Gas is related to Oxygen. Oxygen is related to Blood. Blood is related to Pressure. Pressure is a type of Force. 
Opposite is a synonym of Reverse. Reverse is related to Divide. Divide is related to Separation. Withdrawal is a type of Separation. 
Gas <--RelatedTo--> Oxygen <--RelatedTo--> Blood <--RelatedTo--> Pressure --IsA--> Force 
Opposite <--Synonym--> Reverse <--RelatedTo--> Divide <--RelatedTo--> Separation <--IsA-- Withdrawal 