Physiology: Arterial Blood Pressure and its Control (short & long term) Flashcards
(76 cards)
1
Q
Define blood pressure
A
The outward hydrostatic pressure exerted by blood on the vessel walls
2
Q
Define the systolic and diastolic blood pressures
A
Systolic:
- blood pressure exerted on systemic arteries when the heart contracts
Diastolic:
- blood pressure exerted on systemic arteries when the heart relaxes
3
Q
Define hypertension
A
A Bp > 140/90 mmHg in clinic
or
A daytime average Bp > 135/85 mmHg
4
Q
Define pulse pressure
A
The difference between the systolic and diastolic blood pressures
(systolic)-(diastolic)
5
Q
What is the range for a healthy Bp in adults?
A
90/60 to 120/80
6
Q
Describe how a sphygmomanometer measures Bp
A
- Cuff pressure is made to completely occlude the artery (cuff pressure > systolic pressure)
- Pressure is decreased until sound can be heard (1st Korotkoff sound)
- Sound is due to the turbulent flow of blood through the semi-occluded artery
- Pressure is decreased further until no sound can be heard (5th Korotkoff sound)
- Laminar flow restored
7
Q
What blood pressure is represented by the first Korotkoff sound?
A
The MAX systolic pressure
8
Q
What blood pressure is represented by the fifth Korotkoff sound?
A
The diastolic Bp
9
Q
What pressure gradient drives blood through the systemic circulation?
A
The pressure gradient between the right atrium (pressure nearly 0) and the aorta (high pressure)
Thus it is the MAP that drives blood in the systemic circulation
10
Q
Define mean arterial blood pressure (MAP)?
A
The average arterial blood pressure during a single cardiac cycle
11
Q
Give the 2 formulae for calculating the MAP from blood pressures
A
MAP = [(2x Diastolic)+(systolic)]/3
MAP = Diastolic + (1/3 Pulse Pressure)
12
Q
Give the range for a healthy MAP in an adult at rest
A
70 –> 105 mmHg
13
Q
What is the MAP required to perfuse the vital organs?
A
60 mmHg
14
Q
Give the formulae for calculating the MAP from cardiac output and systemic vascular resistance
A
MAP = CO x SVR
15
Q
Define cardiac output
A
The volume of blood being pumped by each ventricle of the heart per minute
16
Q
Define systemic vascular resistance (SVR)
aka TPR
A
The sum of resistance of all vasculature in the systemic circulation
Mainly from arterioles (~50%)
17
Q
Give the equation for calculating the cardiac output
A
CO = SV x Heart rate
18
Q
Define Total Peripheral Resistance (TPR)
A
The sum of resistance of all vasculature in the systemic circulation
The same as systemic vascular resistance
19
Q
Describe the effect of parasympathetic stimulation of the heart on MAP
A
Vagal stimulation --> heart rate decreases --> cardiac output decreases --> MAP DECREASES
20
Q
Describe the effect of sympathetic stimulation of the heart on MAP
A
Sympathetic stimulation --> heart rate increase --> cardiac output increase --> MAP increase
21
Q
Describe the effect of sympathetic stimulation of the arterioles on MAP
A
Sympathetic stimulation --> vasoconstriction --> SVR increases --> MAP increase
22
Q
Describe the effect of sympathetic stimulation of the veins on MAP
A
Sympathetic stimulation --> vasoconstriction --> venous return increases --> stroke volume increases --> Cardiac output increases --> MAP increases
23
Q
Define postural (orthostatic) hypotension
A
- Failure of Baroreceptor responses causing a drop in Bp
- Due to gravitational shifts in blood, when moving from horizontal to vertical position
24
Q
What mechanism manages acute changes in MAP?
A
The baroreceptor reflex
25
Is the baroreceptor reflex a +ve or -ve feedback mechanism
Negative feedback control
26
Describe a baroreceptor
- Mechanoreceptors
- Sensory neurons
- Detect 'stretch'
27
Where can baroreceptors be found
- Carotid sinuses (carotid baroreceptors)
| - Aortic arch (aortic baroreceptors)
28
Which nerves innervate the baroreceptors?
