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 ‘pO 2 ’, pCO 2 ’ etc, not ‘amount of gas’, ‘less oxygen’, ‘more CO 2 ’ or other vague term.

Partial Pressure and altitude ‘High’ altitude (above 4000m) Lower pO 2 levels (KPa) = fewer oxygen molecules per volume of gas breathed in. Leads to hypoxia (reduced pO 2 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.