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
→No sympathetic innervation/smooth muscle in capillaries so cannot alter radius.
→Less resistance is capillaries because bolus flow reduces viscosity
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.
How does a cardiac output help venous return?
- the circulation is a closed system so the heart pushes the blood further through the vascular system via the arterial side of the capillary system into the venules and veins in the direction of the right side of the heart.
How does breathing help venous return?
the pressure in the chest is negative on inhalation at the same time intra-abdominal pressure rises as the diaphragm moves downwards causing the venous valves in the pelvic veins to close, and the blood moves up into the thorax.
On exhalation, the intra-abdominal pressure decreases and the pelvic veins and inferior vena cava refill.
How does muscle pump help venous return?
every muscle contraction squeezes the veins to push the column of blood in them in the direction of the heart.
When the muscle relaxes, the venous valves prevent the retrograde flow of blood towards the capillaries.
How does venous tone help venous return?
the blood in the veins exerts pressure on the vein’s wall generating tension & maintain pressure.
sympathetic vasoconstriction can mobilise more blood back to the heart.
What are the clinical implications of viscosity factors?
Haematocrit (45% rbc)
Typical blood is 4-5 x H2O- Polycythaemia (high ) - Increased TPR & BP. Decreased flow.
Anaemia (low ) - Low TPR & BP. High HR (baroreceptor reflex) because low blood pressure. High HR to return heart rate to normal
2.Tube diameter(Fahraeus-Lindqvist effect)- Blood viscosity falls in narrow tubes (< 100 m vessels) cells move to centre reducing friction. Blood behaves as if it has a low viscous because cells are concentrated in the centre
Decreased resistance, increased flow, ie low resistance in micro vessels such as capillaries.
- Red cell deformability reduced- Increased , reduced flow eg. sickle cell anaemia crises. Blood behaves as if it has a high viscous. Static flow= high viscocity
- Velocity of blood- Slow venous flow in immobile legs – increased viscosity due to partial clotting.
