Circulation Flashcards
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Define vascular tone
describes degree of constriction of a blood vessel relative to max dilation
What’s vascular tone controlled by?
contractile state of vascular smooth muscle cells (VSMCs)
Tonic sympathetic activity (constriction) + Tonic NO release (dilation)
Which vessels have vascular tone + why?
arteries, arterioles, veins
containing VSMCs
Why regulate vascular tone?
vital target in treating CVD
How do the substances in blood control vascular tone?
Hormones - Adr, Ang II, ADH, ANP
Platelets –TXA2
Immune cells - Histamine
Blood causes stretch
How does the endothelium control vascular tone?
PRODUCES:
Dilators = NO, K+, PGI2
Constrictors = ET-1
How do VSMCs control vascular tone?
responds to:
substances in blood
substances released from endothelium
How do nerves control vascular tone?
Sympathetic vasoconstrictor nerves: NA
Parasympathetic vasodilator nerves: NO, Ach
Sympathetic vasodilator nerves: Ach, VIP
Perivascular sensory nerves nerves: Sub P, CGRP
Where do vasoconstrictor nerves interact at?
interact directly at VSMCs
What do vasoconstrictor nerves interact with?
indirectly
- interact with endothelium
- endothelium interacts with VSMC
How do metabolic factors control vascular tone?
Adenosine, K+, H+
Why have intrinsic or local controls?
Regulate local blood flow to organs/tissues
Vital in regional hyperaemia
eg of intrinsic or local controls?
endothelium, immune cells, platelets, stretch
Why have extrinsic controls?
Regulate TPR to control BP for blood flow
Brain alters blood flow to organs during exercise + thermoregulation
What are extrinsic or external controls + eg?
Nerves: Vasoconstrictors - NA Vasodilators - Ach, NO Hormones: Vasoconstrictor - A, Ang II, ADH Vasodilators - ANP
What does stimulated α1 adrenoceptors do?
vasoconstriction
What does stimulated β2 do?
vasodilation in coronary + skeletal muscle arterioles
Describe how the brainstem control vascular tone?
- sense stress
- feeds into (RVLM) which receives info from CVLM, hypothalamus
- Thoraric spinal cord Intermediolateral (IML)
- short sympathetic preganglionic fibre to sympathetic ganglia/adrenal medulla
- long sympathetic postganglionic fibre
What’s on post-synaptic membrane?
α1 – contraction, α2 – contraction, β2 - relaxation
What’s on pre-synaptic membrane?
AT1 – increase release of NA (RAAS ↑ sympathetic)
α2 – ↓ NA release
K+, adenosine – ↓ NA release for vasodilation
Describe what happens at post-synaptic terminal
- AP down postganglionic fibres
- activate vgccc
- Ca2+ influx
- vesicle release of NA
- NA hit postsynaptic membrane on VSMCs at α1, α2, β2
- α1 causes vasoconstriction
- decrease NA by : reuptake, cleared by capillaries, feedback to α2 on pre-synaptic terminal, adenosine + K+
What’s Rostral Ventral Lateral Medulla (RVLM vasomotor centre) controlled by?
Caudal Ventro Lateral Medulla (CVLM), hypothalamus
Why’s it vital that sympathetics controlled by brainstem + RVLM?
Provides central control of blood flow + BP
Define tonic sympathetic nerve activity
sets vascular tone by firing 1 AP/s
What does fall in sympathetic activity cause?
vasodilation
Roles of sympathetic vasoconstrictor nerves?
-Contract resistance arterioles : produces vascular tone for vasodilatation for increased blood flow
-Distinct RVLM neurones-sympathetic pathways innervate diff tissues : Increased sympathetic nerve stimulation to GI (less blood flow) whilst reduce to skin (more blood flow, cool down)
-Pre-capillary vasoconstriction : ↓capillary pressure due to pressure drop
absorption of interstitial fluid into blood plasma
to maintain blood volume (important in hypovolemia)
Control TPR - maintains arterial blood pressure and blood flow to brain/myocardium since Pa = CO x TPR and BF = Pa / TPR
Controls venous blood volume (at rest 2/3rd blood in veins)
Venoconstriction venous blood volume Venous return
stroke volume via Starling’s law
Roles of sympathetic vasoconstrictor nerves + eg?
