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:

PRESSURE

Systolic P.

Diastolic P.

Blood P.

Partial P.

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 oxy-haemoglobin 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.