Control of resistance of artery tone L2 & L3 Flashcards
What are ‘resistance vessels’?
- arteries and arterioles that contribute most to the TPR
What happens if widespread artery constriction occurs?
- widespread artery constriction raises TPR, upstream pressure rises and downstream pressure falls
What is the tone of a blood vessel?
- 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
What happens if the tone of a blood vessel is increased?
- smooth muscle contracts further - vasoconstriction occurs and the vascular resistance increases
What happens if the tone of a vessel is decreased?
- less contraction of smooth muscle cells
- vasodilation occurs
- vascular resistance decreases - bigger diameter
What factors influence the tone of resistance vessels?
- resistance artery tone is modulated by vasoconstrictor and vasodilator factors
- Extrinsic factors - controlled by the ANS & circulating hormones (humoral stimuli)
- intrinsic factors (LOCAL CONTROL) - brought by the response of smooth muscle to stretch, temperature & locally released chemical factors
What vessel size are mainly under extrinsic control?
- large arteries (except the aorta) and larger veins in comparison to smaller arterioles and venules are under both extrinsic and local control
What vessel’s diameter is not modulated by extrinsic/intrinsic factors?
- capillaries
- as they do not have any smooth muscle
What is autoregulation?
- 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
Give an example of autoregulation
- 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
What is the autoregulation mechanism?
- the myogenic response (myo=muscle)
What is the myogenic response?
- 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
What does the myogenic response to stretching depend on?
- 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
What are examples of metabolic by products that cause vasodilation?
- Co2,
- K+
- Adenosine
What is hyperaemia?
- an increased blood flow - relating to metabolites
What is metabolic / active / functional hyperaemia?
- 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
what is ischemia?
- a shortened blood supply to a tissue/ organ that causes a shortage in oxygen consumption
What is reactive hyperaemia?
- the increase in blood flow after a period of ischemia due to a possible arterial occlusion (blockage of flow through artery)
What are autocoids?
- vasoactive chemicals that are produced locally, released locally and that act locally
- ‘local hormones’ - THEY DONT CIRCULATE
What are examples of autocoids?
- prostaglandins
- platelet-activating factor
What vasodilator substances does the vascular endothelium produce?
- EDRF - the endothelium derived relaxing factor, now known as nitric oxide NO
- also produces prostacyclin - another EDRF
- also the EDHF
What is an example of a vasoconstrictor substance formed in the endothelium?
- Angiotensin converting enzyme (ACE) which is located in the endothelial cell walls produces angiotensin II
- constricts arteries and arterioles
What are examples of stimuli that cause the endothelial cells to produce NO?
- Ach
- shear stress (force per unit area) exerted on the endothelium by the flowing blood
What is EDHF?
- EDHF - endothelial derived hyperpolarising factor
- it is a substance or electrical signal that is synthesized and released by the endothelium
What is the function of EDHF?
- functions to hyperpolarise the vascular smooth muscle cells - in order to cause the vessel diameter to increase & relax
How do the smooth muscle cells and endothelial cells operate as 1 functional unit?
- the cells are electrically coupled by gap junctions
What is the ADRF?
- 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
How are smooth muscle cells innervated by the ANS? brief
- 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
What are the three types of ANS fibres that innervate vascular smooth muscle?
- sympathetic vasoconstrictor fibres
- sympathetic vasodilator fibres
- parasympathetic vasodilator fibres
What main effect do sympathetic adrenergic nerve fibers have?
- 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
What neurotransmitter do the sympathetic adrenergic nerves release?
NA - noradrenaline
this NT can bind to 2 types of receptors
What 2 types of receptors does NA (noradrenaline) bind to in blood vessels? what happens when NA activates each type of receptor?
- 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
What neurotransmitter do the parasympathetic nerves that innervate vascular smooth muscle release?
Ach
How does Ach released by the parasympathetic NS cause vasodilation in vascular smooth muscles?
- Ach released binds to the muscarinic receptors of the vascular smooth muscle
- this causes membrane hyperpolarisation and relaxation
What part of the brain does sympathetic tone originate from?
the medulla oblongata
- influenced by sensory mechanisms involving baroreceptors and chemoreceptors
What hormones cause vasoconstriction?
- noradrenaline
- adrenaline
- angiotensin
- ADH
What hormones cause vasodilation?
- adrenaline
- insulin
- oestrogen
What is pharmacomechanical coupling?
- 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
what is electromechanical coupling?
- 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
