which system operates at a lower pressure: pulmonary or systemic?
Pulmonary
What are the lowest systolic and diastolic pressures?
systolic: ~25mmHg
diastolic: ~10mmHg
The magnitude of oscillation in pressure (pulse pressure) is _____ in the smaller arteries and arterioles
damped
What causes the major drop in pressure after the large arteries?
<0.5mm internal diameter of small muscular arteries
What are the two types of blood flow?
Laminar
Turbulent
Describe Laminar blood flow
– velocity of the fluid is constant at every point and particles move in straight lines without interfering with one another.
Describe Turbulent blood flow
– velocity of the fluid differs at various points and particles flow irregularly while generating whirlpool regions.
When does blood flow not exhibit laminar properties, when it usually does?
At branching points of vessels or when encountering vascular occlusion when they display turbulent flow
What are the two forced endothelial cells lining the interior of blood vessels experience?
Cylindrical strain
Shear stress
Describe cylindrical strain
– vessel wall distention by transmural pressure
Describe shear stress
– adhesive force between blood and surface of the blood vessel
What is shear rate?
What is the equation for this?
the rate at which layers of fluid move past one another. Shear stress is calculated as follows:
Shear stress = shear rate x viscosity
If resistance can not be overcome, what happens?
Blood flow would not be maintained
What does resistance of a tube depend on?
Fluid viscosity (η) Tube length (L) Inner radius of tube (r)
Which variables of resistance remain constant?
Viscosity & tube length usually remain constant, making inner radius the main determinant of vascular resistance
What is Poiseulles’s equation for resistance?
R= 8Lη/πr^4
Halving the radius does what to the resistance?
Increases the resistance by 16x
How can mean blood pressure be calculated?
Cardiac output x Resistance
How can mean blood pressure be calculated?
Stroke volume x Heart rate
List the 5 ways resistance in regulated
- Myogenic theory
- Vascular injury
- Endothelial chemicals
- Systemic circulating hormones
- Autonomic nervous system
How can pressure be regulated by the Myogenic theory?
stretch-sensitive ion channels allow smooth muscle fibres to contract when pressure increases
How can pressure be regulated by vascular injury?
damage to the vessel walls stimulates the release of serotonin from platelets which causes vasoconstriction
How can pressure be regulated by endothelial chemicals?
the endothelium releases both vasorelaxants (nitric oxide and prostacyclin) and vasoconstrictors (endothelins and thromboxane)
How can pressure be regulated by systemic chemicals?
vasodilation can be caused by kinins and atrial natriuretic peptide while vasoconstriction can be caused by vasopressin, noradrenaline and angiotensin II
How can pressure be regulated by the autonomic nervous system?
the sympathetic and parasympathetic branches can increase or decrease heart rate respectively. The sympathetic nervous system causes increased contraction force and vasoconstriction which increases BP whilst the parasympathetic nervous system decreases heart rate and cardiac output to lower BP.
What are Baroreceptors
Stretch receptors found in the aortic arch and carotid sinuses which help regulate blood pressure
Where are low pressure baroreceptors found?
On the vena cava and the right atrium
How can mean systemic arterial pressure be calculated?
Cardiac output x Total peripheral resistance
What are the two readings blood pressure is measured by? When do these take place?
- Systolic blood pressure – during cardiac contraction
- Diastolic blood pressure – resting pressure between heartbeats
What is the difference between systolic blood pressure and diastolic blood pressure known as?
Pulse pressure
Is blood pressure usually measured on right or left arm?
Left
Otherwise it can be taken on other extremities
What are segmental pressures?
Blood pressure redaings taken from different extremities
What can segmental pressure readings help do?
Help identify arterial occlusion or blockage along a limb
What is the mean blood pressure value usually?
approximately DBP + 1/3PP
What is the Windkessel effect?
The damping of blood pressure as blood leaves the heart and enters the aorta before the blood already within them can leave
What does the Windkessel effect help maintain? How?
constant flow and reduces The fluctuation of blood pressure over the cardiac cycle
Possibly due to arterial compliance
Why does blood pressure increase with age?
The arterial compliance is reduced as arteries become stiffer, which reduces the Windkessel effect and increases blood pressure
