Different circulations

Question 1

where do the right and left coronary arteries arise from?

Answer 1

from the aorta immediately above the aortic valve cusps in dilations called the sinuses of Valsalva

Question 2

what do the left arteries supply?

Answer 2

the left ventricle, inter ventricular septum

Question 3

what do the right arteries supply?

Answer 3

right ventricle but distribution varies

Question 4

how is venous blood drained?

Answer 4

95% via the coronary sinus into the right atrium

5% into the cardiac chambers through the anterior coronary and Thebesian veins

Question 5

how long is the coronary circulation? in secs

Answer 5

mean transit time is 6-8 seconds

Question 6

special task of coronary circulation

Answer 6

must deliver oxygen at a high rate to match the basal myocardial demand which is 20x that of skeletal muscle

must increase during exercise

Question 7

structural adaptations

Answer 7

myocardial capillary density is very high to create a large endothelial SA and reduce maximum diffusion distance

myoglobin stores O2 within cardiac myocytes

exercise training increases coronary artery width and number of arterioles and capillaries

Question 8

functional adaptations

Answer 8

endothelial NO helps maintain high basal flow

eNOS inhibitors reduce flow by 60%

FA extraction is high, glucose extraction is unusually low, thus fatty acids are the preferred substrate

metabolic hyperaemia is the dominant flow of vascular regulation, with flow increasing linearly with oxygen consumption

sympathetic vasomotor activity maintains high vascular tone

systole obstructs coronary blood flow

Question 9

explain metabolic hyperaemia

Answer 9

myocardial release of vasodilator substances in proportion to work rate

includes adenosine, H202, H+ and interstitial K+

Question 10

explain sympathetic vasomotor activity

Answer 10

adrenaline is released

acts upon beta 1 adrenergic receptors in the muscle

increases strength of contraction and rate of contraction

b2 receptors on vascular coronary myocytes result in vasodilation increasing coronary flow

Question 11

explain how systole obstructs coronary blood flow

Answer 11

2/3 of the coronary arterial system is within the myocardium (intramural) so systole compresses the vessels, especially during the isovolumetric phase

full flow is only restored in diastole

thus coronary perfusion is driven chiefly by diastolic blood pressuree

Question 12

why are beta blockers useful for heart failure?

Answer 12

increase the length of diastole and thus the time for good perfusion

Question 13

range of blood flow in skeletal muscle

Answer 13

5-10ml/min/100g of tissue at rest

250ml/min/100g during maximal aerobic exercise

Question 14

what is the relationship between work rate and oxygen consumption?

Answer 14

linear, the more work done, the more oxygen used, the more muscle blood flow

Question 15

special task of skeletal muscle

Answer 15

during exercise, circulation must increase to match oxygen consumption due to the increase in work.

skeletal muscle constitutes 40% of body mass and thus its resistance has a huge effect on total peripheral resistance and helps regulate arterial BP

Question 16

structural adaptations of skeletal muscle

Answer 16

capillary density varies dependent on the muscles activity, postural muscles are always active so have a dense capillary network compared to phasic muscles

endurance training stimulates angiogenesis via growth factors

Question 17

functional adaptations of skeletal muscle

Answer 17

vascular tone of resistance vessels is high at rest evident by the 50-100 fold increase in vascular conductance in active muscle

basal tone is non neural in origin

muscles are regulated by the baroreflex that senses arterial BP

metabolic vasodilation of resistance vessels increases and sustains blood flow,

arteriolar dilation due to NO recruits more capillaries

during intense activity, muscle fibres switch to anaerobic metabolism

venous muscle pump enhances perfusion during exercise by intermittent compression of deep veins int he calf, forcing blood out the veins and driving capillary perfusion during relaxation

Question 18

what organ is the least tolerant of ischemia

Answer 18

the brain

Question 19

explain blood vessels of brain

Answer 19

cerebral arterioles are short, thin walled

large cerebral arteries account for the high fraction of vascular resistance with high autonomic innervation

Question 20

special tasks

Answer 20

grey matter has a very high rate of oxidative metabolism and is exquisitely sensitive to hypoxia, so the primary task is to maintain O2 delivery at all costs

Question 21

how does the brain safeguard its blood supply?

Answer 21

the blood brain barrier