Cardiovascular 21 to 30
Describe the phases of the cardiac cycle.
- Atrial systole;
- Isovolumetric ventricular contraction;
- Ventricular contraction;
- Isovolumetric ventricular relaxation;
- Ventricular filling
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
What factors regulate renin secretion?
- sympathetic nervous system, catechols, prostaglandins
- Na and Cl reabsorp, inc BP, angio II, vasopressin
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
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?
What are the factors that influence contractility?
- Drugs +ve / -ve inotropes
- sympathetic tone
- 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
- 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: