Billie-Jo Jenkins's upper airways could explain why her blood was found on her foster father's clothes, the Old Bailey heard.

Blood gathered in the back of her mouth after she was attacked could have been expelled as Sion Jenkins moved her, the jury heard yesterday.

Professor Alan Aitkenhead, of the University of Nottingham, was asked what effect pulling back her shoulder would have on an upper airway obstruction.

He said: "Movement in that way could have two effects which might be of relevance. First is that movement may produce changes in the geometry of the chest and chest wall. That may result in changes in pressure in the lungs if the airway is obstructed.

"Secondly, movement by pushing the shoulder is likely to bring a change in the position of the head in relation to the rest of the body.

"Any change in the position of the head may alter the degree of obstruction to the airways in the back of the throat."

Asked by defence counsel Christopher Sallon QC what effect would occur through a combination of airway pressure and body movement, Professor Aitkenhead said: "It would have caused a release of gas under pressure".

The prosecution claim that Jenkins, 47, former deputy headteacher at William Parker school in Parkstone Road, Hastings, battered 13-year-old Billie-Jo to death with a metal tent peg.

It is alleged he lost control as Billie-Jo painted patio doors at the family home in Lower Park Road, Hastings, on February 15 1997.

Bits of Billie-Jo's flesh in some blood spots found on Jenkins's clothes linked him with the killing, according to the Crown.

However the defence insist that the blood spots were a result of him tending her in her dying moments.

Yesterday a second defence witness Professor Peter Morgan undermined the prosecution's flesh claim by saying they were merely pieces of skin.

Discussion took place about the bizarre discovery of black bin liner stuffed deeply into Billie-Jo's left nostril. Professor Morgan thought it unlikely it could have been breathed up her nostril and said it must have been wedged in with an implement or a finger.

He said the damaged skin could have been forced into the nasal passage and come out of either nostril.

Professor Morgan also said it was not possible to determine whether the bits of skin came from the same place.

Jenkins denies murder. The case continues.