sensors located in the carotid sinus (at the bifurcation or external and internal carotids) and in the aortic arch.
Sense blood pressure and relay information to the brain, so that proper blood pressure can be maintained.
Where do signals from the carotid baroreceptors travel?
glossopharyngeal nerve (Cranial nerve IX)
Where go signals from the aortic arch baroreceptors travel?
vagus nerve (cranial nerve X)
Brief mechanism by which baroreceptors control blood pressure
negative feedback system called a baroreflex occurs as soon as there is a change from the usual mean arterial blood pressure.
What happens once the baroreceptor is stimulated?
- action potential becomes triggered
- directly conducted to the brain stem where central terminations transmit to neurones within the solitary nucleus, which lies in the medulla
- nerve endings present in the tunica adventitia. If there is an increase in mean arterial pressure, the rate of depolarisation of these nerve endings increases.
- lead to action via autonomic nerves to decrease pressure-
- hormones secreted to target the heart and blood vessels
What is the carotid sinus responsive to?
both increases and decreases in arterial pressure
what is the aortic arch responsive to?
only increases in blood pressure
orthostatic hypotension definition
A medical condition wherein a person’s blood pressure falls when standing or sitting
Orthostatic hypotension mechanism
gravity causes blood to pool in the lower extremities, compromises venous return leading to a decreased cardiac output and subsequent lowering of arterial pressure
How is orthostatic hypotension prevented in healthy individuals?
baroreceptor reflex that causes vasoconstriction, pressing the blood from the lower extremities back into the body again
resistance is equal to
length divided by radius ^4
What are arterioles referred to + why?
resistance arterioles- place where there is the greatest fall in pressure
What happens during vasoconstriction?
upstream pressure increases, downstream pressure decreases
What happens when the radius halves?
the resistance increases by a factor of 16
What happens during vasodilation?
upstream pressure decreases, downstream pressure increases
How else flow be increased?
capillary recruitment- more capillaries perfused
Where may tissue perfusion increase rapidly?
salivary glands, skin and skeletal muscle
which tissues always receive close to their maximum possible blood flow?
dynamic range definition
difference between maximum and minimum perfusion
Myogenic mechanism definition
how arteries and arterioles react to an increase or decrease of blood pressure to keep the blood flow within the blood vessel constant
myogenic refers to a contraction initiated by a myocyte itself, not a nerve innervation
Myogenic mechanism stages
- smooth muscle reacts to stretching of the muscle by opening channels
- muscle depolarised
- muscle contracts, which decreases the lumen diameter
- limits the blood flow, increases the blood pressure
Myogenic autoregulation definition
maintain a constant renal blood flow, despite alterations in arterial pressure
Bayliss effect definition
special manifestation of the myogenic tone in vasculature
smooth muscle cells response to stretch
when blood pressure is increased the blood vessels distend, muscle reacts with a constriction
What does the stretch of the muscle open?
stretch mediated ion channels, causing the cell to become depolarised
calcium voltage gated channels open, trigger muscle contraction
Experiment to prove Bayliss effect
reservoir full of water lifted, leading to increase in pressure
in a dead tube, no muscle etc, flow increased and fell similarly to the pressue
arteriole flow dropped back to normal, due to Bayliss effect, despite pressure remaining high
When verapamil added, calcium channel blocked, the arteriole behaved similarly to the dead tube
What does the Bayliss Effect allow?
maintenance of a constant baseline flow
over perfusion definition
tissue receives larger share of cardiac output than is required for maximum oxygen consumption
what tends to be overperfused + why?
kidneys- require blood to filter it and to respire
What tends to be underperfused + why?
heart and brain
better able to extract oxygen from the volume of blood than other tissues
What can override myogenic autoregulation?
Adrenergic regulation of coronary flow
- falling ATP supply leads to increased AMP release from cells
- extracellular adenosine then binds to an alpha 2 receptor
3, causes a raise in intracellular cAMP
- decreases myosin light chain kinase activity
- vasodilation locally, decreases blood pressure
What else can cause vasodilation?
hypercapnia, temperature, autacoids, nitric acid
abnormally elevated carbon dioxide levels in the blood
Stages of vasodilation due to hypercapnia
- more co2, more acidic
- decreases MLCK activity
stages of vasodilation due to temperature
- heat, detected centrally and peripherally
- if centrally, detected by hypothalamus, if peripherally reduces noradrenaline binding to A2
- reduces sympathetic signalling
- greater heat loss due to dilated arterio-venous anastomoses
autocoid definition + examples
locally produced and locally expressed factors that affect physiology
examples:histamine, bradykinin, prostaglandins, leukotrienes
stages of vasodilation due to autocoids
histamine stimulates the synthesis and release of various vascular smooth muscle cell relaxants, such as nitric oxide and endothelium-derived hyperpolarising factor
stages of vasodilation due to nitric oxide
- l-argininine, oxygen and NADPH synthesised endogenously by various nitric acid synthase enzymes.
- nitric acid targets guanylyl cyclase,
- protect cGMP from degradation, allowing signals to be enhanced and thus vasodilation
role of endothelium in regulating flow
stages of vasodilation die to extracellular accumulation of potassium
- small rise in extracellular potassium
- increased potassium permeability
increase of blood flow to different tissues in the body
reactive hyperaemia definition
a period of hyperaemia that follows a brief period of ischaemia, due to the shortage of oxygen and build up of metabolic waste
nerves that innervate blood vessels, which alter their diameter
sympathetic vasoconstrictor stages
- brainstem- nucleus tractus solitarii sends impulse to medulla
- sends impulse to spinal cord
- ACh released from preganglionic neurone
- NA/A released from postganglionic
- alpha 1 adrenergic receptor binds to epinephrine and norepinephrine in the arteriole
- leads to constriction of vessels- increases blood pressure
Experiment to see the effect of baroreceptor innervation
denervate aortic baroreceptor
arterial BP shown to be poorly controlled, irregular
where are low pressure cardiopulmonary receptors found?
large systemic veins, pulmonary artery, walls of atria, ventricles
Function of low pressure receptors
respond to changes in wall tension, which is proportional to the filling state of the low pressure side of circulation.
regulate blood volume
increase in stretch causes
increase in heart rate, decrease in vasopresin- leads to an increase volume of urine excreted, serving to lower blood pressure