Control of Blood Pressure

Question 1

How does cardiac excitation occur?

Answer 1

1% of Cardiac Cells in the Heart are Autorhythmic cells which are responsible for Pacemaker activity

99% Cardiac Cells are contractile

Question 2

What is the SA Node?

Answer 2

Sinoatrial Node - specialised region in the right atrial wall near opening of vena cava

Cells in SA Node normally fire fastest to generate the heartbeat

-Does not have resting potential

Question 3

How does the SA Node achieve the ability of not having a resting potential?

Answer 3

• Slow depolarisation (Slow Na+ influx; Ca2+ influx; reduced K+ efflux)
• ‘Rapid’ depolarisation (Ca2+ influx)
• Repolarisation (K+ efflux)

Potential of the Node always chaning so no resting potential

Question 4

How does the action potential of the ventricular cell occur?

Answer 4

No pacemaker potential (meaning cells remain essentially at rest (-90mV) until excited by electrical activity propagated from the pacemaker

• Rapid depolarisation (Na+ influx)
• Plateau Phase (Ca2+ influx)
• Repolarisation (K+ efflux)

Question 5

How does Cardiac Excitation occur?

Answer 5

-Needs to be efficient and co-ordinated

• Action potentials generated at SA Node
• Rapid excitation through both atria
• excitation reaches AV Node where conduction is SLOW (allow atria to contract and empty blood into ventricles)
• Excitation spreads rapidly down the bundle of His & Purkinje fibres to ensure almost simultaneou s activation of ventricular cells

Question 6

What do the sections of an ECG represent?

Answer 6

P wave - Atrial depolarisation
Ventricular depolarisation (QRS) -this masks atrial repolarisation so no wave for atrial repolarisation
T Wave - Ventricular repolarisation

Question 7

What occurs during Atrial Systole?

Answer 7

AV Valves open & atria empty blood into ventricles

- Atrial excitation and contraction should be complete before onset of ventricular contraction

Question 8

What occurs during Ventricular systole?

Answer 8

Part 1:
- Ventricles contract; rise in pressure closes AV Valve
Part 2:
- Pressure in ventricle rises above aortic pressure therefore aortic valve opens & blood is ejected from heart

Question 9

What occurs during ventricular diastole?

Answer 9

Part 1:
-pressure in ventricles falls below aortic pressure therefore aortic valve closes
Part 2:
-pressure in ventricles falls below atrial pressure therefore AV Valve opens and filling begins

Question 10

What factors are stroke volume influenced by?

Answer 10

1) End Diastolic Volume (Preload)
2) Aortic Pressure (Afterload)
3) Contractility (increased by sympathetic nerve stimulation)

Question 11

What is the equation for Cardiac Output?

Answer 11

Cardiac Output = Stroke Volume x Heart Rate

Question 12

What are the values of Cardiac Output during rest and exercise?

Answer 12

At rest = 5L/min

During Exercise = 22L/min

Question 13

What does cardiac output depend on?

Answer 13

• Stroke Volume and the factors that affect it

- Heart Rate and the factors that affect that

Question 14

What are the factors affecting heart rate?

Answer 14

Intrinsic rate: -Pacemaker (Sino-atrial node)
-Conduction (Atrio-ventricular node)

Influenced by:

• Sympathetic nervous system - Noradrenaline acting on beta-adrenoceptors to cause an increase in heart rate
• Parasympathetic nervous system - acetylcholine acting on muscarinic receptors to cause a decrease in heart rate
• Hormones - adrenaline acting on beta-adrenoceptors to cause an increase in heart rate
• Extra/intracellular ions - eg Potassium

Question 15

What is the average systolic BP? What is it determined by?

Answer 15

120mmHg
determined by:
-Stroke volume (increased stroke volume, increased SBP)
-Aortic elasticity (decrease elasticity, increase SBP)

Question 16

What is the average systolic BP? What is it determined by?

Answer 16

80mmHG
determined by:
-Peripheral resistance (increase TPR, decreased DBP)
-Aortic elasticity (decrease elasticity, decrease DBP)
-Heart rate (decrease HR, decrease DBP)

Question 17

What is the equation for Pulse Pressure?

Answer 17

Pulse Pressure = SBP - DBP = 40

Question 18

What is the equation for mean blood pressure?

Answer 18

Mean BP = DBP + 1/3 Pulse Pressure = approx 93mmHg

Question 19

How does heart rate affect Cardiac Output?

Answer 19

Sympathetic Nervous System leads to release of noradrenaline binding to beta1-adrenoceptors which leads to an INCREASE IN HEART RATE

Parasympathetic nervous system leads to a release of acetylcholine binding to muscarinic receptors which leads to a DECREASE IN HEART RATE

Increasing Heart Rate increases CO
Decreasing Heart Rate decreases CO

Question 20

How does Stroke Volume affect heart rate?

Answer 20

When a ventricular muscle is innervated by the autonomic NS through the sympathetic nervous system which causes the release of Noradrenaline that binds to beta1-adrenoceptors leading to an increase in contractility

Ultimately, sympathetic stimulation causes increase in HR, increase in StV and therefore an increase in CO

Question 21

What are the factors that affect peripheral resistance?

Answer 21

Changes in vessel radius have the greatest influence on resistance:

Vasoconstriction - decrease in diameter therefore increased resistance

Vasodilatation - increase in diameter therefore decreased resistance

Question 22

How are arterioles’ vessel diameter influenced?

Answer 22

Arterioles are innervated by sympathetic NS:

• release of noradrenaline binding to ALHPA1-adrenoceptors causes vasoconstriction
• release of noradrenaline binding to BETA2-adrenoceptors causes vasodilatation

Generally, sympathetic stimulation leads to vasoconstriction and increased TPR

Question 23

Why is control of arterial blood pressure important?

Answer 23

-Driving force for the provision of blood to tissues

Too Low -> inadequate supply of nutrients to tissues

Too High -> excessive strain on the heart and increased risk of vascular damage

Question 24

How is the control of arterial blood pressure achieved?

Answer 24

Effector mechanisms - autonomic nervous system
Pressure sensors - baroreceptors
Central Integration - CNS cardiovascular centres

Question 25

What are the pressure sensors? Where are they and what do they do?

Answer 25

Baroreceptors

• Located in the walls of major arteries and the heart/pulmonary blood vessels
• Sense changes in transmural pressure (stretch)

Question 26

How do Baroreceptors cause change of blood pressure?

Answer 26

• Fall in Blood Pressure
• Sensed by Baroreceptors
• Afferent input to CNA cardiovascular centres
• Information centrally integrated
• Efferent output via autonomic nervous system
• Increased sympathetic drive to heart and blood vessels
• Increase in HR, Stv and TPR
• Rise in Blood Pressure towards Normal

Question 27

How does Postural adaptation cause blood pressure change?

Answer 27

• Moving from supine to upright posture causes pooling of blood in abdomen and limbs
• Reduced venous return, therefore reduced cardiac output
• Reduced arterial blood pressure
• Baroreceptor-Mediated Reflex
• Increased sympathetic output to heart and blood vessels
• Increased HR, Stv and TPR
• Rise in blood pressure towards normal

Question 28

How do you calculate mean arterial BP?

Answer 28

Mean arterial BP = cardiac output x total peripheral resistance