Control of blood pressure

1
Control of blood pressure

• Outline
• Short term control (baroreceptors)
• Location
• Types of baroreceptor
• Baroreceptor reflex
• Other stretch receptors
• Long-term control
• Renin/ angiotensin/ aldosterone system
• Vasopressin
• Atrial natiuretic peptide
• Response to blood loss (shock)

2
Control of blood pressure

• Mean blood pressure is controlled by changing
  total peripheral resistance and or cardiac
  output. P CO x TPR (compare Ohms law)
• Cardiac output is controlled by sympathetic and
  para sympathetic nerves which effect
• heart rate
• force of contraction
• TPR controlled by nervous and chemical means to
  effect constriction/dilatation of
• arterioles and venules

3
Regulation of blood pressure

• How is pressure measured?
• Short term
• Baroreceptors
• Long term
• Kidney via renin angiotensin system

4
Location of baroreceptors

• Baroreceptors sense stretch and rate of stretch
  by generating action potentials (voltage spikes)
• Located in highly distensible regions of the
  circulation to maximise sensitivity

http//www.cvphysiology.com/Blood Pressure/bp012
baroreceptor anat.gif
5
Baroreceptor output(from single fibres)
From Introduction to Cardiovascular physiology.
J.R. Levick. Arnold 4th edition (2003)
6
Two types of baroreceptor

• Type A
• High sensitivity
• High firing rate
• Type C
• Lower sensitivity
• Lower firing rate
• Higher threshold (before firing starts)
• Therefore can deal with higher pressures than
  type A which become saturated

7
Response of single baroreceptor fibre to change
in pressure
From An Introduction to Cardiovascular
Physiology J.R. Levick
8
Baroreceptor reflex
Blood pressure falls
Sensors
Neural integration
Effectors
Increased blood pressure
9
Baroreceptor reflex is a
feedback loop
Example central heating system
Set temperature
Read temperature
Is temperature too high?
Yes
No
Boiler on
Negative feedback
10
Baroreceptor reflex is a
feedback loop
Read pressure
Is pressure too high?
Yes
Increase CO
Increase TPR
Two way negative feedback
11
Positive feedback loop
Unstable
Set temperature
Read temperature
Is temperature too high?
No
Yes
Boiler on
Positive feedback
12
Other stretch receptors

• Coronary artery baroreceptors
• Respond to arterial pressure but more sensitive
  than carotid and aortic ones
• Veno-atrial mechanoreceptors
• Respond to changes in central blood volume
• Lie down, lift your legs and cause peripheral
  vasodilatation
• Unmyelinated mechanoreceptors
• Respond to distension of heart
• Ventricular ones during systole atrial ones
  during inspiration

13
Location of receptors in and near the heart
Nucleus tractus solitarius
Cardiac vagal afferents
Cardiac pain
unmyelinated
myelinated
Spinal cord
Baroreceptors in coronary arteries and aortic arch
Sympathetic afferents unmyelinated nociceptors
From An Introduction to Cardiovascular
Physiology J.R. Levick
14
Other receptors

• Heart chemosensors
• Cause pain in response to ischaemia
• K, lactic acid, bradykinin, prostaglandins
• Arterial chemosensors
• Stimulated in response to
• Hypoxaemia, hypercapnia, acidosis,
  hyperkalaemia
• Regulate breathing
• Lung stretch receptors
• Cause tachycardia during inspiration

too much CO2 too much K
15
Overview of short-term control mechanisms
From Introduction to Cardiovascular physiology.
J.R. Levick. Arnold 4th edition (2003)
16
Long term control of blood pressure

• Involves control of blood volume/sodium balance
  by the kidneys
• Hormonal control
• Renin-angiotensin-aldosterone system
• Antidiuretic hormone (vasopressin)
• Atrial natiuretic peptide
• Pressure natriuresis

17
Renin/angiotensin/ aldosterone system
Reduced renal blood flow
Increased blood volume
Juxtaglomerular apparatus
LV filling pressure)
Fluid re-absorption
Renin
(LV pressure beginning of systole)
Angiotensinogen
Sodium retention
Increased blood volume in the thorax
Angiotensin I
Increased aldosterone secretion
Angiotensin II
vasoconstriction
18
Vasopressin

• Enhances water retention
• Causes vasoconstriction
• Secretion increased by unloading of aortic
  Baroreceptors and atrial sensors

http//www.cvphysiology.com/Blood20Pressure/BP016
.htm
19
Atrial natiuretic peptide

• Increases salt excretion via kidneys
• By reducing water reabsorption in the collecting
  ducts
• relaxes renal arterioles
• inhibits sodium reabsorption in the distal tubule
• Released in response to stimulation of atrial
  receptors

20
Summary of long term BP control

• Cardiac output and BP depend on renal control of
  extra-cellular fluid volume via
• Pressure natriuresis, (increased renal
  filtration)
• Changes in
• Vasopressin
• Aldosterone
• Atrial natiuretic peptide

• All under the control of altered cardiovascular
  receptor signaling

21
Shock
Definition A pathophysiological disorder
characterised by acute failure of the
cardiovascular system to perfuse the tissues of
the body adequately. Levick J.R. An
Introduction to Cardiovascular Physiology

• Symptoms
• Cold, clammy skin
• Muscular weakness
• Rapid and shallow breathing
• Rapid and weak pulse
• Low pulse pressure (and sometimes mean pressure)
• Reduced urine output
• Confusion

22
Types of shock

• Hypovolaemia
• Caused by drop in blood (plasma) volume
• e.g. haemorrhage, diarrhoea, vomiting, injury
• Septic
• Caused by bacterial endotoxins
• e.g. salmonella
• Cardiogenic
• An acute interruption of of cardiac function
• e.g. myocarditis (inflammation of the heart
  muscle) or myocardial infarction
• Anaphylactic
• Caused by allergic reaction

23
Effect of blood loss

• less than 10, no serious symptoms
• e.g. blood transfusion
• 20 - 30 blood loss not usually life threatening
• greater than 30, severe drop in BP and, often,
  death due to impaired cerebral and coronary
  perfusion

24
Response to moderate blood loss(compensated
haemorrhage)

• Blood volume falls therefore pulse pressure and
  stroke volume fall. (Frank-Starling mechanism
  reduced LV contractile force)
• Cardiopulmonary stretch receptor and baroreceptor
  activity falls
• Arterial chemoreceptor activity increases, due to
  hypoxia and acidosis
• ? rapid breathing
• ? release of vasoconstrictors
• Vasopressin, angiotensin etc.

25
Response to moderate blood loss
More serious blood loss can be treated by
transfusion to lessen the effects shown here
26
Uncompensated shock

• If compensation is not sufficient, organ failure
  occurs due to inadequate perfusion
• Heart
• Kidney
• Brain