M3 Topic 6: Blood Pressure

Question 1

Blood pressure

Answer 1

Amount of force that blood exerts on vessel wall

• Measured in millimetres of mercury (mmHg)
• Pumping action of heart generates blood pressure (as blood moves along pressure gradient)

When term ‘blood pressure’ is used w/o additional qualifiers, it is referring to the blood pressure in the large systemic arteries located close to the heart (arterial blood pressure)

Question 2

Where is the systemic blood pressure highest?

Answer 2

In the aorta

• Decreases as blood moves through systemic circulation

Question 3

Reading measurements of arterial blood pressure

Answer 3

As it is pulsatile (increases and decreases in regular manner that reflects contraction and relaxation of ventricles), it is given as two numbers

E.g. for normal blood pressure
- “120 on 80” = 120/80 mmHg
- 120 = the systolic pressure
- 80 = the diastolic pressure

Question 4

Systolic pressure

Answer 4

Pressure in arteries during systole

Question 5

Diastolic pressure

Answer 5

Pressure in arteries during diastole

Question 6

Pulse pressure

Answer 6

Difference between systolic pressure and diastolic pressure

Question 7

How is blood pressure measured?

Answer 7

Done indirectly in brachial artery using inflatable cuff around upper arm with pressure gauge

• Can be done with stethoscope (auscultatory method) or using an automated blood pressure monitor

1. Inflatable cuff wrapped around upper arm, bladder of cuff placed over brachial artery
2. Stethoscope positioned distal to cuff
3. Cuff inflated beyond expected systolic pressure, compressing arm, compressing brachial artery, stopping arterial blood flow. No sound can be heard through the stethoscope at this point
4. Cuff pressure slowly decreased, once cuff pressure drops below systolic pressure, artery begins to open a little bit allowing turbulent movement of blood through brachial artery. This creates Korotkoff sounds. This is the value of the systolic pressure
5. Cuff pressure continues to decrease, once cuff pressure drops below diastolic pressure, no compression of brachial artery, thus laminar flow and no sound. This is the value of the diastolic pressure.

Question 8

Mean arterial pressure (MAP)

Answer 8

Represents average blood pressure that drives blood flow into systemic circulation and tissues

Question 9

MAP formula

Answer 9

MAP = diastolicBP + pulsepressure/3

Question 10

What factors affect MAP?

Answer 10

• Cardiac output (CO)
• Total peripheral resistance (TPR)

Question 11

What causes an increase in MAP?

Answer 11

Increases in CO or TPR

Question 12

What causes a decrease in MAP?

Answer 12

Decreases in CO or TPR

Question 13

Factors that affect CO

Answer 13

Anything that alters HR or SV can also affect MAP as it affects CO

Important factor affecting CO is blood volume

• Increase in blood volume increases VR, EDV and SV to increase CO, which increases MAP
• Decrease in blood volume (due to dehydration, blood loss) decrease VR and SV, decreasing CO, which increases MAP

Question 14

Factors that affect TPR

Answer 14

• Vessel length
• Blood Viscosity
• Vessel diameter

Question 15

TPR analogy

Answer 15

Similar to drinking through a straw

• Longer straw (vessel length) = higher resistance
• Thinner straw (vessel diameter) = higher resistance
• Thicker drink (blood viscosity) = higher resistance

Question 16

Importance of blood vessel diameter in TPR and MAP

Answer 16

Regulation of blood vessel diameter is what occurs in normal physiological circumstances

• Vasoconstriction leads to lower blood vessel diameter, increasing TRP, increasing MAP
• Vasodilation leads to higher blood vessel diameter, decreasing TRP, decreasing MAP

Question 17

Blood viscosity affect in TPR and MAP

Answer 17

• Increase in number of blood vessels in same volume of fluid increases viscosity, increasing TPR, increasing MAP
• Decrease in number of blood vessels in same volume of fluid decreases viscosity, decreasing TPR, decreasing MAP

Question 18

Blood vessel length affect in TPR and MAP

Answer 18

• Increase length = increase TPR = increase MAP
• Decrease length = decrease TPR = decrease MAP

Question 19

How does the ANS regulate MAP

Answer 19

• Sympathetic NS activity
• Parasympathetic NS activity

Question 20

Increase in parasympathetic NS activity affect on MAP

Answer 20

Decrease HR, which decreases CO, decreasing MAP

Question 21

Increase in sympathetic NS activity affect on MAP

Answer 21

• Increase HR, increasing contractility, increasing SV, increasing CO, increasing MAP
• Vasoconstriction in veins, increasing VR/SV, increasing CO, increasing MAP
• Vasoconstriction in arterioles, decreasing blood vessel diameter, increasing TPR, increasing MAP

Question 22

How is the MAP regulated in the short-term?

Answer 22

Baroreceptor reflex

• Neural reflex pathway
• Example of feedback mechanism (homeostatic control of MAP)

Question 23

What does the baroreceptor reflex pathway consist of?

Answer 23

• Receptor = baroreceptors
• Control centre = cardiovascular control centre
• Effectors = changes to CO and TPR

Question 24

Baroreceptors

Answer 24

Monitors and detects changes in blood pressure

• Located in walls of aorta and carotid arteries
• Respond to change in stretch of vessel wall (works as mechanoreceptor)
• Tonically active
• Increase firing rate when arterial pressure increases
• Decrease firing rate when arterial pressure decreases

Question 25

Cardiovascular control centre

Answer 25

Receives signals from baroreceptors, sending it to effector organs (heart, blood vessels) via the ANS - Located in medulla of brain

Question 26

Effector organ response to decrease in arterial pressure (low baroreceptor firing)

Answer 26

- Sympathetic NS activity increase - Parasympathetic NS activity decrease - Leads to increase in HR and SV, increasing CO - Also causes vasoconstriction of blood vessels, causing increase in TPR - All together increases MAP

Question 27

Effector organ response to increase in arterial pressure (high baroreceptor firing)

Answer 27

- Sympathetic NS activity decrease - Parasympathetic NS activity increase - Leads to decrease in HR, which decreases CO, decreasing MAP

Question 28

How is the MAP regulated in the long-term?

Answer 28

Via the kidneys, it is the main organ responsible for regulative blood volume Blood volume = major determinant of CO

Question 29

How do the kidneys regulate blood volume?

Answer 29

Via salt and water excretion - Increase in excretion decreases blood volume, decreasing MAP - Decrease in excretion (increase in retention) increases blood volume, increasing MAP

Question 30

Hormones impact on kidneys and MAP

Answer 30

Hormones affect ability of kidneys to retain salt and water - Salt retention promotes water retention - If hormone increases salt retention, it increases water retention and thus, blood volume is increased

Question 31

Renin-angiotensin system

Answer 31

Most physiologically important hormone system regulating long-term MAP. Activated when arterial pressure decreases - Produces angiotensin II (Ang II) hormone, increasing thirst and releasing two other hormones; Anti-diuretic hormone (ADH) and Aldosterone - ADH increases water retention - Aldosterone increases salt retention All together, increases blood volume, increasing MAP