L12- Control Of Blood Pressure

Front 1

explain the pressure changes through the vasculature...

Back 1

The pressure is lower in the capillaries so they don't rupture or get damaged

Pressure in the lungs is much lower than the systemic circulation, vascular resistance is much lower in the lungs

Front 2

Back 2

In the left ventricle when the heart is beating (contracting and relaxing) the pressure is going from 0 to 120

Front 3

Systolic pressure is determined by...

Back 3

cardiac output

Front 4

Diastolic pressure is determined by...

Back 4

vascular resistance

Front 5

Why does arterial pressure fluctuate between systole and diastole?

Back 5

1) Expansion of elastic arteries during systole

reduces systolic pressure.

2) Elastic recoil during diastole maintains diastolic pressure thus maintaining flow and preventing vessel collaspe.

(if the aorta was rigid, systolic pressure would be very high and diastolic near zero)

Front 6

Pressure is influenced by wall tension and radius.

What is the LaPlace equation:

Back 6

For tubes: Pressure (P) = wall tension (T) / radius (r)

P = T/r

Front 7

For a given wall tension, increased radius means...

Back 7

decreased pressure.

Front 8

Decreased radius means...

Back 8

increased pressure

Front 9

So elastic recoil of arterioles in diastole is an example of decreasing the radius and thus...

Back 9

increasing the pressure (maintaining pressure so it doesn't go too low during diastole)

Front 10

What is the equation for measuring MABP?

Back 10

MABP = diastolic pressure + pulse pressure / 3

Front 11

What is pulse pressure?

Back 11

Pulse pressure = systolic - diastolic

(P1 - P2)

The difference between systolic and diastolic pressures.

Front 12

What are the typical values of systolic and diastolic pressures?

Back 12

diastolic = 80 mmHg

systolic = 120 mmHg

Front 13

What is the name of the equipment used to measure MABP?

Back 13

sphygmomanometer

Front 14

MABP is defined through Darcy's Law.

What is Darcy's law?

Back 14

Pressure = flow x resistance

Front 15

What is the equation that determines MABP?

Back 15

MABP = cardiac output x total peripheral resistance

MABP = CO x TPR

Front 16

What is TPR?

Back 16

The amount of resistance that the heart has to pump/work against for blood to flow in the circuit.

Front 17

Cardiac output is the same from both left and right ventricles, so total flow must be the same in the systemic and pulmonary circulations.

Are pressures the same in the systemic and pulmonary circulations?

Back 17

No - pressures are very different in the systemic and pulmonary circuits because TPR is very different (much lower in the pulmonary vs systemic)

TPR is greater in the systemic circulation compared to the pulmonary - hence greater pressure in the systemic to overcome the resistance.

Front 18

TPR and therefore MABP is regulated by the vascular system.

What vessels are the major determinant of MABP?

Back 18

the arterioles

Front 19

Arterioles are the resistance vessels of the cardiovascular system. This gives them two important functions.

What are these two important functions?

Back 19

1) main site of control of total peripheral resistance (TPR) and therefore a major determinant (together with cardiac output) of mean arterial blood pressure.

2) site of control of relative blood flow through different vascular beds at any given MABP.

Front 20

There are 2 equations relating to MABP, what are these equations?

(one is to measure MABP and one is the determinants of MABP)

Back 20

1) Measuring MABP

= diastolic pressure + pulse pressure / 3

2) determinants of MABP

MABP = CO x TPR

Front 21

On one hand, MABP must be maintained within normal limits in order to ensure adequate perfusion of all parts of the body...

Back 21

not too high risking tissue damage or vessel rupture.

not too low risking vessels collapse and sesation of flow (hypoxia in tissues).

Front 22

However, on the other hand...

Back 22

regional changes in metabolic demand require regional changes in blood flow to meet that demand.

(certain tissues need more oxygen, have a greater physiological need)

Front 23

Arterioles alter TPR and MABP, they are at the point on the graph where changes in diameter relate to the biggest changes in pressure.