- Contract resistance arterioles : produces vascular tone for vasodilatation for increased blood flow
- Distinct RVLM neurones-sympathetic pathways innervate diff tissues : Increased sympathetic nerve stimulation to GI (less blood flow) whilst reduce to skin (more blood flow, cool down)
- Pre-capillary vasoconstriction : ↓capillary pressure due to pressure drop so reabsorption of interstitial fluid into blood plasma maintaining blood volume (vital in hypovolemia)
- Control TPR : maintains arterial BP + flow to brain/myocardium as Pa = CO x TPR and BF = Pa /TPR
- Controls venous blood volume : venoconstriction ↓ venous blood volume for venous return by increasing SV via Starling’s law
eg of vasoconstrictors hormones
Adrenaline, Angiotensin II, Anti-Diuretic Hormone, Endothelin-1 (EN1), Thromboxane (TXA2)
eg of vasodilator hormones
Atrial natriuretic peptide (ANP)
Physiological role of hormonal control of VSMCs?
Controls BP + flow during activity
Maintain BP + flow to brain, heart during haemodynamic crisis (haemorrhage, dehydration)
Pathological role of hormonal control of VSMCs?
Excess production, associated with excess vasoconstriction + vascular disease – hypertension, heart failure
What’s adrenaline?
Released due to sympathetic nerve stimulation
From adrenal glands
Acts on α1-adenoceptors on VSMCs
What’s Angiotensin II (Ang II)?
Formed from RAAS
Potent vasoconstrictor
Acts on AT1 receptors on VSMCs
What’s ADH (Vasopressin/Anti-Diuretic Hormone)
Released from posterior pituitary
Acts on V1-receptors on VSMCs
What’s Endothelin-1 (ET1)?
Released from endothelium
Acts on ET1 receptors on VSMCs
What’s Thromboxane (TXA2)?
Released from aggregating platelets
Acts on TP receptors on VSMCs
Vital vasoconstriction alongside clotting process
How is Ang II produced by RAAS system during drop in CO?
- low CO
- low renal blood flow
- sympathetic nerves β1
- low NaCl load
- produce renin
- produce angiotensinogen in liver
- produce Ang I (decapeptide)
- Angiotensin Converting Enzyme (ACE) on lung vascular endothelium
- produces Ang II (octapeptide)
How much blood does kidney receive?
25% of CO
Why do lungs have ACE?
lungs receive total CO
Effects of Ang II?
- ↑sympathetic - central effect
- ↑Blood volume - adrenal glands release aldosterone NaCl+ H2O Retention
- ↑TPR - vasoconstriction, stimulates sympathetic nerves
What is ADH produced during a haemorrhage?
- drop in blood volume detected by arterial baroceptors + left atrial receptors
- switches off Nucleus Tractus Solitarius (NTS)
- inhibitory pathway switches off CVLM
- switches on positive pathway of hypothalmic nerves
- stimulates magnocellular neurons in SON + PVN
- drives ADH release from posterior pituitary
Effects of ADH?
- ↑blood volume : at kidney V2 receptors, insertion of aquaporin channels in collecting duct so ↑H2O reabsorption
- vasoconstriction : at VSMCs V1 receptors during hypovolemia
Importance of myogenic response?
Maintains local blood flow during changes in local blood pressures
Protective mechanism – BP drops, still good flow – BP high, less flow/damage
What’s the myogenic response?
As pressure increases, more distension, more vasoconstriction but blood flow remains the same
How do vasoconstrictors increase vascular tone?
by activating same G-protein-coupled pathway in VSMCs
What does receptor does NA work on?
α1
What does receptor does Adrenaline work on?
α1
What does receptor does Ang II work on?
AT1
What does receptor does Vasopressin (ADH) work on?
V1
What does receptor does Endothelin-1 (ET-1) work on?
ETA
What does receptor does Thromboxane A2 (TXA2) work on?
TP