Control of Blood Flow Flashcards
What is total peripheral resistance?
TPR controls blood flow and blood pressure
Increase in resistance means need to increase pressure to keep same flow
What controls TPR?
Darcy and Poiseuille’s laws
Myogenic response
Blood viscosity
How does TPR affect flow and blood pressure?
Lower BP:
Decrease in TPR
Decreased blood pressure upstream, but greater flow
Increased BP:
Increase in TPR
Increased blood pressure upstream, but less flow
Hypertension- over constriction of arterioles
Higher arterial BP but less capillary flow- under perfusion
How does blood flow change in response to need?
Brain stem area controlling sympathetic nervous activity to various areas of the body Sedentary: Superior mesenteric dilated Increased flow to intestines Common iliac constricted Decreased flow to legs Exercising: Superior mesenteric constricted Decreased flow to intestines Common iliac dilated Increased flow to legs
What is Poiseulle’s law?
Poiseuille’s law- total peripheral resistance
Poiseuille’s law- describes parameters that govern TPR
Resistance= 8h L/pi r^4
Conductance (G)= pi r4/8h L
How are Darcy’s and Poiseuille’s laws combines?
Flow= Pa- CVP x pr4/8h L
r = radius of vessel
h= blood viscosity
L = vessel length
Blood vessel radius to power of 4 controls TPR
Now we have extended the idea of total peripheral resistance to take into account length, viscosity and radius
Illustrates why radius of the vessel is such an important determinant in changing blood flow
Vessel 2 equals 1/16 of resistance of vessel 1
16 times greater flow in vessel 2 as flow proportional to r4
Vasoconstrictors or dilators produce small changes in vessel radius by affecting smooth muscle have large effects on blood flow
How do arterioles relate to TPR?
Arterioles have the largest pressure drop of 40-50 mmHg amongst vessels
Arteriole radius is tightly controlled by sympathetic nerves providing constant tone dilation vs constriction
What 3 parameters is TPR controlled by?
- Radius (r4)
- Pressure difference across vessels, P1-P2
- Length (L) arterioles are also long vessels
How are capillaries related to TPR?
TPR not controlled by capillaries
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
Individual length of capillaries are short
Less resistance in capillaries because bolus flow reduces viscosity
No sympathetic innervation/smooth muscle in capillaries so cannot alter radius
Capillaries are arranged in parallel, so have low total resistance as RTotal = 1/R1 + 1/R2 etc.
In contrast, arterioles are in series
RTotal = 1/R1 + 1/R2 etc. so total resistance is greater
How is blood flow controlled locally?
Local blood flow through individual organs/tissues is mainly controlled by changes in radius of arterioles supplying a given organ/tissue
Control mechanism of arteriole radius:
Intrinsic- factors entirely within an organ or tissue
Local hormones
Tissue metabolites
Myogenic properties of muscle
Endothelial factors e.g. NO
Extrinsic- factors outside the organ and tissue
Neural e.g. sympathetic nervous system
Hormonal e.g. adrenaline
What is 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 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 depolarise, leading to smooth muscle contraction
How does viscosity relate to blood flow?
Blood flow depends on: Viscosity of blood Vessel diameter Haematocrit Viscosity is a measure of internal friction opposing the separation of the lamina Flow= Pa - CVP x pr4/8h L r = radius of vessel h= blood viscosity L = vessel length Blood vessel radius to power of 4 controls TPR
What is venous blood pressure like?
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 and more compliant- so form blood reservoir
Volume of blood in veins and contractility
Contraction of vessels- expels blood into central veins
Increases venous return/CVP/end-diastolic volume
Increases stroke volume (Starling’s law)
Typical venous pressures:
Limb vein, heart level - 5-10 mmHg
Central venous pressure (entering heart)- 0-7 mmHg
Foot vein, standing- 90 mmHg
How does venous return work?
Venous pressure high at the feet so pressure for return to heart
Also helped by the thoracic pump, skeletal muscle contraction and one-way valves
Stimulation of sympathetic nerves causing vanoconstriction shifts blood centrally
Increases venous return, CVP and end-diastolic pressure
Increased CVP increases preload and so increases stroke volume (Starling’s law)
How does Bernoulli’s law explain arterial blood flow?
Bernoulli theory- mechanical energy of flow is determines by pressure, kinetic, potential energies (r= fluid mass
Energy= Pressure (P)+ kinetic (rV2/2)+ potential (rgh)
Standing
Pressure gradient against flow from the heart to 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
How does venous return work around the body?
Cardiac output - 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.
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 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.
Muscle pump - 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.
Venous tone- the blood in the veins exerts pressure on the vein’s wall generating tension and maintaining pressure. Furthermore, sympathetic vasoconstriction can mobilise more blood back to the heart.
