Control of Blood flow Flashcards
What does TPR control
Blood flow and pressure
Increase resistance = increase pressure to keep same flow
What controls TPR?
Darcy and Poiseulle’s law
Myogenic response
Blood viscosity
Affects of a decrease TPR in flow and BP
Decrease pressure but greater flow
Affects of an increased TPR in flow and BP
Increased pressure upstream but less dlow
Hypertension
Over constriction of arterioles.
Higher arterial BP but less capillary flow - under perfusion
Change in blood flow during excercise
Superior mesenteric constricted - decreased flow to intestines
Common iliac dilated - increased flow to legs
Change in blood flow in sedentary
Superior mesenteric dilated - increased flow to intestines
Common iliac constricted - decreased flow to legs
TPR is controlled by 3 main parameters
1) radius
2) pressure difference across vessels
3) length - arterioles are long vessels
Why is TPR not controlled by capillaries
No sympathetic innervation/smooth muscle - cannot alter radius
Less pressure drop due to less resistance to blood flow
Caoillaries are short
Less resistance because of bolus flow
Capillaries arranged in parallel to low total resistance
What are the control mechanisms of arteriole radius
INTRINSIC - factors entirely within an organ or tissue
EXTRINSIC - factors outside the organ or tissue
Bayliss myogenic response
Maintains blood flow at the same level during changing arterial pressures
IMPORTANT in renal, coronary, cerebral circulation
Having a linear relationship = entail large difference in blood flow with difference in pressure
At high pressures when vessel is stretched - it contracts to reduce flow
Increased distension of vessel
constrict
Decreased distension of vessel
dilate
Viscosity
Measure of internal friction opposing the separation of the lamina
What does blood flow depend on
Viscosity of blood
Vessel diameter
Haematocrit
What occurs to blood viscosity in Polycythaemia
abnormal increased conc. of haemoglobin
HIGH BLOOD VISCOSITY
increase TPR, increase BP, decrease BF
what occurs to blood viscosity in Anaemia
LOW BLOOD VISCOSITY
decrease TPR, decrease BP, increase HR
Effects of tube diameter in blood viscosity
Falls in narrow tubes (<100 micrometer vessels) cells move to centre reducing friction
Decrease resistance, increase BF in micro vessels
Effects of red cell deformability in blood viscosity
Increase blood viscosity
Decrease BF
sickle cell anaemia crises
Function of veins
Thin walled, collapsible, voluminous vessels
Contain 2/3rd 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 – 5-10mmHg
Central venous pressure (entering heart) – 0-7mmHg
Foot vein, standing – 90mmHg
Venous return to heart
Venous pressure high at the feet, so pressure for return to heart. Also helped by thoracic pump and skeletal muscle contraction
Stimulation of sympathetic nerves causing venoconstriction shifts blood centrally
Increases venous return, CVP and end diastolic pressure
Increased CVP increases preload and so increases stroke volume (starling’s law)
Bernoulli theory
Mechanical energy of flow is determined by pressure, kinetic, potential energies
Returning blood to the heart when standing
-90mmHg pressure gradient against flow back to heart from 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
Returning blood to heart – no pressure gradient but kinetic energy
