Cardiovascular 12 – Regulation of the Cardiovascular System
Anil Chopra
1. Describe the local mechanisms that regulate blood flow.
Autoregulation (ability to compensate for changes in pressure intrinsically)
Myogenic theory – smooth muscle fibres in vessel walls will expand and contract
when pressure is increased or decreased. This changes the wall tension.
Metabolic theory – if metabolites accumulate in the blood, then the muscles respond
by causing vasodilation so that the metabolites are washed away quickly.
Injury – if vessels or tissues near the vessels are injured, the smooth muscle responds
by vasoconstricting.
Substances released by endothelium
Endothelin-1 and thromboxane are vasoconstrictors
Nitric oxide and prostacyclin are vasodilators.
Hormones such as:
Bradykinin – causes vasodilation
ANP – atrial natiuretic – causes vasodilation.
Vasopressin, noradrenaline & angiotensin II – cause vasoconstriction.
2 & 3. Describe how blood vessel diameter, heart rate and forces of contraction are all
affected by the autonomic nervous system.
Blood Vessel Diameter
 Blood vessels receive postganglionic sympathetic innervation using
noradrenaline as the neurotransmitter. (Some use nitric oxide).
 There is always some tonic activity and the amount is regulated by the
medulla (depressor and pressor centres).
Heart Rate & Force of Contraction
Sympathetic stimulation results in an increased heart rate and also causes a large
Ca2+ influx into the cells on influx as well as increasing intracellular stores of
Ca2+. This results in an increased force of conduction. Stroke volume is also
increased with increased venous return and decreased intrathoracic pressure (both
increase end-diastolic ventricular volume).
Parasympathetic stimulation results in decreased heart rate and does not increase
contraction force.
4. State the location of the Baroreceptors.
Baroreceptors in the carotid sinus & aortic arch.
5. Define cardiac output, stroke volume, mean systemic arterial pressure and state
the determinants.
Cardiac output: Vol of blood pumped out of the heart per minute
 CO = SV x HR (stroke volume x heart rate)
Stroke volume: Vol of blood ejected on each pump
 SV = EDV – ESV (End diastolic volume – end systemic volume).
Mean systemic arterial pressure: arterial pressure used; NOT average.
 MAP = CO x TPR (cardiac output x total peripheral resistance)
 MAP = DP x 1/3PP (diastolic pressure x 1/3 pulse pressure)
6. Indicate how Baroreceptors control blood
pressure.
Increase in
Blood pressure
Increased Stretch
of baroreceptors
Afferent pathway of
vagus nerve and
glossopharyngeal nerve
Medulla
oblongata
Decrease in
Blood pressure
Decreased Stretch
of baroreceptors
Afferent pathway of
vagus nerve and
glossopharyngeal nerve
Medulla
oblongata
Increases
parasympathetic activity
to heart and decreases
sympathetic activity to
heart and vessels
Increases sympathetic
activity to heart and
decreases
parasympathetic activity
to heart.
7 & 8. Describe the changes in impulse activity in
the carotid sinus nerve, parasympathetic and sympathetic stimulation
nerves following an increase or decrease in blood pressure & construct an
integrated picture of the various systems that control blood pressure and
be able to apply this to specific clinical examples involving blood loss or
fluid overload.
Maintaining arterial pressure
Mean systemic arterial pressure = cardiac output x total peripheral resistance
Arterial pressure
haemorrhage
Cardiac output
Heart rate
CO = HR x SV
Stroke volume
Arterial pressure
Ventricular end
diastolic volume
Venous return
Venous pressure
Firing of
baroreceptors
Parasymp
discharge
to heart
Reflexes
Peripheral
resistance
Cardiac
contractility
Venous tone
Symp discharge
to heart
Symp discharge
to veins
Arteriolar
constrictio
n
Symp discharge
to arterioles
Decrease in
Blood
pressure
Increase in
Blood
pressure