Control of blood pressure and hypertension

Question 1

Describe how arterial blood pressure changes within a cardiac cycle Explain how mean arterial pressure is calculated (2 methods)

Answer 1

BP decreases at first due to diastole

BP then shoots up during systole and then decreases again

Diastolic BP = lowest BP reading

Systolic BP = highest BP reading

Pulse presssure = systolic BP - diastolic BP

Mean arterial BP = diastolic BP + 1/3pulse pressure (as heart spends little time in systole)

OR mean arterial BP = cardiac output x total peripheral resistance

Question 2

Systolic BP is determined by 2 factors; describe them

Answer 2

Stroke volume - the volume of blood being pumped out

Aortic elasticity - Ek is absorbed from blood reducing some of the rise in pressure ; inelastic aorta may cause systolic hypertension

Question 3

Diastolic BP is determined by 3 factors ; describe them

Answer 3

Peripheral resistance

Aortic elasticity - the lower the elasticity the lower the DBP (as Ek absorbed during systole is returned in diastole adding to DBP) Heart rate

Question 4

What are the healthy values for the mean arterial pressure and cardiac output in the systemic circulation of an individual

Answer 4

MAP - 100mmHg ; CO - 5L/min

Question 5

What are the healthy values for the mean arterial pressure and cardiac output in the pulmonary circulation of an individual

Answer 5

Mean arterial pressure - 10mmHg; CO - 5L/min

Question 6

Describe the control mechanisms of arterial blood pressure

Answer 6

Baroreceptors in walls of carotid sinus and aortic arch ; low pressure baroreceptors in pulmonary vessels, atrial-vena caval junctions and ventricular walls

Integration centres in the CNS (in medulla)- processes info from baroreceptors - activation of parasympathetic or sympathetic NS

Effector mechanisms via the autonomic nervous system

Question 7

Briefly describe the action of the parasympathetic and sympathetic nervous system on arterial blood pressure

Answer 7

Parasympathetic

ACh neurotransmitters binds to muscarinic receptors to decrease HR

Sympathetic

Noradrenaline neurotransmitters acts on β₁-adrenoceptors in heart to increase HR and stroke volume ; noradrenaline also acts on α_½ adrenoceptors to cause vasoconstriction in arterioles

Question 8

Describe the integration centre in the medulla

Answer 8

Baroreceptor mediated afferent nerve activity reaches the nucleus of the tractus solitarius

within the NTS there are 3 distinct groups of neurones:

• cardiac vagal nuclei - they send impulses down the vagus nerve
• caudal ventrolateral medulla depressor ; only activated when BP is very high; they depress the activity of the SNS
• rostral ventrolateral medulla pressor ; always active - they act on the SNS ; activity regulated by barorecptor input

Question 9

Which nervous system does the vagus nerve act on?

Answer 9

parasympathetic

Question 10

Explain what happen when BP falls below ideal level

Answer 10

Baroreceptors detect lowered BP

↓impulses sent down vagus = ↓parasympathetic activity

↓inhibiton at rostral ventrolateral medulla

increased sympathetic activity

increased HR/SV/CO via β₁ adrenoceptors

icnreased peripheral resistance via α₁ adrenoreceptors

BP returns to normal

Question 11

Explain what happens when BP increases above ideal levels?

Answer 11

increase detected by baroreceptors

↑impulses sent down vagus nerve

activation of depressor and ↑inhibition at pressor

↓Sympathetic nerve activity

↓HR/StV/CO

↓periperaphal resistance

BP returns to normal

Question 12

Describe the postural reflex

Answer 12

When you go from lying down to standing up, blood pools in legs and abdomen

decreased venous return = ↓CO and mean arterial pressure

activation of baroreceptros and cardiopulmonary receptors

↓vagus nerve activity

↑sympathetic activity in small arteries and arterioles

↑HR/STV/vasoconstriction

in standing positions, you have a lower CO but higher peripheral resistance

Question 13

equation for fluid movement at capillary bed

Answer 13

Fluid movement = filtration forces - re absorption forces = K_f[(Pc+πi) - (πc+Pi)]

