Haemodynamics II

Question 1

What 3 factors control TPR?

Answer 1

1. Poisueille’s law.
2. Myogenic response.
3. Blood viscosity.

Question 2

What is the main vascular cause of hypertension and why is it harmful?

Answer 2

Over-constriction of arterioles > increased TPR > increased BP.

Reduced flow > less blood to tissues - end organ damage.

Question 3

Recall Poisueille’s law and describe the relationship between resistance and conductance.

Answer 3

Resistance = 8ηL / Πr^4.

Conductance = 1 / resistance.

Thus, conductance = Πr^4 / 8ηL.

Question 4

Describe the equation for blood flow in terms of the combined Poisueille’s and Darcy’s laws.

Answer 4

Blood flow = (Pa - CVP) x (Πr^4 / 8ηL).

Flow = pressure difference x conductance.

Question 5

According to Poisueille’s law, doubling the radius of an artery will have what effect on blood flow if blood pressure stays the same?

Answer 5

r^4 = 2 x 2 x 2 x 2 = 16.

Flow will increase by a factor of 16 as resistance decreases by a factor of 16.

Question 6

Arterioles have the largest pressure drop amongst vessels. What is it?

Answer 6

40-50 mmHg.

Question 7

Arteriole radius is tightly controlled by what?

Answer 7

Sympathetic nerve activity.

Question 8

Given that capillaries have a much smaller radius than arterioles, why do capillaries not control TPR? Give 3 reasons.

Answer 8

1. Radius, r^4 - capillaries cannot alter radius as no smooth muscle or sympathetic innervation.
2. Pressure drop (P1-P2) - pressure drop across capillaries is 20-30 mmHg - less than in arterioles, due to less resistance to flow in capillaries:
   Bolus flow in capillaries reduces viscosity, η, which reduces resistance.
   Capillaries are arranged in parallel, so have a low total resistance as Rtotal = 1/R1 + 1/R2 etc.
   Arterioles are arranged in series, so Rtotal = R1 + R2 etc.
3. Length, L - individual capillaries are short.

Question 9

Local blood flow through organs and tissues is controlled by what?

Answer 9

Constriction and dilation of arterioles supplying tissues - i.e. changes in radius.

Question 10

Describe the Bayliss myogenic response and explain its importance as a protective mechanism.

Answer 10

The Bayliss myogenic response maintains blood flow over a range of arterial pressures, and is especially important in the cerebral, renal and coronary circulations. If BP drops, vessels dilate to maintain flow. If BP is too high, vessels constrict to protect finer vessels downstream from damage - excessive flow in fine vessels may cause rupturing - e.g. stroke caused by rupturing of cerebral vessel.

Question 11

Define blood viscosity and list 4 factors that affect it.

Answer 11

Viscosity is a measure of the internal friction opposing separation of the lamina.

Blood viscosity depends on:

1. Velocity of blood.
2. Vessel diameter.
3. Haematocrit.
4. RBC deformability.

Question 12

What is the typical value of viscosity, η, in normal blood?

Answer 12

4-5

Question 13

Describe the Fahraeus-Lindqvist effect.

Answer 13

Blood viscosity decreases as the tube diameter decreases in vessels with diameter < 100μm. This means that resistance is reduced and flow is increased in microvessels e.g. capillaries.

Question 14

Why do sickle-cell anaemia patients suffer from reduced blood flow?

Answer 14

Sickle-cell patients have RBCs with reduced deformability. This increases blood viscosity and consequently resistance, leading to reduced flow.

Question 15

Explain how the haematocrit is altered in anaemia and polycythaemia and the clinical implications for blood flow.

Answer 15

Anaemia - low haematocrit, low η - decreased TPR and BP > baroreceptor reflex causes increased HR.

Polycythaemia - high haematocrit, high η - increased TPR > reduced blood flow.

Question 16

Why is blood flow poor in the veins of immobile legs?

Answer 16

Blood velocity is low when legs are immobile, therefore viscosity increases. Increased viscosity > increased resistance > reduced flow.

Question 17

Describe and explain the difference in compliance between veins and arteries.

Answer 17

Veins are more compliant, meaning their volume increases more given the same pressure applied. This is because veins are thin-walled, with less smooth muscle.

Question 18

What is the effect of venoconstriction on the heart?

Answer 18

Venoconstriction expels blood into central veins > increased CVP > increased EDV > increased SV due to Starling’s law.

Question 19

Give the typical venous pressures of CVP, a limb vein at heart level and a foot vein when standing.

Answer 19

CVP: 0-7 mmHg

Limb vein at heart level: 5-10 mmHg

Foot vein when standing: 90 mmHg

Question 20

How does the thoracic pump increase venous return?

Answer 20

During inhalation the expansion of the thoracic cavity increases intra-abdominal pressure, forcing blood upwards towards the heart. Blood flows faster and right ventricular SV increases as a result.

Question 21

How does the skeletal muscle pump increase venous return and why is this important?

Answer 21

Contraction of skeletal muscle, particularly in the legs, squeezes the veins, increasing return of blood to the right atrium.

During exercise, increased venous return means increased SV.
The skeletal muscle pump reduces high local venous pressures and prevents swelling of feet and ankles.

Question 22

Standing still for a long time or standing up suddenly can lead to fainting. Explain why.

Answer 22

The effect of gravity causes pooling of blood in the lower extremities, reducing venous return and consequently cardiac output. This results in reduced blood pressure and insufficient perfusion to the upper parts of the body, particularly the brain. Vasodilation due to reduced sympathetic tone and a drop in HR and contractility due to enhanced parasympathetic tone may contribute to fainting.

Question 23

When standing, there is a 90 mmHg pressure gradient against flow from the heart to the feet, but blood still flows against this gradient. Explain why.

Answer 23

According to Bernoulli’s law, mechanical energy of flow is determined by kinetic and gravitational potential energy as well as pressure. Blood ejected from the heart has greater kinetic and gravitational potential energy than blood in the feet, and this overcomes the pressure gradient to maintain flow.

Question 24

There is no pressure gradient between central venous blood and blood in the heart, but the heart fills anyway. Explain why.

Answer 24

Blood in the central veins has more kinetic energy than blood in the atria and ventricles. This difference in energy drives flow.