Neural control of blood pressure Flashcards
why do we have a blood pressure control system
- if blood pressure is too low (hypotension) not enough blood and oxygen reaches the organs causing chronic fatigue and inability to exercise
- if blood pressure is too high (hypertension) arteries are overstretched, prolonged stretch causes aneurysms which are when regions of the artery wall are weaker than normal which may burst suddenly
- the physical stress of hypertension is dangerous is atheromatous arteries as they are less compliant, the raised pressure can cause atheromatous plaques to rupture and this causes the formation of a thrombus which can block the artery or travel downstream as an embolus and block smaller downstream arteries
what is hypotension
this is when blood pressure is too low, it is consistency below 90/60
what is hypertension
this is when blood pressure is too high, it is consistently above 140/90
what happens when an atheromatous plaque ruptures
an thrombus is formed
what is a moving thrombus called
an embolus
what are the two systems that regulate blood pressure
- Neuronal system - fast acting
- Hormonal system - slow acting
What part of blood pressure does the neuronal system control
- Neuronal system provides moment to moment regulation for example when you go from lying down to standing up ti automatically regulates the blood flow tot he leg
- vital for maintenance of blood pressure after an haemorrhage
- faster acting
what part of blood pressure does the hormonal system control
- slower acting, works on a time scale of minutes or hours
- uses the renin angiotensin aldosterone system
what receptors are used in the neuronal system in controlling blood pressure
- uses baroreceptors, these are found in the carotid sinus above the internal carotid above the bifurcation of the carotid arteries
- may also be pressure sensors in the aortic sinus which is just above the aortic valve, the artery walls in the sinus have more elastic fibres and are more stretchy than normal artery walls, they distend slightly in each systole
how does the neuronal system control blood pressure
- uses homeostasis and negative feedback
1. the baroreceptors and stretch receptors have stretch that is proportional to the blood pressure,
2. this causes continuous action potentials in sensory nerve endings
3. the baroreceptor nerve fibres travel to the brain in the vagus and glossopharyngeal cranial nerves
4. the nerves feed into the nucleus of the solitary tract (NTS) in the medulla oblongata which is in the lower part of the brainstem
5. this projects to the vasomotor centre which contains the set point for normal blood pressure
what is next of the carotid sinus
- the carotid body which is where the carotid chemoreceptors are found
what is the glossopharyngeal nerve
- it is primarily a visceral (sensory) nerve that contains afferents from the tongue, pharynx, larynx and the carotid sinus
- it enters the brain stem between the pons and medulla
what is the vagus nerve
- it is a mixed parasympathetic nerve that contains a motor output hand sensory afferent nerve fibres nerve fibres arise from the - lungs - gut - pharynx - larynx - carotid sinus
where are the cell bodies of the carotid sinus afferents in the vagus nerve
- In the jugular ganglia
- in the nodose ganglia
where is them nucleus of the solitary tract (NTS)
situated under the dorsal surface of the lower medulla oblongata
what is the nucleus of the solitary tract
- it is an integrating centre for sensory nerve fibres from several visceral organs
what does the nucleus of the solitary tract do
- it projects to the main vasomotor centre in the rostral ventrolateral medulla
- projects to the nucleus ambiguous which contains vagal output neurones
where is the main vasomotor centre
in the rostral ventrolateral medulla
what does the vasomotor centre contain
The vasomotor centre contains a ‘set point’ for blood pressure (normally 120/80 mm Hg) .
how do you work out mean arterial pressure
cardiac output x total peripheral resistance = mean aterial pressure
what happens if blood pressure is too low
- if the stretch is too low this means that the rate of firing of baroreceptors is too low so this means that blood pressure is too low so the output of the vasomotor centre increases
- the NTS signals the vasomotor centre
- the vasomotor centre activates the sympathetic nervous system
- causes contraction of arterioles which raises total peripheral resistance
- the raise in total peripheral resistance increases blood pressure
where do axons of the vasomotor centre project into
- they project down through the spinal cord in the lateral reticulospinal tract to the sympathetic pre-ganglionic neurones in the intermediate part (IML) of the lumbar and thoracic ventral horn
what is the lateral reticulospinal tract
- a bundle of axons that rise from a group of cells int her reticular formation of the medulla of the brainstem
- the vasomotor axons that activate the preganglionic neurones are a small part of the lateral reticulospinal tract
what happens if blood pressure is too high
- means overstretch baroreceptor
- many firing of action potentials which is too high
- inhibits the vasomotor centre
- reduces the sympathetic output flow
- causes the relaxation of arterioles and reduction of the TPR
- lowers blood pressure back to the set level
how do the baroreceptor afferents affect the nucleus ambiguus
- they have an excitatory influence on the nucleus ambiguus of the medulla
- this part is also known as the cardioinhibitory centre
- ## it increases parasympathetic vagal output to the heart
what does the vagus nerve do to the heart
- acts on the SAN slows down the heart and reduces the cardiac output
- a reduction CO leads to reduces BP
How does the baroreceptor reflex work
- stimulation either by an electrical shock to the carotid nerve or by stretch of the nerve endings in the carotid sinus will reduce blood pressure
- this decreases sympathetic outflow in the vasomotor centre and increases the parasympathetic outflow from the nucleus ambiguous to the heart
when is hypotension regarded significant
if symptoms are present such as dizziness and fainting
how much of the population older than 60 have hypertension
50%
what do most people with essential hypertension have
- total peripheral resistance is raised by cardiac output remains normal
- increased peripheral resistance is due to chronic narrowing or constriction of the arterioles
why does chronic narrowing or constriction of the arterioles happen
- increased tonic vasoconstriction via the sympathetic nervous system
- endothelium of the arterioles is damaged in some way which leads to increased constriction
- there is malfunction of the hormonal control system.
in hypertension the set point is
raised
why is the set point raised in hypertension
- One theory is that the stretch receptors in the carotid sinus are less sensitive than normal.
- this means that normal blood pressure will cause a smaller than normal rate of firing of sinus afferents, this is sensed by the vasomotor centre as a low blood pressure and the system will correct this by increasing the sympathetic output raising the blood pressure until the input from the baroreceptors is normal this results in a higher than normal blood pressure
what causes a reduction in sensitivity of baroreceptors
- an atheroma formation in the carotid sinus
- makes walls stiffer and requires more pressure to distend then than normal therefore a normal blood pressure gives a reduced input
- in increased sympathetic drive and decreased parasympathetic drive, resulting in a higher blood pressure.
what happens if atheroma form in arteries supply the vasomotor centre in the rostral medulla
these regions of the brainstem can become relatively hypoxic (low in oxygen).
- this can cause hypertension
