Feedback Control in the CV System

Question 1

Blood Pressure

Answer 1

Outward hydrostatic pressure exerted by blood on blood vessel walls

Question 2

Systemic Arterial Blood Pressure

Answer 2

Systolic Arterial BP (90-120mmHg) - pressure exerted by blood on walls of aorta and systemic arteries when heart contracts

Diastolic Arterial BP (60-80mmHg) - pressure exerted by blood on walls of aorta and systemic arteries when heart relaxes

Question 3

Hypertension

Answer 3

Clinic –> 140/90mmHg or higher
Day time average –> 135/85mmHg or higher

Question 4

Hypotension

Answer 4

90/60mmHg

Question 5

Pulse pressure

Answer 5

Difference between systolic and diastolic BPs

Question 6

Mean Arterial Blood Pressure (MAP)

Answer 6

MAP = (2D+S)/3
MAP + DBP + 1/3 Pulse Pressure

70-105mmHg

Average arterial blood pressure during a single cardiac cycle, which involves contraction and relaxation of the heart

Diastolic part of cardiac cycle is twice as long as systolic part

At least 60mmHg needed to perfuse vital organs but not too high as this will damage blood vessels and place extra strain on the heart and other organs

Question 7

Baroreceptor Reflex

Answer 7

Negative Feedback
–> MAP deviates
–> Detected by the Baroreceptors (mechanoreceptors in carotid & aorta)
–> Informs Medulla (CV control centre)
–> Send instructions to Heart and Blood Vessels
–> Compensatory response
–> Restoration of MAP to desired range

Question 8

Firing Rate in Baroreceptor Afferent Neurons

Answer 8

MAP Increases –> Increased Firing Rate

MAP Decreases –> Decreased Firing Rate

Question 9

CV Control Centre in Medulla

Answer 9

Receives CVS afferent information
Nucleus tractus solitarius (NTS) - first synpase in medulla for CVS afferents
NTS integrates and relays information to other regions that cintrol pathways of two divisions –> Parasympathetic and Sympathetic of ANS to heart and blood vessels

Question 10

Important Relationships

Answer 10

MAP = CO x SVR

CO = volume of blood pumped by each ventrice of the heart per minute

CO = SV x HR

SV = volume of blood pumped by each ventricle of the heart per heart beat

MAP = SV x HR x SVR

SVR = sum of resistance of all vasculature in systemic circulation

Question 11

Heart Rate

Answer 11

Heart is electrically controlled
Electrical signals generated within heart
Can beat rhythmically without an external stimuli –> AUTORHYTHMICITY

Question 12

Autonomic Nervous System

Answer 12

Sympathetic NS
- Increases heart rate -
- Noradrenaline binding on B1 receptors

Parasympathetic NS
- Vagus nerve stimulation reduces heart rate
- ACh binding on Muscarinic M2 receptors

Question 13

Stroke Volume influence by ANS

Answer 13

SV increases if contractile strength of heart is increased

Sympathetic nerves –> Ventricular Myocardium (increases contraction force) –> Increases SV

Parasympathetic Vagus nerve –> Little direct effect on Ventricular contraction

Question 14

Frank-Starling Law of the Heart

Answer 14

Intrinsic control of SV

Question 15

SVR

Answer 15

Arterioles are the major resistance vessels
Smaller diameter which acts to slow blood flow
Are more rigid

50% - Arterioles
20% - Arteries
20% - Capillaries
10% - Veins

SVR regulated by vascular SM
- Vascular SM contraction –> Vasoconstriction –> Increases SVR and MAP
- Vascular SM relaxation –> Vasodilatation –> Decreases SVR and MAP

Vascular SM supplied by sympathetic nerve fibres (Noradrenaline binding to alpha receptors)

Vascular SM partially constricted at rest (Vasomotor tone) –> Tonic discharge of sympathetic nerves

Increased sympathetic discharge –> Increased Vasomotor tone –> Vasoconstriction

Decreased sympathetic discharge –> Decreased Vasomotor tone –> Vasodilatation

No significant parasympathetic innervation of arterial smooth muscles - expect penis and clitoris

Question 16

Baroreceptors only respond to acute changes in arterial BP

Answer 16

Firing decreases if high arterial BP is sustained
Baroreceptors reset –> fire again if there’s an acute change in MAP above new higher steady state level