Question 14

Describe the importance of maintaining bulk flow within narrow parameters

Answer 14

the entire blood plasma is passed through the network of capillaries within a day

if something goes wrong with the regulation of bulk flow, lots of fluid can be lost from the blood and into tissues

Question 15

Describe the factors affecting capillary hydrostatic pressure (Pc)

Answer 15

arteriolar constriction decreased Pc

venular constriction increases Pc

Pc is the most important factor in determining filtration

Question 16

Describe the factors affecting capillary osmotic pressure (πc)

Answer 16

synthesis/breakdown of protein in the liver

capillary permeability to protein

abnormal protein loss due to kidney damage

Question 17

describe the importanice of the lymphatic system in drainage of excess filtered tissue fluid

Answer 17

Excesss filtered fluid and 95% of the protein lost from vascular system is return back to circulation via the subclavian vein

Question 18

Describe the factors affecting hydrostatic pressure of ECF (P_i)

Answer 18

Normally a minor determinant of fluid movement

depends upon : volume of ECF, compliance of organ and effective lymphatic drainage

Question 19

Describe the movement of fluid at the :

arteriolar , capillary and venular ends of the capillary bed

Answer 19

Net filtration at arteriolar end (as Pc>πc)

no net movement mid-capillary

net reabsorption at the venular end (as Pc<πc)

Question 20

Given a condition where:-

Kf = 0.5 ml/min/100g

Pc = 30mmHg, Pi = 1 mmHg

πc = 26 mmHg, πi = 3 mmHg

what is the direction and rate of fluid movement ?

Answer 20

Question 21

Effects of increasing pre-capillary vasocobstruction, venous pressure and hypoproteinaemia on net movement of fluid

Answer 21

Increases pre-capillary vasoconstriction = increased reabsorption

increased venouse pressure = increased filtration= oedema

hypoproteinaemia = increased filtration (due to decreased πc)

Question 22

Causes of oedema

Answer 22

Lymphatic obstruction

increased venous pressure (RVHF)

hypoproteinaemia (renal or liver damage)

hypervolaemia (excess sodium and water)

inflammation

Question 23

filtration out of capillaries is favoured by ….

Answer 23

Filtration out of capillaries is favoured by :

Higher hydrostatic pressure inside capillary(Pc)

Higher osmotic pressure of ECF(πi)

however, πi and Pi are normally negligible so fluid movement actually depends on

increased capillary hydrostatic pressure or P_c= increased filtration

Question 24

reabsorption into capillaries is favoured by …

Answer 24

Lower capillary plasma osmotic pressure (πc)

Higher hydrostatic pressure of interstitial fluid (Pi)

however, πi and Pi are normally negligible so fluid movement actually depends on

increased plasma colloid osmotic pressure or π_c (osmotic pressure due to plasma proteins such as albumin) = increased reabsorption

Question 25

difference between interstitial fluid and extracellular fluid

Answer 25

ECF = tissue fluid + blood plasma; interstitial fluid refers to tissue fluid only

Question 26

what does organ compliance mean?

Answer 26

This physically means that blood vessels with a higher compliance deform easier and increase their volume than lower compliance blood vessels under the same pressure and volume conditions

Question 27

what do P_c, π_i, π_c, P_i mean?

Answer 27

hydrostatic pressure inside capillary plasma(Pc)

osmotic pressure of interstitial fluid(πi)

capillary plasma osmotic pressure (πc)

hydrostatic pressure of interstitial fluid (Pi)

Question 28

What causes an increase in interstitial fluid formation (oedema)

Answer 28

Increased arteriolar pressure

Question 29

identify the 3 main changes when moving from supine to upright position

Answer 29

Reduction in mesenteric flow - due to increased SNS activity

reduction in activity of neurones in the caudal ventrolateral medulla = switching off of depressor

an immediate reduction in venous return