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
Ø Blood vessels receive postganglionic sympathetic innervation using noradrenaline as the neurotransmitter. (Some use nitric oxide).
Ø 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.
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.