Carotid baroreceptors:
- glossopharyngeal nerve (cranial nerve IX)
Aortic baroreceptors:
- Vagus nerve (cranial nerve X)
29
Describe the afferent firing of baroreceptors
Usually fire at normal rate
When MAP increases, firing rate increases
When MAP decreases, firing rate decreases
30
What region of the brain receives the afferent signalling from the baroreceptors?
The cardiovascular control centre in the medulla
31
What is the role of the cardiovascular control centre in the medulla with regards to the baroreceptor reflex?
- Receives afferent signals
- Relays info to other brain regions
- Generates vagal outflow to the heart (if needed)
- Regulates spinal sympathetic neurons
32
What effect can the baroreceptor reflex have on the heart?
- Modify heart rate
| - Modify stroke volume
33
What effect can the baroreceptor reflex have on the vasculature?
Modify systemic peripheral resistance
34
How is heart rate modified by the baroreceptor reflex?
To increase heart rate:
- Sympathetic stimulation
To decrease heart rate:
- Vagal stimulation
35
How is stroke volume modified by the baroreceptor reflex?
To increase SV:
- Sympathetic stimulation of heart
- Sympathetic stimulation of veins (venoconstriction increases venous return, increasing SV)
To decrease SV:
- decreased sympathetic stimulation
- vagal stimulation has little effect
36
How is SVR modified by the baroreceptor reflex?
To increase MAP:
- Increased vasomotor tone
- Vascular smooth muscle contracts
- Causing vasoconstriction
- Increasing SVR
To decrease MAP:
- decreased vasomotor tone
- Vascular smooth muscle relaxes
- Vasodilatation
- Decreasing SVR
37
What type of vessel accounts for the majority of SVR?
Arterioles (~50%)
38
In the baroreceptor reflex what happens to the corrective mechanism after the change in MAP has been corrected?
Negative feedback control stops the overcorrection
39
Describe the 'resetting' of baroreceptors
- Baroreceptors reset what they consider a normal range
- Only stimulating a response again if there is an acute deviation from their current normal range
- Slow changes in MAP can avoid detection of the reflex, and raise its considered normal range
40
Explain why the baroreceptor reflex fails to deal with slow rises in blood pressure
The change is to slow to be picked up the the baroreceptors
Slow enough that the baroreceptors reset their considered normal MAP range to match the raised MAP
41
Describe the constriction of vascular smooth muscle at rest
They are partially constricted
Due to the vasomotor tone
42
What is the vasomotor tone?
The continuous of sympathetic stimulation to vascular smooth muscle
Results in a continuous release or noradrenaline
Can vary in strength to modify vasoconstriction and vasodilation
43
Describe the parasympathetic innervation of arterial smooth muscle
There is no significant innervation of arterial smooth muscle
(except in the penis and clitoris)
44
What type of control mechanisms control long-term changes on MAP
Hormonal control of blood volume
45
Describe the fluid compartments of the body
Intracellular:
- Intracellular fluid
Extracellular:
- Interstitial fluid
- blood plasma
46
Describe what happens to the interstitial fluid volume if the blood plasma volume decreases
It decreases as compensatory mechanisms shift fluid from the interstitium to the the plasma compartment
47
List the 2 main factors that affect extracellular fluid (ECF) volume
- Water excess/deficit
| - Na+ excess/deficit
48
What do hormones regulate in order to control the ECF volume
Hormones control the extracellular fluid volume (including Plasma Volume) by regulating the Water and Salt Balance
49
List the hormone systems that regulate ECF volume
- Renin-Angiotensin-Aldosterone system (RAAS)
- Natriuretic peptides (NPs)
- Antidiuretic hormone (ADH
(aka Arginine vasopressin)
50
Describe how aldosterone is produced by the Renin-Angiotensin-Aldosterone System (RAAS)
- Juxtaglomerular cells in the kidneys release renin
- Renin stimulates the formation of angiotensin I from angiotensinogen
- Angiotensin I is converted to angiotensin II by the enzyme 'angiotensin converting enzyme (ACE)'
- Angiotensin II stimulates the release of aldosterone from the adrenal cortex
51
Where is angiotensinogen produced?
