Crdiovascular control 2 Flashcards
Define stroke volume
The volume of blood pumped out of the heart in each beat.
What is venous volume distribution affected by
Venous volume distribution affected by peripheral venous tone, gravity, skeletal muscle pump & breathing
Describe central venous pressure (pressure in the right atrium)
Central venous pressure (mean pressure in the right atrium) determines the amount of blood flowing back to the heart.
What does the amount of blood flowing back to the heart determine
The amount of blood flowing back to the heart determines stroke volume (using Starling’s Law of the Heart)
What is meant by the tone of a blood vessel
How constricted they are
What are the consequences of venous constriction
In veins, constriction reduces compliance and venous return
What does constriction determine in the arterioles
In arterioles, constriction determines:
Blood flow to downstream organs
Mean arterial blood pressure
The pattern of blood flow to organs
What is flow primarily altered by
Changes in the radius of the blood vessel.
Describe the suck-pump analogy of the heart
The heart pumps out how much blood returns (Frank-Starling relationship).
Describe the two different methods of regulating blood flow
Local mechanisms are intrinsic to the smooth muscle (or closely associated)- Important for reflex local blood flow regulation within an organ.
Systemic mechanisms are extrinsic to the smooth muscle- circulating hormones and the autonomic nervous system.
What happens in the systemic mechanisms
It affects the constriction or distension of the blood vessels all over the body.
Without any compensatory mechanisms, what would you expect to happen if perfusion pressure decreased
Resistance to increase (passive constriction as the intravascular pressure falls)
Flow to decrease
What is meant by autoregulation
Autoregulation is the intrinsic capacity to compensate for changes in perfusion pressure by changing vascular resistance)
Flow increases and resistance decreases in response to a decrease in perfusion pressure.
Describe the myogenic theory for autoregulation
Myogenic theory states that smooth muscle fibres respond to tension in the vessel wall (e.g. as pressure rises, fibres contract; stretch sensitive channels involved)
Hence low pressure, less constriction, dilation
Describe the metabolic theory for autoregulation
Metabolic theory (as blood flow decreases, metabolites accumulate and and vessels dilate; subsequent increased flow ‘washes’ metabolites away) Feedback mechanism.
How can injury influence blood flow
serotonin release from platelets causes constriction, hence reducing the volume of blood lost.
List the local (endothelial derived hormones) involved in the regulation of blood flow
Nitric Oxide
Prostacyclin
Thromboxane A
Endothelins
List the circulating (non-endothelial derived hormones) involved in the regulation of blood flow
Kinins Atrial natriuretic peptide (ANP) Vasopressin Noradrenaline/Adrenaline Angiotensin 2
Describe nitric oxide
potent vasodilator produced from arginine. NO diffuses into vascular smooth muscle cells.
Describe prostacyclin
cardioprotective vasodilator synthesised from prostaglandin precursor (PGH2) – also has antiplatelet and anticoagulant effects
Describe thromboxane A
vasoconstrictor synthesised from prostaglandin precursor (PGH2) – also heavily synthesized in platelets (amplify platelet activation)
Describe endothelin
vasoconstrictors generated from the nucleus of endothelial cells – has minor vasodilator effects but principally a vasoconstrictor
Describe Kinins
hormones that bind to receptors on endothelial cells and stimulate NO synthesis – vasodilator effects
Describe Atrial natriuretic peptide
secreted from the atria in response to stretch – vasodilator effects to reduce BP
Describe Vasopressin
secreted from posterior pituitary in response to high blood osmolality. Binds to V1 receptors on smooth muscle to cause vasoconstriction
Describe adrenaline/noradrenaline
secreted from adrenal gland and causes vasoconstriction
Describe angiotensin 2
potent vasoconstrictor product from the renin-angiotensin-aldosterone axis. Also stimulates SNS activity and ADH secretion.
Describe the use of ACE inhibitors as an anti-hypertensive treatment
Reduces the production of angiotensin 2, thus the vessels are constricted less, reducing BP.
What is the SNS important in controlling
SNS is important for controlling the circulation
What is the PNS important in controlling
PNS is important for controlling the heart rate
Describe the neurotransmitters used in the pre-ganglionic neurons and the post-ganglionic neurones
Pre-ganglionic fibres use ACh as their neurotransmitter
PNS post ganglionic NT = ACh
SNS post ganglionic NT = NA
Where do the sympathetic pre-ganglionic neurons synapse
In the paravertebral ganglion.
