This came up in  Question 28.1 from the first paper of 2011. The anatomy of the pulmonary artery catheter  is discussed in greater detail elsewhere. This is a brief summary, of which the most important aspect is really this diagram:

When handling one of these things in a viva, one may be expected to mention some of the safety features.

Safety features of the PA catheter include the following:

• The syringe plunger is limited to a maximum volume of 1.5ml
• The syringe port can be locked in the closed position, preventing accidental wedging
• The transparent sliding lock device allows the catheter depth to be manipulated without increased risk of infection
• 10cm interval markings on the catheter allow depth of insertion to be reliably documented
• The catheter is made of PVC, which softens at body temperature, reducing the risk of injury to intracardiac structures
• Lumens have standard colours across manufacturers, to prevent confusion:
  • Red = balloon
  • Yellow = distal PA
  • White = RA (31cm)
  • Blue = RA (30cm)