Control of blood flow Flashcards
What three things mainly control TPR?
→Poiseulle’s Law
→ myogenic response
→blood viscosity
What does an increase in resistance mean?
→ Pressure must be increased to maintain the same flow
What is the equation for blood flow? (CO)
→ Pressure gradient / TPR
What is hypertension?
→ over constriction of arterioles
→ Higher arterial BP but less capillary flow - under perfusion
What are the changes in blood flow when sedentary?
→ Superior mesenteric dilated
increased flow to intestines
→ Common Iliac constricted
decreased flow to legs
What are the changes in blood flow when exercising?
→Superior mesenteric constricted
decreased flow to intestines
→ Common iliac dilated
increased flow to legs
What does Poiseuille’s Law describe?
→ Parameters that govern TPR
→Illustrates why the radius of a vessel is such an important determinant in changing blood flow.
What is the state of the BP and blood flow in arteries and arterioles in a normal situation?
→In a normal situation, the arteries have a greater BP than the arterioles.
→The pressure drop between the arteries and arterioles causes blood flow.
What is the state of BP and blood flow when the arterioles are dilated?
→With the arterioles dilated, there is a decrease in TPR. →These leads to decreased BP upstream, but greater blood flow.
What is the state of BP and blood flow when the arterioles are constricted?
→With the arterioles constricted, there is an increase in TPR.
→ This leads to increased BP upstream, but less blood flow.
What is the equation for conductance ?
Conductance (G) = (πr^4)/(8ηL)
r: radius of vessel
η: blood viscosity
L: vessel length
(blood vessel radius to the power of 4 controls TPR)
What is Poiseuille’s and Darcy’s Law combined?
→CO = Pa - CVP x (πr^4)/(8ηL)
What is the r^4 effect ?
→Double the radius of vessel 2 compared to vessel 1
→ The change in r^4 is 16 x
→ 16x greater flow in vessel 2 as flow is proportional to r^4
How do vasoconstrictors or dilators have large effects on blood flow?
→ Vasoconstrictors or dilators produce small changes in the vessel radius by affecting smooth muscle
→ these have large effects on blood flow
What is the pressure in arterioles?
→40-50 mmHg amongst vessels
→ largest pressure drop
What is the arteriole radius controlled by?
→ Tightly controlled by sympathetic nerves providing constant tone
→dilate and constrict
What parameters is TPR controlled by?
→ Radius r^4
→ Pressure difference across vessels (P1-P2)
→ Length (L)
Why do arterioles control TPR and not capillaries?
→ Capillaries are arranged in parallel so they have a low total resistance
→ R total = 1/R1 + 1/R2
→ arterioles are in series so total resistance is greater
→ R total = R1+R2
How is local blood flow through individual organs/ tissues controlled?
→ Controlled by changes in radius of arterioles supplying a given organ/ tissue
What are the control mechanisms for the arteriole radius?
→Intrinsic
factors are within the organs or tissues
→Extrinsic
factors are outside the organ or tissue
What is Baylis’s myogenic response?
→ Increased distension of a vessel makes it constrict
→ Decreased distension of a vessel makes it dilate
What is the function of Baylis’s myogenic response?
→ It maintains blood flow at the same level during changing arterial pressure
In what circulations is Baylis’s myogenic response important?
→renal, coronary and cerebral circulation
What happens when the muscle is stretched to make it contract ?
→ Ion channels open which depolarize leading to muscle contraction
What is blood viscosity?
→Viscosity is the measure of internal friction opposing the separation of the lamina.
What does blood flow depend on?
→ Viscosity
→ Vessel diameter
→ Haematocrit
How much of the blood volume is at rest in systemic veins and venules?
60%
What do veins function as?
→ reservoir
→ blood can be diverted from it in times of need
What are properties of veins?
→ they are thin walled and collapsible, voluminous vessels
How much of the blood volume do veins contain?
→2/3rds of blood volume
What are veins innervated by?
→ smooth muscle which is innervated by sympathetic nerves
what is the difference between arterial muscle and vein muscle?
→ Vein muscle is thinner and more compliant so forms a reservoir
What does contraction of venous vessels do?
→ Expels blood into central veins
→Increases venous return/CVP/ End-diastolic volume
→ Increases SV
What are the venous pressures in the limb veins at heart level?
→ 5-10 mmHg
What is the central venous pressure?
→ 0-7mmHg
What is the venous pressure in the foot vein while standing?
→ 90mmHg
Why is the venous pressure high at the feet?
→ so high pressure is generated for return to the heart
What is venous pressure helped by in the feet?
→ thoracic pump and skeletal muscle contraction
what does stimulation of sympathetic nerves causing venoconstriction cause?
→ Shifts blood centrally
→ Increase in venous return, CVP and end diastolic pressure
→ Increased CVP increases preload and so increases SV
What is the Bernoulli theory?
→ Mechanical energy of flow is determined by pressure, kinetic + potential energies
How does blood flow from the heart to the feet in terms of kinetic energy, potential energy + pressure?
→There is a -90 mmHg pressure gradient against the flow from the feet back to the heart.
→The ejected blood has greater kinetic energy at the heart than the feet (more velocity).
→greater potential energy at the heart than at the feet (more height).
→The greater kinetic/potential energies overcome the pressure gradient to maintain flow.