How do arterioles alter TPR and MABP?

Back 23

1) more branching of vessels

2) decreasing radius

Front 24

Describe the effect of branching on resistance and thus MABP...

Back 24

1) As vessels continue to branch, the number of resistances in parallel increases, thus the segment resistance drops,

1/Rtotal = 1/R1 + 1/R2

more vessels = less resistance = lower pressure

Front 25

However, resistance is inversely proportional to r4, what does this mean?

Back 25

so small decreases in radius will cause relatively large increases in vascular resistance.

(as arterioles branch, their radius gets smaller and resistance increases)

Front 26

So in arterioles there is a balance between increasing resistance due to decreased radius and decreased resistance due to increased branching and this alters MABP.

Back 26

Key point:

Arterioles can actively alter their radius through constriction or dilation of the smooth muscle layer -> this alters segement resistance.

Front 27

why is pressure lower in the pulmonary circuit compared to the systemic circuit?

Back 27

pressure is lower in the pulmonary because vessels are shorter and wider.

Front 28

describe the changes in pressure...

Back 28

1) in the large arteries pressure is high

2) internal pressure starts to decrease in the arterioles because of the dramatic reduction in radius, increasing resistance.

3) In veins, pressure is lowered due to the wider lumen, decreased resistance.

Front 29

Vasoconstriction of the smooth muscle in arterioles does what?

Back 29

vasoconstriction decreases the radius of arterioles which increases the internal pressure.

Front 30

Name 3 vasoconstrictor stimuli...

Back 30

1) noradrenaline

2) angiotensin II

3) endothelin

Front 31

Vasodilation relaxes smooth muscle, what effect does this have on pressure?

Back 31

Vasodilation reduces internal pressure because resistance is decreased.

Front 32

List 4 vasodilator stimuli...

Back 32

1) Adrenaline (in skeletal, liver and muscle arterioles)

2) Acetylcholine

3) Histamine

4) Others acting via nitric oxide

Front 33

Briefly describe the mechanism of smooth muscle contraction...

Back 33

1) smooth muscle contraction is induced by an increase in intracellular calcium ions.

2) calcium ion increase is primarily via the activation of phospholipase C, which triggers both release of stored calcium ions and influx from outside the cell.

3) calcium ions enter via non-selective cation channels, which cause depolarisation, thus also opening voltage-gated calcium channels.

4) depolarisation may spread through gap junctions to other muscle cells more distant from the source of the original stimulus.

Front 34

Briefly describe the mechanism of smooth muscle relaxation...

Back 34

1) smooth muscle relaxation is induced by a decrease in intracellular calcium ions

2) calcium ions are decreased primarily via the opening of potassium channels, which causes hyperpolarisation. Also via production of cAMP and cGMP.

3) hyperpolarisation closes voltage-gated calcium channels, thus reducing calcium ion concentration.

Front 35

Blood flow velocity is inversely proportional to total cross-sectional area.

(so as cross sectional area increases, blood flow velocity decreases)

Describe this...

Back 35

1) Arteries branch into aterioles which branch into capillaries.

2) At each branching point, the cross sectional area of individual vessels decreases and blood velocity decreases

3) BUT as the number of parallel vessels increases, the total cross-sectional area increases.

So, as the total cross-sectional area increases, blood velocity decreases.

Front 36

In terms of vessels, describe this

(blood flow velocity being inversely proportional to the total cross-sectional area)...

Back 36

Blood slows down at the capillary bed

because blood is traveling further away from the heart and because the total cross-sectional area of the capillary bed is increased and so blood is more distributed ---> motion/velocity is slower.

Front 37

However, blood flow velocity needs to increase in veins in order to go back to the heart.

How is blood flow velocity increased in veins?

Back 37

1) capillaries merge into venules which merge into veins

2) At each point of merger, total cross-sectional area decreases

3) blood flow velocity increases. ---> speeds up to get back to the heart.

Front 38

How is velocity calculated?

Back 38

velocity = distance traveled / unit time.

