Lecture 3 Flashcards
What is haemodynamics?
Movement of blood around the body. Blood is modelled as a fluid.
What is the difference between serum and plasma?
Plasma contains anticoagulants-prevent clotting.
Centrifuged: (from bottom to top)
RBC’s, buffy coat (WBC’s), plasma (containing clotting factors)
Serum does NOT contain anticoagulants.
What is viscosity?
Thickness of blood.
Does whole blood viscosity change?
It is uncommon.
Some diseases increase the viscosity (making it thicker)
-polycythaemia (RBC’s- increased conc of Hb in blood due to increase in RBC’s or reduction of plasma)
-thrombocythaemia (more platelets)
-leukaemia (WBC’s-cancer)
Lead to thick sludgy blood which can lead to dry gangrene in peripheries
Does plasma viscosity change much?
Minor changes are more common than whole blood changes, as can be raised by inflammation etc.
-changes due to acute plasma proteins
(CRP, fibrinogen)
What is measured to indicate inflammation?
Levels of CRP (C-reactive protein)
What order of vessels does the blood flow through?
Aorta, arteries, arterioles, capillary, venules, vein, vena cava
It moves from high to low pressure.
What are the types of blood flow?
Laminar flow: organised, streamline fashion, smooth, no energy lost. SILENT. (Moves quickest in the centre of the vessel due to less resistance)
Turbulent flow: disorganised, irregular movement, energy lost. NOISY ( can be cause due to obstruction of diameter of vessel or vessel branching)
What is the rate of flow compared with pressure in turbulent/laminar flow?
Laminar flow: as pressure increases, flow increases directionally proportional to it.
Turbulent flow: once it reaches a critical point, the flow is not directionally proportional to the pressure. The flow increases at a slower rate compared to the pressure.
What is flow?
Volume transferred in a particular time (L/min)
-it is pulsatile
What is pressure?
Force being applied for a measured area (mmHg/pascals)
How do you link flow and pressure?
Flow= K(change in pressure)
K- conductance (ease of flow) ( hard to calculate mathematically so we use resistance)
What is resistance?
Measure of difficulty of flow
Reciprocal of K (1/K)
What is Darcy’s Law?
Flow = change in pressure/resistance
Therefore a change in resistance or pressure causes a change in flow. Mainly resistance.
By rearranging Darcy’s Law how do you work out resistance?
R = change in pressure/flow (the difference of pressure needed to move one unit of flow)
Resistance is inversely related to flow. As resistance goes up, flow goes down.
Therefore as you increase pressure, the resistance increases.
At high resistance, the flow decreases.
What contributes most to resistance?
Diameter (most important as length doesn’t change and viscosity is kept within a narrow range), length, viscosity of the fluid
-any small change in the radius will have a large effect (constriction/dilation)
Why is there a large pressure jump between the arteries and arterioles?
Due to large change in resistance
- arterioles play largest role in regulating TPR
- resistance in aorta is low (large diameter and short)
Why is the resistance in pulmonary circulation much lower than in the systemic circulation?
Shorter, wider vessels= lower resistance
-right side of heart has thin wall, pressure required is much lower in pulmonary circulation
What is velocity?
Distance fluid moves in a given time (cm/s) (speed at which is moves)
Flow- amount moving
Velocity- how quickly it is moving
What is the equation linking flow and velocity?
Flow= velocity x cross sectional area (piR2)
At constant flow, velocity is inversely related to radius squared
i.e. if the radius gets smaller, velocity increases
Which vessel has the highest cross sectional area?
Capillaries due to the vast quantity of them. Velocity is much slower here because the OVERALL radius is large due to the cross sectional area.
In what vessels is velocity the highest?
Aorta/vena cava. Because they have the smallest overall radius
Why do you want slow velocity in the capillaries?
To allow for exchange of materials between the tissue and the vessel.
What is peak systolic pressure?
Pressure when the ventricles are contracting, ejecting blood out of ventricles
What is the end diastolic pressure?
The pressure in the ventricles after contraction, when they haved filled fully
What is the pulse pressure?
The difference between the peak systolic pressure and the end diastolic pressure. PP= SBP-DBP
What is total flow?
The cardiac output
Using the equation R= change in pressure/flow, what is the equation linking TPR, aortic pressure and CO?
Total flow: cardiac output
Pressure difference: mean aortic pressure - central venous pressure (0)
Resistance: systemic resistance (TPR)
Therefore
TPR = mean aortic pressure/CO
What effects pulse pressure?
SV: increase in SV, increases pulse pressure
Compliance of the arterial system: lack of stretch-no compliance-pressure would need to be huge. But aorta allows stretch allowing pressure to be reduced and pulsatile effect is lowered.
- haemorrhage: reduce in diastolic pressure changing the PP
- ageing (atherosclerosis:stiffening of the vessel-reduced compliance) increasing systolic pressure, increasing PP
What does it mean if you have a strong bounding pulse?
You have a high pulse pressure, indicative of SV.
What is the pulse?
A shock wave that arrives slightly before blood itself.
What causes increased pulse pressure? (Bounding/strong pulse)
- bradycardia (slowing of heart, so the curve carries on as a contraction takes longer, therefore reaching a lower end diastolic pressure)
- vasodilatation (decrease TPR so diastolic pressure changes)
- elite athletes (systolic increases, diastolic decreases)
How do you measure blood pressure?
- can use a cannula
- measure BP non-invasively
Turbulent flow can be hear- it is not the heart
What happens to blood beyond a stenosis?
Velocity increases, flow decreases
What can be heard/felt in turbulent flow?
Heard: bruit
Felt: thrill
How do you create turbulent flow and why would you want to?
How do you read BP?
- want to because you can hear the turbulent flow which can be used to estimate BP
- apply cuff and increase pressure and closes off vessel temporarily
- release pressure in the cuff gradually allowing blood to start flowing again
- blood moving through vessel is turbulent, and gradually the flow starts to become laminar again as you loosen the cuff
How do you listen and calculate the BP?
- pump cuff to above the systolic pressure (around 120/125) to close vessel
- you start to hear turbulent flow (Korotkoff sounds) and this fades in and then fades out again.
- start to hear it at systolic pressure
- when it fades away this is the diastolic pressure
Measure it over your Brachial artery in the arm
How does the electronic machine estimate BP?
It contains a pressure sensor feeling for vibrations to see how far to increase cuff to shut off vessel etc.
-less accurate than manual way
Things you must do when taking manual BP:
-correct size cuff
(Too small:overestimate BP Too big:underestimate BP)
-positioning of the cuff
-measure both arms (higher value you use as reference arm)
-patient sat, legs UNCROSSED, at rest, arm supported
-repeated several times and mean taken of two closest values
How does gravity effect BP?
Pressure below heart: greater
Pressure above the heart: lower
-pressure gradient from heart to foot when standing
-pooling of blood below heart due to standing of blood in venous system
(If patient lying down on back pressure is same across the whole body)
Where is the blood pressure taken at the level of?
Assume the heart, as BP in different parts of the body can be different
What is postural hypotension?
Dizzy upon standing
- pooling of blood
- Baroreceptors have not kicked in yet
- brain not being supplied efficiently
Which patients suffer more from postural hypotension?
Elderly
