Blood Pressure and Control Mechanisms

Question 1

How is blood pressure generated and maintained?

Answer 1

*Ventricular Contraction
* Elastic rebound evens & maintains pressure

Question 2

What are the factors affecting blood pressure?

Answer 2

• Cardiac Output
• Peripheral Vascular resistance
• Volume

Question 3

What are the important equations in regards to mean arterial pressure (MAP)?

Answer 3

*MAP = CO (Cardiac Output) x PVR (Peripheral Vascular Resistance)
*Cardiac Output = Stroke Volume x Heart Rate

Question 4

What are the factors affecting cardiac output and their equations?

Answer 4

*Heart rate
*Stroke volume= end diastolic volume - end systolic volume
*Cardiac output (CO)= HR X SV

Question 5

What are factors affecting heart rate?

Answer 5

*Autonomic innervation
*Hormones
*Fitness levels
*Age

Question 6

What are factors affecting stroke volume?

Answer 6

*Heart size
*Fitness levels
*Sex
*Contractility
*Duration of contraction
*Preload (end diastolic volume)= amount of stretch during diastole when the ventricles fill with blood
*Afterload (resistance)= the pressure against which the heart must work to eject blood during systole

Question 7

What are factors affecting contractility?

Answer 7

Starling’s Law of the Heart (The Frank-Starling Mechanism)

Question 8

What does resistance depend on in a closed system?

Answer 8

*The length of the tube
*Radius of the tube
*Viscosity of the fluid

Question 9

What is the effect of increased blood volume (hypervolemia)?

Answer 9

• Increased BP
  *Increased preload = increased stroke volume= increased cardiac output= increased blood pressure

Question 9

What is the effect of decreased blood volume (hypovolemia)?

Answer 9

*Decreased blood pressure
*Compensatory mechanisms
*Hypovolaemic shock

Question 10

What are factors affecting blood volumes?

Answer 10

Oedema = accumulation fluid in interstitial space
Disruption in fluid exchange between blood and tissues
Factors disrupting balance
1. Increased hydrostatic pressure – eg heart failure – back pressure
2. Decreased plasma protein concentration – osmotic balance disrupted
3. Increase in interstitial proteins

Question 11

What is vascular tone?

Answer 11

• Refers to the degree of constriction experienced by ablood vessel relative to its maximally dilated state (max lumen= maxcapacity).
• All arterial and venous vessels under basal conditions exhibit some degree of smooth muscle contraction that determines the diameter, and hence tone, of the vessel.
  *Vascular Tone: balance between contraction and dilatation in a vessel
  *Contraction and dilatation depends on:
  1) Composition of the vessels
  2) Control by Intrinsic and extrinsic factors

Question 12

What effects smooth muscle contraction?

Answer 12

• The mechanisms by which Extrinsic and Intrinsic mechanism influences either constrict or relax blood vessels involve a variety of signal transduction mechanisms that ultimately influence the interaction between actin and myosin in the smooth muscle promoting contraction/relaxation.
• Calcium changes inside the muscle cells are coupled to contraction/relaxation of the cell.

Question 13

What are the control mechanisms of the cardiovascular system?

Answer 13

• Nervous/cytokine/autoregulatory
• ANS/RAAS
• NO, PGs etc..

Question 14

How do baroreceptors regulate heart rate?

Answer 14

*Stretch-sensitive mechanoreceptors
*Found in Aortic arch and carotid sinus
Increased/Decreased Blood Pressure -> Increases/Decreases Stretch on Vessel Wall -> Increased/Decreased firing rate from Baroreceptor -> Medullary Cardiovascular Control Centre -> Altered Sympathetic / Parasympathetic Activity

Question 15

What does the Sympathetic Nervous System do to the heart rate?

Answer 15

• SNS increases (+) heart’s actions leading to:
• Increased rate/chronotropy (tachycardia)
• Increased force of contraction/inotropy
• Increased relaxation during diastole (lusitropy)
• Slight Increased impulse conduction (dromotropy)
• Excitability is also increased
  *Postganglionic fibres from ganglion chain
• Innervate atria, especially SA node, ventricles, including conduction system

Question 16

What does the Parasympathetic Nervous System do to the heart rate?

