The Circulation Flashcards
CIrculation; Flow; Vascular starling forces; Lymphatic system; Law of LaPlace; Vascular endothelium; Vascular endothelium drugs
What are the two main physical principles of the circulatory system?
Double circulation - systemic and pulmonary, each with different pump
Pressure gradient - generated by heart, causes movement of blood
What is the function of the circulatory system?
Transport oxygen, CO2, nutrients, hormones, metabolites, cells and immunity related molecules around the body
Thermoregulation
What is the general order of the circulatory vessels?
Arteries -have a low sfa -store little blood but at high fluctating pressure Arterioles -site of drop in BP -can be constricted to control flow of blood Capillaries -highest total sfa -form networks/beds for diffusion Venules -connect veins to capillary beds Veins -holds the most blood and at the lowest pressure
Define laminar flow
Velocity of fluid constant at any one point
Flows in layers
Fastest closest to centre of lumen (some friction with endothelial cell lining)
High level of shear stress - promotes endothelial cell survival and alignment in direction of flow to promote secretions to allow vasodilation and anticoagulation
Define turbulent flow
Associated with pathophysiological changes to endothelial lining
Erratic flow
Forms eddys
Prone to pooling
may lead to occlusion
Low shear stress promotes endothelial proliferation allows secretions to promote endothelial proliferation, apoptosis and shape change
What does the parabolic velocity profile show?
Shows how flow further from the wall = increased velocity
Tangent at any point on the parabolic profile = shear rate
shear rate x velocity = shear stress
What is shear stress?
Governs how well endothelial cells work
shear rate x viscosity
How is turbulent flow used in the measurement of blood pressure?
Release of BP cuff leads to turbulent flow
Can be heard with stethoscope
What is Poiseuille’s equation and its importance?
resistance = (8×length×viscosity)/(π×r^4)
length and viscosity are effectively constant so only radius changes rapidly
halving radius decreases flow 16 fold ∴ capillaries and arterioles have the most resistance to flow
What is vascular compliance?
The relationship between transmural pressure and vessel volume
change in volume/change in pressure
Dependent on vessel elasticity
Venous>arterial
Define elastance
Inverse of compliance
ΔPressure/Δvolume
Produced by elastin fibres in the vascular wall which leads to recoil upon stretching
Blood vessels with high compliance will have low elastance
arterial>venous
∴arteries recoil to maintain pressure whilst veins distend
What is the Windkessel effect?
Dampening effect of the change in pressure during ejection
Blood enters aorta faster than it is leaving ∴ 40% of stroke volume is stored by elastic arteries
when ejection ends and aortic valve closes, elastic arteries recoil and send off all stored blood
∴aortic pressure falls slowly and blood isnt lost all at once
How can compliance be modulated?
Externally - by stockings to reduce pooling in veins
Internally - by the Renin-angiotensin-aldosterone system, endogenous vasodilators/constrictors and vasoactive drugs
What is the effect of gravity on flow?
pulls blood towards ground
standing causes increased hydrostatic pressure ∴blood trasiently pools in veins due to high compliance
∴reduced venous return ∴ reduced CO and BP (starlings law)
Why do healthy individuals not faint when they stand up?
Standing activates SNS
constrict venous smooth muscle to stiffen veins
constrict arteries to increase resistance and maintain BP
increase HR and force of contraction to maintain CO
What is the relationship between the Law of LaPlace and blood flow?
increased radius increases flow but also leads to higher wall stress ∴ thicker wall is needed to produce higher tension as the radius increases for a given pressure
What is an aneurysm?
Balloon like distension increases radius of vessel
For same pressure, tension increases
If weakened fibres cannot supply needed force then will distend in negative feedback cycle until RUPTURE
What is the microcirculation?
Specific to each organ
1st order arterioles
Covered in smooth muscle
Branch to capillaries via terminal arterioles to enter tissues
Pre-capillary sphincters control blood flow to capillary bed
Venules leave tissue to collect blood and deliver to the heart
What is a fluid circuit?
Flow rate=pressure gradient/resistance
mainly affected by vessel radius - poiseuilles eqn
Where is the pressure gradient formed within the microcirculation?
Between arteriole and capillary
What would decrease the Windkessel effect?
decrease in arterial compliance
Where is the pressure gradient for a fluid circuit found?
between arteriole and capillary
Why do the radii of arterioles vary?
Local intrinsic control - matching blood flow to specific tissue needs
Extrinsic control - regulating BP
What is the formula for organ flow?
ΔP(MAP)/Resistance of organ
What is active hyperemia?
Increase in organ blood flow associated with increased metabolic activity of an organ or tissue
e.g. when activity of muscle increases, metabolism within that muscle increases ∴ so does oxygen consumpton, local tissues detect these increases and send a signal to the arterioles, arterioles vasodilate and increase oxygen reaching cells
What is myogenic vasoconstriction?
Local tissue responds to changes in physical environment like temperature changes ∴ if things get colder, this is sensed and sends signals that cause arterioles to vasoconstrict and reduce heat loss
Also respond to changes in pressure e.g. if big increase in BP to organ that doesn’t require extra flow, arterioles vasoconstrict to help direct greater blood flow to other organs
What is the general structure of a capillary?
Single endothelial cell wall
Diameter big enough form 1 cell
Extensive branching
minimises diffusion distance, maximises diffusion time and maximises sfa
