Hemodynamics 1 Flashcards
What are the main abbreviations?
CO-cardiac output
MAP- mean arterial pressure
NE- norepinephrine/noradrenaline
TPR-total peripheral Resistance
Give an overview of Bulk flow law/Darcy’s law
Flow = 🔼P/ R
Give an overview of Poiseuille’s law
Poiseuille’s law which states that flow (Q) is equal to pressure (P) [ the change in pressure, or the difference between mean arterial pressure, or the difference between mean arterial pressure (MAP) and central veinous pressure (CVP)] divided by resistance (R)
What is the importance of resistance?
Resistance arterioles controls flow of blood to organs
Parallel vs series
What is the relationship between velocity, flow, and cross-sectional area?
As the blood vessels cross sectional area increases, flow increases, the velocity decreases
What is the study of hemodynamics ?
- blood flow
- pressure
- resistance
What are the rules of the cardiovascular system in relation to hemodynamics?
- Why does blood flow round the circulation? Bulk flow from high to low pressure states
- What factors affect flow?
Volume of blood that passes a given point per unit time (L/min). Determined by pressure gradients and resistance
-Resistance opposes blood flow. Depends on length of the tube, Radius of the tube (4th power) and viscosity of blood
- What determines velocity of blood flow?
How far a volume of blood travels per unit of time. (mm/sec). Determined by cross-sectional area, of the flow rate is constant
What is pressure?
Driving force to move fluid through a tube of constant diameter/radius and known length
What is resistance ?
Used to regulate flow in body
What is flow?
Flow(Q) is volume/time
What are the types of pressure in cardiovascular physiology?
- driving pressure
- transmural pressure
- hydrostatic pressure (important in capillaries)
What is driving pressure?
Pressure difference along the length of the vessel
-responsible for blood flow
What is transmural pressure?
Pressure difference across a blood vessel wall
Influences vessel diameter because vessels are compliant
What is hydrostatic pressure?
Important in capillaries
-pressure created by weight of blood
Increases when moving from supine to standing
How can the pressure gradient be calculated with MAP and right atrial pressure?
Blood flows around the systemic circulation because of the pressure gradient between the aorta and right atrium
🔼P= driving force= mean arterial pressure- right atrial pressure
🔼P (systemic) =85-0(85)
🔼P(pulmonary)= 25-8(17)
What is the systemic/pulmonary blood flow?
Systemic blood flow= pulmonary blood flow= 5 L/min
How can the pressure gradient be calculated with flow and resistance?
🔼P= flow x resistance
Systemic vascular resistance= 0.02 mmHg x min/ml
Pulmonary vascular resistance = 0.003 mmHg x min/ml
Pressure difference between an upstream and downstream points due to resistance between these points
Flow through the systemic and pulmonary circulations= cardiac output
🔼P(systemic circulation) = total peripheral resistance(SVR)
Explain the relationship between CO and TPR
CO= MAP/TPR MAP= CO x TPR
MAP is influences by both the heart (via CO ) and the vasculature (via TPR)
-If CO increases then MAP increases of TPR is constant
-If TPR increases then MAP increases if CO is constant
What is total peripheral resistance?
This is equal to systemic vascular resistance to blood flow offered by all the systemic vasculature
TPR= MAP- CVP- CO(*CVP about 0)
If MAP is 100 mmHg, and CVP is zero, and CO is 6 L/min- what is the value of the TPR? 100/6= 16.7 mmHg x min/ml
However TPR is not determined by MAP and CO
Explain Poiseuille’s law
Relates:
- Resistance to fluid flow in a tube to
- Geometry of tube
- Properties of fluid
Darcy’s law: flow= 🔼P/ R
Poiseuille defined: Flow= 🔼P x (pixr^4)/8nL
Poiseuille defined: resistance is inversely proportional to (8x viscosity x length)/ (pi x radius^4)
Poiseuille’s law resistance is inversely proportional to n/viscosity
Therefore= flow is inversely proportional to radius ^4
How does radius/resistance affect flow?
Poiseuille’s law
Resistance most sensitive to radius : 1/r^4
(Decrease radius/constriction —: increase resistance)
Small changes in vessel diameter results in large changes in flow
What are the deviations from Poiseuille’s law?
Law can apply in general to blood flow in body
Equation has following assumptions, and applies under the below conditions:
- Incompressible: valid- blood is fluid which cannot be compressed
- Laminar flow: blood flow is usually laminar
- Rigid, cylindrical, unbranched tubes: vasculature is not
- Non-pulsatile flow: blood flow pulsatile flow
- Newtonian fluid: blood is non-Newtonian. Hematocrit (% red blood cell volume in blood) alters viscosity and thus flow
What are the factors affecting resistance?
- resistance inversely proportional to viscosity(n)
- viscosity= degree of slipperiness of 2 layers of fluid
- viscosity is inversely proportional to hematocrit
- hematocrit= packed cell volume/ total blood volume
- When hematocrit is high, the resistance to blood flow is increased (more work is expended driving blood round the circulation)
How is blood flow between organs arranged ? What is the consequence of this?
