Haemodynamics Flashcards
What are haemodynamics ?
The physical law governing pressure and flow in blood vessels
What is the equation used to calculate flow
Flow = ΔP/R ΔP = pressure gradient between arteries and veins R = resistance of blood vessels to the flow of blood through it
What are the factors aftecting flow rate
Flow is directly proportional to :
Pressure gradient (ΔP)
(r4) radius
flow is indirectly proportional:
viscosity (η)
length of tube (L)
What is the main determinant of vascular resistance ?
which vessels are described as resistance vessels
since L and η usually remain constant ; resistance is mainly determined by radius of vessels
small arteries and arteriokes have the greatest capacity to change radius and hence are called resistance vessels ; they are the most important vessels in determining resistance ; importnat in dissipating high pressure by the time blood enters capillaries
Equation for total peripheral resistance
(arterial pressure - venous pressure)/cardiac output
venous pressure is usually zero
Equation for renal vascular resistance
(Arterial pressure - venous pressure)/renal blood flow
venous pressure is usually zero
Why is turbulent flow problematic ?
Turbulent flow (mainly due to hypertension) needs higher driving pressure to achieve the same flow = higher resistance
Define arterial and venous compliance
Arterial compliance = provides smoothing of blood flow
venous compliance = provides capacity for storage of blood for when demand increases
the extent of elasticity determines compliance - the change in volume for a given change in pressure
factors affecting venous return
Constriction of veins (assisted by skeletal muscle pump and the pressure gradient provided by respiration )
venous valve competence
What 3 factors affect blood vessel radius ?
Active tension exerted by smooth muslce
passive elastic properties of wall (due to elastin and collagen)
blood pressure inside vessel
What dones vascular tone mean?
what do the terms vasomotor and venomotor refer to?
Degree of constriction or dilatation
vasomotor tone refers to arterioles and arteries
venomotor tome refers to venules and veins
Describe the factors affecting vascular smooth muscle contraction
Hormones :
catecholamines - noradrenaline(dilates when acting on β2 receptors and constricts when acting on α½ receptors )/adrenaline (constrict)
peptides - vasopressin/angiotensin (constrict) and bradykinin (dilate)
intrinsic mechanisms :
endothelium derived vasodilators - PGI2/NO/EDHF
endothelium derived vasoconstrictors - endothelin (pathologically relevant)
metabolites - ↑[lactate] =dilatation / ↑[O2] = constriction of systemic vessels(opposite effect in pulmonary vessels)/↑[CO2] = dilatation of systemic vessels(opposite effect in pulmonary vessels)
autoregulation
How is the velocity of blood flow calculated
Distance/time or flow(ml/min)/cross sectional area (cm2)
Describe auto regulation of vascular smooth muscle
particularly important in cerebral vasculature ; necessary to maintain a constant blood supply to brain
when arterial pressure increases in cerebral arterioles , the smooth muscle in the media responds by constricting = increased resistance = increases arterial pressure is counteracted
opposite happens if arterial pressure drops
Role of extrinsic and intrinsic vascular control mechanisms
Extrinsic Mechanics (hormones/nerves) primarily control the systemic arterial blood pressure (overall driving force to push blood out of heart and around body)
intrinsic mechanisms are more important in ensuring the correct fraction of the cardiac output is delivered to the different organs
Describe the medullary ischaemic reflex
when there is a reduction in cerebral blood flow(e.g. due to tumour), medullary ischamia occurs = increase in BP to restore
Describe the factors influencing blood flow to the heart
Heart receives small proportion of cardiac output
major neural influence from sympathetic stimulation of beta-mediated increases in HR and stroke volume
major influence from metabolism ; hypoxia,hypercapnia (high CO2) and adenosine all cause vasodilation
mechanical influence ; no flow during systole due to compression of coronary vessels ; flow occurs during diastole
describe the factors affecting blood flow to the skin
Highly influenced by temperature
A-V anastomoses are specialised structures found in the skin that allow bypass of capillary beds ; blood flows from arterioles to venules directly ; they have have dense innervation (via alpha vasoconstriction)
major sympathetic role ; blood flow can be directed away from skin and towards vital organs in cold temperatures
Describe the factors affeating blood flow to the skeletal muscle
At rest : neural influence is the most important α vasoconstriction, some β vasodilatation
during exercise : metabolites have the major influence - most importantly adrenaline
special feature - active hyperaemia (increased blood flow)
Describe the factors affecting blood flow to the mesenteric beds and liver
The mesenteric beds are those passing through the digestive system
intestinal : moderate α vasoconstriction from neural influence ; influenced by local peptides ; increased blood flow post meal ; neurohormonal vasoconstriction in aim to deliver more blood to other organs can be problematic in haemorrhagic/septic shock
hepatic: important α venoconstriction, autoregulation of hepatic artery
Describe the factors affecting blood flow to the kidneys
Neural influence ; important α vasoconstriction; some β dilation ; renin secreting cells have β adrenoceptors
autoregulation is highly important in regulating excretory function
constriction caused by noradrenaline/adrenaline/angiotensin
dilatation caused by vasopressin and dopamine
Describe the factors affecting blood flow to the lungs
Minor neural influence
flow is greatly influenced by changes in alveolar pressure and lung volume
hypoxia/hypercapnia cause vasoconstriction - as this slows down blood flow (more time for gas exchange)
NO causes vasodilation
when alveolar pressure>intravascular pressure, flow is reduced bc capillaries are compressed
lung inflation reduces resistance in extra-alveolar vessels (opposite effect in intra-alveolar vessels)
