Control of resistance of artery tone L2 & L3

Question 1

What are ‘resistance vessels’?

Answer 1

• arteries and arterioles that contribute most to the TPR

Question 2

What happens if widespread artery constriction occurs?

Answer 2

• widespread artery constriction raises TPR, upstream pressure rises and downstream pressure falls

Question 3

What is the tone of a blood vessel?

Answer 3

• the tone of a blood vessel relates to the degree of constriction relative to its maximally dilated state
• all vessels have some amount of smooth muscle contraction at rest (basal conditions) that determine the diameter & hence tone of a vessel

Question 4

What happens if the tone of a blood vessel is increased?

Answer 4

• smooth muscle contracts further - vasoconstriction occurs and the vascular resistance increases

Question 5

What happens if the tone of a vessel is decreased?

Answer 5

• less contraction of smooth muscle cells
• vasodilation occurs
• vascular resistance decreases - bigger diameter

Question 6

What factors influence the tone of resistance vessels?

Answer 6

• resistance artery tone is modulated by vasoconstrictor and vasodilator factors

1. Extrinsic factors - controlled by the ANS & circulating hormones (humoral stimuli)
2. intrinsic factors (LOCAL CONTROL) - brought by the response of smooth muscle to stretch, temperature & locally released chemical factors

Question 7

What vessel size are mainly under extrinsic control?

Answer 7

• large arteries (except the aorta) and larger veins in comparison to smaller arterioles and venules are under both extrinsic and local control

Question 8

What vessel’s diameter is not modulated by extrinsic/intrinsic factors?

Answer 8

• capillaries
• as they do not have any smooth muscle

Question 9

What is autoregulation?

Answer 9

• intrinsic ability of an organ to maintain constant blood flow despite changes in pressure
• NB it occurs independently of the nervous system
• different organs have different degrees of autoregulation

Question 10

Give an example of autoregulation

Answer 10

• eg let’s say the blood pressure falls
• this will cause the walls of arterioles and arteries to constrict which will bring the pressure back to normal

Question 11

What is the autoregulation mechanism?

Answer 11

• the myogenic response (myo=muscle)

Question 12

What is the myogenic response?

Answer 12

• it is based on the ability of the vascular smooth muscle (small arteries and arterioles) to constrict and narrow the diameter or dilate and widen the diameter in response to changes in blood pressure

Question 13

What does the myogenic response to stretching depend on?

Answer 13

• the activation of specific mechanoreceptor channels that increase the cells permeability to Na+ and K+ - causing depolarisation & Ca2+ entry which leads to smooth muscle contraction

Question 14

What are examples of metabolic by products that cause vasodilation?

Answer 14

• Co2,
• K+
• Adenosine

Question 15

What is hyperaemia?

Answer 15

• an increased blood flow - relating to metabolites

Question 16

What is metabolic / active / functional hyperaemia?

Answer 16

• the increase of organ blood flow that is associated with the increase of metabolic activity of an organ or tissue
• eg the increase of blood flow associated with skeletal muscle contraction (functional) - flow increases because of the increase in oxygen consumption stimulates the production of vasodilating substances

Question 17

what is ischemia?

Answer 17

• a shortened blood supply to a tissue/ organ that causes a shortage in oxygen consumption

Question 18

What is reactive hyperaemia?

Answer 18

• the increase in blood flow after a period of ischemia due to a possible arterial occlusion (blockage of flow through artery)

Question 19

What are autocoids?

Answer 19

• vasoactive chemicals that are produced locally, released locally and that act locally
• ‘local hormones’ - THEY DONT CIRCULATE

Question 20

What are examples of autocoids?

Answer 20

• prostaglandins
• platelet-activating factor

Question 21

What vasodilator substances does the vascular endothelium produce?

Answer 21

• EDRF - the endothelium derived relaxing factor, now known as nitric oxide NO
• also produces prostacyclin - another EDRF
• also the EDHF

Question 22

What is an example of a vasoconstrictor substance formed in the endothelium?

Answer 22

• Angiotensin converting enzyme (ACE) which is located in the endothelial cell walls produces angiotensin II
• constricts arteries and arterioles

Question 23

What are examples of stimuli that cause the endothelial cells to produce NO?

Answer 23

• Ach
• shear stress (force per unit area) exerted on the endothelium by the flowing blood

Question 24

What is EDHF?

Answer 24

• EDHF - endothelial derived hyperpolarising factor
• it is a substance or electrical signal that is synthesized and released by the endothelium

Question 25

What is the function of EDHF?

Answer 25

• functions to hyperpolarise the vascular smooth muscle cells - in order to cause the vessel diameter to increase & relax

Question 26

How do the smooth muscle cells and endothelial cells operate as 1 functional unit?

Answer 26

• the cells are electrically coupled by gap junctions

Question 27

What is the ADRF?

Answer 27

• adipocyte derived relaxing factor
• this ‘factor’ (unknown substance) is released from adipose tissue and causes a vasodilator response
• activates K+ channels in vascular smooth muscle cells

Question 28

How are smooth muscle cells innervated by the ANS? brief

Answer 28

• outermost layer of smooth muscle cells in the media layer are innervated
• but remember smooth muscle cells are connected to e/o via gap junctions so they don’t all have to be innervated

Question 29

What are the three types of ANS fibres that innervate vascular smooth muscle?

Answer 29

1. sympathetic vasoconstrictor fibres
2. sympathetic vasodilator fibres
3. parasympathetic vasodilator fibres

Question 30

What main effect do sympathetic adrenergic nerve fibers have?

Answer 30

• they have mostly a vasoconstrictor effect, as NA has a greater affinity for alpha adrenoceptors
• they are tonically active - meaning they fire AP’s all the time

Question 31

What neurotransmitter do the sympathetic adrenergic nerves release?

Answer 31

NA - noradrenaline

this NT can bind to 2 types of receptors

Question 32

What 2 types of receptors does NA (noradrenaline) bind to in blood vessels? what happens when NA activates each type of receptor?

Answer 32

• 2 types of receptors - alpha and beta adrenoceptors
• NA causes vasoconstriction when it binds to alpha adrenoreceptors
• NA causes vasodilation when it binds to beta adrenoceptors

Question 33

What neurotransmitter do the parasympathetic nerves that innervate vascular smooth muscle release?

Answer 33

Ach

Question 34

How does Ach released by the parasympathetic NS cause vasodilation in vascular smooth muscles?

Answer 34

• Ach released binds to the muscarinic receptors of the vascular smooth muscle
• this causes membrane hyperpolarisation and relaxation

Question 35

What part of the brain does sympathetic tone originate from?

Answer 35

the medulla oblongata

• influenced by sensory mechanisms involving baroreceptors and chemoreceptors

Question 36

What hormones cause vasoconstriction?

Answer 36

• noradrenaline
• adrenaline
• angiotensin
• ADH

Question 37

What hormones cause vasodilation?

Answer 37

• adrenaline
• insulin
• oestrogen

Question 38

What is pharmacomechanical coupling?

Answer 38

• process that involves an agonist (causes physiological response) binding to a receptor (eg NA and alpha 1 adrenoceptor) that is coupled to a mechanical response eg contraction
• depolarization independent - dosent rely on membrane potential change

Question 39

what is electromechanical coupling?

Answer 39

• the coupling of an electrical affect with a mechanical response ie contraction
• electrically dependent
• involves TRP channels & Na+ and Ca2+ entering cell causing depolarisation and contraction