Regulating Arteriolar Resistance

Question 1

What is Darcy’s Law?

Answer 1

Flow = pressure difference / Resistance

Question 2

What factors influence resistance and how do they effect flow?

Answer 2

Viscosity & Vessel length increase resistance, therfore decrease flow
Vessel radius exponentially decreases resistance so increases flow

Question 3

Which resistance influencing factor does the body use to regulate flow?

Answer 3

The vessel radius because its easily altered

Question 4

If we apply darcy’s law to systemic circulation what do we get?

Answer 4

CO = MAP/TPR or MAP = CO x TPR

Question 5

The radius of arterioles is altered to direct blood to certain capillaries beds when needed, why should one constrict for every one that relaxes?

Answer 5

Because opening lots of channels will increase vascular volume thus decreasing MAP which will cause overall blood flow to drop and some tissues will become hypoxic

Question 6

Whats the differnce between intrinsic and extrinsic mechanisms?

Answer 6

Intrinsic are to do with meeting the selfish needs of a single tissue
Extrinsic mechanisms ensure the whole body TPR and thus MAP remains normal

Question 7

What are the types of extrinsic mechanisms?

Answer 7

Neural & Hormonal

Question 8

How do neural extrinsic mechanisms work?

Answer 8

Sympathetic Fibres release norepinephrine

• > Binds to alpha1-receptors in arterioles
• > Arterioles constrict
• > Flow through tissues decrease and TPR rises

Question 9

How do hormonal extrinsic mechanisms work?

Answer 9

The adrenal medulla releases epinephrine

• > Binds to alpha1-receptors-
• > arterioles constrict
• > Flow decreases and TPR rises

Question 10

What TPR reducing effect does epinephrine have?

Answer 10

In some tissues like skeletal & cardiac muscle epinephrine activates beta2-receptors causing arteriolar dilation which reduces TPR

This is part of the redirecting of blood to important tissues during fight or flight

Question 11

What other hormones are involved in long term control of MAP

Answer 11

Angiotensin II
Vasopressin (ADH)
Atrial & brain Natriuretic Peptide (prodiuretic)

Mainly involved in renal fluid absorption/excretion

Question 12

What are the types of intrinsic (local) mechanisms?

Answer 12

Active (metabolic) hyperaemia
Pressure (flow) autoregulation
Reactive Hyperaemia
Injury Response

Question 13

How does active or metabolic hyperaemia work?

Answer 13

Increased metabolic activity of a tissue means a higher conc. of metabolites passes into its blood supply.
In response the endothelium produces EDFR (endothelium derived relaxing factor)
EDFR causes arteriolar dilation so that flow increases and the metabolites are washed out

Question 14

What type of homeostatic mechanism is metabolic hyperaemia/

Answer 14

A negative feedback mechanism

Question 15

What is pressure or flow autoregulation?

Answer 15

When MAP suddenly decreases flow will decrease and metabolites accumulate in vessels.
Again EDFR is produced -> Arteriolar dilation -> normal flow restored

The same response as metabolic hyperaemia but designed to maintain blood flow when MAP changes instead of altering blood flow to match metabolic activity

Question 16

What is the method of reactive hyperaemia?

Answer 16

Extreme pressure autoregulation
An occlusion of the vessels in a tissue means theres no pressure driving the flow. Therefore theres a massive metabolite build up -> Loads of EDFR -> Extreme dilation to try and shift the occlusion.
When the occlusion is removed (by whatever method) the vessels are very dilated so theres a huge abnormal flow through that tissue.

Question 17

What is an injury response?

Answer 17

C fibres release substance P
Acts on mast cells stimulating Histamine release
triggers arteriolar dilation which increases flow and vessel permeability to that area.
Essentially the start of an inflammatory response.

Question 18

Some areas of circulation regulate arteriolar differently, which areas are these?

Answer 18

Coronary Circ.
Cerebral Circ.
Pulmonary Circ.
Renal Circ.

Question 19

The coronary circulation is differnet in that it recieves most of its blood supply during dyastole rather than systole, Why?

Answer 19

During heart contraction (systole) the heart muscle squashes the coronary arteries
Also during systole the aortic valve occludes the aortic sinuses so blood cant pass into the coronary arteries

Question 20

How does the coronary circulation cope with increased demand of exercise?

Answer 20

It shows excellant active hyperaemia
It also expresses a lot of B2 receptors so when epinephrine is released the arterioles dilate

These 2 mechanisms swamp any sympathetic arteriolar constriction caused by alpha-1 receptors, specifically in the heart.

Question 21

Why is the cerebral circulation different and how does it do it?

Answer 21

It has to remain stable no matter so it shows amazing presssure autoregulation in order to stay constant no matter what happens to the rest of the body.

Question 22

the main function of the renal circ. is filtration, what does filtratin rate depend on?

Answer 22

Pressure so is heavily effected by MAP

Question 23

How does the renal circ prevent you excreting all your water every time your BP increases?

Answer 23

It shows excellant pressure autoregulation so the renal circ. pressure can be a bit independant of the MAP.

Question 24

Summarize the extrinsic effect?

Answer 24

Mainly neural/hormonal
Controls arteriolar radius in order to regulate whole body TPR
Exists to maintain an adequate MAP

Question 25

Summarise the intrinsic effects?

Answer 25

Concerned with matching local blood flow to the needs of a tissue.
Metabolic hyperaemia, pressure autoregulation, reactive hyperaemia, injury response.