Control of blood flow Flashcards
What does Darcys Law state?
The role of pressure on flow.
Blood flow = Pa - CVP /TPR
Rearrange darcys equation to include G and what does G state?
G = Conductance is the reciprocal of TPR (how easily it flows)
CO = Pa - CVP x G
What does TPR control?
• TPR controls blood flow and blood pressure - Increase in resistance means need to increase pressure to keep same flow.
What controls TPR?
- Darcy’s and Poiseuille’s laws
- Myogenic response
- Blood viscosity
Describe how pressure, blood flow and TPR changes when during normal artery blood pressure, a lower artery blood pressure and higher artery blood pressure
Normal blood pressure in the artery:
The pressure drops between arteries and arterioles causes normal blood flow in capillaries
Lower blood pressure in the artery:
Vasodilation in the arteriole decreased TPR, decreased blood pressure upstream, but greater flow
Increased blood pressure in artery:
Vasoconstriction in the arteriole, increased in TPR, increased blood pressure upstream but less flow
What is hypertension?
Over constriction of the arterioles, higher arteriole blood pressure upstream but less flow.
Describe Changes in blood flow in response to changes in need
• Brain stem areas controlling sympathetic nervous activity to various areas of the body. These are coordinated by the brain stem and the medulla at the top of the spinal cord. Sedentary • Superior mesenteric dilated Increased flow to intestines • Common iliac constricted Decreased flow to legs • Superior mesenteric constricted Decreased flow to intestines Exercising • Common iliac dilated Increased flow to legs
What factors control TPR?
The radius of the vessel, blood viscosity and the length of the vessel all effect the resistance.
Why do small changes in radius size have a huge effect on TPR?
Small changes in radius have a huge effect of the resistance to flow. This is why contraction and relaxation have a big influence on TPR. It is because it is power of 4.
The longer the vessel the greater the…….?
TPR
What does Poiseuille’s Law state?
Describes parameters that govern TPR
What is Poiseuille’s equation for his law?
Resistance = (8 x blood viscosity x vessel length) / (Pi x radius^4)
What is the equation of Conductance (G)
Reciprocal of Poiseuille’s law equation
What is Darcys Law equation taking into account G
CO = Pa - CVP x G
Combine Darcy’s equation and Poiseuille’s equation
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Describe the r4 effect
Vasoconstrictors or dilators produce small changes in vessel radius by affecting smooth muscle have large effects on blood flow.
What is the pressure drop in arterioles?
• Arterioles have the largest pressure drop of 40-50 mmHg amongst vessels.
How is the arteriole radius controlled?
• Arteriole radius is tightly controlled by sympathetic nerves providing constant tone dilation vs constriction
TPR controlled by 3 main parameters:
- radius (r4)
- Pressure difference across vessels, P1-P2.
- Length (L) arterioles are also long vessels
Capillaries have a much smaller radius than arterioles so why do arterioles control TPR?
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How does the body control local blood flow?
Give some examples of intrinsic and extrinsic factors
Local blood flow through individual organs/tissues is mainly controlled by changes in radius of arterioles supplying a given organ/tissue
Intrinsic:
• Factors entirely within an organ or tissue (Allow response to local factors)
Extrinsic
- Factors outside the organ or tissue (see Nervous & hormonal control of blood vessel lecture)
- If we have a tissue that is metabolising and producing a lot of CO2 and ions so the pH decreases and from adenosine from the breakdown of ATP. These things will cause arterioles to dilate. Allowing more blood into the tissue.
Describe the bayliss myogenic response
- If we have a blood vessel of any given size we would expect a relationship between pressure and flow – a reasonably straight line.
- At very low pressure they start of with a linear relationship
- As the pressure increases, they don’t want the high pressure and flow to reach the capillaries. So, as pressure and flow increase it stretches the arterioles. This causes ion channels to open which depolarise and cause constriction, to limit flow.
- Increased distension of vessel makes it constrict. Decreased distension of vessel makes it dilate.
- Having a linear relationship would entail very large differences in blood flow with differences in pressure
- Maintains blood flow at the same level during changing arterial pressures. Vary important in renal, coronary, cerebral circulation.
- Stretching of the muscle causes ion channels to open, which then depolarize, leading to muscle contraction.
What does blood flow depend on?
- Viscosity of blood
- Vessel diameter
- Haematocrit
What factors can affect blood viscosity?
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Why is SCA bad?
In sickle cell the blood cells cannot squish through the small vessels, they might build up and cause a blockage increasing viscosity and lower flow.
Describe blood in the venous system
- 60% of blood volume at rest is in systemic veins and venules
- Functions as blood reservoir
- Blood can be diverted from it in times of need
- e.g. exercise, haemorrhage
Describe the general features of veins
Describe the volume in veins and their contractility
Describe the typical venous pressures
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Describe the venous pressure-volume curve and its parameters
- Venous pressure high at the feet so pressure for return to heart. Also helped by thoracic pump and skeletal muscle contraction.
- Stimulation of sympathetic nerves causing venoconstriction shifts blood centrally.
- Increases venous return, CVP & end-diastolic pressure.
- Increased CVP increases preload and so increases stroke volume (Starling’s law).
Bernoulli’s law explains blood flow
• How can blood flow from heart to feet in this situation?
• How does blood fill the ventricles when CVP is so low?
• Flow cannot be only determined by pressure alone…
• Bernoulli theory – mechanical energy of flow is determined by pressure, kinetic, potential energies (ρ = fluid mass)
• Energy = Pressure (P) + kinetic (ρV2/2) + potential (ρgh)
Standing
• -90 mmHg pressure gradient against flow back to heart from feet
• Ejected blood has greater kinetic energy at heart than feet (more velocity, V).
• Also, greater potential energy than at heart than feet (more height, h)
• Greater kinetic/potential energies overcome pressure gradient to maintain flow
• But flow to feet easily compromised – clinically important
• Returning blood to heart - no pressure gradient but kinetic energy
