Session 5

Question 0

Describe what is meant by ‘laminar’ and ‘turbulent’ flow

Answer 0

Laminar flow - a gradient of velocity from the middle to the edge of the vessel (velocity highest at centre, stationary at edge)
Turbulent flow - velocity gradient breaks down, fluid tubules over (increased flow resistance)

Question 1

Define the terms ‘flow’ and ‘velocity’ with respect to the movement of fluids through tubes and state the relationship between them

Answer 1

Flow - the volume of fluid passing a given point per unit time
Velocity - rate of movement of fluid particles along the tube
Flow = velocity x area

Question 2

Describe what is meant by ‘viscosity’, and the effect of viscosity upon flow

Answer 2

Viscosity - the extent to which fluid layers resist sliding over one another
The higher the viscosity, the slower the central layers will flow, and the lower the average velocity.

Question 3

Describe the effects of change in tube diameter on flow rate

Answer 3

Viscosity determines the slope of the gradient of velocity.
At a constant gradient, the wider the tube, the faster the middle layers move.
Mean velocity is proportional to the cross sectional area of the tube.

Question 4

Define the term ‘resistance’ to flow and state the factors which affect flow resistance

Answer 4

Resistance - pi r^4

Resistance increases as viscosity increases.

Question 5

Describe the relationship between pressure, resistance and flow

Answer 5

If flow is fixed, the higher the resistance the greater the pressure change from one end of the vessel to the other.
If pressure is fixed, the higher the resistance the lower the flow.

Question 6

Describe the effects of combining flow resistances in series and in parallel

Answer 6

For vessels in series, resistances add together.

For vessels in parallel, the effective resistance is lower, as there is more than one path for the current to flow down.

Question 7

Describe the pattern of flow resistance and pressure over the systematic circulation

Answer 7

Over the whole circulation flow is the same at all points.
Arteries are low resistance, arterioles are high resistance, venules and veins are low resistance.
The pressure within arteries is high because of the high resistance of the arterioles.

Question 8

Describe how the distensibility of blood vessels affects the relationship between flow and pressure

Answer 8

Blood vessels have distensible walls, and the pressure within the vessel generates a transmural pressure across the wall which stretches the vessel.
As the vessel stretches, the diameter of the lumen increases, so resistance falls and flow increases.
As the pressure within a distensible vessel falls, the walls eventually collapse, and blood flow ceases before the driving pressure falls to zero.

Question 9

Describe how the distensibility of blood vessels produces the property of capacitance

Answer 9

As vessels widen with increasing pressure, more blood transiently flows in than out. This allows distensible vessels to store blood.
Veins are the most distensible vessel.

Question 10

Define the terms ‘systolic’, ‘diastolic’ and ‘pulse pressure’

Answer 10

Systolic - maximum arterial pressure e.g. 120 mmHg, affected by how hard the heart pumps, total peripheral resistance, stretchiness of arteries.
Diastolic - minimum arterial pressure e.g. 80 mmHg, affected by systolic pressure, total peripheral resistance
Pulse pressure - difference between systolic and diastolic arterial pressure e.g. 40 mmHg
Average pressure - diastolic + 1/3 pulse pressure

Question 11

Define the term ‘total peripheral resistance’

Answer 11

Total peripheral resistance - the sum of the resistance of all the peripheral vasculature in the systemic circulation

Question 12

Draw and describe a typical arterial pressure wave form

Answer 12

Contraction of he ventricles generates a pulse wave which propagates along the arteries faster than blood. This is felt where arteries come close to the surface.
Dicrotic notch - a slight dip in the wave due to pressure in the left ventricle falling below aortic pressure, subsequent backflow
Dicrotic wave - a slight rise in the wave due to the recoil of blood off the closed aortic valve

Question 13

Describe what is meant by ‘vasomotor tone’ and list the main factors that affect it

Answer 13

Vasomotor tone - continuous contraction of muscle
Produced by the sympathetic nervous system (a1 receptors).
Tone is antagonised by vasodilator factors.
Actual resistance is determined by a balance between the two.

Question 14

Describe how vasodilator metabolites modify vasomotor activity to permit local control of blood flow

Answer 14

Metabolically active tissues produce vasodilator metabolites e.g. H+, K+, adenosine.
These metabolites cause the relaxation of local smooth muscle, lowering resistance and increasing blood flow.

Question 15

Describe reactive hyperaemia

Answer 15

If circulation to an organ or limb is cut off for a minute or two then restored, a large amount of blood enters after a period of no blood flow.
This is because the organ or limb has continued metabolising and producing vasodilators during the period of no circulation, so when circulation is restored, the local arterioles dilate maximally and blood flow is very high.

Question 16

Define the terms ‘central venous pressure’ and ‘venous return’

Answer 16

Central venous pressure - the pressure in the great veins

Venous return - the rate of flow of blood back to the heart, limits cardiac output