Arteries And The Circulation System

Question 1

Different types of circulation

Answer 1

Arterial circulation: arteries carry blood away from the heart
Venous circulation: Veins carry blood to your heart
Capillary circulation: capillaries connect arterial and venous circulatory subsystems - thin walls - transfer of nutrients
Pulmonary circulation: movement of blood from heart to lungs and back to heart again - both arterial and venous

Question 2

Blood cells and plasma

Answer 2

Cells
- Erythocytes (RBCs)
- immune system cells (WBCs)
- clotting cells
Plasma
- liquid component of blood

Question 3

Blood Pressure and Gravity

Answer 3

• measured in Hg with respect to atmospheric pressure
• doesn’t tell us the force pushing blood around body
• transmural pressure - pressures between two places in the body are typically 10kPa but can be 25kPa in limbs
• pressure = p0 + hpg where h=0 at level at heart

Question 4

Fluid flow and continuity equation

Answer 4

If the fluid cannot be compressed and there is no accumulation and viscosity, for a given volume, mass in = mass out
Energy balance
- mechanical work done + kinetic energy + potential energy must be constant if viscous effects ignored
- mechanical work = PV
- kinetic energy = 1/2mv^2
- potential energy = mph

E/V=P + 1/2pu^2 + pgh

Question 5

Shear modulus and viscosity

Answer 5

Shear modulus G = shear stress/shear strain

Viscosity = shear stress/velocity gradient

Question 6

Poiseuille flow

Answer 6

• model of flow when viscosity is present
• fluid is stationary at walls and increases to the centre
• dp/L = 8uQ/piR^4
  -dp/L is pressure gradient down the tube
• Q is flow rate
  -u is viscosity
  -R is total tube radius

Quantitatively valid under conditions of:
- steady state flow
- fully developed flow
- tube uniformity

Question 7

Shear stress at vessel wall

Answer 7

• indicates how much force is being exerted on vessel wall
• high levels may damage RBCs, the endothelium or platelets
• may initiate clot formation

Question 8

Dynamics of blood flow

Answer 8

• from Poiseulle’s equation we can see that resistance (dp/LQ) varies as r^-4
• therefore smaller vessel = greater resistance
• changes in small artery resistance will have much larger impact on total resistance of system
• fluids through vessels work with same relationships to resistors (Rt=R1+R2)(1/Rt=1/R1 + 1/R2)

Question 9

Critical stenosis

Answer 9

• narrowing of an artery that results in significant reduction in blood flow

50% reduction in radius = 16x resistance