Haemodynamics

Question 1

What are haemodynamics ?

Answer 1

The physical law governing pressure and flow in blood vessels

Question 2

What is the equation used to calculate flow

Answer 2

Flow = ΔP/R ΔP = pressure gradient between arteries and veins R = resistance of blood vessels to the flow of blood through it

Question 3

What are the factors aftecting flow rate

Answer 3

Flow is directly proportional to :

Pressure gradient (ΔP)

(r⁴) radius

flow is indirectly proportional:

viscosity (η)

length of tube (L)

Question 4

What is the main determinant of vascular resistance ?

which vessels are described as resistance vessels

Answer 4

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

Question 5

Equation for total peripheral resistance

Answer 5

(arterial pressure - venous pressure)/cardiac output

venous pressure is usually zero

Question 6

Equation for renal vascular resistance

Answer 6

(Arterial pressure - venous pressure)/renal blood flow

venous pressure is usually zero

Question 7

Why is turbulent flow problematic ?

Answer 7

Turbulent flow (mainly due to hypertension) needs higher driving pressure to achieve the same flow = higher resistance

Question 8

Define arterial and venous compliance

Answer 8

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

Question 9

factors affecting venous return

Answer 9

Constriction of veins (assisted by skeletal muscle pump and the pressure gradient provided by respiration )

venous valve competence

Question 10

What 3 factors affect blood vessel radius ?

Answer 10

Active tension exerted by smooth muslce

passive elastic properties of wall (due to elastin and collagen)

blood pressure inside vessel

Question 11

What dones vascular tone mean?

what do the terms vasomotor and venomotor refer to?

Answer 11

Degree of constriction or dilatation

vasomotor tone refers to arterioles and arteries

venomotor tome refers to venules and veins

Question 12

Describe the factors affecting vascular smooth muscle contraction

Answer 12

Hormones :

catecholamines - noradrenaline(dilates when acting on β₂ receptors and constricts when acting on α_½ 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

Question 13

How is the velocity of blood flow calculated

Answer 13

Distance/time or flow(ml/min)/cross sectional area (cm²)

Question 14

Describe auto regulation of vascular smooth muscle

Answer 14

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

Question 15

Role of extrinsic and intrinsic vascular control mechanisms

Answer 15

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

Question 16

Describe the medullary ischaemic reflex

Answer 16

when there is a reduction in cerebral blood flow(e.g. due to tumour), medullary ischamia occurs = increase in BP to restore

Question 17

Describe the factors influencing blood flow to the heart

Answer 17

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

Question 18

describe the factors affecting blood flow to the skin

Answer 18

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

Question 19

Describe the factors affeating blood flow to the skeletal muscle

Answer 19

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)

Question 20

Describe the factors affecting blood flow to the mesenteric beds and liver

Answer 20

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

Question 21

Describe the factors affecting blood flow to the kidneys

Answer 21

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

Question 22

Describe the factors affecting blood flow to the lungs

Answer 22

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)