Factors Affecting Flow Through Tubes Flashcards
What is the definition of pressure (P)?
Forced exerted per unit area
What is the definition of pressure gradient (∆P)?
The difference in forces exerted (per unit area) at either end/side of an object e.g. a tube or membrane
What is the definition of flow (Q)?
The volume of fluid passing a given level of the circulation/airways per unit time (usually measured in ml/s or L/min) e.g. cardiac output (~5L/min output of each side of heart at rest)
What is the definition of resistance (R)?
A force that tends to oppose the flow of a substance
Describe the pressure + resistance in the systemic + pulmonary systems?
Systemic: High resistance so oxygenated blood is pushed through at higher pressures
Pulmonary: Low resistance so deoxygenated blood is pushed through at lower pressures
Explain the concept of pressure drop in the systemic circulation.
Pressure in large arteries = 100 mmHg -> decreases gradually through muscular arteries, arterioles, capillaries, venules -> veins where it is 0-8 mmHg finally (flow will remain at 5L/min at every level)
Drop is different in different parts of circulation; reflects difference in resistance to flow
What vessels is the majority of our blood stores within?
Venous side of circulation
What route does the blood take in the systemic circulation and how does it move?
Left ventricle of heart pumps blood from aorta/major arteries -> resistance vessels (arteries/arterioles) -> exchange vessels capillaries -> capacitance vessels veins -> right atrium of heart
How does air flow come into/out of the lungs?
Expansion of lungs, lung volume increases, dropping the pressure and causing a pressure gradient for atmospheric air to come in
Compression of lungs decreases volume + increases pressure allowing air to move down a pressure gradient out of lungs into atmosphere
How is blood flow (Q) related to pressure gradient (∆P)?
Blood flow (Q) is generated by a pressure gradient (∆P)
Flow is proportional to pressure difference/ pressure gradient between ends of vessels/airways if everything else is equal
How does flow (Q) and resistance (R) affect the pressure gradient (∆P)?
For a given pressure gradient, flow (Q) is determined by resistance (R) of vessel/airway (depends on radius)
Pressure gradient = flow x resistance (∆P = QR)
OR
Arterial BP = CO X SVR/TPR (clinical application)
How can we maintain blood flow (Q) despite an increase in resistance (R)?
Rearrange equation ∆P = QR -> Q = ∆P/R
If R is doubled, to maintain Q, we must also double ∆P so we need to double the pressure gradient
What is the relationship between flow + velocity?
Although flow of blood/air will be the same through the circulation/airways, its velocity will vary as flow is volume whilst velocity is speed
What is the definition of velocity (V)?
The rate of movement of fluid particles along a vessel/airway (measured in cm/s)
What will happen to velocity + the pressure gradient if flow through a system is constant?
Velocity will vary along the length if cross sectional area/radius of vessel/airway changes so velocity will be inversely proportional to radius (A) (A = πr^2)
Pressure gradient must increase for smaller radius’
-> constant flow (ml/s)
How can a decreased radius of a vessel/airway be detected clinically?
A decreased radius will increase velocity causing more turbulent flow which can be heard through noises when listening to patients chest through a stethoscope for e.g.
What will happen to the radius (r) of the vessel/airway + the velocity (V) of flow if the pressure gradient is constant?
Laminar flow means width of tube greatly affects its resistance so reduction in r will reduce V in middle of tube = V reduced 4x
So mean V is proportional to r^2
Flow will fall by 16x
What is laminar flow?
Fluid will not move with same velocity across width of tube, its flow is laminar so velocity is lowest/essentially stationary at edges of tube + highest at centre of tube where edge effect is weakest = successive flow of layers
What is Poiseuille’s Law?
Flow (Q) = pressure gradient (∆P)/resistance (R)
But R depends on its radius (R), length (l) + viscosity (μ) of fluid too so R = 8μl/πr^4
Overall Q = ∆Pπr^4/8μl SO flow is proportional to fourth power of radius of tube
What does Poiseuille’s Law mean clinically in terms of pathology’s?
