P: Arterial blood pressure

Question 1

Determinants of mean arterial pressure:

Answer 1

• Rate of inflow (Qh) of blood into arteries during ventricular systole
• Rate of outflow (Qr) through arterioles into capillaries.
• Qh > Qr = MAP increases
• Qh < Qr = MAP decreases
• Increases in CO or peripheral resistance increases MAP
• Compliance of arteries: rigid arteries increase MAP, elastic arteries decreases rate of MAP increase

Question 2

Where is resistance to blood flow the greatest?

Answer 2

In arterioles and capillaries, due to narrow diameter.

Question 3

Total peripheral resistance:

Answer 3

Resistance to blood flow through entire arteriolar system –> determines rate of blood flow out of arterial system.

Question 4

Mean arterial pressure equation

Answer 4

cardiac output x total peripheral resistance.

Question 5

How stroke volume determines pulse pressure:

Answer 5

• During ventricular ejection, arterial blood volume increases to V2
• Blood pressure increases to P2
• During diastole, peripheral runoff reduces volume to V1 and pressure to P1.

(V2-V1) = (P2-P1) = pulse pressure

Question 6

Why do lower compliant arteries have higher blood pressure?

Answer 6

• Ejected blood exerts higher pressure on rigid walls than elastic walls
• Ps (systolic pressure) is increased in rigid arteries so pulse pressure also increases
• Aortic compliance decreases with age, Ps and pulse pressure increase as we age
• Compliance with SV is a determinant of pulse pressure.

Question 7

To measure systolic + diastolic pressure:

Answer 7

• Inflatable cuff & sphygmomanometer measures systolic + diastolic pressure
• Cuff placed over brachial artery
• As cuff is inflated, it exerts increasing inwards pressure on artery
• When cuff pressure > systolic pressure, artery is entirely closed and blood flow stops.
• Cuff pressure is slowly dropped, and artery partially opens
• Blood flow restarts in turbulent fashion through pinched artery
• Causes thumping noise (Korotkoff sounds) which can be detected with stethoscope placed on artery
• When cuff pressure < diastolic pressure, artery is fully open, blood flow is smooth and Korotkoff sounds disappear.

Question 8

Vascular smooth muscle structure

Answer 8

• No sarcomeres, so not striated
• Filaments form lattice structure around cell periphery
• Long thin filaments attach to cytoplasmic structures - dense bodies
• Smooth muscle contraction dependent mostly on inward diffusion of extracellular Ca2+ through voltage-gated Ca2+ channels
• Calmodulin acts as a Ca2+ sensor –> Ca2+/calmodulin complex binds and activates myosin light chain kinase

Question 9

Intrinsic control of resistance:

Answer 9

• Vascular smooth muscle undergoes myogenic regulation
• Rapid increase in blood pressure induces reflex contraction of arterioles
• Rapid decrease in blood pressure induces -reflex dilation of arterioles
• Constant blood flow is maintained

Question 10

Absence of myogenic regulation examples:

Answer 10

• Increase capillary blood pressure: hydrostatic pressure
• Elevated hydrostatic pressure increases capillary filtration causing fluid accumulation in feet and lower legs.

Question 11

Endothelial-mediated regulation

Answer 11

• Endothelium also releases substances that trigger contraction/ relaxation of VSM
• Increased blood flow causes shear stress to endothelium & induces releases release of nitric oxide (synthesised from L-arginine)
• NO diffuses into VSM cells and activates guanylyl cyclase which increases intracellular [cGMP] –> decreases intracellular calcium in VSM, inducing dilation of blood vessels.

Question 12

Extrinsic control of peripheral blood flow:

Answer 12

• Post-ganglionic sympathetic nerves innervate VSM of arterioles and veins
• Increased nerve activity mostly causes vasoconstriction while a decrease induces vasodilation.
• Noradrenaline binds to a-adrenergic receptors on VSM cells inducing vasoconstriction
• Adrenaline binds to B2-adrenergic receptors on VSM cells in skeletal blood vessels, inducing vasodilation.

Question 13

alpha-adrenergic receptors:

Answer 13

• Activate phospholipase C which produces 2nd messengers which increase intracellular calcium
  
  • Results in contraction

Question 14

B2-adrenergic receptors

Answer 14

Activates adenylate cyclase, increasing [cAMP]I, which inhibits MLCK.

Question 15

Sympathetic nerve stimulation:

Answer 15

• Reduces blood flow into tissues by constriction of arterioles
• Decreases tissue blood volume by constriction of venules
• Increase in movement of blood towards heart (venous return)
• Redistribution of blood from venous into arterial system
• Reduces capillary hydrostatic pressure.
  -Absorption of interstitial fluid into capillaries increases.

