9. Regulation of the Cardiovascular System Flashcards
What is central venous pressure?
- Mean pressure in the right atrium
- Determines the amount of blood flowing back to the heart
- If high, less blood comes back to the right atrium
What are 3 ways of regulating flow?
- Local mechanisms
- Hormonal
- Autonomic nervous system
Describe the local mechanisms to regulate blood flow
- Autoregulation = intrinsic capacity to compensate for changes in perfusion pressure by changing vascular resistance
- e.g. if BP drops, autoregulation causes a gradual decrease in resistance and increase in flow
- Injury to the vessel can change autoregulation - platelets aggregate and release serotonin (vasoconstrictor)
- Release of NO, prostacyclin, thromboxane A2 and endothelins from the endothelium
What are the 2 theories for autoregulation?
• Myogenic theory
- smooth muscle fibres contract and pressure rises to keep flow constant
- due to stretch sensitive channels being involved
• Metabolic theory
- vessels supplying a particular vascular bed cotnracts
- flow decreases so vascular bed produces more metabolites
- metabolites feed back on the vessel causing vasodilation
- increased flow - metabolites washed away
Describe the systemic regulation of blood flow by (3 types of) hormones
- Kinins - e.g. bradykinin, interact with the Renin-Angiotensin System, tend to relax VSMCs
- ANP (Atrial Natriuretic Peptide) - circulating peptides, as the cardiac atria stretch, they secrete more ANP => vasodilation
- Circulating Vasoconstrictors - Vasopressin (ADH) secreted from posterior pituitary, Angiotensin II formed by increased renin secretion from kidney, Noradrenaline released from adrenal medulla
Describe the design of the Autonomic Nervous System, comparing the Parasympathetic and Sympathetic nerves
• Regions going down the CNS - cranial, thoracic, lumbar + sacral
Parasympathetic • cranial + sacral • long pre-ganglionic fibre and short post-ganglionic fibre • ganglion beside the SA node • ACh transmitter throughout • Nicotinic synapse at ganglion • Muscarinic synapse at effector • important in regulating heart rate
Sympathetic
• thoracic + lumbar
• short pre-ganglionic fibre and long post-ganglionic fibre
• pre-ganglionic fibre secretes ACh at ganglion
• post-ganglionic fibres secrete NA at effector
• important in controlling flow
Which vessels do sympathetic nerve fibres innervate?
All vessels except capillaries
Why is the distribution of sympathetic nerve fibres uneven?
- More innervate vessels supplying the kidney, gut, spleen and skin
- Fewer innervate the skeletal muscle and brain
- More potential to constrict the blood going to these places
- Blood can therefore be diverted to other more important organs
How does concentration affect the types of receptors adrenaline binds to?
- Normally - Beta 2 adrenoreceptors - vasodilation
* High concentration - Alpha adrenoreceptors - override vasodilatory effects and produce vasoconstriction
Where is the vasomotor centre located and what does it do (including the differences between the lateral and medial portions)?
- Located bilaterally in the reticular substance of the medulla
- Lower third of the pons
- Consists of a vasoconstrictor area (pressor), vasodilator area (depressor), Cardioregulatory Inhibitory Area (heart rate)
- Transmits impulses distally through the spinal cord to almost all blood vessels
- Higher centres in the brain (e.g. hypothalamus) can exert excitatory and inhibitory effects on the VMC
- Lateral portions: control heart activity - heart rate and contractility
- Medial portions: transmit signals via the vagus nerve to the heart - decreases heart rate
- Anticipatory response to exercise - heart rate and ventilation goes up slightly before exercise because of the higher sensors in the brain
Why do the parasympathetic pathways slow down the heart rate?
- Acetylcholine decreases the gradient of the pacemaker potential
- Potential takes longer to reach threshold and fire
- Heart rate slows down
Why do the sympathetic pathways increase the heart rate?
- Adrenaline and noradrenaline increase the gradient of the pacemaker potential
- Threshold is reached more quickly
- Heart rate increases
What happens to the heart rate if you cut the sympathetic nerves?
- Lose the ability to increase the heart rate
* Heart rate falls
What is the normal resting heart rate, and what would be the normal rate without innervation?
- Normal - 70bpm
* No innervation - 100 bpm
What 2 factors can increase the force of contraction
- Starling’s Law
* Sympathetic activity
How does noradrenaline lead to the contraction of muscle cells?
- NA binds to Beta 1 adrenoreceptors
- Increase in cAMP which activates Protein Kinase A
- PKA phosphorylates the LTCCs and the SR calcium release channel + SERCA
- More calcium influx and more calcium taken back up into the stores
- Increased contraction
- Altered heart rate and strength of contraction
What effect does parasympathetic activity have on the strength of contraction?
None
How do baroreceptors in the aortic arch and carotid bodies feed back to the vasomotor centre?
- Aortic arch - Vagus Nerve (C10)
* Carotid Sinus - Glossopharyngeal Nerve (C9)
What do the different baroreceptors respond to?
- Stretch of smooth muscle
- NOT BP
- Carotid respond to pressure between 60 and 180 mmHg
- Most sensitive around 90-100 mmHg (around mean BP)
What is reciprocal innervation?
Innervation so that the contraction of a muscle is accompanied by the simultaneous inhibition of an antagonistic muscle
How does reciprocal innervation work?
- Receptor detects an increase in pressure
- Nerve activity increases and fires down the afferent nerve directly to the parasympathetic nerve synapse
- Decrease in sympathetic activity - due to increased activity of the inhibitory interneurone
- Slows down tonic activity
In which direction does the afferent input, parasympathetic nerves and sympathetic nerves go, with reference to reciprocal innervation?
- Afferent input towards the brain
- Stimulates the parasympathetic nerves to the heart
- Simultaneously inhibits sympathetic innervation to heart, arterioles and veins
How does the vagus nerve have both an afferent and efferent activity?
- Afferent activity from baroreceptor to the VMC
* Efferent activity from VMC to the heart
What effect does decreased sympathetic stimulation have on the heart and blood vessles?
- Decreased heart rate and stroke volume
* Vasodilation