If the radius of a tube is halved (e.g. atherosclerosis, stenosis), flow will decrease by 16 times
-> Increased velocity by 4x (murmurs, bruits or wheezes) + pressure will rise by 16x (hypertension)
What is the clinical application of Poiseuille’s Law?
Intravenous cannulas (if radius of canula halved, flow of fluid falls by 16x) Endotracheal tubes Disease states that change radius of vessel/airway e.g. atherosclerosis/asthma
What components are in blood and how do they affect viscosity of blood?
Cellular components e.g. cells + proteins affect viscosity + flow and because of laminar flow,red cells get borne along in most rapid moving stream in centre of blood vessels
Therefore, if you increase RBCs, viscosity will increase limiting flow itself
What are 2 clinical problems that can occur as a result of cellular components affect on blood viscosity?
Altitude training will increase RBCs as more oxygen needs to be delivered to tissues -> increases viscosity -> increasing resistance + decreasing flow
Myeloma = higher levels of plasma proteins -> increases viscosity of blood -> increasing resistance + decreasing flow
What causes flow to become turbulent?
High velocity
Low viscosity of blood
High vessel diameter
What happens if flow becomes turbulent?
Layers of lamina flow will break up, flow becomes disordered + this gives rise to sound
What is Reynold’s number?
Summarises conditions of turbulence where number < 2300 indicates laminar flow whereas number > 4000 indicates turbulent flow (2300-4000 are transition numbers)
What is the equation for Reynold’s number?
NR = pDv/n
p = density D = diameter v = mean velocity n = viscosity
What are the different noises turbulent flow can make and where do they occur?
Murmurs in heart
Bruits in arteries e.g. renal
Wheeze in airways
What will turbulent flow cause to happen in the vessels?
Resistance to flow is increased so results in damage to endothelium lining blood vessels
Where does the most resistance in the circulation come from? Why?
Arterioles
Arranged in series rather than parallel
Resistances comes just as __ ____ do.
Electrical resistances
What happens if the resistance of the arterioles ride?
Stroke work of the heart must increase to maintain cardiac output (flow of blood around the body)
What are the different pressures exerted upon blood vessels?
Intravascular = blood inside them exerting force on internal walls Extravascular = interstitium pressure pushing against external wall
What is transmural pressure?
P (intravascular) - P (extravascular)
Tends to stretch the vessel if +ve, will collapse it if -ve + diameter will be maintained just above 0
Blood vessels are not rigid, they are distensible. What vessels are the most distensible? What does this mean for them?
Veins
Particularly compliant + hold ~67% of circulating blood volume
What will happen to resistance of rigid tubes + distensible tubes if pressure is increased?
Rigid: walls do not move so resistance is constant
Distensible: Stretches walls lowering resistance
What will happen if the pressure drops in a distensible tube?
If pressure falls towards 0, vessels will collapse + flow will cease (why transmural pressure of vessels must be > 0 to permit vessel to be open)
What is the benefit of blood vessels being distensible?
Gives them capacitance so vessels widen with increasing pressure, allowing more blood to flow in than out so vessel will hold more blood the more compliant it is
What is the rule for parallel arrangements of capillaries? Why is the resistance not that much greater than for the aorta despite smaller vessel radius?
Parallel arrangement of multiple capillaries reduces overall resistance
Flow through each vessel will be inversely proportional to its resistance
E.g. if there is 3 tubes paralleling each other + their resistance is R1, R2 + R2 the equation for total resistance would be:
1
____________________________________
1/R1 + 1/R2 + 1/R3
SO resistance will always be lower than any component of the system
What is the rule for series arrangements of vessels?
Adds resistance
Flow through each tube will be the same
E.g. if there is 3 tubes connected to one another + there resistance is R1, R2 + R2 you would just add these to get the total resistance SO total resistance will always be higher than one component of the system