Note- it is different to blood flow which is the volume moved / unit time

Front 39

Why is blood flow velocity important?

Back 39

Low capillary flow velocity maximizes time available for exchange of substances between blood and interstitial fluid. (facilitates gaseous exchange in capillaries)

Front 40

How is MABP controlled in the SHORT-term?

Back 40

The baroreceptor reflex

This is a homeostatic mechanism, which responds very rapidly.

Front 41

What does a short-term response mean?

Back 41

Short-term responses to changes occurring over seconds to hours in duration.

Front 42

what does a long-term response mean?

Back 42

longer-term changes (several hours to days in duration) - they're dealt with through changes in blood volume.

Front 43

MABP is a physiological variable that must be maintained within normal physiological limits.

What does this require?

Back 43

1) A sensor or sensors e.g pressure transducers

2) an integrating centre or 'control centre'

3) an effector or effectors e.g pump control or resistance control

Front 44

what is the afferent pathway?

Back 44

from sensors to the integrating centre

Front 45

what is the efferent pathway?

Back 45

from the integrating centre to the effectors

Front 46

the baroreceptor reflex ( short-term regulation of MABP) is an example of what type of feedback?

Back 46

neuronal negative feedback

Front 47

what are the sensors and integrating centre of the baroreceptor reflex?

Back 47

-the sensors are the baroreceptors in the carotid sinus and aortic arch

- the integrating centre = cardiovascular centre in the medulla

Front 48

what are the names of the afferent nerves? (from sensors to the cardiovascular centre)

Back 48

Glossopharyngeal nerve and vagus nerve

Front 49

what are the names of the efferent nerves (from medulla to the effectors)

Back 49

vagus nerve (parasympathetic) and sympathetic cardiac nerves

Front 50

what do the barorecptors actually detect?

Back 50

baroreceptors sense stretch (mechanoreceptors)

increased pressure causes stretch, stretch is detected by baroreceptors, which triggers action potentials.

Front 51

Baroreceptors respond very quickly to changes in MABP and to changes in pulse pressure, by changing their action potential firing rate.

An increased MABP means an...

Back 51

increased firing rate of baroreceptors

(reduced MABP causes reduced firing rate of baroreceptor action potentials)

Front 52

Increased pulse pressure (reduction in diastolic pressure and an increase in systolic pressure) leads to an...

Back 52

increased firing during systole.

Front 53

Arterioles are effectors in the baroreceptor reflex...

Back 53

- Arterioles have many sympathetic nerve endings innervating their smooth muscle layer.

- This innervation is particularly strong in skin, kidneys, gut and spleen.

- sympathetic nerves release noradrenaline which acts primarily on a-adrenergic receptors to cause vasoconstriction.

noradrenaline ---> a-adrenergic receptors---> vasoconstriction.

Front 54

veins are also effectors in the baroreceptor reflex...

Back 54

-veins are also innervated by the sympathetic NS

-noradrenaline also constricts veins

-increases venous pressure

-helps push blood back up to the heart against the force of gravity

-partial restoration of venous return

Front 55

what happens to the firing of the vagus nerve when you want to reduce BP?

(want to reduce BP when you have increased firing of carotid sinus nerves)

Back 55

increased parasympathetic (vagus nerve) firing (reduced HR)

reduced sympathetic stimulation (reduced contractility)

reduced vasoconstriction in vascular nerves

effect = reduced CO and TPR = reduced MABP

Front 56

what happens to the firing of the vagus nerve when you want to increase BP?

(want to increase BP when you have a decreased firing of carotid sinus nerves)

Back 56

reduced parasympathetic (vagus) nerve firing = increase in HR

increased sympathetic cardiac nerves = increased contractility

increased vasoconstriction in vascular nerves

effect= increased CO and TPR = increased MABP

Front 57

is the vagus nerve sympathetic or parasympathetic?

Back 57

Parasympathetic

hence why when you want to reduce BP, you increase the firing of the vagus nerve to lower HR and when you want to increase BP, you reduce the firing of the vagus nerve to increase HR)

Front 58

Blood pressure is controlled by a negative feedback mechanism.

