Lecture 5 - Haemodynamics

Question 1

What are the 3 physical factors that govern blood flow?

Answer 1

Flow, which depends on the pressure gradient, and this depends on the resistance.

Question 2

What is blood flow defined as?

Answer 2

Blood flow is the quantity of blood passing a given point in the circulation in a given period and expressed in ml/min

Overall blood flow in total circulation is 5000ml/min (the CO)

Question 3

What equation describes laminar flow

Answer 3

F=P/R

Question 4

What are the 3 patterns of blood flow

Answer 4

1. Laminar: occurs in most arteries, arterioles, venules, veins
2. Turbulent: Ventricles, sometimes aorta
3. Bolus: capillaries (when RBCs pass through single file a bolus of plasma is formed between each RBC)

Question 5

Describe laminar flow

• Layers of blood

Answer 5

When laminar flow occurs the different layers of blood flow at different rates, creating a parabolic profile. This sliding motion of one laminar layer over the other is called shear.

The shear stress is due to friction of molecules against each other. (normal shear stress releases 0.5-1.5Pa).

Question 6

Describe turbulent blood flow

Answer 6

Turbulent blood flow occurs when the pressure driving the fluid is progressively increased, so much so that flow no longer increases linearly, and increases as the square root of the pressure.

Question 7

What is the equation for turbulent blood flow?

Answer 7

The critical value for the reynold number (Re) is ~2000, Re in most blood vessels are less than the critical value.

Re is normally high in

• Left ventricle
• Aortic root
• Exercise
• pregnancy
• Anaemia
  
  • can be detected with stethescope in heart murmors, or in vessels (bruit)
• Re is abnormally increased in athersclerosis, since walls become hardened (less distensible to absorb energy)

Question 8

What governs resistance?

Answer 8

A small change in radius has a large impact upon resistance

Question 9

What is the main site of resistance in the CVS?

Answer 9

Arterioles and the smallest arteries are the main site of resistance to blood flow

Question 10

Why do parallel circuits have lower resistances than systemc

Answer 10

Parallel resistances add to give an overall smaller resistance, whereas resistances in series do not.

Question 11

What are the benefits of parallel flow in the systemic circulation?

Answer 11

• Guaruntees that all tissues recieve fresh oxygenated blood.
• Allows independent variation of blood flow through different organs depending on their need.

Question 12

In relation to pressure, how do the aorta and large arteries function?

Answer 12

These vessels reduce the fluctuations in flow and pressure generated by the intermittent ejection of the stroke volume.

• This is accomplished by the highly elastic walls of these vessels.
• During systole energy is stored in the vessel wall as the elastic wall is stretched, this energy is then released during diastole - the elastic structure squeezes back, pushing the blood stored in the extra space through the circulation, this ensures perfusion can continue even though the heart is relaxed
  
  • this maintains blood flow during diastole
  • this also prevents large swings in arterial BP between heart beats and results in a more or less steady blood flow into the smaller arteries

Question 13

What is pulse pressure?

And what factors determine it?

Answer 13

Pulse pressure = systolic pressure - diastolic pressure

• determined by stroke volume of left ventricle and
• arterial stiffed (Reduced compliance)

Pulse pressure = stroke volume/compliance

• When arteries harden due to athersclerosis this reduces compliance, so pulse pressure can double with aging.

Question 14

Describe the pulse velocity compared to the blood velocity

Answer 14

the pulse travels at 4-5m/s in young people, and 10-15m/s in older people

This is much quicker than blood velocity (0.2m/s)

The transmission velocity increases with stiffness

• Hypertension
• Ageing

The pulse lag between central and peripheral arteries can be used to estimate human arterial distensibility

Question 15

Describe veins

Answer 15

• Veins have all three distinct layers. The walls are thinner than arteries, so they often appear collpased in histological slides.
• Veins also have less smooth muscle and elastin than arteries
• Veins are highly distensible, so they are called capacitance vessels that act as blood reservoirs.
  
  • this is because they have a high compliance at normal operating pressures, as they can release or store blood in response to small changes in pressure

Question 16

What are the 4 determinants of venous pressure?

Answer 16

1. Sympathetic innervation
2. Blood volume
3. Respiratory pump
4. Skeletal muscle pump

Question 17

What causes varicose veins?

Answer 17

Veins have one-way valves that prevent backflow of blood.

Varicose veins are veins that have become dilated and tortuos because of incompetent (leaky) valves.