Regulation of resistance, pressure and flow in arteries

Question 1

What is Darcy’s law?

Answer 1

Flow = delta P / R

Question 2

What does Poiseuille’s law tell us?

Answer 2

That varying radius is a powerful way of controlling resistance. Varying radius of arterioles is used to control flow through individual vascular beds.

Question 3

What do we get when Poiseuille’s law is applied to systemic circulation?

Answer 3

MAP - CVP (0) = CO x TPR so…

MAP = CO x TPR

CO is 5L/min, CVP is 0 and MAP is 90 mmHg.

Question 4

Which 2 mechanisms keep MAP in the correct range?

Answer 4

Control of smooth muscle surrounding arterioles.

1) Intrinsic mechanisms: meeting needs of each individual tissue.
2) Extrinsic mechanisms: ensures that TPR of whole body stays in correct range.

Question 5

What happens when resistance is reduced through a vascular bed?

Answer 5

Increases flow through that vascular bed. However, reducing TPR also reduces MAP which can have serious consequences on eg. brain.

Question 6

What are the 4 mechanisms of intrinsic (local) control?

Answer 6

1) Active (metabolic) hyperaemia.
2) Pressure (flow) autoregulation.
3) Reactive hyperaemia.
4) Injury response.

Question 7

Describe active (metabolic) hyperaemia.

Answer 7

High metabolic activity causes high concentration of metabolites eg. more CO2, K etc. These high concentrations are sensed by capillary endothelium and EDRF (smooth muscle relaxation) causes arterioles to dilate. High blood flow can then wash out metabolites so that they go back to normal concentration. This is an adaptation to match blood supply to the needs of that tissue.

Question 8

Describe pressure (flow) autoregulation.

Answer 8

Same mechanism as active (metabolic) hyperaemia but caused by different factors. Low MAP causes reduced blod. Metabolites accumulate and this triggers EDRF. Arterioles dilate and blood flow returns to normal. An adaptation to ensure tissue still receives blood supply despite changes in MAP.

Question 9

Describe reactive hyperaemia.

Answer 9

Occlusion (blockage) of a vessel causes a subsequent increase in blood supply - an extreme version of pressure autoregulation.

Question 10

Describe the injury response.

Answer 10

C-fibres detect injury (small, unmyelinated neurones). Mast cells degranulate and release histamine which causes arteriolar dilation. Higher blood flow and higher permeability to allow blood born leukocytes etc to get to the site of injury.

Question 11

What are the 2 mechanisms of extrinsic control?

Answer 11

Neural and hormonal.

Question 12

Describe extrinsic neural control.

Answer 12

Sympathetic: nerves release NA which binds to A1 receptors and causes smooth muscle constriction. Therefore, less flow to tissues and this tends to increase TPR. Parasympathetic: usually no effect as they don’t innervate smooth muscle around blood vessels (exceptions are salivary glands and genetalia).

Question 13

What are the 4 mechanisms of extrinsic hormonal control?

Answer 13

Adrenaline, angiotensin 2, vasopressin (ADH), atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP).

Question 14

Describe extrinsic hormonal control.

Answer 14

1) Adrenaline: released from adrenal medulla and binds to A1 receptors; causes arteriolar constriction and increases TPR; however in some tissues also binds to B2 and causes arteriolar dilation; more flow through that tissue and decreased TPR; important during exercise as blood is redirected to parts of body that need it most.
2) Angiotensin 2: produced in response to low blood volume, causes arteriolar constriction and increases TPR.
3) Vasopressin (ADH): produced in response to low blood volume, causes arteriolar constriction and increases TPR.
4) ANP and BNP: produced in response to high blood volume, cause arteriolar dilation and decrease TPR.

Question 15

Where are the 4 special areas?

Answer 15

Coronary, cerebral, pulmonary and renal.

Question 16

Describe the 4 special areas.

Answer 16

1) Coronary: every time heart contracts (systole) it cuts off its own blood supply; during exercise shows excellent active (metabolic) hyperaemia; B2 receptors swamp any sympathetic arteriolar constriction.
2) Cerebral: pressure autoregulation; needs to be perfused no matter what.
3) Pulmonary: low O2 causes arteriolar constriction (opposite to other tissues) which ensures blood is directed to better ventilated parts of the lung.
4) Renal: pressure autoregulation as its main function is filtration which depends on pressure; changes on MAP would have massive changes on blood volume.

Question 17

Which sounds can be auscultated?

Answer 17

Korotkoff using a sphygmomanometer and stethescope. Listening for turbulence in the blood.

Question 18

What should be heard when measuring BP?

Answer 18

Silence, tapping (systolic pressure), thumping, muffled, silence (diastolic).

Question 19

What are the advantages of using a sphyg to measure BP?

Answer 19

Cheap, non-invasive and no sterile technique needed.

Question 20

Why does arterial pressure increase with age?

Answer 20

Loss of elasticity in the aorta.

Question 21

Which pressure does TPR mostly effect?

Answer 21

Diastolic. During exercise, systolic pressure > as heart is contracting more strongly. However, diastolic pressure falls as arteries in skeletal muscle are dilated.

Question 22

Describe pressure and flow in arteries.

Answer 22

Pressure falls throughout the vascular tree. 95 to 90 mmHg through arteries. 90 to 40 mmHg through arterioles - large drop in pressure as they control where blood goes. Pressure very low in capillaries.

Question 23

Describe pressure and flow from arteries to veins.

Answer 23

There is a small pressure difference pushing blood back through to veins - 5 to 20 mmHg. This is systemic filling pressure which pushes blood back to the heart to fill it up again. Pulmonary circulation pressure is 1/5 of systemic.

Question 24

Are flow and velocity the same in all vessels?

Answer 24

Flow is but velocity (speed) is not.

Question 25

Describe pressure and flow in veins.

Answer 25

Pressure is low, therefore the pressure driving blood back to the heart is low. Veins are distensible and collapsible - external influences affect flow.

Question 26

What 5 external influences affect flow in veins?

Answer 26

Gravity, skeletal muscle pump, respiratory pump, venomotor tone and systemic filling pressure.

Question 27

How does gravity affect flow in veins?

Answer 27

Can cause postural hypotension - venous distension in legs ( SVC –> RA. More pressure is added from the feet to get blood back to the heart. When we stand, 1/2 L blood accumulates in legs - drop in MAP makes you faint so that blood comes back to heart. Upside down? Opposite effect.

Question 28

What effect do skeletal muscle and respiratory pumps have on veins?

Answer 28

Skeletal muscle: muscle contraction squeezes blood from veins back to heart; blood can’t go backward due to valves. Only during rhythmic, not static, exercise.

Respiratory: breathing faster/deeper draws more blood back to heart and > EDV.

Question 29

What effect does venomotor tone and systemic filling pressure have on veins?

Answer 29

Venomotor tone: state of contraction of smooth muscle surrounding venules and veins. Under sympathetic control and mobilises capacitance.

Systemic filling pressure: pressure created by ventricles and transmitted through vascular tree to veins. Main thing pushing blood from capillaries back to heart.