1- special circulations

Question 1

where do coronary arteries arise from?

Answer 1

base of aorta

Question 2

what is oxygen demand of cardiac muscle like?

Answer 2

high, especially during exercise

Question 3

what are special adaptations of coronary circulations?

Answer 3

• high capillary density
• high basal blood flow
• high oxygen extraction under resting conditions

Question 4

how can extra oxygen (when required) be supplied to the heart?

Answer 4

can be supplied by increasing coronary blood flow
- there is high oxygen extraction (75% over 25% whole body average) under resting conditions

Question 5

what is coronary blood flow controlled by?

Answer 5

• intrinsic mechanisms
• extrinsic mechanisms

Question 6

how does intrinsic mechanisms control coronary blood flow?

Answer 6

• decreased pO2 (partial pressure oxygen) causes vasodilation of coronary arteries (as an attempt for more O2 to get to tissues)
• metabolic hyperaemia matches flow to demand
• adenosine (from ATP) is a potent vasodilator (heart work harder breaks down ATP to make adenosine eventually)

Question 7

what is metabolic hyperaemia?

Answer 7

process by which the body adjusts blood flow to meet the metabolic needs of its different tissues in health and disease

Question 8

how does extrinsic mechanisms control coronary blood flow?

Answer 8

• coronary arteries supplied by sympathetic vasoconstrictor nerves BUT:
  • vasoconstriction over-ridden by metabolic hyperaemia as a result of increased heart rate & stroke volume
  • so sympathetic stimulation of the heart results in coronary vasodilation despite direct vasoconstriction affect (called functional sympatholysis)
  • circulating adrenaline activates beta 2 adrenergic receptors which cause vasodilation (beta 2 receptors in blood vessels)

Question 9

what is functional sympatholysis?

Answer 9

when sympathetic stimulation of the heart results in coronary vasodilation despite direct vasoconstriction effect

Question 10

explain how sympathetic stimulation of heart affects coronary blood flow?

Answer 10

so sympathetic stimulation itself →decreased coronary blood flow

sympathetic stimulation also →increased circulating adrenaline → increased coronary blood flow

sympathetic stimulation also→increased SV & HR →increased CO which leads increased coronary blood flow
the increased SV & HR →increased cardiac work →increased metabolsim → decreased pO2 →increased adenosine →increased coronary blood flow

increased metabolism →metabolites like K+, pCO2, H+ →increased coronary blood flow

= this means that overall even though sympathetic stimulation itself decreases coronary blood flow due to all the other factors it actually leads to increased coronary blood flow

Question 11

when does peak coronary artery flow occur?

Answer 11

in diastole because:

during systole compression the coronary arteries are compressed as the muscular walls are squeezing & limiting blood flow, in diastole the heart is relaxed and the pressure within heart muscle is lower which creates a pressure gradient favoring flow into coronary arteries, perfusion pressure is optimal in diastole meaning more oxygen & nutrient delivery to myocardium

Question 12

what does shortening diastole effect on coronary flow?

Answer 12

very fast heart rate = shortening diastole = decreased coronary flow

Question 13

when does most of coronary blood flow and myocardial perfusion occur?

Answer 13

occurs in diastole when the subendocardial vessels in left coronary artery are not compressed

Question 14

what percentage of adult body mass if skeletal muscle?

Answer 14

40%

Question 15

how does skeletal muscle blood flow increase during exercise?

Answer 15

• during exercise, local metabolic hyperaemia overcomes sympathetic vasoconstrictor activity
• circulating adrenaline causes vasodilation (beta 2 adrenergic receptors)
• plus increased cardiac output during exercise these could increase skeletal muscle blood flow in many folds

Question 16

does skeletal muscle vascular bed impact blood pressure?

Answer 16

yes - skeletal muscle vascular bed has a big impact
- during resting conditions, blood flow to skeletal muscles is low

Question 17

what is the skeletal muscle pump?

Answer 17

• large veins in limbs lie between skeletal muscles
• contraction of muscles aid in venous return
• one way venous valves allow blood to move forwards towards the heart

Question 18

what does skeletal muscle pump help with?

Answer 18

help reduces the chance for postural hypotension & fainting

Question 19

what happens if skeletal muscle pump defective?

Answer 19

blood pools in lower limb veins if venous valves become incompetent (varicose veins)

Question 20

does varicose veins lead to reduction in cardiac output?

Answer 20

no - usually doesn’t lead to reduction of cardiac output because of chronic compensatory increase in blood volume

Question 21

in pulmonary circulation is resistance higher in pulmonary or systemic circulation?

Answer 21

systemic circulation resistance higher - pulmonary resistance is only 10% of that of systemic circulation

Question 22

is pulmonary artery blood pressure typically high, average or low?

Answer 22

low blood pressure - usually 20-25 over 6-12 mmHg

Question 23

what are special adaptations of pulmonary circulations?

Answer 23

• pulmonary capillary pressure is low (8-11 mmHg) compared to systemic capillary pressure (17-25 mmHg) = this helps less blood move from pulmonary to systemic circulation
• absorptive forces exceed filtration forces - protects against pulmonary oedema
• hypoxia causes vasoconstriction of pulmonary arterioles. COMPLETE OPPOSITE EFFECT OF HYPOXIA ON SYSTEMIC ARTERIOLES (purpose = would help divert blood from poorly ventilated areas of lung)