Flashcards in Flow through Tubes and Peripheral Circulation Deck (29):
How would you describe the way vessels are arranged as they bifurcate.
Vessel types are arranged in parallel with each other.
How does the body ensure blood always flow in the right direction?
Blood pressure must always be higher at previous vessel. In arteries this is achieved through elastic recoil and the smooth muscle contraction and in the venous system by muscle contraction and valves.
When the flow is fixed, what is velocity of blood related to?
Velocity is inversely proportional to the cross sectional area. Note capillaries are in parallel so the velocity is slower than that of the aorta because the overall cross sectional area is greater.
When the driving force is fixed what is flow related to?
Flow is related to the resistance of the vessels (cross sectional areas) and the velocity which is related to the viscosity of the fluid.
Describe Laminar flow
Laminar flow is characterised by a gradient of velocity – fastest flow in the centre and slowest around the edge to the point of being stationary.
What reason cause flow to become turbulent?
As mean velocity increases flow eventually becomes turbulent (after reaching a critical value) because velocity the gradient breaks down and fluid tumbles over each other. Flow is also turbulent when viscosity is low or the lumen of the vessel is irregular. This turbulent flow generates a sound called a bruit.
Viscosity is the inability of the layers of laminar flow to slide over each other.
When flow rate isn't fixed what does velocity relate to?
Viscosity determines the velocity gradient across the vessel. For a given gradient the wider the vessel the faster it allows the middle layers to move and so velocity is proportional to the cross sectional area when the flow isn’t fixed.
What is flow equal to?
Flow is equal to the product of the mean velocity and the cross sectional area.So Flow is proportional to the Pressure difference x r^4/viscosity x length.
Why is flow's relation with radius important for young children?
Their airways can become compromised much more easily because - a 2mm reduction in an adult may not be noticed but for a baby could be considerable and even enough for the airways to collapse.
Describe the symptoms and causes of Hyperviscosity syndrome.
Hyperviscosity Syndrome causes easier clotting, decreased retinal function and seizures due to reduced cerebral blood flow. Casues - abnormally high plasma protein levels such as high IgM in Waldenstrom Macroglobulinaemia - treated by plasmaphresis and abndormally high RBC or WBC count such as Polycythemia - treated by phebotomy.
Why might functional cardiac murmurs appear in people with anaemia?
Anaemia leads to low viscosity and so high velocity, this causes the breakdown of the velocity gradient a so turbulent flow.
How does Pressure relate to flow and resistance?
Pressure of fluid in a tube is equal to Flow multiplied by resistance. Can be re-arranged which shows us that Resistance decreases to the 4th power of an increase in radius.
How does vessels being in parallel vs series effect overall resistance?
Resistance of Blood vessels in series add up, in parallel resistance becomes equal to 1/((1/R)+(1/R)). This means that our capillaries offer little resistance in reality because they are all in parallel so have little pressure drop. Veinsalso have a low pressure drop because they have low resistance.
How is the high pressure in large arteries achieved?
High reisistance in the arterioles allows for arterial high pressure before them.
Describe compliance and capacitance.
Many vessels are distensible or compliant. As the tubes distend with increasng pressure this means they can act as a store of blood which is called capacitance. Capacitance is a measure of volume increase per unit pressure and so is C=V/P.
What impact does compliance have on flow rate?
Compared to a rigid tube compliance allows for faster flow rate because resistance is low and cross sectional area is high.
What percentage of blood is found in the veins and why?
Veins have a large compliance and about 60% of blood can be found in the venous system.
What is TPR (total peripheral resistance)
TPR is the sum of all arteriolar resistance.
Describe the compliance in arteries.
Arteries do still have compliance but very little whose only purpose is to store mechanical energy of contraction in elastic recoil of their walls to maintain pressure through arteries, having more flowing into arteries than out smooths out the pressure waves of contractions. This is known as the Windkessel effect.
What would reduced compliance in arteries and veins cause
Reduced compliance can lead to essential hypertension.
What is Pulse pressure
Pulse pressure is the systolic minus the diastolic pressure.
What is mean artereial blood pressure
MaBP is the diastolic pressure plus 1/3 of pulse pressure- diastolic predominates as you spend more time in diastole than systole (or 2/3 diastolic pressure plus 1/3 sytolic pressure).
What are resistance vessels?
Arterioles and capillary bed sphincters control how much blood passes through the capillary netowork they are called resistance vessels.
Why do arterioles have a high resistance?
The arterioles generally have a high resistance due to a proportionally large amount of smooth muscle.
What is vasomotor tone and describe the difference between vasodilation and vasodilatation.
Vasomotor tone is controlled by the SNS through circulating hormones. At rest vasomotor tone is high due to low demand. Vasodilation is the reversal of vasoconstriction whilst vasodilatation is when vessels dilate past their norm.
What is reactive hyperaemia/autoregulation?
Vasomotor tone is also achieved by the release of vasodilator metabolites such as H+, CO2, K+ and Adenosine. The increased flow returns things to normal.
How is venous pressure controlled?
Pressure in veins is mainly controlled by the the volume coming in and out. This itself is controlled by the cardiac output and our bodies metabolic demand.