cardiovascular mechanics 3 Flashcards
what are the main aspects of design of the system?
- circulation is designed to transport blood around the body - regulate temperature - deliver nutrients and hormones - two circulations present - and a muscular pump that generates a pressure gradient - the diffusion is only effective over short distances
how do arteries and arterioles respond to resistance?
- extensive smooth muscle - regulates diameter and the resistance to blood flow
how do veins and venuoles respond to resistance?
- they are very compliant - they act as a reservoir for blood volume
why does blood flow?
- it is blood pressure that drives the circulation - the pressure difference between the aorta and the vessels
what is the equation for resistance in fluids?
- ΔP = Q x R Q = volumetric flow, R = resistance
how can we replace the terms of - ΔP = Q x R ?
blood pressure = cardiac output x resistance
what assumptions does blood pressure = cardiac output x resistance make?
- steady flow - rigid vessels - the right atrial pressure is negligible
how is regulation of flow achieved?
- variation in resistance in the vessels - the blood pressure remains constant through
how does pressure change across the circulation?
- the pressure falls across the circulation - due to frictional pressure losses - small arteries and arterioles are most resistant to flow
what factors decide the resistance of a tube to flow?
- Fluid viscosity (, eta) 2. The length of the tube (L). 3. Inner radius of the tube (r)
what is poiseulles equation?
- small changes in vascular tone results in large changes to flow halving the radius decreases by 16 times
what is the cardiac output at rest? what is the cardiac output with exercise?
- rest = 5L/min - exercise = 20L/min
what is laminar flow?
- the velocity of fluid is constant - flows in layers - fastest flow closer to the lumen
what is turbulent flow?
- blood flows erratically and is prone to pooling - Associated with pathophysiological changes to the endothelial lining of blood vessels
why is the flow fastest in the lumen?
- there are adhesive forces between the fluid and the surface - the velocity of layers increases as the distance form the wall increases
sketch a velocity profile:
what is the shear rate
what is shear stress?
shear rate is the gradient of the velocity profile at any point
shear stress is the shear rate multiplied by the viscosity
what does laminar shear stress do?
- high shear stress
- promotes endothelial cell survival
- the cells are aligned in direction flow
- Secretions promote vasodilation and anticoagulation
what does disturbed shear stress do?
- low shear stress
- promotes endothelial proliferation
- apoptosis and shape change
- Secretions promote vasoconstriction, coagulation and platelet aggregation
how is blood pressure measured?
- measured on the upper arm
- slow deflation of the cuff causes turbulent flow
- this is heard with a stethoscope
how to work out pulse pressure?
how to work at MAP?
pulse pressure = SBP - DBP
MAP = DBP + 1/3 pulse pressure
why do ventricular and aortic pressures differ?
- when the aortic valve shuts the ventricular pressure falls rapidly the aortic pressure falls slowly
- we can explain this by the elasticity of the aorta and the large arteries which try to buffer the change in pulse pressure
how does the aorta manage blood from the ventricles?
- during ejection, blood enters the aorta faster than it leaves the aorta
- 40% of SV is stored in arteries
- when the aortic valve closes the ejection ceases and due to the recoil of elastic arteries the pressure falls slowly and there is diastolic flow
- If arterial compliance decreases , arteries become stiffer , the pulse pressure gets higher
what is the law of laplace?
tension = pressure x radius
what does high circumferential stress cause?
vessel distension
how do aneurysms form?
- vessel walls can weaken causing a balloon-like distension
how does the law of laplace relate to aneurysms?
- vascular aneurysms increase the radius of the vessel
- this means that for the same internal vessel there is a greater radius therefore greater wall tension
- so more force is needed
- however the muscle is too weak to provide the force
so the aneurysm will continue to expand till it ruptures
how does venous compliance compare to arterial compliance?
Venous compliance is 10 to 20 times greater than arterial compliance at low pressures.
what does increases smooth muscle contraction do the vein?
decreases venous volume and increases venous pressure.
so with contraction, the compliance goes down
what does standing do?
increases hydrostatic pressure in legs
- Blood transiently pools in the veins due to their high compliance -> reduced venous return
- reduce cardiac output and blood pressure
might cause fainting as not enough blood is going to the head
what are problems with standing?
- incompetent valves cause dilated superficial veins in the leg
- Prolonged elevation of venous pressure causes oedema in feet
