Special Circulations Flashcards
What are the approximate blood pressures in the heart, pulmonary artery, and aorta?
Right atrium: 0-8mmHg
Right ventricle: 15-30mHg/0-8mmHg
Pulmonary artery: 15-30mmHg/4-12mmHg (elastic recoil)
Left atrium: 1-10mmHg
Left ventricle: 100-140mmHg/1-10mmHg
Aorta: 100-140mmHg/60-90mmHg (elastic recoil)
Describe the blood supply to the lungs.
Bronchial circulation: part of systemic circulation; meets metabolic requirements of the lungs
Pulmonary circulation: blood supply to the alveoli; required for gas exchange (must accept entire cardiac output)
What are the features of the pulmonary circulation? What adaptations promote efficient gas exchange?
LOW PRESSURE, LOW RESISTANCE
Short, wide vessels; lots of capillaries; little smooth muscle in arterioles
- high density of capillaries in alveolar wall (high s.a.)
- short diffusion distance (thin alveolar wall)
- optimal ventilation:perfusion (blood diverted from alveoli which are not well perfused e.g. mucus plug, diseased)
What is the difference between ventilation and perfusion?
VENTILATION = amount of air getting to the alveoli
PERFUSION = amount of blood getting to the alveoli/lungs
What is the effect of acute and chronic hypoxic pulmonary vasoconstriction?
Acute: optimises gas exchange by maintaining the ventilation:perfusion. Vasoconstriction of pulmonary vessels to poorly ventilated alveoli.
Chronic (e.g. altitude, lung disease): increase in vascular resistance —> chronic pulmonary hypertension —> high afterload on right ventricle —-> right-sided heart failure
How does the hydrostatic pressure in the vessels of the lung vary in the orthostatic position?
Apex: vessels collapse during diastole (low hydrostatic pressure)
Heart level: vessels continuously patent
Base: vessels distended by gravity (high hydrostatic pressure)
What is the effect of exercise on pulmonary blood flow?
Increased cardiac output —> small increase in pulmonary arterial pressure —> increase O2 uptake in lungs —> increased blood flow —–> reduced capillary transit time
What does an increase in capillary pressure cause?
Oedema
e.g. due to mitral valve stenosis, left ventricular failure
remember: capillary hydrostatic pressure is influenced more by the venous pressure in the systemic circulation
& fluid tends to move out at arterial end and in at venous end
What are the features of the cerebral circulation? What adaptations promote efficient gas exchange?
Circle of Willis: anastomoses between basilar & internal carotid arteries
O2 supply: cut off for 4s - unconsciousness; 4min+ - irreversible damage
- high capillary density (large s.a. & low diffusion distance)
- high basal flow rate (x10 than rest of body)
- high O2 extraction
Brainstem regulates other circulations:
Myogenic autoregulation = maintains cerebral perfusion during hypotension by changing blood vessel resistance (>50mmHg)
Metabolic factors: panic hyperventilation (hypocapnia -> vasoconstriction -> dizziness/fainting)
What factors indicate high metabolic activity and cause vasodilation?
Hypercapnia High potassium Adenosine Hypoxia Metabolic acidosis Inorganic phosphates High osmolarity
What is Cushing’s reflex?
Impaired blood flow to vasomotor control region of brainstem increases sympathetic vasomotor activity to help maintain cerebral blood flow
e.g. due to increased intracranial pressure (tumour, haemorrhage)
(low HR & irregular heart beat)
What are the features of the coronary circulation? What adaptations promote efficient gas exchange?
Right & left coronary arteries arise from aortic sinuses and fill mainly in diastole
note: coronary arteries are functional end arteries and have few arterio-anastomoses (prone to atheromas)
- high capillary density (higher than skeletal muscle)
- short diffusion distance
- continuous production of NO by coronary endothelium maintains a high basal flow)
What are the features of the skeletal muscle circulation? What adaptations promote efficient gas exchange?
Baroreceptor reflex maintains blood pressure by causing sympathetic vasoconstriction
- postural muscles have a high capillary density
- high vascular tone permits lots of vasodilatation
- at rest only 50% of capillaries are perfused; allows for increased recruitment of capillaries with exercise
What are the features of the cutaneous circulation? What adaptations promote efficient gas exchange?
Apical skin has arterovenous anastomoses which are under neural control (sympathetic vasoconstrictor fibres) (NOT regulated by local metabolites)
e.g. reduction in core temperature -> increased sympathetic tone in AVAs -> reduced blood flow to apical skin
increase in core temperature -> reduced sympathetic tone in AVAs -> low resistance shunt to venous plexus & increased blood flow to apical and non-apical skin -> increase in dissipating heat
note: in non-apical skin ACh is used to cause vasodilatation
Why is the pressure of the left atrium normally higher than the pressure of the right atrium? Which part of the circulation therefore has the largest pressure drop?
Low resistance in pulmonary circulation, therefore less pressure drop when blood enters the left atrium (similar blood pressure in veins as in left atrium)
Systemic arterioles have the largest pressure drop (why?)