Question 16

Vasomotor centre:

Answer 16

• Located in reticular substance of medulla and lower 1/3 of the Pons
• Transmits impulses via vagus nerve to heart and sympathetic impulses via spinal cord and peripheral sympathetic nerves to heart and arteries, arterioles and veins.

Question 17

Rostral Ventrolateral medulla (RVLM)

Answer 17

• Stimulates sympathetic nerve activity
• Neurons here send fibres to spinal cord which excite pre-ganglionic neurons of sympathetic nervous system
• Maintains both sympathetic cardiac & vasoconstrictor tone
• Stimulation of RVLM increases BP and HR

Question 18

Caudal Ventrolateral medulla (CVLM)

Answer 18

• Fibres from these neurons project to RVLM and inhibit its activity, causing vasodilation.
• Depressor region
• Vasodilator area

Question 19

Nucleus ambigus (NA, AMB) and dorsal motor nucleus of vagus (DMV)

Answer 19

• Origin of vagal projections to the heart
• Vagal centre.

Question 20

Nucleus of tractus solitarius (NTS)

Answer 20

• NTS receives sensory nerve signals from circulatory system mainly through vagus and glossopharyngeal nerves
• These nerves in turn are stimulated by baroreceptors and cardiopulmonary receptors
• NTS sends output to NA and CVLM
• Stimulation of NTS reduces sympathetic outflow and increases vagal outflow.

Question 21

NTS in medulla receives sensory impulses from:

Answer 21

• Baroreceptors
• Cardiopulmonary receptors
• Chemoreceptors
• Other brain regions

Question 22

How are sudden changes in blood pressure detected?

Answer 22

• Stretch-sensitive mechanoreceptors (baroreceptors) are located in walls of carotid artery and aortic arch
• An increase in blood pressure causes the walls of these blood vessels to stretch
• Baroreceptors become more active in response

Question 23

How do arterial baroreceptors work?

Answer 23

• Stretch receptors located in carotid sinuses and aortic arch
• Afferent pathways: Carotid sinus nerve, glossopharyngeal nerve, NTS, vagus nerve
• Collectively known as Buffer nerves, cause abrupt changes in BP

Question 24

Sudden reduction in blood pressure causes sympathetic nerve activity to increase and causes:

Answer 24

• Increase cardiac output by increasing heart rate + stroke volume
• Contract venous smooth muscle, promoting venous return
• Stimulate contraction of arterial smooth muscle to increase peripheral resistance
• Vasoconstriction in kidney arterioles also minimises urine formation

Question 25

Sudden elevation in blood pressure stimulates vagus nerve activity which:

Answer 25

• Reduces cardiac output by slowing heart rate
• Inhibition of sympathetic nerve activity reduces cardiac output, causes relaxation of arterial smooth muscle to decrease peripheral resistance
• Kidneys stimulated to excrete more water as urine, reducing total blood volume.

Question 26

Cardiopulmonary baroreceptor locations

Answer 26

atria, ventricles and pulmonary vessels

Question 27

Cardiopulmonary baroreceptor functions (what does it mainly act on and how):

Answer 27

• Initiate reflex that lowers arterial blood pressure in response to changes in blood volume
• Mainly acts on kidneys to reduce blood volume by increasing urine output
• Done by reduction of sympathetic nerve activity to kidney, inhibition of release of angiotensin, aldosterone and vasopressin (antidiuretic hormones).

Question 28

How do saline infusions increase heart rate?

Answer 28

• Increase blood volume increases atrial pressure causing rapid increase in HR
• Mediated by atrial stretch receptors that transmit afferent signals via vagus nerves to NTS
• Efferent signals are transmitted back through vagal and sympathetic nerves to increase heart rate and contractility
• Transfer of blood into arterial circulation
• Prevents damming of blood in veins, atria and pulmonary circulation.

Question 29

Bainbridge reflex:

Answer 29

Increased blood volume increases atrial pressure, causing rapid increase in HR

Question 30

How are bainbridge and baroreceptor reflexes antagonistic?

Answer 30

• Increase in blood volume = bainbdirge reflex predominates
  
  • Blood volume decreases: baroreceptor reflex dominates

Question 31

Chemoreceptor locations:

Answer 31

Central medulla and carotid sinuses & aortic arch

Question 32

Chemoreceptor reflex

Answer 32

increase in heart rate is a secondary effect of an increase in ventilation.

Question 33

Anterior hypothalamus function

Answer 33

decreases BP and HR

Question 34

Posterior and lateral hypothalamus function:

Answer 34

• Alerting reaction
• Increased BP + HR, vasodilation in skeletal muscle
• Vasoconstriction in skin and splanchnic organs

Question 35

Cerebral cortex

Answer 35

• Involved in motor functions
  
  • Cause vasoconstriction of skin, splanchnic and renal vessels
  • Vasodilation in skeletal muscles