The supply of blood flow is arranged In parallel arrangement
Resistance in parallel= 1/Rt = 1/R1 + 1/R2
How are blood vessels arranged WITHIN AN ORGAN? what is the consequence of this?
Within each orga, the blood vessels (arteries) divide into arterioles and inti capillaries, then reunite into venules and veins
Total resistance is equal to the sum of resistances of individual components Rt= R1+ R2+ R3
Within each organ, the resistances are in series however capillary networks are regarded as in parallel networks
What is the region of cardiovascular with the greatest fall of pressure ?
Resistance arterioles
Flow from: high pressure (aorta, arteries)—> low presssure (capillaries)—> lowest pressure (right atrium)
Arteriole: bigger drop in pressure gradient 🔼P. 50-70% drop. Resistance greater!
Capillary: smaller drop in pressure gradient (🔼P) resistance less!
This is because resistance is greatest in capillaries have the smallest diameter than arterioles (R inversely proportional to 1/r^4)
Resistance of ONE capillary > resistance of ONE arteriole
When is resistance highest?
In capillaries when in series
Total resistance = R1+R+R3
How can the total resistance from artery to vein is calculated?
Sum of the resistance of the individual components
Why does the capillary bed not offer greater resistance to flow than arteriolar network ?
Because there is a huge number of capillaries arranged IN PARALLEL (capillary bed)
The resistances are ‘IN PARALLEL’. Flow can occur through paths with least resistance. Flow in and out remains the same
Total resistance across a parallel arrangement is less than the sum of the individual components
C total= 1/R total
Total conductance= sum of the individual conductances
Contrast resistance in one capillary and one arteriole in series
Resistance in ONE capillary > ONE arteriole in SERIES although resistance of a single capillary is greater than the resistance of a single arteriole because the capillaries are arranged in parallel they as a group, offer less resistance to the flow of blood
Contrast resistance in arterioles and capillaries
Resistance in arterioles > capillaries(plural) due to PARALLEL arrangement of capillaries
PRESSURE drop is greatest across resistance arterioles
Consequently, the pressure drop across arterioles is about 50 mmHg (I.e. from about 80 to about 30 mmHg) whereas the pressure drop across capillaries is about 18 mmHg ( I.e. about 30 mmHg to about 12 mmHg
What are the basic consequences of vasodilation of resistance arterioles?
Short term consequences: elevated pressure at capillaries, increased edema
Clinical examples: septic shock, or anaphylaxis—> massive vasodilation. However, there is eventually pooling of blood in capacitance vessels (venules, veins), hypotension, reduced tissue perfusion, changes in heart rate etc. (medical emergency
Exercise- massive reduction of resistance- increased blood flow muscles. Redistribution of blood flow to other tissues to maintain MAP
What is the clinical correlation of vasodilation?
Graves’ disease and vasodilation of resistance arterioles
-Immune disorder—> over production of thyroid hormones
- Hyperthyroidism, basal metabolism elevated
- Increased metabolism associated with arteriolar vasodilation
- Reduced arteriolar resistance —> reduced dampening of pulsatile arterial Pressure in capillaries
- pulsatile flow in capillaries- observed in fingernail beds of patients with graves
What are the short term consequences of vasoconstriction ?
🔼P increased in arterioles
In overall 🔼P not changed (arteries to RA)
Short term: reduced pressures at capillaries
Clinical examples: pre-eclampisa—> massive vasoconstriction (endothelin). Reduced pressure in capillaries, reduced tissue perfusion
What is the importance of capacitance?
Venules/veins- capacitance vessels—> affect preload (filling of heart, EDV)
Arterioles- resistance vessels—> affect afterload (peripheral resistance, TPR)
MAP= (CO x TPR) + CVP
CO= MAP/TPR
What is main quality of parallel blood flow?
The mean pressure in each major artery supplying individual organs is the same
Pressure head at A= pressure head at B= Pressure head at C
There is very little drop in pressure between the aorta and the large arteries supplying the various organs
Blood distribution in parallel
How much blood flows to organs?
Flow through each organ is a fraction of the total blood flow.
Skeletal muscle receives 20%
Brain receives 13%
How can flow to organs be regulated?
By changing resistance- altering the diameter of the vessel
Easier to measure pressure than flow (vol/time)in body
Poiseuille’s law- change in radius radically changes resistance
R inversely proportional to 1/r^4
Some examples:
-skeletal muscle blood flow during exercise
- cutaneous blood flow during cold stress
- hypertensive patient- protecting the kidneys
How can hormones control resistance?
Sympathetic NS: innervates and stimulates arterioles causing vasoconstriction
Other vasoconstrictions: endothelin and serotonin
Vasodilatirs- nitric oxide, histamine, adenosine
Since, resistance inversely proportional to 1/r^4
Small decrease in arteriole radius causes a large increase in resistance
-Arterioles are called “resistance vessels”
