Under Pressure!
Different sorts of pressure!
• The key concept in the Transport exam
will be
• PRESSURE
• The SI unit of pressure = KPa
• ( although medical profession still use
mmHg)
Distinguishing the types of
pressure you need to know:
Systolic P.
Diastolic P.
Blood P.
Partial P.
PRESSURE
Hydrostatic P.
Osmotic P.
Systolic pressure
• The pressure generated when the
VENTRICLES contract.
• Systolic pressure forces blood out of the
ventricles and into the arteries.
• The elastic walls of the arteries stretch,
then recoil.
• Can be felt as a ‘pulse’.
• N.B. arteries themselves do not ‘contract’.
Diastolic Pressure
• The ‘resting’ pressure
measured when the
heart muscles are
relaxed.
• During this time the
heart chambers are
refilling.
• Raised diastolic
pressures are
symptomatic of
‘hypertension’ – why?
Blood pressure
• The wave of pressure moving along an
artery causes the walls to expand.
• A ‘pulse’ is felt.
• Collagen and elastic fibres in the walls
then recoil and help to maintain the
pressure.
• However, some energy is invariably lost
and BP progressively falls as it nears the
capillaries. (see text book diagram)
Common misconceptions:
• ‘the arteries contract to create a pulse’
• (they don’t: they merely respond passively
to the passing surge of pressure,
generated by the heart, then recoil.)
• ‘capillaries have low pressure because
they are further from the heart.’
• (no: pressure is lost as the artery walls are
stretched, then recoil again.)
Partial Pressure
• Important in questions concerning oxyhaemoglobin dissociation curves.
• Eg. Bohr effect, altitude, myoglobin, foetal
haemoglobin, exercising tissues etc.
• Always express answers using ‘pO2’,
pCO2’ etc, not ‘amount of gas’, ‘less
oxygen’, ‘more CO2’ or other vague term.
Partial Pressure and altitude
• ‘High’ altitude (above 4000m)
• Lower pO2 levels (KPa)
• = fewer oxygen molecules per volume of
gas breathed in.
• Leads to hypoxia (reduced pO2 in blood)
Typical vague comment: ‘there is less
oxygen at altitude…’ – what should it be??
Hydrostatic Pressure
• The pressure in a fluid.
• Eg in a mammal, due to systolic
contraction of the heart.
• HP forces plasma out from the blood to
become tissue fluid around cells.
• High HP results in swelling and oedema.
• N.B. Lymph vessels need to return excess
tissue fluid.
Osmotic pressure
• Pressure due to the presence of solutes.
• Eg in a mammal, this would be plasma
proteins that exert an osmotic ‘pull’
towards them (OP).
• If the blood is low in plasma proteins ( eg
starvation) then there is less tendency for
water to diffuse back from tissues – so
they swell.
Net Filtration
• At the arterial end of a capillary:
• Blood pressure is relatively high…
• HP > OP therefore fluid is filtered out…
• At the venous end of a capillary:
• Blood pressure has dropped…
• HP < OP therefore fluid is returned to the
blood.
HP, OP and oedema
• Explain why a PEM disease like
Kwashiorkor results in oedema of the
tissues.