Control of Blood flow

Question 1

What does TPR control

Answer 1

Blood flow and pressure

Increase resistance = increase pressure to keep same flow

Question 2

What controls TPR?

Answer 2

Darcy and Poiseulle’s law
Myogenic response
Blood viscosity

Question 3

Affects of a decrease TPR in flow and BP

Answer 3

Decrease pressure but greater flow

Question 4

Affects of an increased TPR in flow and BP

Answer 4

Increased pressure upstream but less dlow

Question 5

Hypertension

Answer 5

Over constriction of arterioles.

Higher arterial BP but less capillary flow - under perfusion

Question 6

Change in blood flow during excercise

Answer 6

Superior mesenteric constricted - decreased flow to intestines
Common iliac dilated - increased flow to legs

Question 7

Change in blood flow in sedentary

Answer 7

Superior mesenteric dilated - increased flow to intestines

Common iliac constricted - decreased flow to legs

Question 8

TPR is controlled by 3 main parameters

Answer 8

1) radius
2) pressure difference across vessels
3) length - arterioles are long vessels

Question 9

Why is TPR not controlled by capillaries

Answer 9

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

Question 10

What are the control mechanisms of arteriole radius

Answer 10

INTRINSIC - factors entirely within an organ or tissue

EXTRINSIC - factors outside the organ or tissue

Question 11

Bayliss myogenic response

Answer 11

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

Question 12

Increased distension of vessel

Answer 12

constrict

Question 13

Decreased distension of vessel

Answer 13

dilate

Question 14

Viscosity

Answer 14

Measure of internal friction opposing the separation of the lamina

Question 15

What does blood flow depend on

Answer 15

Viscosity of blood
Vessel diameter
Haematocrit

Question 16

What occurs to blood viscosity in Polycythaemia

Answer 16

abnormal increased conc. of haemoglobin
HIGH BLOOD VISCOSITY
increase TPR, increase BP, decrease BF

Question 17

what occurs to blood viscosity in Anaemia

Answer 17

LOW BLOOD VISCOSITY

decrease TPR, decrease BP, increase HR

Question 18

Effects of tube diameter in blood viscosity

Answer 18

Falls in narrow tubes (<100 micrometer vessels) cells move to centre reducing friction
Decrease resistance, increase BF in micro vessels

Question 19

Effects of red cell deformability in blood viscosity

Answer 19

Increase blood viscosity
Decrease BF
sickle cell anaemia crises

Question 20

Function of veins

Answer 20

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

Question 21

Volume of blood in veins and contractility

Answer 21

Contraction of vessels – Expels blood into central veins
Increases venous return/CVP/ end- diastolic volume
Increases stroke volume (starling’s law)

Question 22

Typical venous pressures

Answer 22

Limb vein, heart level – 5-10mmHg
Central venous pressure (entering heart) – 0-7mmHg
Foot vein, standing – 90mmHg

Question 23

Venous return to heart

Answer 23

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)

Question 24

Bernoulli theory

Answer 24

Mechanical energy of flow is determined by pressure, kinetic, potential energies

Question 25

Returning blood to the heart when standing

Answer 25

-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