Answer 16

• PNS decreases (-) heart’s actions leading to:
• Decreased rate/chronotropy (bradycardia), dromotropy
• Slight decreased inotropy, lusitropy
• Vagus nerves (Xth)
• Right to SA, Left to AV, but left/right innervation sometimes overlaps
• Some atrial muscle, less so ventricles (hence why less control of inotropy/ lusitropy)

Question 17

What receptors are used in which nervous system?

Answer 17

*Sympathetic
Noradrenaline – adrenoreceptors
Alpha and Beta
*Parasympathetic
Acetylcholine - muscarinic receptors
*Ganglia (both)
Acetylcholine – Nicotinic receptors

Question 18

What effect does adrenaline have on adrenergic receptors?

Answer 18

• α1 stimulated by noradrenaline –vasoconstriction
• Neurotransmitter effect
• Low [adren]: β2-selective -> mostly vasodilatation
• High [adren]: also stimulates α1, andβ1 receptors, producing
• vasoconstriction (mediated byα1 receptors)
• increased heart rate & contractility

Question 19

What is the stimulus for renin release?

Answer 19

1 Decrease in sodium chloride concentration
2 Decreased rate of blood flow through the macula densa of kidneys
3 Baroreceptor detected decrease in blood pressure

Question 20

What is renin’s mode of action?

Answer 20

Converts angiotensinogen (synthesised in liver) into angiotensin I (ATI)

Question 21

What are the forms of Androtensin Converting Enzyme?

Answer 21

*ACE and ACE2
*Expressed predominantly in lung
*Converts ATI into Angiotensin II (ATII)
*Inhibited by ACE inhibitors (ACEI) e.g. captopril

Question 22

What is the actions of angiotensin II?

Answer 22

1. Vasoconstrictor
2. ADH (vasopressin) release
3. Increased Sympathetic nervous system activity
4. Increased sodium/water reabsorption
5. Aldosterone Release

Question 23

What do ATI receptors do?

Answer 23

Vasoconstriction which increases the blood pressure

Question 24

What are the actions of ATII?

Answer 24

*Stimulates ADH (Vasopressin) release
*Stimulates AT1 receptors in hypothalamus -> Release of ADH -> Increased water reabsorption
*Increased SNS activity
*Aldosterone release

Question 25

What is aldosterone?

Answer 25

*Mineralocorticoid
*Steroid hormone released form adrenal gland
*From adrenal cortex (zona glomerulosa)

Question 26

How does hormonal/ paracrine regulation work?

Answer 26

*Release of a number of mediators e.g nitric oxide, endothelin , prostaglandins etc.
*Vascular endothelium

Question 27

What is the action of Nitric Oxide?

Answer 27

• Vasodilation
• Controls platelet aggregation, immunomodulatory

Question 28

What stimulates the production of Nitric Oxide?

Answer 28

• Acetylcholine –
• Histamine, bradykinin, serotonin
• Shear stress on blood vessels
• Synthesised from arginine by nitric oxide synthase (NOS)

Question 29

What is the action of Endothelin?

Answer 29

*Vasoconstriction
*Positive inotropic/chronotropic effects
*Opposite effect to NO

Question 30

What are the 3 isoforms of Endothelin and their receptors?

Answer 30

*Three isoforms - ET-1, ET-2, ET-3
*ET-1 main for cardiovascular effects(produced by endothelium and also VSMC (vascular smooth muscle cells) in inflammation)
*ETa and ETb

Question 31

What does ECE stand for?

Answer 31

Endothelin converting enzyme

Question 32

What is the action of Prostaglandin I2 (PGI2; prostacyclin) and what is it synthesised from?

Answer 32

*Vasodilator
*Inhibits platelet aggregation
*Anti-inflammatory effects
*PGI2 = synthesised from arachidonic acid