What do the SNS fibres innervate
SNS fibers innervate the heart and ALL VESSELS except capillaries and precapillary sphincters and some metarterioles
Why is it difficult to predict the effects of the SNS on a specific vascular bed
SNS innervation elsewhere is variable:
Heavily innervated: kidneys, gut, spleen and skin
Poorly innervated: skeletal muscle and brain
How does noradrenaline cause vasoconstriction
Noradrenaline preferentially binds to α1 adrenoceptors to cause smooth muscle contraction and vasoconstriction.
Where is the vasomotor centre located
VMC is located bilaterally in the reticular substance of the medulla and the lower third of the pons
What is the vasomotor cortex composed of
The VMC is composed of a vasoconstrictor (pressor) area, a vasodilator (depressor) area and a cardioregulatory inhibitory area
How does the vasomotor transmit impulses
The VMC transmits impulses distally through the spinal cord to almost all blood vessels
Lateral portions of VMC controls heart activity by influencing heart rate and contractility
Medial portion of VMC transmits signals via vagus nerve to heart that tend to decrease heart rate
Explain how the vasomotor centre can take into account thinking about events
Many higher centers of the brain such as the hypothalamus can exert powerful excitatory or inhibitory effects on the VMC
Anticipatory response- adrenaline is released when preparing for exercise or a stressful event.
Describe the nervous control of vessel diameter
Blood vessels receive SNS post-ganglionic innervation Neurotransmitter = noradrenaline (NA)
Always some level of tonic activity which can be:
Increased – causing vasoconstriction
Decreased – causing vasodilation
Generally no PNS innervation to vasculature
What can increase heart rate
SNS stimulation
Increased plasma adrenaline
What can decrease heart rate
PNS stimulation
What does the fact that heart rate increases when both the sympathetic nerves and parasympathetic nerves are cut indicate
That PNS activity is always lowering activity- and has a more dominant effect at rest.
Describe how the SNS can control the force of contraction
Noradrenaline binds to B1 receptors on heart
Increased cAMP and protein kinase A
PKA phosphorylates L-Type calcium channel, SR Ca2+ release channel and Ca2+ ATPase
Ca2+ influx increased
Ca2+ uptake into intracellular stores increased
Ca2+ release from intracellular stores increased
More calcium binding to troponin- greater force of contraction
What are the intrinsic factors that can increase stroke volume
Increased venous return — increased atrial pressure – increased EDV
Increased respiratory movements — decreases intrathoracic pressure — increased EDV.
What are the extrinsic factors that can increase stroke volume
Plasma adrenaline
SNS efferent to heart
Describe the baroreceptors
Mechanoreceptors in carotid sinus change their firing rate in response to changes in pressure
Mechanoreceptors in aortic arch change their firing rate in response to changes in pressure
Aortic arch- vagus nerve afferents
Carotid sinus- Glossopharyngeal nerve efferents
What do baroreceptors respond to
Stretch, the more they stretch (higher BP) the more they fire
Describe baroreceptor activity
Carotid sinus baroreceptors respond to pressures between 60 and 180 mmHg.
Baroreceptors respond to changes in arterial pressure
Baroreceptors reflex is most sensitive at pressures around 90-100 mmHg- physiological BP
- Greatest change in impulse rate per unit change in pressure occurs about this point
Describe reciprocal innervation
Afferent nerves bifurcate- PNS branch and SNS branch
PNS branch linked to heart
Bifurcation -series of inhibitory interneurons
Tonic activity from SNS to blood vessels and heart
Explain how increased baroreceptor firing decreases blood pressure
Increased afferent input from increased baroreceptor activity stimulates parasympathetic nerves to heart
It also simultaneously inhibits sympathetic innervation to heart, arterioles and veins
Decreasing HR and causing vasodilation of the blood vessels.
Describe how the BP reflex activity is controlled by carotid sinus activity
Increase blood pressure – increases baroreceptor stretch – increased firing of carotid sinus nerve
Increased firing of vagus nerve
Decrease firing of sympathetic cardiac and vasoconstrictor nerves(resistance and capacitance vessels).
What are the consequences of a haemorrhage
Decreased blood volume Decreased venous pressure Decreased venous return Decreased arterial pressure Decreased EDV Decreased SV Decreased Cardiac Ouput Decreased BP
How does the body respond to a haemorrhage
Reduced baroreceptor firing
Increased SNS discharge to heart, reduced PNS discharge
Increase in contractility- increased stroke volume- increased cardiac output- increased mean arterial pressure
Increased SNS discharge to veins Increased venous tone Increased venous pressure Increased venous return Increased EDV Increased cardiac output- increased MAP
Increased SNS discharge to arterioles
Arteriolar constriction
Increased Total Peripheral Resistance — Increased MAP
