4. Cardiovascular mechanisms 3 Flashcards
Function of pulmonary circulation
allows blood to be oxygenated
Function of systemic circulation
Pumps blood to rest of body
Why do small arteries and arterioles have extensive smooth muscle in their walls?
to regulate their diameters and resistance to blood flow
What do capillaries provide?
Large SA
Thin walls for diffusion to occur
Which vessels are described as capacitance vessels?
Veins and venules
highly compliant
act as a reservoir for blood volume
Equation for MAP (including cardiac output and resistance)
MAP = Cardiac output x Resistance MAP = Q x R
What 3 assumptions mean the equation for MAP can only give an approximation?
(eq. Q x R)
Steady flow (which does not occur due to the intermittent pumping of the heart)
Rigid vessels
Right atrial pressure is negligible
How does pressure change with distance through circulation?
Pressure falls
Due to viscous pressure losses.
Small arteries and arterioles present most resistance to flow.
High pressure in arteries, markedly drops in arterioles, further drops in capillaries.
Pressure difference allows blood to flow through capillaries
What 3 factors influence the resistance of a tube to flow?
Fluid viscosity
Length of the tube
Inner radius of the tube
Poiseuilles equation
R= (8 x L x fluid viscosity) / pi x r^4
What can be ignored in poiseuilles equation when calculating resistance in circulatory system?
Viscosity- doesn’t change
Length- doesn’t change
What is the simplified version for poiseuilles equation when calculating resistance in circulatory system?
Resistance = 1 / r^4
How is blood flow distribution determined?
By metabolic need
Laminar flow
Linear, Healthy
Velocity of the fluid is constant at any one point and flows in layers
Blood flows fastest closest to the centre of the lumen.
Flows slower near walls due to resistance from hitting side walls of vessel
Turbulent flow
Less healthy
Blood flows erratically, forming eddys, and is prone to pooling
Associated with pathophysiological changes to the endothelial lining of the blood vessels
What gives blood a parabolic velocity profile?
Adhesive forces between fluid and surface slow flow
Velocity of layers increases as distance from wall increases
How is the shear rate determined?
Gradient of velocity profile
Shear stress equation
Shear rate x Viscosity
What does shear stress govern?
How well endothelial cells work
Endothelial cells release molecules that govern resistance to flow and govern laminar flow
Laminar shear stress
High shear stress
Promotes endothelial cell survival and quiescence
Cells aligned in direction of flow
Secretions promote vasodilation and anticoagulation
GOOD
Disturbed shear stress
Low shear stress
Promotes endothelial proliferation, apoptosis and shape change
Secretions promote vasoconstriction, coagulation and platelet aggregation
BAD
What causes turbulent flow to increase in carotid arteries?
Age
What does slow deflation of a cuff when measuring BP cause?
Turbulent flow, which can be heard with a stethoscope
What causes the dichroic notch when graphing BP?
Closure of aortic valve
Equation for pulse pressure
PP= SBP - DBP
Equation for MAP (including DBP and PP)
MAP = DBP + 1/3 (PP)
When the aortic valve closes, why does ventricular pressure fall rapidly but aortic pressure fall slowly?
Windkessel effect:
Elasticity in aorta allows blood flow to be sustained, less pulsatile, maintains diastolic pressure
During ejection blood enters the aorta and other downstream vessels at what relative speed?
Enters faster than it leaves them
What happens of arterial compliance decreases? (become stiffer e.g. with age)
The damping effect of the Windkessel effect is reduced
Pulse pressure increases.
Circumferential stress
= Tension (T) / Wall thickness (h)
Equation for pressure gradient
P1 (start of vessel) - P2 (end of vessel)
Pathological example of Law of Laplace
Vascular aneurysms increase radius of the vessel.
So, for the same internal pressure, the inward force exerted by the muscular wall must also increase.
However, if muscle fibres have weakened, the force needed can’t be produced, so the aneurysm will continue to expand until it ruptures
Compliance
The relationship between transmural pressure and vessel volume
Depends on vessel elasticity
Venous compliance relative to arterial compliance at low pressure
Venous compliance 10-20x greater
What can decrease radius and venous volume, thereby increasing venous pressure
Increased innervation and increasing smooth muscle contraction
Describe the effects of gravity when standing up
Increases hydrostatic pressure in legs
Blood transiently pools in veins due to their high compliance, reducing venous return
What is the approximate increase in hydrostatic pressure in vessels due to gravity?
100mmHg
How does pressure ensure unidirectional flow?
Pressure gradient from left heart to right heart is maintained
What is the difference between the blood pressure in the ankle compared to the hand?
BP in the ankle is much greater than in the hand
Why don’t we faint every time we stand up due to blood pooling in our leg veins?
Standing causes activation of the SNS, which stiffens and constricts the veins resulting in greater venous return to the heart.
Constricted arteries means there is an increase in TPR to maintain BP.
Also a slight increase in HR and force of contraction.
What are the 2 pumps that facilitate the movement of venous blood back to the heart?
Skeletal muscle pump
Respiratory pump
