Circulation

Question 1

What are the roles of arterioles in systemic circulation?

Answer 1

The main site of systemic vascular resistance

Question 2

What is the role of the capillaries in systemic vascular circulation?

Answer 2

site of exchange of gas, nutrients and water between blood and tissues

Question 3

What is the role of the veins in systemic vascular circulation?

Answer 3

they are the capacitance vessels - hold most of the blood volume during rest
passageways of blood from tissues to heart

Question 4

What is the role of the arteries in systemic vascular circulation?

Answer 4

The passageways of blood from the heart to tissues

Question 5

What causes an increase in SVR and MAP?

Answer 5

vasoconstriction of smooth muscles

Question 6

Resistance to blood flow is directly proportional to…

Answer 6

n.L/r4 
blood viscosity(n) and length of blood vessel(L); and inversely proportional to the radius of blood vessel (r) to the power 4:

Question 7

What are the extrinsic controls of the vascular smooth muscles?

Answer 7

• Vasomotor tone
• sympatheic fibres and the neurotransmitter noradrenaline acting on A receptors
• adrenaline (acting on A receptors causes vasoconstriction, acting in B2 receptors causes vasodilation)
  A receptors supply skin, gut & kidney arterioles and B2 receptors supply cardiac and skeletal muscle arterioles
• Angiotension II - vasoconstriction
• ADH vasopressin causes vasoconstriction

Question 8

What is vasomotor tone?

Answer 8

• it is caused by tonic discharge of sympathetic nerves resulting in continuous release of noradrenaline
• therefore increasing sympathetic discharge increases vasomotor tone resulting in vasoconstriction and hence decreasing the vasomotor tone resulting in vasodilatation

Question 9

What are the intrinsic controls of the vascular smooth muscles?

Answer 9

• the intrinsic controls can over-ride the extrinsic control mechanisms
• Chemical
  local metabolites
  local humoral agents
  nitric oxide
• Physical
  temperature
  myogenic response
  sheer stress

Question 10

What is the effect of local metabolites on vascular smooth muscle?

Answer 10

vasodilatation and metabolic hyperaemia are caused by 
decreased local PO2
increased PCO2
increased local H+ (decreased pH)
increased extracellular K+
increased osmolarity of ECF
adenosine release from ATP

Question 11

What are the local humoral agents which can affect vasodilatation?

Answer 11

can sometimes be released as a result of tissue injury and inflammation

the following humoral agents cause relaxation of smooth muscle - vasodilatation
histamine
bradykinin
nitric oxide (NO)

Question 12

What are the properties of nitric oxide?

Answer 12

• it is continuously produced by the vascular endothelium from the amino acid L-arginine through enzymatic action of NO synthase
• it is a potent vasodilator - with a short half-life important in regulating blood flow and maintaining vascular heath
• NO diffuses from the vascular endothelium into the adjacent smooth activating the formation of cGMP and serving as a second messenger for smooth muscle relaxation

Question 13

What is the effect of sheer stress on the vascular endothelium?

Answer 13

• dilatation of arterioles causes sheer stress in the arteries upstream making them dilate. Increasing blood flow to metabolically active tissue
• increased flow causes sheer stress
• there is release of calcium in vascular endothelial cells which activate NOS and hence vasodilatation

Question 14

What mechanism do chemical stimuli use to induce NO formation?

Answer 14

receptor stimulated NO formation

- many vasoactive substances work through stimulation of NO formation

Question 15

What are the local humoral agents which can affect vasoconstriction?

Answer 15

serotonin
Thromboxane A2
leukotrienes
endothelin (a potent vasoconstrictor released by endothelial cells, it is produced in response to various agents which cause vasoconstriction)

Question 16

What is the role of the endothelium in the regulation of vascular diameter?

Answer 16

• it has a important role in maintaining vascular health
• the endothelial damage and dysfunction can be caused by HBP, high cholesterol, diabetes and smoking
• endothelial produced vasodilators are anti-thrombotic, anti-inflammatory, anti-oxidants
• endothelial produced vasoconstrictors are pro-thrombotic, pro-inflammatory, pro-oxidants

Question 17

What is the effect of temperature on vascular smooth muscles?

Answer 17

Cold - causes vasoconstriction

Warmth - causes vasodilatation

Question 18

What is the myogenic response to stretch?

Answer 18

important in tissues like brain and kidneys helps with regulation of local blood flow

• rise in MAP causes resistance vessels to constrict and limit flow
• fall in MAP causes resistance vessels to dilate and increase flow

Question 19

What is the autoregulation of celebral blood flow?

Answer 19

the myogenic response to MAP which keeps celebral blood flow constant over a wide range of the MAP

Question 20

What factors increase venous return?

Answer 20

increased respiratory pump
increased skeletal muscle pump
increased blood volume
increased venomotor tone

Question 21

What is the effect of increased venous return?

Answer 21

increased arterial pressure
increased end diastolic volume
increased stroke volume in accordance to the Frank- Starling curve

Question 22

What is the venomotor tone?

Answer 22

• venous smooth muscle is supplied by sympathetic nerve fibres
  stimulation of these fibres gives venous constriction
• increased venomotor tone increases ve ous return SV and MAP

Question 23

What is the respiratory pump in venous return?

Answer 23

in inspiration intrathoracic pressure decreases and intraabdominal pressure increases

• there is increased pressure gradient for venous return - suction effects moving the blood in the veins to the heart
• increased rate and depth of breathing causes increased venous return to the heart

Question 24

What is the skeletal muscle pump in venous return?

Answer 24

large veins in limbs lie between skeletal muscles, the contraction of these muscles aids venous return
- One way venous valves allow blood to move forward towards the heart

Question 25

What is the acute response to exercise?

Answer 25

- increase in sympathetic nerve activity - HR and SV increase - CO increase - vasomotor nerves reduce flow to kidneys and gut through vasoconstriction - cardiac and skeletal muscle have vasodilatation because metabolic hyperaemia overcomes the vasomotor drive, there is increased blood flow in proportion to metabolic activity - pulse pressure increases because CO increases systolic BP and metabolic hyperaemia decreases SVR and decreases diastolic BP - there is a post exercise hypotensive response

Question 26

What is the effect of sympathetic stimulation on the heart?

Answer 26

- increases heart rate by increasing the rate of firing of the SA node and decreases AV nodal delay - increases force of contraction - peak ventricular pressure rises, so the contractility of the heart at a given EDV also rises the frank-starling curve is shifted to the left

Question 27

What is the effect of noradrenaline on pacemaker cells?

Answer 27

a positive chronotropic effect - the slope of the pacemaker potential increases causing the threshold to be reached quicker - there is a higher frequency of action potentials

Question 28

What is the chronic response to exercise?

Answer 28

- regular aerobic exercise helps reduce BP Chronic cardiovascular responses to regular exercise may include: Reduction in sympathetic tone and noradrenaline levels Increased parasympathetic tone to the heart Cardiac remodeling Reduction in plasma renin levels Improved endothelial function: increased vasodilators decreased vasoconstrictors decreased Arterial stiffening