Hemodynamics 1

Question 1

What are the main abbreviations?

Answer 1

CO-cardiac output

MAP- mean arterial pressure

NE- norepinephrine/noradrenaline

TPR-total peripheral Resistance

Question 2

Give an overview of Bulk flow law/Darcy’s law

Answer 2

Flow = 🔼P/ R

Question 3

Give an overview of Poiseuille’s law

Answer 3

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)

Question 4

What is the importance of resistance?

Answer 4

Resistance arterioles controls flow of blood to organs

Parallel vs series

Question 5

What is the relationship between velocity, flow, and cross-sectional area?

Answer 5

As the blood vessels cross sectional area increases, flow increases, the velocity decreases

Question 6

What is the study of hemodynamics ?

Answer 6

• blood flow
• pressure
• resistance

Question 7

What are the rules of the cardiovascular system in relation to hemodynamics?

Answer 7

1. Why does blood flow round the circulation? Bulk flow from high to low pressure states
2. 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

3. 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

Question 8

What is pressure?

Answer 8

Driving force to move fluid through a tube of constant diameter/radius and known length

Question 9

What is resistance ?

Answer 9

Used to regulate flow in body

Question 10

What is flow?

Answer 10

Flow(Q) is volume/time

Question 11

What are the types of pressure in cardiovascular physiology?

Answer 11

• driving pressure
• transmural pressure
• hydrostatic pressure (important in capillaries)

Question 12

What is driving pressure?

Answer 12

Pressure difference along the length of the vessel

-responsible for blood flow

Question 13

What is transmural pressure?

Answer 13

Pressure difference across a blood vessel wall

Influences vessel diameter because vessels are compliant

Question 14

What is hydrostatic pressure?

Answer 14

Important in capillaries

-pressure created by weight of blood

Increases when moving from supine to standing

Question 15

How can the pressure gradient be calculated with MAP and right atrial pressure?

Answer 15

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)

Question 16

What is the systemic/pulmonary blood flow?

Answer 16

Systemic blood flow= pulmonary blood flow= 5 L/min

Question 17

How can the pressure gradient be calculated with flow and resistance?

Answer 17

🔼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)

Question 18

Explain the relationship between CO and TPR

Answer 18

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

Question 19

What is total peripheral resistance?

Answer 19

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

Question 20

Explain Poiseuille’s law

Answer 20

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

Question 21

How does radius/resistance affect flow?

Answer 21

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

Question 22

What are the deviations from Poiseuille’s law?

Answer 22

Law can apply in general to blood flow in body

Equation has following assumptions, and applies under the below conditions:

1. Incompressible: valid- blood is fluid which cannot be compressed
2. Laminar flow: blood flow is usually laminar
3. Rigid, cylindrical, unbranched tubes: vasculature is not
4. Non-pulsatile flow: blood flow pulsatile flow
5. Newtonian fluid: blood is non-Newtonian. Hematocrit (% red blood cell volume in blood) alters viscosity and thus flow

Question 23

What are the factors affecting resistance?

Answer 23

• 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)

Question 24

How is blood flow between organs arranged ? What is the consequence of this?

Answer 24

The supply of blood flow is arranged In parallel arrangement

Resistance in parallel= 1/Rt = 1/R1 + 1/R2

Question 25

How are blood vessels arranged WITHIN AN ORGAN? what is the consequence of this?

Answer 25

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

Question 26

What is the region of cardiovascular with the greatest fall of pressure ?

Answer 26

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

Question 27

When is resistance highest?

Answer 27

In capillaries when in series Total resistance = R1+R+R3

Question 28

How can the total resistance from artery to vein is calculated?

Answer 28

Sum of the resistance of the individual components

Question 29

Why does the capillary bed not offer greater resistance to flow than arteriolar network ?

Answer 29

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

Question 30

Contrast resistance in one capillary and one arteriole in series

Answer 30

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

Question 31

Contrast resistance in arterioles and capillaries

Answer 31

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

Question 32

What are the basic consequences of vasodilation of resistance arterioles?

Answer 32

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

Question 33

What is the clinical correlation of vasodilation?

Answer 33

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

Question 34

What are the short term consequences of vasoconstriction ?

Answer 34

🔼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

Question 35

What is the importance of capacitance?

Answer 35

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

Question 36

What is main quality of parallel blood flow?

Answer 36

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

Question 37

How much blood flows to organs?

Answer 37

Flow through each organ is a fraction of the total blood flow. Skeletal muscle receives 20% Brain receives 13%

Question 38

How can flow to organs be regulated?

Answer 38

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

Question 39

How can hormones control resistance?

Answer 39

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”