Pressure and flow in arteries and veins Flashcards
Describe the Korotkoff sounds made on auscultation to measure BP
Silence
Tapping - systolic BP
Then aortic valve closes (dicrotic notch) -> thumping
Then muffled/silent - diastolic BP
What is the function of elastic arteries?
Dampen pressure variations
State 4 factors that affect the pressure wave
Stroke volume
Velocity of ejection
Elasticity of arteries
Total peripheral resistance
State normal arterial pressure
120/80mmHg
What happens to arterial pressure, especially pulse pressure with ageing?
Why is this?
Arterial pressure, especially pulse pressure increases.
Due to loss of aortic elasticity
Describe the effect of exercise on arterial pressure
Arterial pressure increases since velocity of ejection increase and the artery can’t respond that quickly
State three factors that affect arterial pressure
Age
Exercise
Time of day
Define pulse pressure
The difference between systolic and diastolic pressure
Describe pressure changes through arteries and veins
Arteries have a low resistance conduit - 95-90mmHg
Arterioles are resistance vessels - 40mmHg
Pressure already low when it gets to capillaries - 20mmHg
Small pressure difference pushing blood back into veins - 5mmHg
What name is given to the small pressure change pushing blood from the periphery back to the heart?
Systemic filling pressure
What is the fraction of pulmonary circulatory pressure in relation to systemic circulatory pressure
1/5
What is the relationship between velocity, cross section and surface area?
The bigger the cross section, the smaller the surface area and the great the velocity.
Therefore blood flows fastest through the aorta and vena gave which have the largest cross section but smallest surface area. It flows slowest through capillaries which have the smallest cross section and the largest surface area.
Is the change in pressure driving blood back to the heart big or small?
The change in pressure driving blood back to the heart from the periphery is small since venous pressure is low.
What characteristics of venous vessels make them vulnerable to external influences?
Collapsible and distensible
List the 6 factors that affect pressure and flow in veins
Gravity CVP (central venous pressure) Skeletal muscle pump Respiratory pump Venomotor tone Systemic filling pressure
Does gravity affect flow from arteries to veins?
No, but it causes venous distension in the legs
Describe the effect of gravity on venous pressure between lying down and standing.
Lying down - 20mmHg at head, 0mmHg at arms and 20mmHg at feet
Standing: -20mmHg at head, 0mmHg at arms and 100mmHg at feet
How much blood can accumulate in the feet due to the effects of gravity when standing?
1.5l
Describe the effects of gravity on venous return.
What are the clinical consequences of this?
Reduced EDV Reduced preload reduced stroke volume reduces CO reduces MAP Clinical consequence: postural hypotension - less blood reaches brain -> not enough O2 -> fainting
What is used to estimate CVP?
JVP/jugular collapse
Describe the mechanism of the skeletal muscle pump
Muscle contracts to squeeze blood back to the heart and increasing EDV.
Rhythmic exercise is better than static explaining why travellers are more prone to DVT and varicose veins
Describe the mechanism of the respiratory pump
During inspiration, there is an increase in thoracic pressure which reduces the pressure gradient between the veins and the heart, leading to a reduced EDV
Describe the effect of venomotor tone on venous return
Venomotor tone - state of contraction of smooth muscle surrounding veins and venules which mobilises their capacitance -> increased EDV
What triggers an increase in venomotor tone?
Increased sympathetic tone
Define systemic filling pressure
Pressure created by ventricles and transmitted through vascular tree to veins
What increases systemic filling pressure?
Exercise
