Control of Blood Flow Flashcards
What is the Darcy’s law equation
What does total peripheral resistance 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 Poiseulle’s laws
Myogenic response
Blood viscosity
What is conductance
The reciprocal of TPR
G = 1/TPR
Describe how peripheral resistance impacts arterioles when it is normal, increased and decreased
- Decrease in TPR. Decreased blood pressure upstream, but greater capillary flow
- Increase in TPR. Increased blood pressure upstream, but less capillary flow
- When total peripheral resistance is at a normal level and flow is normal, blood pressure is also normal
Hypertension is widespread over constriction of arterioles and leads to a higher arterial BP but less capillary flow ie under perfusion.
Explain how there are changes in blood flow in response to changes in need
When sedentary - the superior mesenteric is dilated - increased flow to intestines and the common iliac is constricted so there is decreased flow to legs
When exercising - the superior mesenteric is constricted - decreased flow to intestines and the common iliac dilated so there is increased flow to legs
What is Poiseuille’s Law
Describes the parameters that govern total peripheral resistance
Combine Darcys and Poiseuilles laws
This shows that resistance is affected by the difference in pressure, length of vessel, viscosity of blood and radius
What is the r^4 effect
Vasoconstrictors or dilators produce small changes in vessel radius by affecting smooth muscle having large effects on blood flow
Describe the pressure changes in arterioles
- Arterioles have largest pressure drop of 40-50 mmHg amongst vessels
- Arteriole radius is tightly controlled by sympathetic nerves providing constant tone dilation vs constriction
State the 3 main parameters that control total peripheral resistance
- radius (r^4)
- 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
- Less pressure drop across capillaries due to less resistance to blood flow in capillaries
- No smooth muscle in capillaries so cannot alter radius
- Individual capillaries are short
- Less resistance in capillaries because bolus flow reduces viscosity
- Capillaries are arranged in parallel, so have a low total resistance WHEREAS arterioles are in series so total resistance is greater
What are the control mechanisms for arteriole radius
- Intrinsic - factors entirely within an organ or tissue
(Allow response to local factors), e.g. local hormones, tissue metabolites, myogenic properties of muscle, endothelial factors like nitric oxide - Extrinsic - factors outside the organ or tissue
(Nervous & hormonal control of blood vessels), e.g. neural eg. sympathetic nervous system and hormonal eg. adrenaline
Describe the Bayliss myogenic response
- 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. Very important in renal, coronary, cerebral circulation
- Stretching of the vessel causes ion channels to open, which then depolarize, leading to smooth muscle contraction
Define viscosity
Viscosity is a measure of internal friction opposing the separation of the lamina
What does blood flow depend on
- Viscosity of blood
- Vessel diameter
- Haematocrit
Describe what happens when changes happen to the following factors:
- haematocrit
- tube diameter
- red cell deformability reduced
- velocity of blood
Describe the veins
- Thin-walled, collapsible, voluminous vessels
- Contain 2/3rd of blood volume
- Contractile – contain smooth muscle, innervated by sympathetic nerves but thinner than arterial muscle & more compliant – so form blood reservoir
What is the relationship between blood in veins & contractility
Contraction of vessels – expels blood into central veins
– Increases venous return
– Increases stroke volume (Starling’s law)
What are the typical venous pressures
Limb vein, heart level 5-10 mmHg
Central venous pressure (entering heart) 0-7 mmHg
Foot vein, standing 90 mmHg
Describe what happens in veins where there is high or low pressure
- When there is very little pressure veins don’t really constrict they just fold up. Some pressure in them and they will distend
- Venous pressure high at the feet so there is pressure to return blood to the heart. Also helped by thoracic pump, skeletal muscle contraction & one way valves
- Sympathetic nerve stimulus can contract them and shift blood centrally increasing CVP, diastolic pressure and preload
- 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)
What is Bernoulli’s law
The role of pressure, kinetic, potential energies in flow
How does blood flow from heart to feet
- There is a pressure gradient against flow from the heart to the feet
- Ejected blood has a greater kinetic energy at the heart than at the feet - more velocity
- It also has greater potential energy at the heart than at the feet - more height
- Greater kinetic/potential energies overcome the pressure gradient to maintain blood flow
How does cardiac output contribute to 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 contribute to 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 contribute to venous return
The deep venous system is embedded in muscles. Due to this, 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 contribute to venous return
The blood in the veins exerts pressure on the veins wall generating tension & maintaing pressure. Furthermore, sympathetic vasoconstriction can mobilise more blood back to the heart
