mechanics 3

Question 1

blood pressure =

Answer 1

cardiac output x resistance

Question 2

fluid circuit equation

Answer 2

delta P = Q x R

Question 3

what are the assumptions of the blood pressure equation?

Answer 3

steady flow (doesn’t occur due to the intermittent pumping of the heart)

rigid vessels

right atrial pressure is negligible

Question 4

features of laminar flow

Answer 4

velocity of the fluid is constant at any point and flows in layers

blood flows fastest closest to the centre of the lumen

Question 5

features of turbulent flow

Answer 5

blood flows erratically, forming eddies and is prone to pooling

associated with pathophysiological changes to the endothelial lining of the blood vessels

Question 6

what happens at laminar shear stress

Answer 6

high shear stress
promotes endothelial cell survival and quiescence
cells aligned in direction of flow
secretions promote vasodilation and anticoagulation
GOOD

Question 7

what happens at disturbed shear stress

Answer 7

low shear stress
promotes endothelial proliferation
promotes apoptosis and shape change as well
secretions promote vasoconstriction, coagulation and platelet aggregation
BAD

Question 8

pulse pressure =

Answer 8

SBP - DBP

Question 9

MAP =

Answer 9

DBP + 1/3PP

Question 10

why do ventricular and aortic pressures differ?

Answer 10

once the aortic valve closes ventricular pressure falls rapidly but aortic pressure falls slowly

• due to the elasticity of the aorta and large arteries which act to buffer the change in pulse pressure
• elasticity of vessel is related to its compliance

Question 11

arterial compliance and pulse pressure

Answer 11

during ejection blood enters the aorta and other downstream elastic arteries faster than it leaves them
when the aortic valve closes, ejection ceases but due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in the downstream circulation
if arterial compliance decreases, the damping effect of the windkessel effect is reduced and the pulse pressure increases

Question 12

effect of pressure on vessel walls

Answer 12

transmural pressure causes a tension force in the wall that can be described by the law of la place

Question 13

what effect do aneurysms have?

Answer 13

increase radium of the vessel
for the same internal pressure, the inward force exerted by the muscular wall must also increase
however, if the muscle fibres have weakened, the force needed cannot be produced and so the aneurysm will continue to expand, often until it ruptures

Question 14

compliance properties of arteries and veins compared

Answer 14

venous compliance is 10 to 20 times greater than arterial compliance at low pressures

Question 15

increasing smooth muscle contraction

Answer 15

decreases venous volume and increases venous pressure

Question 16

effect of standing up

Answer 16

increases hydrostatic pressure in the legs as a result of gravity
blood transiently pools in the veins due to their high compliance - leads to reduced venous return
without compensatory mechanisms this would reduce cardiac output and blood pressure

Question 17

muscle and respiratory pumps

Answer 17

Muscle Pump – Return of blood in the upright posture is assisted by contraction of skeletal muscle (A) which compresses veins.
§ Respiratory Pump – During inhalation, there is a relative negative pressure in the thorax which effectively sucks blood into the central veins by reducing extra-vascular pressure in the thorax and increasing it in the abdominal cavity (differences in pressure = flow).
§ BOTH of these depend on vein valves as they aren’t constant.

Question 18

name two conditions that occur due to standing:

Answer 18

1. Varicose veins – bad valves.

2. Oedema – Prolonged increased venous pressure.