Lecture 15: Regulation Of Peripheral Blood Flow

Question 0

How is arteriolar radius controlled?

Answer 0

Radius is dependant on contractile state of smooth muscle in arteriolar wall

• vasoconstriction: increased contraction ➡ decreased radius
• vasodilation: decreased contraction ➡ increased radius

Question 1

Smooth muscle contraction. Remind me how it works again?

Answer 1

When Ca2+ enters the cell it binds to calmodulin to form the Ca-calmodulin complex. This activates MLCK which causes the phosphorylation of myosin light chain ➡ this makes myosin ATPase active ➡ crossbridge cycling ➡contraction

Question 2

What is hyperemia?

Answer 2

Higher than normal blood flow

Question 3

Arteriolar radius and distribution of blood flow

Answer 3

• Flow of blood through an organs is regulated by changing arteriolar resistance in that organ.
• distribution of cardiac output between different organs can be changed according to need
• arteriolar resistance is regulated both intrinsically and extrinsically

Question 4

What is the functions of varying arteriole radius?

2

Answer 4

1. Regulating mean arterial pressure (extrinsic)

2. Controlling blood flow to individual capillary beds (intrinsic)

Question 5

List the intrinsic factors influencing arteriolar radius?

Answer 5

1) arteriolar tone
2) active hyperemia
3) reactive hyperemia
4) myogenic response

Question 6

Regulation of arteriolar resistance
Intrinsic regulation
A) arteriolar tone

Answer 6

Arteriolar tone= contraction of arteriolar smooth muscle in the absence of extrinsic input

• spontaneous, slow depolarisation of smooth muscle cells ➡ calcium influx and contraction
• makes it possible to locally decrease resistance below resting value

Question 7

Regulation of arteriolar resistance
Intrinsic regulation
B) active hyperemia
Also what are the mechanisms of active hyperemia?

Answer 7

Active hyperemia= increased blood flow in response to increase metabolic activity

• increased blood flow to contracting muscles, secreting glands, active intestinal epithelium
• changes assosiated with increased metabolic activity generally cause vasodilation.
  
  • ⬆carbon dioxide
  • ⬆potassium
  • ⬆ hydrogen ions
  • ⬇ in tissue oxygen concentration

Mechanism of active hyperemia
CBF slide 17

Question 8

Regulation of arteriolar resistance
Intrinsic regulation
C) reactive hyperemia
Also what are the mechanism of reactive hyperemia

Answer 8

Reactive hyperemia= increased blood flow die to previous restriction in blood flow eg blockage
-quickly restores normal chemical balance to deprived tissue
-changes assosiated with blockage of blood flow cause vasodilation ie metabolises increases and oxygen decreases
-Changes associated with release of blockage cause vasoconstriction ie increased blood flow due to lower resistance, metabolises removed, oxygen increases
-blockages can be
-pathological (eg clots)
-physiological (eg blockage of flow during cardiac or skeletal muscle contraction.)
Mechanisms of reactive hyperemia?
CBF slide 20

Question 9

What is the differences between active vs reactive hyperemia?

Answer 9

Slide 22

Question 20

Arteriolar radius
Intrinsic regulation
Myogenic response. Explain

Answer 20

Myogenic response = contraction of arteriolar smooth muscle in response to stretch.
-Stretch sensitive Ca2+ channels open➡ contraction
-maintains a relatively constant flow even in different arterial pressures because of they are sensitive to stretch.
-safeguards blood supply to brain, kidneys, heart
Eg ⬆arteriolar stretch ➡⬆arteriolar SM contraction ➡⬆ resistance ➡⬇flow (negative feedback)

Question 21

Arteriolar radius
Extrinsic regulation
Neuronal regulation ➡ sympathetic control of arteriolar radius

Answer 21

Postganglionic sympathetic neuron ➡ noradrenaline ➡ a adrenergic receptors ➡ vasoconstriction.
There is a baseline (tonic) activity of the sympathetic NS all the time.
Increased sympathetic activity ➡ increased noradrenaline release➡ vasoconstriction.
Decreased sympathetic activity ➡ decreased noradrenaline released➡ vasodilation.

Question 22

Arteriolar radius
Extrinsic regulation
Hormonal control of radius
What are the 3 main hormones involved in regulation of arteriolar radius?

Answer 22

Adrenaline ➡ vasoconstriction or dilation depending on receptor
Antidiuretic hormone
Angiotensin 2
Both function to vasoconstrict

Question 23

How does adrenalin control arteriolar radius?

Answer 23

Released from the renal medulla in response to sympathetic activity.
-action depends on type of adrenergic receptors in different vascular beds
-GI tract, mostly a (alpha) receptors ➡ vasoconstriction
-skeletal muscles, mostly B (beta) receptors ➡ vasodilation
BETA MAKES IT BIGGER!

Question 24

ADH, Angiotensin 2 and control of arteriolar radius

Answer 24

• Both stimulate vasoconstriction of arterioles
• both also involved in increasing water reabsorption in the kidney ➡ blood volume
• affects peripheral resistance and blood volume, therefore regulators of arterial pressure

Question 25

Interplay between intrinsic and extrinsic regulation.
What is the deciding factor in determining blood flow to an organ:
Can intrinsic regulation override extrinsic regulation?

Answer 25

Extrinsic mechanisms maintain stable arterial pressure, coordinate circulatory system as a whole
-intrinsic mechanisms give the individual organ the blood supply it needs
Coronary and cerebral arterioles:
-sparse sympathetic innervation and efficient myogenic response, intrinsic mechanisms dominate ➡ blood supply matches need.

Working skeletal muscles:
-intrinsic regulation dominates ➡ blood supply matches need
Other organs:
-lack well developed intrinsic mechanisms, at the mercy of extrinsic regulation
-blood supply restricted when cardiovascular system is under stress, to maintain MAP

Question 26

Learn the table on slide 33

Answer 26

Do it

Question 27

What are the structures controlling blood flow through capillary beds

Answer 27

Capillaries themselves don’t have smooth muscle
-arterioles, metarterioles and precapillary sphincter regulate blood flow through capillary beds.
Pre capillary sphincters: smooth muscle rings, contract and act as valves regulating capillary blood flow
Metarterioles: shut, allows capillary bypass.
Slide 34 for image

Question 28

Precapillary sphincter control flow through capillary beds

Answer 28

• Contribute to slow, intermittent flow through capillaries
• situation-dependant bypass of capillary beds eg shut down GI capillary beds between meals
• extrinsic (sympathetic) and dominant local control