The liver
52
Name the enzyme that converts angiotensin I --> angiotensin II
Angiotensin converting enzyme (ACE)
53
Where is aldosterone released from?
The adrenal cortex
54
Describe the role of angiotensin II in regulating blood pressure
1) It causes systemic vasoconstriction, increasing the SVR, thus increasing the MAP
2) Stimulates the release of aldosterone from the adrenal cortex. Increases water and Na+ retention
3) Stimulates thirst. Water intake increases --> plasma volume increases --> MAP increases
4) Stimulates ADH release. Increasing plasma volume
55
Describe the role of aldosterone in regulating blood pressure
Aldosterone:
- Increases Na+ and water reabsorption in the kidneys
- Decreases Na+ and water excretion
- Plasma volume increases, thus Bp increases
56
What stimulates the release of renin from the juxtaglomerular cells in the kidneys (starting the RAAS)?
- Low plasma volume and low Bp
- Decreased [Na+] in the renal tubular fluid
- stimulation of renal sympathetic nerves
57
Where is Angiotensin converting enzyme (ACE) produced?
pulmonary vascular endothelium
58
What is the rate limiting step for RAAS?
The secretion of renin from the kidneys
59
Describe the natriuretic peptide control of Bp
```
Increase Na+ and water excretion in the kidneys
-->
Reducing blood plasma
-->
BP decrease
```
Also decrease renin release, decreasing Bp
Also act as vasodilators (decrease SVR, decrease Bp)
60
What causes Natriuretic Peptides to be released?
- Cardiac distension (hypervolemic state)
| - Neurohormonal stimuli
61
What inhibits the release of renin from the juxtaglomerular cells in the kidneys?
Natriuretic peptides
62
Describe the role of the Natriuretic Peptides
To lower blood pressure and plasma volume
Acts as a county-regulatory system for the RAAS
63
Name the 2 types of Natriuretic Peptide released by the heart
- Atrial Natriuretic Peptide (ANP)
| - Brain-type Natriuretic Peptide (BNP
aka ventricular-type)
64
Describe Atrial Natriuretic Peptide (ANP)
- 28 amino acid peptide
- synthesised and stored by atrial myocytes
- released in due to atrial distension (hypervolemic state)
65
Describe Brain-type Natriuretic Peptide (BNP)
- 32 amino acid peptide
| - synthesised by heart ventricles, brain and other organs
66
Describe the synthesis of BNP
```
Prepro-BNP
-->
Pro-BNP
-->
BNP
```
67
Describe the role of BNP in diagnosing heart failure
- Serum BNP and NT-pro-BNP can be measured to diagnose heart failure
68
Describe the synthesis and storage of antidiuretic hormone (ADH)
ADH is derived from a prohormone precursor that is synthesised in the hypothalamus
- ADH is stored in the posterior pituitary
69
What is another name for antidiuretic hormone (ADH)
vasopressin
70
What stimulates the secretion of ADH from the posterior pituitary?
1) increased extracellular fluid osmolality
2) reduced extracellular fluid volume
3) increased plasma osmolality
* (osmolarity increases due to loss of water volume, thus ADH responds to changing water levels)
71
How is plasma osmolarity monitored?
By osmoreceptors
- Mostly in the brain close to the hypothalamus
72
Describe the mechanism of action of ADH
```
Acts in the kidney tubules to increase water reabsorption
-->
Increases ECF and plasma volumes
-->
Cardiac output increases
-->
Bp increases
```
Also causes vasoconstriction
-->
Increases SVR and Bp
73
Describe the main role of ADH
To regulate plasma and interstitial fluid volumes and osmolarities
74
Describe the main role of RAAS
Long term regulation of MAP
75
Why don't you check both carotid pulses simultaneously?
It would generate enough baroreceptor stimulation to cause hypotension that would lead to syncope or presyncope
76
Describe why checking the carotid pulse causes baroreceptor stimulation
finger pressure causes partial occlusion of the artery
-->
causes a back pressure
-->
back pressure reaches the carotid sinus, causing it to stretch
-->
The increased stretch is detected by carotid baroreceptors
