6 - Cardiovascular control 2 Flashcards
Equation for stroke volume?
SV = end-diastolic vol - end-systolic vol
Equation for cardiac output?
Q = SV x HR
Equation for blood pressure? (MAP)
MAP = Q x total peripheral resistance (TPR)
What is venous volume distribution affected by?
Peripheral venous tone
Gravity
Skeletal muscle pump
Breathing
Where is central venous pressure measured?
The right atrium
How is flow controlled in veins?
Veins are compliant. They can constrict to reduce venous return.
In arterioles, what does constriction determine?
Blood flow to downstream organs
Mean arterial blood pressure
Pattern of blood flow to organs
What is the main way flow is changed by? What equation shows this?
Vessel radius.
Flow = change in pressure/resistance
Resistance = 1/r^4
What type of mechanisms are intrinsic to smooth muscle?
Local mechanisms - local blood flow regulation within an organ.
What type of mechanisms are extrinsic to smooth muscle?
Systemic mechanisms - e.g.
Circulating hormones
Autonomic nervous system
What is autoregulation? What are the theories behind it?
The intrinsic capacity to compensate for changes in perfusion pressure by changing vascular resistance.
E.g. if pp falls, without autoregulation flow will fall. With autoregulation, resistance will decrease so that flow can increase.
Theories for autoregulation are myogenic and metabolic.
What is myogenic theory?
Smooth muscle fibres respond to tension in the vessel wall.
E.g. as pressure rises, fibres contract. Involves stretch sensitive channels.
What is metabolic theory?
As blood flow decreases, metabolites accumulate and the vessels dilate. Subsequent increased flow washes the metabolites away
What does injury do to vessel radius?
Serotonin release from platelets causes constriction.
Name 4 LOCAL (endothelium derived) hormones?
Nitric Oxide (NO) Prostacyclin Thromboxane A2 (TXA2) Endothelins (ET)
Name 5 CIRCULATING (non-endothelium derived) hormones?
Kinins Atrial natriuretic peptide (ANP) Vasopressin (ADH) Noradrenaline/Adrenaline Angiotensin II
How does Nitric oxide work?
VASODILATOR
Made from arginine
Diffuses into vascular smooth muscle cells
How does Prostacyclin work?
VASODILATOR
Made from prostaglandin precursor (PGH2)
Anti platelet and anticoagulant effect too.
How does thromboxane A2 work?
VASOCONSTRICTOR
Made from prostaglandin precursor (PGH2)
Heavily synthesised in platelets
How do endothelins work?
VASOCONSTRICTOR
Made from nucleus of endothelial cells.
Have minor vasodilator effects.
How do kinins work?
Hormones that bind to receptors on endothelial cells.
Stimulates NO synthesis.
VASODILATOR effects.
How does Atrial natriuretic peptide (ANP) work?
Secreted from atria in response to stretch.
VASODILATOR effects to reduce BP.
How does Vasopressin work?
Secreted from posterior pituitary in response to high blood osmolality. Binds to V1 receptors on smooth muscle to cause VASOCONSTRICTION.
How does noradrenaline/adrenaline work?
Secreted from adrenal gland. Causes VASOCONSTRICTION.
How does angiotensin II work?
VASOCONSTRICTOR from renin-angiotensin-aldosterone axis.
Stimulates SNS activity and ADH secretion.
What is the broad function of the parasympathetic nervous system?
Rest and digest - heart rate
What is the broad function of the sympathetic nervous system?
Fight or flight - circulation
Where does the parasympathetic system arise from?
The cranial and sacral part of the spinal cord.
Where does the sympathetic system arise from?
The thoracic and lumbar vertebra.
Where do sympathetic neurones synapse?
Sympathetic chain paravertebral ganglia.
What neurotransmitter is used in ALL pre-ganglionic fibres?
Acetylcholine
What neurotransmitters are used in post-ganglionic fibres?
Sympathetic: Noradrenaline
Parasympathetic: Acetylcholine
Which fibres are longer in the sympathetic and parasympathetic system - pre or post ganglionic?
Sympathetic: Shorter pre, longer post (because of paravertebral ganglia)
Parasympathetic: Longer pre, shorter post
Sympathetic fibres initiate the heart and all blood vessels except what?
capillaries and pre capillary sphincters.
some metarteroles.
What do sympathetic fibres heavily initiate? What is weakly innervated?
Heavily: Kindeys, gut, spleen, skin
Lightly: Skeletal muscle, brain
What does noradrenaline bind to?
a1 adrenoceptors to cause smooth muscle contraction and vasoconstriction.
What is the VMC? What do the different parts do?
Vasomotor centre - part of brain.
In bottom part of pons and medulla.
Lateral parts control: Heart rate and contractility
Medial parts control: Decreases heart rate via vagus nerve
What are the 3 different areas in the VMC?
Vasoconstrictor (pressor) area
Vasodilator (depressor) area
Cardioregulatory inhibitory area
What is baseline SNS discharge?
The sympathetic system is always innervating the vessels. Increased and decreased levels of innervation are what causes vasoconstriction and vasodilation.
Summary: What extrinsic factors affect vessel radius?
Circulating hormones
Sympathetic nervous system
Summary: What intrinsic (local) factors affect vessel radius?
Oxygen Metabolites Potassium Osmolarity H+ CO2
Which nerves increase HR and which decrease HR?
Sympathetic increases HR
Parasympathetic decreases HR
How does noradrenaline have an affect on the force of contraction, what is the mechanism?
- Noradrenaline binds to B1 receptors on heart cell
- Increased cAMP
- Increased protein kinase A
- PKA phosphorylates the L-type calcium channels, SR calcium release channels and calcium ATPase.
- Calcium influx, uptake and release is all increased = more forceful contraction.
What extrinsic things increase stroke volume?
Sympathetic nervous stimulation
Plasma adrenaline
What intrinsic things increase stroke volume?
End diastolic volume (Starling’s law)
Influenced by:
Increased venous return & atrial pressure
Increased respiratory movements = Decreased intrathoracic pressure
How is arterial pressure detected?
Baroreceptors (-ve feedback). Respond to stretch. Fire more when they’re stretched more.
Where are baroreceptors found? How do they send info?
Mechanoreceptors are in the carotid sinus and aortic arch.
Carotid sinus: Via glossopharyngeal nerve (PNS afferent)
Aortic arch: Vagus nerve (PNS afferent and efferent)
At what pressures can baroreceptors sense and when are they most sensitive?
They can respond to pressures between 60-180mmHg.
They are most sensitive at 90-100mmHg.
How do baroreceptors change blood pressure?
Baroreceptor recognises increase in BP.
Increased baroreceptor activity
Increased afferent input
Increased PNS stimulation to heart to SLOW HR.
INHIBITS sympathetic stimulation of heart and blood vessels = slower HR and vasodilation (tonic activity decreases)
What 4 things can increase venous return?
- Increased Skeletal muscle pump
- Increased Blood volume
- Increased sympathetic nerve stimulation
- Increased respiratory movements
How is arterial blood pressure maintained?
See poster
