mechanics 3 Flashcards
(18 cards)
blood pressure =
cardiac output x resistance
fluid circuit equation
delta P = Q x R
what are the assumptions of the blood pressure equation?
steady flow (doesn’t occur due to the intermittent pumping of the heart)
rigid vessels
right atrial pressure is negligible
features of laminar flow
velocity of the fluid is constant at any point and flows in layers
blood flows fastest closest to the centre of the lumen
features of turbulent flow
blood flows erratically, forming eddies and is prone to pooling
associated with pathophysiological changes to the endothelial lining of the blood vessels
what happens at laminar shear stress
high shear stress
promotes endothelial cell survival and quiescence
cells aligned in direction of flow
secretions promote vasodilation and anticoagulation
GOOD
what happens at disturbed shear stress
low shear stress
promotes endothelial proliferation
promotes apoptosis and shape change as well
secretions promote vasoconstriction, coagulation and platelet aggregation
BAD
pulse pressure =
SBP - DBP
MAP =
DBP + 1/3PP
why do ventricular and aortic pressures differ?
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
arterial compliance and pulse pressure
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
effect of pressure on vessel walls
transmural pressure causes a tension force in the wall that can be described by the law of la place
what effect do aneurysms have?
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
compliance properties of arteries and veins compared
venous compliance is 10 to 20 times greater than arterial compliance at low pressures
increasing smooth muscle contraction
decreases venous volume and increases venous pressure
effect of standing up
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
muscle and respiratory pumps
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.
name two conditions that occur due to standing:
- Varicose veins – bad valves.
2. Oedema – Prolonged increased venous pressure.
