Control of blood flow Flashcards
What is Atherosclerosis
Atheroscelerosis- build up of fat in the blood vessels which can reduce blood flow to the heart
What is the equation for flow in Darcy’s law
Flow= Arterial pressure (Pa)/ Central venous pressure (CVP) x Total peripheral resistance (TPR)
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 and Poisuelle’s laws
-myogenic response (reflexive contraction)
-blood viscosity
What is the equation for the G (conductance is the reciprocal of TPR)
G= 1/ TPR
What is the equation for the combined flow equation and G equation
Flow= Pa - CVP x G
Explain the relationship with normal blood flow with TPR in blood vessels
In the artery, if there is normal BP this will cause normal TPR in the arterioles and will result in normal flow in the capillaries.
-In the artery, if there is a lower BP this will cause decreased TPR in the arterioles so can lead to vasodilation and increased flow in the capillaries
-In the artery, if there is increased BP this will cause an increased TPR in the arterioles so there is vasoconstriction. Therefore there is decreased flow in the capillaries.
What does Sedentary brain stem area do for blood flow
Sedentary- involved in the resting process and causes vasodilation
What does superior mesentric do
Superior mesenteric which increases flow to the intestines by dilating blood vessels
What does the common iliac do
Common iliac causes vasoconstriction of blood vessels causes decreased flow to the legs
Explain what the superior mesenteric does whilst exercising
Superior mesenteric, causes vasoconstriction so decreased flow to intestines
What does the common iliac do when exercising
common iliac causes vasodilation so increased flow to the legs
What is the equation for total peripheral resistance in Poiseuille’s law
TPR= 8 x blood viscosity (h) x Length (L)/ 3.814 x radius of vessel (r)^4
What is the equation for conductance
Conductance (G)= 3.814 x r^4/ 8 x blood viscosity (h) x length (L)
Explain the r4 effect (radius effect)
Vasocontriction or dilators produce small changes in vessel radius by affecting smooth muscle have large effects on blood flow
What is arteriole radius is tightly controlled by
Arteriole radius is tightly controlled by sympathetic nerves providing constant time dilation vs constriction
What is TPR controlled by?
TPR is controlled by 3 main parameters:
1) Radius (r4)
2) Pressure difference across vessels (P1-P2)
3) Length (L) arterioles are also long vessels
Capillaries have a much smaller radius than arterioles so why do arterioles control TPR?
Capillaries are arranged in parallel formation so have low total resistance. In contrast arterioles are in a series so total resistance is greater
What is local blood flow controlled by
local blood flow through individual organs/tissues is mainly controlled by changes in radius of arterioles supplying a given organ/tissues.
What are control mechanisms of arteriole radius?
Control mechanisms of arteriole radius:
Intrinsic- factors entirely within an organ or tissue:
-tissue metabolites
-local hormones
-myogenic properties of muscles
-endothelial factors (NO which causes vasodilation)
-hormonal
Extrinsic- factors outside the organ or tissue:
-neural (sympathetic nervous system)
-hormonal (e.g. adrenaline)
What is distension
Distension- refers to the act of expanding or swelling due to internal pressure.
Explain Bayliss myogenic response
This refers to increased distension of a vessel- causing vasoconstriction and vice versa.
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.
Stretching of the vessel causes ion channels to open (which then depolarise) leading to smooth muscle contraction
What does blood flow depend on
Blood flow depends on:
1) Viscosity of blood
2) Vessel diameter
3) Haematocrit
What is the definition of viscosity
viscosity- a measure of internal friction opposing the separation of the lamina
What is the equation for flow linking blood viscosity
flow= Arterial pressures - Central venous pressure x 3.148 x radius^4/ 8 x blood viscosity x vessel length
What are the clinical implications of haemotocrit
Clinical implications:
-polycythaemia (high viscosity)- increased TPR + BP and decreased flow
-anaemia (low viscosity)- low TPR and BP. High HR
What are the clinical implications of tube diameter
Clinical implications:
-blood viscosity falls in narrow tubes, cells move to centre reducing friction
-decreased resistance, increased flow
What are the clinical implications for reduction in red cell deformability
Clinical applications:
-Increased blood viscosity, reduced flow
What are the clinical applications for the velocity of blood
Clinical applications:
-slow venous flow in immobile legs
-increased viscosity due to partial clotting
Explain characteristics of the veins
Veins:
-thin walled, collapsible, voluminous vessels
-contains 2/3rd of blood volume
-contractile- contains smooth muscle, innervated by sympathetic nerves but thinner arterial muscle and more compliant
Explain the volume of blood in veins and contractility
Contraction of vessels- expels blood into central veins:
-increases venous return
-increases stroke volume
Explain venous pressure at different parts of the body
Venous pressure high at the feet so pressure for blood return to the heart.
This is helped by thoracic pump, skeletal muscle contraction and one way valves
What happens to veins when there is very little pressure
when there is very little pressure, veins just fold up and collapse
How can sympathetic nerves cause an increase of CVP, diastolic pressure and preload
Stimulation Of sympathetic nerves causing vasoconstriction shifts blood centrally.
-increased venous return, CVP and end diastolic pressure
-increased CVP increases preload and so increases stroke volume (Starling’s law)
Explain Bernoulli’s theory
Bernoulli Theory- mechanical energy of flow is determined by pressure, kinetic, potential energies (e.g. fluid mass)
What is the equation for energy in Bernoulli’s law
energy= pressure (P) + kinetic (pV2/2) + potential energies (pgh)
Explain arterial blood flow when standing
Pressure gradient against flow from heart to feet. Ejected blood has greater kinetic energy at the heart compared to the feet.
-greater potential energy at heart than feet due to greater height so greater kinetic/ potential energies overcome pressure gradient to maintain flow
Explain venous return in cardiac output
Cardiac output- the circulation is a closed system- heart pushes blood further through vascular system.
This is via the arterial side of the capillary system into the venules and veins
Explain venous return through breathing
Breathing- 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 pelvis to close.
The blood moves up into the thorax, during exhalation the intra abdominal pressure decreases and the pelvic veins and inferior vena cava refill
Explain venous return through muscle pump
Muscle pump- the deep venous system is embedded within the muscles. Every muscle contraction squeezes the veins to push the column of blood in the direction of the the heart.
When muscle relaxes, venous valves prevent the retrograde of blood towards the capillaries
Explain venous return through venous tone
venous tone- the blood in the veins exerts pressure on the vein’s wall generating tension and maintaining pressure
What are 3 types of drugs related to vasoconstriction/ vasodilation?
1) Nitrates
2) ARBs
3) ACE inhibitors
Explain what nitrates do
Nitrates:
Vasodilators- lower BP
Explain what ARBs do
Angiotensin receptor blockers:
Prevents vasoconstriction caused by ACE
Explain what ACE inhibitors do
Angiotensin converting enzyme inhibitor:
-prevents vasoconstriction caused by ACE
