Session 9: Special Circulations

Question 1

normal pressures in pulmonary circulation

Answer 1

artery: 12-15mmHg
capillaries: 9-12mmHg
veins: 5mmHg

Question 2

ventilation/perfusion ratio and maintenance

Answer 2

optimal value is 0.8 (V=4, Q=5) and is altered by diverting blood away from alveoli not well ventilated to reduce perfusion

hypoxic pulm. vasoconstriction occurs, where alveolar hypoxia leads to vasoconstriction of pulm. vessels -> greater flow to well ventilated areas and less flow to poorly ventilated areas

chronic Hyp. pulm. Vconst. (eg from high altititude) leads to chronic inc. in vasc. resistance -> inc. afterload on r/vent. -> r/vent. failure

Question 3

forces involved in formation of tiss. fluid in lungs and systemic circulation

Answer 3

starling forces: hydrostatic pressure of blood w/in cap.s (greater in lower lung when standing up) pushes fluid out

• oncotic/colloid osmotic pressure exerted by large molecules eg plasma proteins drains fluid out
  ven. press. is low therefore only a small amount of fluid leaves the cap.s -> if ven. press. inc. too much it can lead to pulmonary oedema, spread throughout lungs when lying down or just the base when standing

Question 4

relationship between coronary blood flow and myocardial oxygen demand

Answer 4

coronary circulation must be able to meet inc. demand as cardiac work can inc. five-fold

almost linear relatioship until very high demand leads to flow starting to level off

Question 5

features of coronary circulation

Answer 5

r/ and l/ coronary arteries arise from r/ and l/ aortic sinuses

these fill in diastole as in systole the press. is too high for filling

Question 6

consequences of partial/total occlusion of coronary arteries

Answer 6

partial: angina when exercising as inc. HR -> dec. diastole time -> less filling of coronary arteries
total: MI

Question 7

factors influencing blood flow in the brain

Answer 7

secure steady blood flow necessary as neurones are v. sensitive to hypoxia -> syncope after few sec.s and irreversible damage in ~ 4 mins

This is ensured: structurally (anastomoses between basilar and internal carotid arteries) and functionally:

• myogenic autoregulation: hypotension -> vasoconstriction and vice versa in hypertension, maintains cerebral flow when BP alters
• brainstem regulates other circulations as it contains the M.O.
• metabolic regulation: inc. PCO₂ (hypercapnia) -> vasodilation and vice versa - panic hyperventilation leads to hypocapnia and vasoconstriction -> dizziness and falling
• regional activity: inc. neuronal activity inc. blood flow which shows on fMRI, adenosine in particular is a powerful vasodilator
• cushing’s reflex: inc. in intracranial press (eg tumour, haemorrhage) leads to inc. cerebral blood flow -> inc. SyNS vasomotor activity -> inc. arterial BP and helps to maintain cerebral blood flow

Question 8

factors that influence blood flow through skin

Answer 8

most blood flow to skin is not nutritive and much is through arteriovenous anastomoses (AVAs) present in apical/acral skin w/ high SA:Vol. eg hands, feet, nose, ears

eg core temp. inc -> dec. SyNS innervation -> dec. vasomotor drive to AVAs -> dilation -> blood flow and heat loss

Question 9

factors that influence blood flow through skeletal muscle

Answer 9

inc. in flow mainly brought about by opening up more cap.s under the influence of vasodilator nervous activity and local metabolites -> these dec. SyNS activity and dec. vasoconstriction

flow can be inc. > 20x in active muscle, at rest about half of cap.s are shut off by pre cap. sphincters