Feedback Control in the Cardiovascular System

What is Blood Pressure?

Hydrostatic pressure excreted by blood on blood vessel walls

Measure systemic atrial blood pressure by Systolic & Diastolic BPs

What is Systolic arterial blood pressure?

Pressure exerted by blood on walls of aotra & systemic arteries when heart contracts

What is Diastolic arterial blood pressure?

Pressure exerted by blood on walls of aorta & systemic arteries when heart relaxes

What is the ideal normal systolic blood pressure?

90-120mmHg

What is the ideal normal diastolic arterial blood pressure?

60-80mmHg

What is Pulse Pressure?

Difference between systolic & diastolic blood pressure

Normal range between 30 & 50mmHg

E.g systolic is 120 & diastolic is 80

PP = 40

What is mean arterial blood pressure?

Average arterial blood pressure during a single cardiac cycle which involves contraction & relaxation of heart

Relaxation is twice as long as contraction

What is the simplest way to calculate MAP?

(2x Diastolic) + Systolic divided by 3

Why must MAP be regulated within a narrow range?

Pressure high enough to perfuse vital organs like brain, heart & kidneys

Low enough to not damage blood vessels or place an extra strain on heart & other organs like brain, kidneys & eyes

What are baroreceptors?

Mechanoreceptors which are sensitive to stress

Have nerve endings

What happens to the firing rate of baroreceptors when MAP increases or Decreases?

Increases when MAP increases

Decreases when MAP decreases

Carotid & aortic baroreceptor pressure

What do baroreceptors do?

Send afferent impulses to Cardiovascular control centre in medulla of brain

CCC receives CVS afferent information

Nucleus tractus Solitarus is site of 1st synapse for all CVS afferent in medulla

NTS integrates & relays info to other regions that control pathways of the two divisions of the ANS to heart & blood vessels

MAP could be regulated by regulating what?

Heart rate

Stroke volume

Systemic Vascular Resistance

Increasing these increases MAP

What is heart rate?

Heart is an electrically controlled muscular pump

Electrical signal which control heart are generated within the heart

Modified by the autonomic nervous system

What is autorhythmicity?

Heart is capable of beating rhythmically in absence of external stimuli

How is stroke volume modified by the ANS?

Increases if contractile strength of heart is increased

Sympathetic nerves innervate the ventricular myocardium & stimulation increases force of contraction & increases stroke volume

Vagus nerve has little direct effect - no ventricular contraction

What is intrinsic control of Stroke Volume?

Through Frank-Starling mechanisms or Starling’s Law of heart

What is Systemic Vascular Resistance?

The resistance that must be overcome for blood to flow around circulatory system

What are features of Systemic Vascular Resistance?

Regulated by vascular smooth muscle

Arterioles are main site of SVR

Contraction of Vascular smooth muscle causes vasoconstriction & increases SVR & MAP, …relaxation causes vasodilation, decreasing SVR & MAP

How is SVR controlled by Vascular Smooth Muscle?

VSM supplied by Sympathetic nerve fibres

Neurotransmitter is noradrenaline acting on a receptors

VSM are partially constricted at rest - Vasomotor tone

Caused by tonic discharge of sympathetic nerves resulting in continuous release of noradrenaline

How is SVR controlled by the ANS?

Increased sympathetic discharge will increase vasomotor tone - vasoconstriction → increase SVR & MAP - pressure upstream, … decreased will decrease vasomotor tone - vasodilation → Decrease SVR & MAP

No significant parasympathetic innervation of arteria, smooth muscles - except penis & clitoris

What is meant by baroreceptors only respond to acute changes in arterial blood pressure?

Firing decreases if high arterial blood pressure is sustained - Re-set → only fire again if there is an acute change in MAP above new higher steady state level

Can’t supply info about prevailing steady state arterial BP

Control of MAP in longer-term is largely by control of BV mainly hormones