Arterioles and resistance Flashcards
What is the relationship between flow and pressure gradient?
Flow is directly proportional too pressure gradient.
There is only flow into veins and capillaries if the pressure before is greater.
Pressure gradient is the difference between arterial and venous pressure.
What is the relationship between flow and resistance?
Flow is inversely proportional to resistance.
Blood flow is faster in the middle of the vessel than the outside, lamina flow, and there is friction in between the layers.
What determines resistance?
Resistance to flow in a single vessel is determined by:
Tube length - longer = higher resistance
Viscosity - thickness, less RBCs less thick blood, less resistance.
Radius - wider, less resistance.
What is the relationship of resistance?
Resistance is directly proportional to viscosity and length of tube.
Resistance is inversely proportional to the π radius^4.
What is the graph of flow and radius?
Relative flow against relative radius.
Relative flow against π radius^4 is a linear relationship.
See picture.
If a renal artery diameter is reduced to 25% of its original value, what will the resistance along the length of the artery be increased to?
R ∝ 1/r^4
= 1/(0.25)^4
= 1/0.0039
= 256-fold
What would be the effect on resistance if the radius was increased by 50%?
Doubling the radius would increase it 16-fold.
What is Poiseuille’s equation?
Flow = pressure gradient x πr^4 / viscosity x length
What is the reason for Poiseuille’s equation?
Changing the flow by opening the arterioles and changing resistance or viscosity, without changing blood pressure.
What is an example of using Poiseuille’s equation?
Radius of vessel 2 is 2x that of vessel 1.
Resistance in vessel 2 is 1/16 of vessel 1.
So the flow in vessel 2 is 16x that of vessel 1.
What is PAOD?
Peripheral arterial occlusive disease.
Atherosclerotic plaques in large and medium arteries.
The radius is reduced, so flow is reduced.
There is ischaemia distal to the stenosis (narrowed artery).
This is particularly painful during exercise when lactic acid is produced.
Tissue hypoxia (less oxygen) causes pain - intermittent claudication (muscle pain).
What are the types of blood flow?
There are concentric layers of flow.
Flow is fastest at the vessel centre.
It is slower, stick round the outside because platelets stick to the sides of vessels.
What is laminar flow?
Normal pattern of flow.
Highly efficient.
Follows Poiseuille’s law.
Cannot be heard.
What is turbulent flow?
Occurs where flow velocity is high.
Inefficient.
Cannot apply Poiseuille’s law.
Vibrations can be heard as Korotkoff sounds and murmurs.
It requires a greater driving pressure.
When is there high flow velocity?
At large artery branches
Pregnancy
Exercise
Anaemia
Valve defects/ arterial stenosis - the cross-sectional area is reduced, so velocity must increase to compensate for the same cardiac output.
How is blood pressure taken?
When cuff pressure is increased above systolic pressure, there is no flow, because the pressure outside is greater than outer the artery (occluded).
Then pressure is released slowly, and blood starts to flow - this is systolic pressure, and Korotkoff sounds can be heard because the artery is constricted.
Pressure is released and flow becomes more turbulent, until the pressure is less than diastolic pressure, and flow becomes laminar, not heard.
How can resistance to flow be altered?
Vasodilation - increases flow.
Vasoconstriction - decreases flow.
Arterioles are always slightly constricted, so that the radius can be both dilated and constricted.
What is vasoconstriction and vasodilation controlled by?
Endothelial factors
Local mechanisms
Central neural mechanisms
Hormonal mechanisms
How is TPR affected by vasoconstriction?
Constriction of arterioles to multiple organs increases TPR and therefore increases ABP (ABP = COxTPR).
e.g. in a haemorrhage, to maintain bp.
Constriction of an arteriole to one organ just decreases flow to that organ.
e.g. to the skin in cold conditions.
What is endothelial control of vasodilation?
Stimuli to endothelial cells by bradykinin, and neurotransmitters and other factors can cause nitric oxide release from the endothelium.
This causes vasodilation, by decreased Ca2+ in the smooth muscle cell.
What is endothelial control of vasoconstriction?
Endothelins, produced by angiotensin II and trauma cause an increase in Ca2+ in the smooth muscle cell.
This causes the cells to contract and vasoconstrict.
How do local factors cause functional hyperaemia?
When the tissues have increased metabolism, it produces metabolic waste - adenosine, lactate, CO2, H+ and K+, which are vasodilators.
The more products, the more vasodilation, and is proportional to need.
Functional hyperaemia.
How do local factors change during exercise?
In exercise, the heart needs more oxygen, so needs more blood flow through the coronary arteries.
The increased adenosine produced vasodilates the coronary arteries and increases blood flow.
How do local factors cause reactive hyperaemia?
A reduction in flow causes the muscles to contract so that flow stops.
The muscle is still metabolising and producing metabolic waste, but there is a physical barrier to the muscle opening.
When the reduction is removed, there are lots of metabolites, so the tissue massively dilates - reactive hyperaemia.
What is autoregulation against blood pressure?
The brain, kidneys and heart always require high blood flow, so if blood pressure goes down, they are autoregulated so that flow does not change.
Resistance vessels dilate at low pressure to maintain optimal flow.
How do the cerebral and kidney vessels autoregulate?
If blood pressure decreases, they vasodilate to maintain the same flow.
If blood pressure increases, they vasoconstrict.
It will get to a point where no more vasodilation can occur, so if blood pressure continues to decrease, they will go unconscious, and will need to lie down to increase venous return.
What are central neural mechanisms of blood pressure?
Most vascular smooth muscle have tonic vasomotor tone (constricted) due to ongoing sympathetic nerve activity.
Decreases sympathetic activity, the arteriole vasodilates.
Increases sympathetic activity, arteriole vasoconstricts.
Very little parasympathetic activity.
How does the sympathetic system act on the blood vessels?
Noradrenaline is released and binds to a1 receptors, which become excited and causes vasoconstriction.
Noradrenaline can also bind to B2 receptors on the skeletal muscle to cause vasodilation, but it binds weakly so the a1 receptors predominate.
What is the effect of plasma adrenaline?
B2 receptors cannot distinguish between plasma adrenaline and noradrenaline released.
When there is low plasma adrenaline, it causes dilation by B2 receptors.
When there is high plasma adrenaline, it causes vasoconstriction.
Circulating adrenaline predominates over noradrenaline.
What are hormonal mechanisms for vasoconstriction?
ADH (vasopressin) acts on V1 receptors on blood vessels and causes vasoconstriction.
ADH is released during a haemorrhage to reabsorb water and increase circulating volume.
Angiotensin II increases Na+ and water reabsorption. It acts on AT1 receptors to cause vasoconstriction.
What is the balance of change in resistance?
Local factors outweigh all other factors - neural, autoregulation, endothelial, hormonal.
