Regulation of Arteriolar Resistance

Question 1

What does Poiseuille’s law state?

Answer 1

When a fluid is pumped through a closed system, its flow is determined by the pressure developed by the pump and the resistance to that flow

Question 2

What is resistance to flow caused by?

Answer 2

Frictional forces within the fluid

Question 3

What does resistance to flow depend on?

Answer 3

Viscosity of the fluid and the dimensions of the tube

Question 4

What is the equation relating flow, pressure and resistance?

Answer 4

Flow = change in pressure/resistance

Pressure = flow x resistance

Question 5

What does varying the radius of the resistance vessels control?

Answer 5

Controls TPR and regulates MAP

Question 6

What is the varying radius of the arterioles used to control?

Answer 6

Flow through the individual vascular beds

Also affects change in pressure

Question 7

You can’t affect the flow through the vascular beds without affecting what?

Answer 7

MAP

Question 8

In order to keep blood flow to each vascular bed sufficient and to keep MAP in the right range, what is needed?

Answer 8

Some resistance

Question 9

How is resistance in vessels achieved?

Answer 9

Two levels of control over the smooth muscle surrounding the arterioles;
Intrinsic mechanisms, concerned with meeting the needs of each individual tissue
Extrinsic mechanisms, concerned with ensuring that the TPR of the whole body stays in the right range

Question 10

How are extrinsic mechanisms causing resistance controlled neurally?

Answer 10

Neural control - sympathetic nerves release norepinephrine which binds to alpha-1 receptors and causes arterioles constriction, which decreases the flow through that tissue and increases TPR
Parasympathetic nerves usually have no effect

Question 11

What is hyperaemia?

Answer 11

Increase in organ blood flow associated with increased metabolic activity of an organ or tissue

Question 12

What is pressure autoregulation?

Answer 12

Maintenance of constant blood flow despite changes in MAP

Question 13

What is reactive hyperaemia?

Answer 13

Large transient increase in blood flow that occurs following a brief period of ischaemia/release of occlusion (extreme version of autoregulation)

Question 14

What is the basic sequence of events of the injury response?

Answer 14

Mast cells release histamine in response to an injury
This causes arteriolar dilation, increased blood flow and increased permeability
Aids the delivery of blood-borne leukocytes etc. to the injured area

Question 15

What hormones affect the extrinsic control of resistance?

Answer 15

Epinephrine/adrenaline
Angiotensin II
Vasopressin
Atrial natriuretic peptide and brain natriuretic peptide

Question 16

How does epinephrine affect resistance?

Answer 16

Released from adrenal medulla
Binds to alpha-1 receptors
Causes arteriolar constriction so tends to decrease flow through that tissue and increase TPR

In some tissues e.g. cardiac muscle it also activates beta-2 receptors to cause arteriolar dilation, increase flow and increase TPR

Question 17

How does angiotensin II affect resistance?

Answer 17

Produced in response to low blood volume
Causes arteriolar constriction
Causes increased TPR

Question 18

How does vasopressin affect resistance?

Answer 18

Released in response to low blood volume

Causes arteriolar constriction and increased TPR

Question 19

How do ANP and BNP affect resistance?

Answer 19

Released in response to high blood volume

Causes arteriolar dilation and decreased TPR

Question 20

What are the intrinsic control mechanisms that affect resistance?

Answer 20

Active hyperaemia
Pressure autoregulation
Reactive hyperaemia
Injury response

Question 21

How does active hyperaemia affect resistance?

Answer 21

Increased metabolic activity causes increased concentration of metabolites
Triggers release of EDRF
Causes arteriolar dilation
Increased flow to wash out metabolites
Adaptation made to match blood supply to the metabolic needs of that tissue

Question 22

How does pressure autoregulation affect resistance?

Answer 22

Decreased MAP causes decreased flow
Metabolites accumulate
Triggers release of EDRF
Arterioles dilate and flow restored to normal
Adaptation made to ensure that a tissue maintains blood supply despite changes in MAP

Question 23

What are the main features of the coronary circulation?

Answer 23

Blood supply interrupted by systole
Has to cope with increased demand during exercise
Shows excellent active hyperaemia
Expresses beta-2 receptors

Question 24

What are the main features of the cerebral circulation?

Answer 24

Needs to be kept stable at all times

Shows excellent pressure autoregulation

Question 25

What are the main features of the pulmonary circulation?

Answer 25

Decreased oxygen causes arteriolar constriction
Opposite response to that of most tissue
This ensures that blood is directed to the best ventilated parts of the lungs

Question 26

What are the main features of the renal circulation?

Answer 26

Main function is filtration which depends on pressure
Changes in MAP have big effects on blood volume
Shows excellent pressure autoregulation