If the medulla responds to increased MABP by decreasing sympathetic output and increasing and increasing parasympathetic output, the result is to lower TPR and CO therefore lowering MABP back to normal.

Conversely...

Back 58

If the medulla responds to reduced MABP by increasing sympathetic output and decreasing parasympathetic output, the result is to raise TPR and CO, therefore raising MABP back to normal.

Front 59

When you stand up (moving from supine to standing position), the baroreceptor reflex is stimulated to stop blood pressure falling too low.

Describe the baroreceptor reflex when moving from supine to standing position...

Back 59

1) moving from a supine to a standing position

2) gravity causes blood to pool in the legs

3) reduced venous return

4) reduced stroke volume (starlings law)

5) reduced MABP

6) reduced baroreceptor firing

7) increased sympathetic and decreased parasympathetic output

8) increased TPR, increased HR and improved venous return

9) MABP returned to normal

Front 60

So when you move from supine to standing...

Back 60

1) SV reduces

2) HR increases in response to reduced SV

3) CO decreases

4) relative TPR increases

5) initially MABP decreases but then increases to above normal due to large increase in TPR

(without the baroreceptor reflex, orthostatic hypotension results (dizziness, fainting)

Front 61

An increase in HR cannot fully compensate for reduced SV, so CO is still reduced.

True or false?

Back 61

True- HR increase is not sufficient along to increase MABP (need TPR to increase)

Front 62

The baroreceptors have a role in maintaining MABP during exercise.

During exercise, what happens to the metabolic demand and blood flow?

Back 62

Metabolic demand of working muscles increases several-fold

Blood flow to the muscles must also increase several-fold and this is achieved through local vasodilation (in the muscle)

Also, the heat generated triggers thermoregulatory reflex resulting in further vasodilation (skin)

Front 63

Since muscle and skin are two of the largest vascular beds in the body, this vasodilation (skin and muscles) greatly lowers TPR, and since MABP = CO x TPR, this would also dangerously lower MABP unless corrected for.

How is a large regional change in blood flow achieved whilst maintaining MABP?

Back 63

1) the brain 'exercise centres' feed-forward and muscle mechanoreceptor stimulation and muscle metabolite accumulation

2) signals sent to the medulla cardiovascular centre

3) reduced parasympathetic output --> increased HR and SV --> increased CO

4) increased sympathetic output --> vasoconstriction in other body parts (kindney and GI tract) --> increased venous return --> partial restoration of TPR

= maintainence of MABP

Front 64

Remember there would not be an increase in cardiac output without an increase in venous return...

Back 64

1) increased sympathetic output

2) venous constriction

3) increased venous pressure

4) increased venous return

5) starling mechanism

6) increased SV

7) increased CO

Front 65

During exercise, you need to redistribute blood flow from areas that have a low metabolic demand to areas that have a high metabolic demand.

List the changes in regional blood flow that occur during exercise...

Back 65

1) brain - similar blood flow but smaller % share of total blood

2) skeletal muscle - 6-fold increase, tripling of % share of total blood

3) skin - doubling of blood flow but similar % share

4) kidneys - 1/3 reduction of blood flow and a smaller % share

5) gut etc - 1/4 reduction, % share very much smaller

Front 66

the largest blood flow increase is seen in the skeletal muscles.

So blood flow increases in the skin and skeletal muscles and decreases in...

Back 66

kidneys and gut (stays the same largely in the brain)

Front 67

Facial emersion in cold water triggers...

Back 67

1) reduction/drop in HR

(increased MABP- but blood is directed to brain and heart, in response to selective peripheral and muscular vasoconstriction)

vasoconstriction of the periphery and vasodilation of vessels supplying the brain and the heart- response to increase blood flow to the brain and heart

Front 68

what happens to HR during inspiration?

Back 68

increases during inspiration

Front 69

what happens to HR during expiration?

Back 69

decreases during expiration