Flow through Tubes Flashcards
What are the two major subdivisions of the vascular system?
Systemic circulation and Pulmonary circulation
How are the systemic and pulmonary circulations arranged?
In series with one another
What is each subdivision of the circulation composed of?
Arteries Arterioles Capillaries Venules Veins
How are most vessels of a given type arranged?
In parallel with each other
What must happen in order for blood to keep flowing?
Blood pressure must fall all the way from the aorta to the venae cavae
How is flow of blood driven through vessels?
By the gradient of pressure
What is flow proportional too?
The pressure difference between the ends of a vessel
What is the result of a higher pressure difference on flow?
Higher flow
What is the flow for a given pressure gradient determined by?
The resistance of a vessel
What is the resistance of a vessel determined by?
The nature of the fluid and the vessel
Define flow
The volume of fluid passing a given point per unit time
Define velocity
The rate of movement of fluid particles along the tube
How does flow differ at different points along a vessel?
It doesn’t
When can velocity vary along the length of a vessel?
If the radius of the tube changes
What is the relationship between velocity and cross sectional area at a given flow?
They are inversely proportional
What is the velocity of vessels with a small cross sectional area at a given flow?
High
What is the velocity of vessels with a large collective cross sectional at a given flow?
Low
What vessels have a large cumulative cross sectional area?
Capillaries
What happens as arteries branch?
The total cross sectional area of the vascular bed increases, and thus so does flow
How can the flow be described in most blood vessels?
Laminar
What happens in laminar flow?
There is a gradient of velocity from the middle (highest), to the edge, where fluid is stationary
What happens as mean velocity increases?
Flow eventually becomes turbulent
What happens in turbulent flow?
The velocity gradient breaks down as fluid tumbles over, and the flow resistance greatly increases
What is flow determined by in a vessel with constant pressure?
Mean velocity
What does mean velocity depend on?
Viscosity of fluid
Radius of tube
How does fluid move in laminar flow?
In concentric layers, with middle edges moving faster than the outer layers, and therefore the layers must slide over one another
What is the extent to which fluid layers resist sliding over one another known as?
Viscosity
What does a higher viscosity result in?
Slower flow of central layers, and a lower average velocity
What does viscosity determine?
The slope of the gradient of velocity
What does a wider tube result in at a constant gradient?
A faster middle layer
What is the relationship between mean velocity and cross sectional area of the tube?
Proportional
What is flow the product of?
Mean velocity and cross sectional area
How can flow be calculated?
Poiseuille’s Law
What is Poiseuille’s Law?
Q = π.∆P.r 4
8.n.L
Where
·
Q=
flow
·
π/8 =
constant of proportionality
·
∆P =
inflow and outflow pressure difference
·
r =
radius of tube
·
n =
viscosity of fluid
·
L =
length of tube
What does Poiseuille’s Law say that flow is, when blood flow is steady and laminar in blood vessels larger than arterioles ?
Proportional to the difference between inflow and outflow pressures
Proportional to the fourth power of the radius
Inversely proportional to the length of vessel
Inversely proportional to the viscosity of the blood
Why does Poiseuille’s Law only work in vessels larger than arterioles?
Because otherwise the vessel is so small that the flow doesn’t work fully for them
Whos airways are particularly prone to compromised flow?
Childrens, as they are narrow
What is childrens airways being narrow relevant to?
Narrowing of airways in childhood asthma
Bronchiolitis being primarily a disease of children
The need to avoid emotionally upsetting a sick child who is already fighting for breath, as upsetting them leads to more narrowing of airways
The need to ventilate an intubated child during surgery
Why is the narrower childs airway relevant in intubation?
Because a 2mm reduction in diameter of a child’s trachea caused by an inserted tube reduces the flow more than a 2mm reduction in diameter of an adults trachea, as it’s smaller and so 2mm is a larger proportional reduction
What can hyper-viscosity syndrome (HVS) be caused by?
Abnormally high plasma protein levels
Abnormally high RBC or WBC count
Give an example of a HVS caused by abnormally high plasma protein levels
High IgM in Waldenstrom macroglobulinaemia
What % of HVS cases doesWaldenstrom macroglobulinaemia account for?
85%
How isWaldenstrom macroglobulinaemia treated?
Plasmapheresis
Give an example of a HVS caused by abnormally high RBC or WBC count
Polycythemia
How is polycythemia treated?
Phlebotomy
What must be done to fully stop HVS?
The underlying condition must be treated, otherwise HVS reoccurs
When can functional cardiac murmurs occur?
In severe anaemias, as a result of high blood flow velocities and reduced viscosity of blood
Why do severe anaemias cause functional cardiac murmurs?
Because of the low blood cell count
What does flow of fluid along tubes obey?
Ohms law
What is Ohms law?
V = IR
What happens to resistance as viscosity increases?
It increases
What happens to resistance as the radius increases?
It decreases to the fourth power the radial increase
What happens to resistance when blood vessels are connected together?
They combine
How do you work out the total resistance for vessels in series?
The total resistance equals the sum of all the individual resistances
What is the effective resistance for vessels in parallel?
Lower for that in series
How do you work out the total resistance for vessels in parallel?
The reciprocal of the total resistance equals the sum of the resistance of the individual resistances
Why do capillaries offer little collective resistance?
On account of their parallel arrangement
What happens to the pressure change at a higher resistance, if the flow is fixed?
There is a greater pressure change from one end of the vessel to the other
What happens to flow at higher resistances, if the pressure is fixed?
The lower the flow
How does the flow change over the whole circulation?
It doesn’t- over the whole circulation, flow is the same at all points
What resistance are arteries?
Low
How big is the pressure drop over arteries?
Small
What resistance are arterioles?
High
How big is the pressure drop over arterioles?
High
What resistance are capillaries?
Individual capillaries are high resistance, but many are connected in parallel and so the overall resistance is low
How big is the pressure drop over capillaries?
Small
What resistance are veins and venules?
Low
Why is pressure within arteries high?
Because of the high resistance of arterioles
What does a higher arteriolar resistance cause for a given total flow?
Higher arterial pressure
When does flow become turbulent?
If flow velocity is high
If viscosity is low
If lumen of vessel irregular
What can cause irregularity of the lumen?
Irregular narrowing, e.g. atherosclerosis
Give an example of where flow may become turbulent
Aorta
What does turbulent flow generate?
Sound
What is sound caused by turbulent flow called?
Bruit
How can a bruit be heard?
On auscultation
When can a bruit be heard?
If cardiac valves become stenoic (narrowed)
When atherosclerotic blockages obstruct a carotid artery, a renal artery, a hepatic artery, or a femoral artery
What can blood vessel walls?
Distend
What does pressure within vessels generate?
Transmural pressure between inside and outside
What is the result of the transmural pressure?
It tends to stretch the tube
What happens as a blood vessel stretches?
Resistance falls
What is the result of a higher pressure in a distensible vessel?
The easier it is for blood to flow through it
What happens as vessels widen with increasing pressure?
More blood transiently in than out
What can distensible vessels do?
Store blood- they have capacitacnce
What are the most distensible blood vessels?
Veins
