Describe the phases of the cardiac cycle.

  • Atrial systole;
  • Isovolumetric ventricular contraction;
  • Ventricular contraction;
  • Isovolumetric ventricular relaxation;
  • Ventricular filling

Relate the aortic pressure to the phases of the cardiac cycle.

cardio 12


Describe how blood flow is regulated at the level of the endothelium.

  • Vasodilators: prostacyclins, NO, kinins
  • Vasoconstrictors: endothelin, thromboxane, serotonin

What other general effects do endothelins have on the cardiovascular system?

  • Positive inotrope and chronotrope
  • Rise in ANP/renin/aldosterone
  • Decreased GFR and renal blood flow


Describe how the renin-angiotensin system regulates blood pressure and flow.

  • Fall in renal blood flow leads to renin
  • Renin, angiotensin I > II
  • Vasoconstrictor

What factors regulate renin secretion?

  • Stimulate:
  • sympathetic nervous system, catechols, prostaglandins
  • Na and Cl reabsorp, inc BP, angio II, vasopressin
  • Inhibit:


What changes in arterial blood pressure do baroreceptors respond to?

  • Carotid sinus (rise or fall)
  • Aortic arch (rise)

What happens when the baroreceptors detect a fall in arterial pressure?

  • decreased firing rate of Hering’s nerve
  • CN IX transmits to vasomotor centre
  • decreased parasympathetic outflow to heart
  • increased sympathetic outflow to heart
  • increased sympathetic outflow to vessels
  • increased heart rate, contractility
  • Arteriolar and venous constriction

What is the Set Point?

  • Neutral MAP for vasomotor centre Around 100 mm Hg


Discuss the central neural control affecting arteriolar tone.

  • Presence of a vasomotor centre situated in the CNS medulla with both vasoconstrictor and vasodilatory areas
  • Medullary vasomotor centre is influenced by peripheral baroreceptors, peripheral chemoreceptors and higher neural centres
  • Noradrenergic vasoconstrictor fibres descend from medullary vasomotor centre via spinal cord to the smooth muscle in the walls of arterioles
  • Peripheral baroreceptors in carotid sinus and aortic arch respond largely to changing blood pressure and act to inhibit vasoconstrictor centre
  • Peripheral chemoreceptors in carotid bodies and aortic bodies respond to hypoxia and act to excite the vasoconstrictor centre

Describe the Volume (atrial stretch) reflex.

  • Atrial stretch results in reflex afferent arteriolar renal dilatation


Discuss the hormones that influence arteriolar tone.

  • Adrenaline is released from the adrenal medulla in response to sympathetic stimulation. It acts via
  • alpha-1 receptors to constrict arterioles in most areas. It also acts via Beta 2 receptors to vasodilate muscle and liver blood vessels
  • Noradrenaline although largely a neurotransmitter, is released from the adrenal medulla in response to sympathetic stimulation. It acts via alpha 1 receptors to constrict arterioles
  • Angiotensin II is a generalised arteriolar constrictor. It is formed from angiotensin I in the lung
  • Vasopressin is a potent arteriolar constrictor. It is released from the posterior pituitary
  • Bradykinin is a tissue hormone that causes arteriolar dilatation
  • Histamine is produced by basophils and mast cells and causes arteriolar dilation
  • Serotonin
  • Adrenomedullin


Discuss the local factors that affect arteriolar tone.

  • Arteriolar tone changes to regulate local blood flow across a range of blood pressures. Two theories by which this occurs: myogenic or metabolic
  • Myogenic theory – distension of vessel with increasing pressure stretches the vascular smooth muscle leading to contraction of the muscle
  • Metabolic theory – vasodilator metabolites accumulate in tissues when blood flow falls leading to relaxation of vascular smooth muscle
  • Vasodilators include local hypoxia and acidosis, CO2 build up, heat, potassium, lactate, histamine, adenosine
  • Serotonin causes localised vasoconstriction after vessel injury
  • Prostacyclin (vasodilatation) and thromboxane (vasoconstriction) after local vessel injury
  • Endothelium Derived Relaxing Factor (nitric oxide) Many vasodilators act by activating EDRF
  • Endothelin – vasoconstrictor


What are the parameters that define cardiac output?

HR x Stroke Vol

What factors influence stroke volume?

  • afterload
  • preload
  • contractility

What are the factors that influence contractility?

  • Hypoxia
  • Drugs +ve / -ve inotropes
  • pH
  • sympathetic tone
  • hypercapnoea
  • myocardial damage


Describe the mechanical events that occur during the cardiac cycle.

  • Diastole-slow filling, decreasing in rate
  • Atrial systole
  • Closure of mitral and tricuspid valves
  • Isovolumetric Ventricular contraction
  • Opening of pulmonary and aortic valves
  • Ventricular ejection
  • Protodiastole
  • Isovolumetric relaxation
  • Opening of AV valves and commencement of diastole


What is the normal Central Venous pressure at rest.

  • Pressure in Right Atrium = 0 (range –5 to +5)

Describe the factors that determine Central Venous Pressure.

  • Balance between venous return, and ability of heart to pump out of RA
  • Factors affecting venous return:
  • Gravity, intraabdominal pressure (eg pregnancy), hypo/hypervolaemia, venodilation (drugs/fainting), sympathetic tone (venoconstriction), arteriodilation (sepsis, drugs, anaphylaxis), resistance to venous return (tamponade, tumour)
  • Myocardial contractility, Hypertrophy (Athlete) Cardiac Failure, Myocardial Infarction (RV), Arrhythmias, Atrial Fibrillation (Volume & filling time, and contractility), Resistance to RV = Pulm valve stenosis, PE, LVF, Hypoxia, tension pneumothorax
  • Factors affecting ability of heart to pump blood:

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