Pressure and Flow in arteries and veins Flashcards
How are a sphygmomanometer and a stethoscope used to measure arterial pressure?
Cuff is inflated - constricts brachial artery
First audible tapping noise through stethoscope is because peak of systolic Bp is greater than pressure of cuff - small blip of turbulent blood flow passes stethoscope
Noise muffles as cuff pressure is reduced because flow becomes more laminar
Sounds disappear completely since blood flow through artery has returned to normal.
Last audible sound defined as diastolic pressure
What are the advantages/disadvantages of this method of measurement of blood pressure?
Disadvantages:
Accuracy
Discontinuous – only for instantaneous measurement
Needs care – constant surveillance, not automated
Advantages:
Cheap and non-invasive
What is the effect of high diastolic pressure on the aorta?
higher total peripheral resistance so aorta finds it harder to release blood
What is the effect of elastic vessels on pressure variations?
Dampens them down
What is the pressure wave affected by?
- stroke volume
- velocity of ejection
- elasticity of arteries
- total peripheral resistance (If TPR increases, stroke volume will go down (more energy is “wasted” building up sufficient pressure to open the aortic valve))
Define pulse pressure
Pulse pressure = difference between systolic and diastolic pressure readings
What feature of arteries allows maintainance of high pressure during high systolic pressure?
Elastic fibres
How does pressure change from arteries - arterioles - capillaries - venules?
Decreases
What is name given to the blood pressure left to bring the blood back to the heart?
Systemic filling pressure
What is normal arterial pressure?
“Normal” arterial pressure
= 120/80 mmHg
Arterial pressure (especially pulse pressure) increases with age
What is the pressure drop through the arteries?
arteries (from ~ 95 to 90 mmHg)
- low resistance conduit
What is the drop in blood pressure throught the arterioles?
Large drop through arterioles (from ~ 90 to 40 mmHg)
- resistance vessels
Why is it good that blood pressure is low when blood reaches the capillaries?
Capillaries are thin-walled
What is the small pressure difference pushing blood back through the veins?
From ~ 20 to 5 mmHg
- systemic filling pressure
What is the pulmonary pressure in relation to the systemic pressure?
Pulmonary circulation is 1/5 systemic
Where is velocity fastest?
aorta and vena cava - slowest in capillaries
Look
Vessels of veins are distensible and collapsable what does this mean?
External influences affect flow
Does gravity affect driving pressure from arteries to veins?
NO
What is the effect of gravity in venous blood in the leg?
Causes venous distension
Decreased:
- end diastolic volume
- preload
- stroke volume
- cardiac output
- mean arterial blood pressure (orthostatic hypotension)
What is the effect of gravity in the neck?
Causes venous collapse in the neck - can be used to estimate central venous pressure
Where in the body has -20 mmHg?
Head and neck
Where in the body is +100 mmHg?
Feet
If pressure in the arteries always remains constant what does this mean for the driving force of the blood?
Always remains the same
What is the pressure that causes venous collapse?
Pressure of the interstitial fluid
What is the purpose of valves?
Ensures directional flow
What is the effect of sustained muscle contraction in the legs?
Stops blood flow due to sustain compression of vein
Less venous return
(Decreased end diastolic volume, preload, stroke volume, cardiac output, mean arterial blood pressure)
What is meant by the respiratory pump?
During inspiration, decreases in intrathoracic pressure draws air into the trachea, bronchi, and lungs and draw blood into the vena cava and right atrium of the heart
How do varicose veins arise?
Result from poor valves – more pooling of blood and high pressure in deep veins
What is meant by venomotor tone?
State of contraction of the smooth muscle surrounding the venues and veins
- mobilises the capacitance
What are all the factors that affect venous pressure and blood flow in veins?
- Gravity
- skeletal muscle pump
- respiratory pump
- venomotor tone
- systemic filling pressure
